I have been looking at the "Tire Placard" AKA Vehicle Certification Label for a number of 5th wheel trailers. I learned a few things.
A couple of manufacturers have upgraded the tires on the newer units. Some went from ST235/85R16 LR-E to ST235/85R16 LR-F. The inflation went from 80 to 110 so they are clearly offering improved load capacity or giving you a better margin above the tire capacity.
If you have an older 5th wheel and have either ST235/80R16 LR-E or ST235/85R16 LR-E you might want to check and confirm if your wheels are rated for 110 psi. If so, you might consider moving on up to LR-F and doing the slight up-size if you have the needed clearance as the 85 series tire is a bit larger in OD.
I also found one model that went from 16" to 17.5" wheels. It appears that the new RVIA 10% load margin requirement achieved some better than expected results. While this may not help out those with older trailers just knowing there are wheel & tire options available that are approved by your RV company could be considered good news.
On a related matter, I wonder how many of you have ever read the section on Tires in your owner's manual. I was surprised to see that some manuals have pretty good and detailed info. They cover stuff like weighing each side of the RV and not just the total on the 4 or 6 tires. While this isn't as good as learning the individual load on each tire a side to side comparison may identify some load imbalance. Since I suggest that trailers run the tire sidewall inflation we are not looking at calculating a minimum inflation. BUT we still want to be sure you have at least a 15% margin on load, so you should use the heavier side of the RV when doing your"Safety margin" calculations.
I also found some clear info on "cold inflation" and operation speed which was better than I expected. Tire life and expected replacement age were not uniform brand to brand but some of you may be surprised to see that some manufacturers actually tell you the number of years use you should expect from your tires.
Hope to meet a few of you at FMCA GLAMARAMA in Michigan in a couple weeks or in Gillette, Wy in July. Safe travels.
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Showing posts with label Trailers. Show all posts
Showing posts with label Trailers. Show all posts
Friday, May 25, 2018
Thursday, February 22, 2018
Tire inflation not the same for all trailers
I have written a number of times on the topic of Interply Shear. This is the force that is tring to tear the belts of radial tires apart.
There are some highly technical papers on the topic and you can review tham after a simple Google search on the term. You can also look here on my blog for the posts where the term is tied to a post by simply checking the list of topics on the left side of my blog. Basically this force comes about when the belts in a radial tire are forced to change shape. This means either when a tire rolls and the footprint changes from curved to flat as the footprint rolls into contact with the ground. This force increases when external forces from cornering are also applied to tires.
So why do trailers seem to have such high Interply Shear forces? Well it's not all trailers as the cornering forces of single axle trailers are much lower than the forces of tandem or triple axle trailers.
A short time later I saw a video from Keystone RV Company of wheel lug nut torque (you should watch the full video some time)
While watching their video I recall some special high side load tests we ran in our tire test lab.
Here is a short out-take from the longer Keystone video that shows what happens to multi-axle trailer tires.
In the video you can see how the two tires are fighting each other with one bending in as the other bends out. You can imagine that if the turn is made on gravel at some point the high sideload would result is a sudden breakaway or slip. That's what I observed on the gravel turn.
Back at work I had some high powered Finite Element computer programs run to simulate the side load of a multi axle trailer and the results showed that the side loads on a trailer could be 24% higher than on a standard vehicle even with identical radius and loads on the tires.
This post shows why tires get different side loading.
Further analysis showed that increasing the tire inflation could lower the extra shear, but sadly not eliminate it.
Bottom line
In a number of posts I have recommended that Motorhomes set their inflation based on measured static load plus a margin of at least 10% additional PSI. This would also apply to single axle trailers. BUT for Tandem and triple axle trailers I strongly recommend that the tire cold inflation be set to the inflation molded on the tire sidewall associated with the tire maximum load capacity. I also recommend that the measured static tire loads on these trailers be no greater than 85% of the tire maximum with a 20% margin being better.
There are some highly technical papers on the topic and you can review tham after a simple Google search on the term. You can also look here on my blog for the posts where the term is tied to a post by simply checking the list of topics on the left side of my blog. Basically this force comes about when the belts in a radial tire are forced to change shape. This means either when a tire rolls and the footprint changes from curved to flat as the footprint rolls into contact with the ground. This force increases when external forces from cornering are also applied to tires.
So why do trailers seem to have such high Interply Shear forces? Well it's not all trailers as the cornering forces of single axle trailers are much lower than the forces of tandem or triple axle trailers.
What tipped me off to this was an observation at a campground that happened to have a freshly smother gravel driveway and a multi-axle trailer happened to make a 180° turn as I was walking by. I noticed that the gravel marks were not a smoth curve but there was a series of turns interupted by discontinuities.
A short time later I saw a video from Keystone RV Company of wheel lug nut torque (you should watch the full video some time)
While watching their video I recall some special high side load tests we ran in our tire test lab.
Here is a short out-take from the longer Keystone video that shows what happens to multi-axle trailer tires.
In the video you can see how the two tires are fighting each other with one bending in as the other bends out. You can imagine that if the turn is made on gravel at some point the high sideload would result is a sudden breakaway or slip. That's what I observed on the gravel turn.
Back at work I had some high powered Finite Element computer programs run to simulate the side load of a multi axle trailer and the results showed that the side loads on a trailer could be 24% higher than on a standard vehicle even with identical radius and loads on the tires.
This post shows why tires get different side loading.
Further analysis showed that increasing the tire inflation could lower the extra shear, but sadly not eliminate it.
Bottom line
In a number of posts I have recommended that Motorhomes set their inflation based on measured static load plus a margin of at least 10% additional PSI. This would also apply to single axle trailers. BUT for Tandem and triple axle trailers I strongly recommend that the tire cold inflation be set to the inflation molded on the tire sidewall associated with the tire maximum load capacity. I also recommend that the measured static tire loads on these trailers be no greater than 85% of the tire maximum with a 20% margin being better.
Friday, February 16, 2018
Changing tires on a trailer - NEW load capacity requirement
On many of the RV forums I monitor that focus on trailer application, there is a recurring question about changing tires. Some wonder about going up in Load Range (Ply Rating) some wonder about changing the "Type" tire P > ST, or P> LT, or ST > LT. Others want to change the tire dimensions. While there are many reply posts, I do note that not everyone offering answers has worked as a tire design engineer. It takes years of working with the engineering and scientific knowledge before you can be given the responsiblity to develop a new tire capable of passing various company and DOT regulations and be produced for sale in the tens or hundreds of thousands.
While I have tried to provide answers, I seem to end up saying the same things over and over so this post is intended to be a go-to post for those asking tire change questions.
First, it is important that the owner know the ORIGINAL tire size including the Type and Load Range and the recommended inflation from the RV company along with the GAWR for their specific Trailer. Finally if considering a change we need to ensure that the new tires can properly support the ACTUAL load on the new tires.
I will start off assuming the owner is keeping the tire dimensions the same i.e. 225/75R15 > 225/75R15. Note I said "dimensions" not the "size" as a tire engineer "size" to me includes the Type + Dimensions + Load Range
For P > ST or P > LT you need to remember that application of a P-type tire on trailer required that the RV company "De-rate" the load marked on the tire sidewall Sidewall/1.10 = load capacity of a P-type tire on a trailer.
For ST > LT you will probably need to increase the dimensions and or Load Range to achieve sufficient load capacity.
The general rule of thumb "Any replacement tire MUST be capable of supporting equal or greater load than the original tire".
Another rule: You need to ensure that any tire you use is capable of supporting your actual MEASURED load, Not the load your neighbor said he has and not an estimate or the measured load someone posted on a forum. The load measurement ideally should be obtained with your trailer at its heaviest i.e. fuel, water, propane, clothes etc If you can't get individual one side weights DO NOT assume a 50/50% side to side load split. While some trailers may be balanced at 49/51% but some have been found as much as 10% off balance i.e. 40/60% As a rule of thumb I suggest you use at least an assumed 47/53% splits you would use the 53% figure.
If making an investment in new size tires & wheels you really need to learn the real loads before making the change or you may discover you bought tires you should not be using.
You will need to consult the published Load & Inflation tables for your old and new tire to confirm load capacity numbers. I have THIS post with links to many different tire companies. Be sure you understand how to read the tables as while most provide load capacity per tire, some load figures are per axle. DO NOT use the "Dual" load numbers as these only apply when there are two tires mounted as a pair on each end of an axle.
Comment on valves. I always recommend that whenever changing tires, even if you are just replacing with same size and type, that rubber valves be replaced with bolt on metal valves and if you already have metal bolt in valves that you get the various rubber gaskets and "O" rings replaced as these rubber parts age out just as tires age out and it's awful to read about a $2 valve failing which can result in hundreds or thousands of dollars in damage and costs.
Finally, some new info that all RV owners should consider:
Late last year RVIA updated the tire type and load spec such that "based on the rating of the axle the tires have to be 10% greater than the axle rating," You will note that RVIA decided to ignore the reality of load unbalance.
Clearly, if you are getting new tires it makes sense to incorporate this new safety margin in your calculations.
I want to thank my fellow RV owner and tire design engineer CapriRacer for doing a bit of technical editing on this post. He also has a blog on tires.
Next week I will do a post on trailer tire inflation.
If you find this post helpful and happen to see someone posting questions about changing tires please consider posting a link to this post as I don't see every tire question posted by all RV owners.
While I have tried to provide answers, I seem to end up saying the same things over and over so this post is intended to be a go-to post for those asking tire change questions.
First, it is important that the owner know the ORIGINAL tire size including the Type and Load Range and the recommended inflation from the RV company along with the GAWR for their specific Trailer. Finally if considering a change we need to ensure that the new tires can properly support the ACTUAL load on the new tires.
I will start off assuming the owner is keeping the tire dimensions the same i.e. 225/75R15 > 225/75R15. Note I said "dimensions" not the "size" as a tire engineer "size" to me includes the Type + Dimensions + Load Range
For P > ST or P > LT you need to remember that application of a P-type tire on trailer required that the RV company "De-rate" the load marked on the tire sidewall Sidewall/1.10 = load capacity of a P-type tire on a trailer.
For ST > LT you will probably need to increase the dimensions and or Load Range to achieve sufficient load capacity.
The general rule of thumb "Any replacement tire MUST be capable of supporting equal or greater load than the original tire".
Another rule: You need to ensure that any tire you use is capable of supporting your actual MEASURED load, Not the load your neighbor said he has and not an estimate or the measured load someone posted on a forum. The load measurement ideally should be obtained with your trailer at its heaviest i.e. fuel, water, propane, clothes etc If you can't get individual one side weights DO NOT assume a 50/50% side to side load split. While some trailers may be balanced at 49/51% but some have been found as much as 10% off balance i.e. 40/60% As a rule of thumb I suggest you use at least an assumed 47/53% splits you would use the 53% figure.
If making an investment in new size tires & wheels you really need to learn the real loads before making the change or you may discover you bought tires you should not be using.
You will need to consult the published Load & Inflation tables for your old and new tire to confirm load capacity numbers. I have THIS post with links to many different tire companies. Be sure you understand how to read the tables as while most provide load capacity per tire, some load figures are per axle. DO NOT use the "Dual" load numbers as these only apply when there are two tires mounted as a pair on each end of an axle.
Comment on valves. I always recommend that whenever changing tires, even if you are just replacing with same size and type, that rubber valves be replaced with bolt on metal valves and if you already have metal bolt in valves that you get the various rubber gaskets and "O" rings replaced as these rubber parts age out just as tires age out and it's awful to read about a $2 valve failing which can result in hundreds or thousands of dollars in damage and costs.
Finally, some new info that all RV owners should consider:
Late last year RVIA updated the tire type and load spec such that "based on the rating of the axle the tires have to be 10% greater than the axle rating," You will note that RVIA decided to ignore the reality of load unbalance.
Clearly, if you are getting new tires it makes sense to incorporate this new safety margin in your calculations.
I want to thank my fellow RV owner and tire design engineer CapriRacer for doing a bit of technical editing on this post. He also has a blog on tires.
Next week I will do a post on trailer tire inflation.
If you find this post helpful and happen to see someone posting questions about changing tires please consider posting a link to this post as I don't see every tire question posted by all RV owners.
Friday, January 12, 2018
Why do trailer tires fail more often than tow vehicle tires?
Here is the question posted on a travel trailer forum.
Why so many trailer tire failures and so few, if any, tow vehicle tire failures?
This post could be considered Part 2 of my post of last week.
The primary reason IMO is staring everyone in the face and the information is molded onto the tire sidewall so you do not need to be an Einstein, but almost everyone chooses to ignore the science so they can save a few bucks.
Fact: Tire load capacity is a function of air volume (tire size) and inflation pressure. This is well established and is confirmed by every Load & Inflation chart printed. Increase size or increase pressure and you increase load.
If we all drove around on tires with LT level sizing and inflation there would be almost no failures other than those caused by road hazard or punctures.
BUT
People want softer ride, better fuel economy and lower cost so alternatives were introduced into the market.
LT tires can be considered the standard for tires designed to carry load at highway speeds.
P type tires have a higher load capacity than identical size LT tire but if you check the actual load on most car tires, you will see they have 15% to 25% or more "Reserve Load" capacity (more capacity than needed). Inflation is set for cars based not on load but ride, handling and fuel economy. You also know that P tires must have their load capacity de-rated when P tires are used in truck, SUV or trailer application. When P type tires are run with zero reserve load you will probably get more failures. (See Ford Explorer crashes.)
ST tires have a higher claimed load capacity than LT tires and TT companies use the smallest (lowest cost) tire that can, most of the time, meet the specification for load.
How then is it that ST tires are rated to carry more load?
The load formula for ST tires is based on a 65 mph Max operation speed and an expected shorter life. The formula was developed back when bias tires that wore out at less than 15,000 miles were the norm
Since 1990 when ST tires were still "Alpha" sized ( H78-15ST) the load formula has not been changes as far as I can determine.
The science indicates that if you want tire durability on your trailer as you get on your truck maybe you need to consider running the same type tires i.e. LT type and of course that means you also need to use LT Load & Inflation limits.
Here is some info I posted Sept 14 2011
"Here are some facts from the Tire & Rim Association industry standards book
P235/75R15 105S (Standard Load -35 psi @ max load)
2028# 35 psi 112mph on a Passenger car
1844# 35 psi 112mph on a SUV or P/U or trailer (no Dual)
LT235/75R15 LRC
1512#single 1377# Dual 50 psi 85mph
ST235/75R15 LRC
2340#single 2040# Dual 50 psi 65mph"
NOTE I did not mention the actual load on a tire but only the specified max. Weight data indicates many TT have one or more tire in overload.
Why so many trailer tire failures and so few, if any, tow vehicle tire failures?
This post could be considered Part 2 of my post of last week.
The primary reason IMO is staring everyone in the face and the information is molded onto the tire sidewall so you do not need to be an Einstein, but almost everyone chooses to ignore the science so they can save a few bucks.
Fact: Tire load capacity is a function of air volume (tire size) and inflation pressure. This is well established and is confirmed by every Load & Inflation chart printed. Increase size or increase pressure and you increase load.
If we all drove around on tires with LT level sizing and inflation there would be almost no failures other than those caused by road hazard or punctures.
BUT
People want softer ride, better fuel economy and lower cost so alternatives were introduced into the market.
LT tires can be considered the standard for tires designed to carry load at highway speeds.
P type tires have a higher load capacity than identical size LT tire but if you check the actual load on most car tires, you will see they have 15% to 25% or more "Reserve Load" capacity (more capacity than needed). Inflation is set for cars based not on load but ride, handling and fuel economy. You also know that P tires must have their load capacity de-rated when P tires are used in truck, SUV or trailer application. When P type tires are run with zero reserve load you will probably get more failures. (See Ford Explorer crashes.)
ST tires have a higher claimed load capacity than LT tires and TT companies use the smallest (lowest cost) tire that can, most of the time, meet the specification for load.
How then is it that ST tires are rated to carry more load?
The load formula for ST tires is based on a 65 mph Max operation speed and an expected shorter life. The formula was developed back when bias tires that wore out at less than 15,000 miles were the norm
Since 1990 when ST tires were still "Alpha" sized ( H78-15ST) the load formula has not been changes as far as I can determine.
The science indicates that if you want tire durability on your trailer as you get on your truck maybe you need to consider running the same type tires i.e. LT type and of course that means you also need to use LT Load & Inflation limits.
Here is some info I posted Sept 14 2011
"Here are some facts from the Tire & Rim Association industry standards book
P235/75R15 105S (Standard Load -35 psi @ max load)
2028# 35 psi 112mph on a Passenger car
1844# 35 psi 112mph on a SUV or P/U or trailer (no Dual)
LT235/75R15 LRC
1512#single 1377# Dual 50 psi 85mph
ST235/75R15 LRC
2340#single 2040# Dual 50 psi 65mph"
NOTE I did not mention the actual load on a tire but only the specified max. Weight data indicates many TT have one or more tire in overload.
Tuesday, February 28, 2017
Question on tread scrubbing on trailer tires.
Got this question:
"I have a question regarding interply shear or scrubbing common on double or triple axle fifth wheels. I have a double axle and try to avoid sharp turns and U-turns as much as possible, but I see visible signs of scrubbing on the tread of my tires. It looks like flat spots on the edge of the tires. After a turn, I can go back and see the rubber I've scrubbed and left on the road. It's frustrating. I have the axles aligned and tires balanced once a year and have individual wheels weighed occasionally. I try to keep side-to-side and front-to-back weights within a couple hundred pounds. I currently carry 85%-90% of load capacity, or about 3300-3400 lbs per tire.
"My question is: Would going to the next load range up (from G to H) or going to a harder compound tire reduce this problem? I currently use Goodyear G614RST tires, size LT235/85R16. I've heard in the past that Michelin uses a harder compound in their tires, which makes for a little stiffer ride, but might this overcome some of the effects of scrubbing? I feel I could get a lot more miles out of my RV tires if I could reduce the flat spots or sculpting caused by unavoidable scrubbing."
My answer:
Axle alignment or wheel balance isn't the problem. There is a sketch in this post on interply shear that shows why the tire tread scrubs. The center of tire rotation is not pointed to the center of the turn radius so the tires are always being dragged around every turn. It is just worse on tight turns.
Lowering the percentage of max load capacity is a good idea. Don't forget that it is the air pressure that determines the load capacity not the Load Range (G to H). You will gain nothing from a Load Range change if you do not also increase the air pressure. You do need to confirm the wheel max psi capacity which, for some wheels, is not easy to do as some wheel manufacturers do not have high pressure ratings easily available.
Regarding tread compound: Sometimes it isn't just the hardness of the rubber but also the tread pattern that can affect scrubbing wear.
In general, the tread scrub is a function of dragging a trailer around.
Send your questions to me, Roger, at Tireman9 (at) gmail.com
Subscribe to the weekly RVtravel.com newsletter. News, advice and information for RVers since 2001. Learn more or subscribe.
##RVT783
"I have a question regarding interply shear or scrubbing common on double or triple axle fifth wheels. I have a double axle and try to avoid sharp turns and U-turns as much as possible, but I see visible signs of scrubbing on the tread of my tires. It looks like flat spots on the edge of the tires. After a turn, I can go back and see the rubber I've scrubbed and left on the road. It's frustrating. I have the axles aligned and tires balanced once a year and have individual wheels weighed occasionally. I try to keep side-to-side and front-to-back weights within a couple hundred pounds. I currently carry 85%-90% of load capacity, or about 3300-3400 lbs per tire.
"My question is: Would going to the next load range up (from G to H) or going to a harder compound tire reduce this problem? I currently use Goodyear G614RST tires, size LT235/85R16. I've heard in the past that Michelin uses a harder compound in their tires, which makes for a little stiffer ride, but might this overcome some of the effects of scrubbing? I feel I could get a lot more miles out of my RV tires if I could reduce the flat spots or sculpting caused by unavoidable scrubbing."
My answer:
Axle alignment or wheel balance isn't the problem. There is a sketch in this post on interply shear that shows why the tire tread scrubs. The center of tire rotation is not pointed to the center of the turn radius so the tires are always being dragged around every turn. It is just worse on tight turns.
Lowering the percentage of max load capacity is a good idea. Don't forget that it is the air pressure that determines the load capacity not the Load Range (G to H). You will gain nothing from a Load Range change if you do not also increase the air pressure. You do need to confirm the wheel max psi capacity which, for some wheels, is not easy to do as some wheel manufacturers do not have high pressure ratings easily available.
Regarding tread compound: Sometimes it isn't just the hardness of the rubber but also the tread pattern that can affect scrubbing wear.
In general, the tread scrub is a function of dragging a trailer around.
Send your questions to me, Roger, at Tireman9 (at) gmail.com
Subscribe to the weekly RVtravel.com newsletter. News, advice and information for RVers since 2001. Learn more or subscribe.
##RVT783
Thursday, February 23, 2017
Q&A on 5th wheel trailer and tow vehicle, and an announcement
Dear Roger,
"I pull a fifth wheel trailer. When I am towing, I inflate all tires on the truck and fifth wheel to their max load pressure. The truck tires are load E, Max psi is 80. The fifth wheel tires are load G, full steel belted with max pressure 110psi. Weighing the rig at Cat scales shows me that the weights on the axles are not heavy enough to warrant using the maximum pressures. I would like to inflate the tires so that the psi matches the actual load. Is there a way to calculate this? I have scoured the internet and cannot seem to find any tables that would give me a clear answer. Thank you for your advice."
My answer:
Hi. There is much information in this blog that can answer your question and provide good background. But I'll provide some info here so you can get on the road with proper inflation. You can then review the info on the blog as time permits.
First we need to separate the tow vehicle from the trailer as the tires are different and the forces on the tires are also different.
Trailer. Due to the fact that trailer tires are basically being dragged around corners rather than steered, there are forces called "Interply Shear" that place much higher forces on the belts of the radial tires when in trailer application as opposed to when on a motorized vehicle such as a motorhome or tow vehicle. You did not offer the actual measured loads on the trailer tires or the tire size so I can only offer general guides. Be sure the load capacity of the tires at the inflation you are running is at least 10% greater than the measured load with 15% better. Were the LR-G tires the OE size and load range or did you upgrade to larger or higher?
Tow vehicle:
If you are running the OE size and load range your owners manual should give recommended minimum inflation levels for a loaded vehicle. Many pickup trucks offer "full load and light load" condition inflation recommendations. I recently (Feb. 15) did a post on tow vehicle tire inflation. I suggest you review it and let me know if you still have questions.
You also asked about load and inflation tables. Here are 5 posts on that topic.
* * *
Now, some of you may feel that a more detailed reply would have been in order, and I agree. However, it's very difficult to provide a more specific answer without more information. This brings me to the "Announcement" portion of this post.
With almost 260 posts in this blog, I've covered most features and topics but after reviewing the posts some people still have questions. So I'll do my best to try and provide specific answers. There are a few specifics that I will need in order to provide a useful reply.
With thousands of possible tire/vehicle combinations there is no way I can know the specifics of them all. So I will need your help. When you send a question please provide as many of the following details as possible:
•Tire size, including the letters in front of the numbers if any.
•Load range or the tire max inflation.
•Measured load on each tire position or each axle. If scale weights are not available then GAWR from the tire sticker/placard
•Specific application, i.e., trailer, motorhome, car, pickup
•Tire DOT if the question is about tire age or country of origin
Send your questions to me, Roger, at Tireman9(at)gmail.com
Subscribe to the weekly RVtravel.com newsletter. News, advice and information for RVers since 2001. Learn more or subscribe.
"I pull a fifth wheel trailer. When I am towing, I inflate all tires on the truck and fifth wheel to their max load pressure. The truck tires are load E, Max psi is 80. The fifth wheel tires are load G, full steel belted with max pressure 110psi. Weighing the rig at Cat scales shows me that the weights on the axles are not heavy enough to warrant using the maximum pressures. I would like to inflate the tires so that the psi matches the actual load. Is there a way to calculate this? I have scoured the internet and cannot seem to find any tables that would give me a clear answer. Thank you for your advice."
My answer:
Hi. There is much information in this blog that can answer your question and provide good background. But I'll provide some info here so you can get on the road with proper inflation. You can then review the info on the blog as time permits.
First we need to separate the tow vehicle from the trailer as the tires are different and the forces on the tires are also different.
Trailer. Due to the fact that trailer tires are basically being dragged around corners rather than steered, there are forces called "Interply Shear" that place much higher forces on the belts of the radial tires when in trailer application as opposed to when on a motorized vehicle such as a motorhome or tow vehicle. You did not offer the actual measured loads on the trailer tires or the tire size so I can only offer general guides. Be sure the load capacity of the tires at the inflation you are running is at least 10% greater than the measured load with 15% better. Were the LR-G tires the OE size and load range or did you upgrade to larger or higher?
Tow vehicle:
If you are running the OE size and load range your owners manual should give recommended minimum inflation levels for a loaded vehicle. Many pickup trucks offer "full load and light load" condition inflation recommendations. I recently (Feb. 15) did a post on tow vehicle tire inflation. I suggest you review it and let me know if you still have questions.
You also asked about load and inflation tables. Here are 5 posts on that topic.
* * *
Now, some of you may feel that a more detailed reply would have been in order, and I agree. However, it's very difficult to provide a more specific answer without more information. This brings me to the "Announcement" portion of this post.
With almost 260 posts in this blog, I've covered most features and topics but after reviewing the posts some people still have questions. So I'll do my best to try and provide specific answers. There are a few specifics that I will need in order to provide a useful reply.
With thousands of possible tire/vehicle combinations there is no way I can know the specifics of them all. So I will need your help. When you send a question please provide as many of the following details as possible:
•Tire size, including the letters in front of the numbers if any.
•Load range or the tire max inflation.
•Measured load on each tire position or each axle. If scale weights are not available then GAWR from the tire sticker/placard
•Specific application, i.e., trailer, motorhome, car, pickup
•Tire DOT if the question is about tire age or country of origin
Send your questions to me, Roger, at Tireman9(at)gmail.com
Subscribe to the weekly RVtravel.com newsletter. News, advice and information for RVers since 2001. Learn more or subscribe.
Monday, March 30, 2015
When and Why some need to "De-Rate" their tires
Had a question from a TT owner about De-rating the tire load. He wanted to use P-Type tires on his trailer.
Can you explain why anyone would need
to de-rate the weight rating just because a tire is used on a trailer? I
don't understand why if the rating is 2510 lbs on one vehicle why
wouldn't it be the same on another at the same pressure?
Now if you don't like the simple answer here is a longer approximation.
When tires are designed there are certain things that can be expected. First off they are expected to pass all the regulatory tests. Some of these tests will be at or near the tire's max rated load. None of these tests are for 60,000 miles but may only last 2,000 miles at the max when running in 100F temperature on a curved drum (which is much harder than running on flat surface) and the tire is never be allowed to cool down as it is also run at say 50 mph. Outdoor testing may be at 100% load and 30,000 miles but the test car does come to a stop for 30 min or so every few hours. The test track however has no sharp turns or pot holes.
If a tire can survive the above in all probability it can run 60,000 on regular highways BUT occasionally at higher speeds and with occasional pot holes etc. We know that there is a "reserve load" of say 10% to 20% between the tire's load capacity at the placard inflation in normal car operation and there may only be 10% or less of the time the tire is asked to run anywhere near it's max load.
Now if that same tire is put on a small pick up or SUV the % of the time it may be required to run at or near it's max load may be 50 % or more.
It was learned many years ago with the introduction of station wagons that if we were to avoid premature tire failures something needed to be done. At the time there were only passenger and true truck tires so the only option was to reduce the load allowed when the passenger tires were placed in station wagon service. Later on, when SUVs were invented they replaced station wagons in the market place so the definition of "Multi-purpose" vehicles was coined and since pick up trucks were "civilized" to be more like cars and less like trucks and occasionally passenger type tires were applied to these trucks. Again the percentage of time the tire was asked to operate at its max load capability was much greater than when the same tire was placed on a car.
The "de-rating" was established at a factor of 1.10
Now in reality early RV trailers were single axle but would spend an even greater portion of the time at their max load. To counter that their service life was much less so the 1.10 factor was applied to passenger tires in trailer service.
If you look at the rated load capacity of a true LT type tire vs a P type at the same inflation you will see that the LT is rated at about 25% lower load capacity. This is because vehicles that are expected to be used as trucks ie a major part of their life fully loaded, the reality of the physics involved dictates that if you want to get 40 to 60,000 mile life you must reduce the load. This lower capacity is one reason there is no De-Rating for LT tires when placed on trailers.
It may help if you think of tire life as a formula in the form of
LIFE = 100,000 - (A x %of time at 100%load) -(B x % of time at 90% load) - (C x % of time at 80% of max load)-(D x %of time at speed greater than 50 mph)- (E x % of time at speed greater than 65 mph)- (F x number of 4" deep pot holes) etc, etc, etc.
If you think of A, B C etc as large numbers such as 50,000 or 20,000 I think you can see that it is easy to "consume" the finite tire life of 100,000.
Now in reality there is no such formula as it would have hundreds for factors, each of which is almost impossible to establish without hundreds of thousands of dollars research.
Hope this helps you understand why the De-Rating formula for P Type tires when placed in "mult-purpose or trailer service is Load Max New = (Load Max on tire)/1.10
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Sunday, March 22, 2015
Repeat failures on Airstream trailer. Why?
Saw this question of an RV forum on trailers
"We just acquired 16" Sendel wheels and a set of XPS Ribs for our 2009 28' International to help deal with our tendency to have a front left wheel blowout. We have been through a set of Goodyear Marathons and a set of Towmarks (e-rated, 80 psi cold max). I have not had access to individual scales, but I have reason to believe that the left front is carrying more weight because we have 130 lbs of extra battery forward in the coach on the left side. We will be addressing that by replacing all 260 lbs of battery with 84 lbs of lithiums. This will result in an overall tongue weight reduction of close to 100+, but most importantly, it will take at least 50 pounds off the left front, too.
The question is proper inflation. Our Airstream weighs 5960 on the axles when the WD hitch is engaged. Our tire dealer divided by four, which of course, is not accurate, as we can guess that there is much more weight on that particular wheel. But I'm getting input to inflate to less than 80psi when checking on a cool or cold early morning based on the reasoning that the 80psi recommendation is for cold pressure at around 68F. The theory is that as the ambient pressure rises, that same air will increase in pressure, with the coach just sitting there. Therefore, if one inflated to the max 80psi at 68F, that would be way over a "cold" tire pressure taken later in the day when the temp hits 100F. This is what they say all there 18 wheeler customers do when running on a day that they know will be considerably hotter later--and I've had the same advice from three different tire dealers.
Would our resident tire expert care to "weigh in" on this subject? It would be most appreciated.
"We just acquired 16" Sendel wheels and a set of XPS Ribs for our 2009 28' International to help deal with our tendency to have a front left wheel blowout. We have been through a set of Goodyear Marathons and a set of Towmarks (e-rated, 80 psi cold max). I have not had access to individual scales, but I have reason to believe that the left front is carrying more weight because we have 130 lbs of extra battery forward in the coach on the left side. We will be addressing that by replacing all 260 lbs of battery with 84 lbs of lithiums. This will result in an overall tongue weight reduction of close to 100+, but most importantly, it will take at least 50 pounds off the left front, too.
The question is proper inflation. Our Airstream weighs 5960 on the axles when the WD hitch is engaged. Our tire dealer divided by four, which of course, is not accurate, as we can guess that there is much more weight on that particular wheel. But I'm getting input to inflate to less than 80psi when checking on a cool or cold early morning based on the reasoning that the 80psi recommendation is for cold pressure at around 68F. The theory is that as the ambient pressure rises, that same air will increase in pressure, with the coach just sitting there. Therefore, if one inflated to the max 80psi at 68F, that would be way over a "cold" tire pressure taken later in the day when the temp hits 100F. This is what they say all there 18 wheeler customers do when running on a day that they know will be considerably hotter later--and I've had the same advice from three different tire dealers.
Would our resident tire expert care to "weigh in" on this subject? It would be most appreciated.
You are correct that simply dividing by 4 does not give you the actual load on each tire.
The worksheet on THIS page will show you how to do the math.
I have a blog post with an example how to get your weights for free. Some folks have had success in contacting their State Police and asking if they could go to a location where they are checking trucks.
But setting aside the need to confirm your load balance.
I suggest that to lower the internal structural forces that are trying to tear the belts apart, you need to set your "cold" inflation to the max on the tire sidewall. The "why" of this was covered in this post. My position was developed based on the work done by Turner & Ford as seen in the book "Mechanics of Pneumatic tires" pub by U.S. Department of Transportation, National Highway Traffic Safety Administration, 1981 .
Now to your specific question on how to set the "cold" pressure.
I covered pressure vs temperature in THIS post and when you review that post you will see that the pressure in a tire will change by about 2% for each 10°F change so in tour example of 68 vs 100F I would expect about 6% change. The intention of the "Set the pressure in the morning" is to not use a warm (higher than ambient) tire pressure when it has been in the Sun.
I think the reality is that if you check your tires in early AM before you start your travels and before the tire is in full Sunlight, you will be fine for the rest of that day, no matter what the temperature is over the rest of the day.
Tires generate heat internally (not on their surface as many think). This heat is transferred to the moving air around the tire and wheel until a steady state is reached where the heat being generated equals the heat flowing away from the tire.
Hope this helps
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Monday, June 23, 2014
Can I change from ST to LT tires on my Trailer or 5ver?
This question is a real "Hot Button" on a number of RV forums and blogs. People ask this question because they want a better or alternate selection of brands or they want to improve the durability of their tires.
The answers given seem to range from "Sure, Why not" to "Absolutely Not, Never do it" and some will even offer that they think you are breaking some law if you make any change from the type, size, Load Range or cold inflation from the OE tire information shown on the tire Placard.
As we all should know by now answers about tires are never simple and straight forward and changing tires is definitely one of the more involved answers.
First off, I am not a lawyer but an Engineer and as such I form opinions based on data and facts. So here is the answer based on my 40 years experience as a Tire Design Engineer.
there are some things you MUST do to ensure that any change you make will actually improve your probability of having better tire durability.
My plan is to provide an outline of the steps you need to take before you make any change. I will try and include each step and each of the points of data you need to collect and evaluate. If you skip a step you may end up with a less durable tire selection which could lead to tire failure, RV damage and even an accident.
Before we start you need to consider that the most conservative approach is to make no change and to simply use the tire Construction (Bias or Radial), Type (ST or LT) and size and Load Range and inflation as specified on the placard and specification documents. This represents the RV manufacturers recommendation based on a number of assumptions as well as some legal regulations the RV MFG must follow plus in many or some cases a desire on the Mfg part to keep their costs as low as possible.
So if you still want to move forward here are the steps you need to take:
1. You need to know the actual load on each tire. This is important because A. we will be basing some decisions on the tire loading and B. It is possible that there is a significant unbalance in the tire loading which may be the cause of poor tire durability. With sufficient unbalance it may be impossible to provide a tire selection that would lower the probability of having problems.
To learn the actual individual tire loads you need to either find a company such as RVSEF or agency that has individual scales or to follow the steps outlined on worksheets such as This one or This one. I have hears some people say that they have been able to get the individual tire loads from their state police or state DOT.
2. Knowing the ACTUAL LOAD on each tire you need to confirm that no single tire is loaded more than the max load molded on the tire sidewall. This is an absolute rule. If any tire is overloaded you should not move the trailer until you either change the load or change the tire.
3. Assuming no tire is overloaded we want to make sure that all tires on en axle are inflated to the same inflation. For multi-axle trailers you can lower the internal tire structural shear forces (the forces trying to tear the tire apart) by running the inflation molded on the tire sidewall. Sometimes this is stated as the Max PSI and other times it is stated as the PSI for the max load. For our purposes we will consider this the proper cold inflation you should always run.
4. We should have "Headroom" or "Reserve Load" or "Safety Margin" on the tire loading. I suggest 15%. However I know that for many trailers 15% extra capacity above the actual load is very hard to do. Especially since some RV MFG manage to so under-size the tires that even when the trailer is empty they may not have 15% margin.
5. If you don't have at least a 10% margin I would strongly suggest you need to consider changing tires to something with more load capacity when inflated tot he sidewall PSI.
OK so you have some homework to do. We will continue this in the next post.
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The answers given seem to range from "Sure, Why not" to "Absolutely Not, Never do it" and some will even offer that they think you are breaking some law if you make any change from the type, size, Load Range or cold inflation from the OE tire information shown on the tire Placard.
As we all should know by now answers about tires are never simple and straight forward and changing tires is definitely one of the more involved answers.
First off, I am not a lawyer but an Engineer and as such I form opinions based on data and facts. So here is the answer based on my 40 years experience as a Tire Design Engineer.
Yes you may be able to change tires
there are some things you MUST do to ensure that any change you make will actually improve your probability of having better tire durability.
My plan is to provide an outline of the steps you need to take before you make any change. I will try and include each step and each of the points of data you need to collect and evaluate. If you skip a step you may end up with a less durable tire selection which could lead to tire failure, RV damage and even an accident.
Before we start you need to consider that the most conservative approach is to make no change and to simply use the tire Construction (Bias or Radial), Type (ST or LT) and size and Load Range and inflation as specified on the placard and specification documents. This represents the RV manufacturers recommendation based on a number of assumptions as well as some legal regulations the RV MFG must follow plus in many or some cases a desire on the Mfg part to keep their costs as low as possible.
So if you still want to move forward here are the steps you need to take:
1. You need to know the actual load on each tire. This is important because A. we will be basing some decisions on the tire loading and B. It is possible that there is a significant unbalance in the tire loading which may be the cause of poor tire durability. With sufficient unbalance it may be impossible to provide a tire selection that would lower the probability of having problems.
To learn the actual individual tire loads you need to either find a company such as RVSEF or agency that has individual scales or to follow the steps outlined on worksheets such as This one or This one. I have hears some people say that they have been able to get the individual tire loads from their state police or state DOT.
2. Knowing the ACTUAL LOAD on each tire you need to confirm that no single tire is loaded more than the max load molded on the tire sidewall. This is an absolute rule. If any tire is overloaded you should not move the trailer until you either change the load or change the tire.
3. Assuming no tire is overloaded we want to make sure that all tires on en axle are inflated to the same inflation. For multi-axle trailers you can lower the internal tire structural shear forces (the forces trying to tear the tire apart) by running the inflation molded on the tire sidewall. Sometimes this is stated as the Max PSI and other times it is stated as the PSI for the max load. For our purposes we will consider this the proper cold inflation you should always run.
4. We should have "Headroom" or "Reserve Load" or "Safety Margin" on the tire loading. I suggest 15%. However I know that for many trailers 15% extra capacity above the actual load is very hard to do. Especially since some RV MFG manage to so under-size the tires that even when the trailer is empty they may not have 15% margin.
5. If you don't have at least a 10% margin I would strongly suggest you need to consider changing tires to something with more load capacity when inflated tot he sidewall PSI.
OK so you have some homework to do. We will continue this in the next post.
Subscribe to the weekly RVtravel.com newsletter or one of our other newsletters about RVing. Great information and advice. Now in our 14th year. Learn more or subscribe.
Saturday, May 31, 2014
What is the Best Trailer Tire?
I frequently get asked "What is the Best tire for my trailer?
I don't want another blowout"
My answer:
I previously did a post on "Best Tire".
Sorry there is not enough data to provide a more focused answer. I am sure some are hoping I would provide the one size and one brand that will solve all their tire problems.
I would point everyone back to this link from an RV forum with data from NHTSA. As you can see the numbers of RV owners that make the minimal effort to file a complaint sends the message to NHTSA that there are no significant problems with RV tires. I also know from looking at some of the complaints the quality of the information is spotty even if a complaint was filed.
Examples include wrong size, wrong tire brand, missing or incomplete DOT serial. Expecting information on actual load, speed etc is unrealistic and expecting an accurate description of the real failure is just a dream.
Engineers react to and work with facts. I have been trying to educate a few folks on another forum that melted body ply cord, as seen in tires with a Run Low Flex Failure
is physical evidence of a tire having been run under-inflated at highway speed. The discussion ended with one poster accusing me of hiding the facts and lying to keep my job. Of course he had no idea why the body cord melted but was certain I didn't know either. That ended my participation on that thread.
I know of no data to indicate that anyone is intentionally making "bad" tires. I do know of data that confirms the majority of RVs have tires in overload and that a significant percentage of tires on the highway are under-inflated. I share with others the personal observations that a good number of trailers are exceeding the speed limit of their tires.
Multi-axle trailers place unique and high side loading on tires. This cause internal forces that contribute to the short life for tires in this application. This is a fact based on engineering and all tires are subjected to this type of structural load. It is just a fact that multi axle trailers are worse than single axle RVs.
Bottom Line
If you want to minimize the probability of having a tire failure on your multi-axle trailer, all I can suggest is you read my post on "Best tire", inflate your tires to the sidewall inflation 100% of the time, at least once confirm your side to side and axle to axle unbalance is not giving you an overloaded tire, use TPMS w/metal valve stems and do a free spin inspection at least once a year.
and cover your tires with WHITE cover whenever parked in the sun for more than a day.
You will probably still only get 4-5 years out of your tires because of the internal stress load but hopefully they will be 5 years with no problems.
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newsletters about RVing. Great information and advice. Now in our 14th
year. Learn more or subscribe.
Wednesday, November 20, 2013
"Interply Shear" and other Techno Babble
I have been putting this topic off for quite some time as I was worried about overloading you with too much "Techno-babble" but I find myself having to constantly repeat this information in individual posts on various RV forums I monitor so figured it would be easier to do a complete post that I could direct people that want to understand why the loading of some trailer tires is much more complex than the average person thinks.
To make this less painful I will give the Bottom Line info first, so those not interested, can stop reading before I put them to sleep.
BOTTOM LINE
When a radial tire is loaded, the belts and body have to bend from a round shape to a flat shape in the area that contacts the road. In addition when you turn a corner the forces generated to move the RV sideways have to be transferred through the tire structure.
This causes additional bending of the belt and body structure. The more the bending the higher the stretching of the rubber. With enough stretch, microscopic cracks form and existing cracks get bigger. Eventually with enough cycles and enough force the cracks may grow and join up with the possibility of tire components separating which could lead to a tire failure. You can lower the stretching if you lower the bending and you can lower the bending if you increase the inflation.
So now on to the Engineer Speak and Techno Babble
If you own a multi-axle trailer these forces can be much higher than those seen on a tow vehicle, motorhome or car, where the tires are not close together but at the corners of the vehicle.
I found an excellent video that shows the results of these forces at Keystone RV. Watch the section from time 0:46 to 1:07 and note that the tires on one axle bend inboard while the others are forced outward.
Special consideration for multi-axle trailers. Warning, this gets technical.
When not driving in a straight line there are special side loads on multi-axle trailers because the tires are fighting each other because they are not "pointed" to the center of the radius of the turn. These loads cause interior structural tearing. Sometimes 24% higher loads than those seen in tires on non-trailer application. Initially tearing is at the microscopic level but with time and repeated cycles these forces grow which can lead to small cracks at the belt edges as seen here at the arrows.
If not spotted these cracks continue to grow to almost the full width of the tread as seen below.
If you are lucky you will see the bulge in the tread as seen here and now you know this tire has failed and MUST be removed AT ONCE as the separation can grow and can cause a belt to come off the body of a tire.
You can lower these forces by either decreasing the load 24% on the tire (probably not something you want to do or may not be able to do) or you can increase the inflation to stiffen the structure and decrease the slip-angle. In this case you could increase the tire inflation from the minimum inflation needed for the static load to the inflation associated with the max tire load as molded on the tire sidewall. BUT you need to be sure you are not exceeding the max rating of the wheel.
So the best recommendation I can give to trailer owners is to run the inflation molded on the tire sidewall. For owners of a TV or motorhomes, I recommend you run the inflation needed to carry the actual measured tire load plus at least a 10% margin.
To make this less painful I will give the Bottom Line info first, so those not interested, can stop reading before I put them to sleep.
BOTTOM LINE
When a radial tire is loaded, the belts and body have to bend from a round shape to a flat shape in the area that contacts the road. In addition when you turn a corner the forces generated to move the RV sideways have to be transferred through the tire structure.
So now on to the Engineer Speak and Techno Babble
If you own a multi-axle trailer these forces can be much higher than those seen on a tow vehicle, motorhome or car, where the tires are not close together but at the corners of the vehicle.
I found an excellent video that shows the results of these forces at Keystone RV. Watch the section from time 0:46 to 1:07 and note that the tires on one axle bend inboard while the others are forced outward.
Special consideration for multi-axle trailers. Warning, this gets technical.
When not driving in a straight line there are special side loads on multi-axle trailers because the tires are fighting each other because they are not "pointed" to the center of the radius of the turn. These loads cause interior structural tearing. Sometimes 24% higher loads than those seen in tires on non-trailer application. Initially tearing is at the microscopic level but with time and repeated cycles these forces grow which can lead to small cracks at the belt edges as seen here at the arrows.
If not spotted these cracks continue to grow to almost the full width of the tread as seen below.
If you are lucky you will see the bulge in the tread as seen here and now you know this tire has failed and MUST be removed AT ONCE as the separation can grow and can cause a belt to come off the body of a tire.
You can lower these forces by either decreasing the load 24% on the tire (probably not something you want to do or may not be able to do) or you can increase the inflation to stiffen the structure and decrease the slip-angle. In this case you could increase the tire inflation from the minimum inflation needed for the static load to the inflation associated with the max tire load as molded on the tire sidewall. BUT you need to be sure you are not exceeding the max rating of the wheel.
So the best recommendation I can give to trailer owners is to run the inflation molded on the tire sidewall. For owners of a TV or motorhomes, I recommend you run the inflation needed to carry the actual measured tire load plus at least a 10% margin.
Friday, September 28, 2012
What should trailer owners do?
Lets call the owner "Tex. He wrote:
"What is confusing is that tire pressure charts don't apply here. This is an older trailer and I may be trading it soon, but say I were going to keep it and put 17.5" wheels on it. Would I inflate to the maximum of 150 psi or would I refer to a pressure chart?"
My reply: Tire charts are based on the ability of a single tire to carry a specific load at a specific inflation and speed on a test drum while running straight ahead. Most vehicles have four tires, one at each corner. A few like some one-ton pickups and vans might have dual tires but the load in dual application is reduced based on years or real life experience that indicates that if you run tires in a close side by side basis you cannot get the same life and durability as when those tires are run on opposite ends of the same axle.
Now multi-axle trailers have been around for a long time but again most of these vehicles run the tires at the reduced "dual" load and also are probably running at the max pressure the tire is rated for. The use of multi-axles on RVs presents a different problem. The loads are not reduced to the "dual" load and the pressure on most RVs is not maintained at the max for the tire. In fact most RV trailers are running one or more tires in an overloaded condition.
RV manufacturers produce many units that are significantly out of balance side to side as they leave the production line. Since cost cutting is a major objective for both the manufacturer and the RV buyer, larger tires are seldom provided and in some cases games are played by de-rating the GAWR to allow smaller (less expensive) tires to be used. I have even seen a case where the RV manufacturer got a letter from the tire importer claiming the tire was capable of carrying more load than was standard for that size. Of course this "special" information is not provided to the RV owner, who will probably buy a replacement tire that shows the correct size but is not rated to carry the same load.
The manufacturers do not take into consideration the side to side unbalance, as to do so would require them to provide larger (more expensive) tires. The other thing RV industry does not take into consideration is the forces to the tire structure due to running close axle spacing. Engineering analysis shows that when turning corners the forces trying to tear the tire apart can be over 20% higher in multi-axle applications than with tires at the corners of the vehicle.
The only options for the trailer owner are to up-size the tires (if there is room) or up-rate and increase inflation (if a higher Load Range tire and stronger wheels are available). Lacking the above being sure to run the tire at the inflation shown on the sidewall (i.e. max) will slightly decrease but not eliminate the overload forces.
No, you should not run 150 psi. I doubt you will find that is the rating for the 17.5 tires or rims. What you should do is consult the load inflation tables and after getting accurate loads on each individual tire run no less than the minimum inflation for the heaviest tire on each axle. If you want to be more realistic and possibly see better tire durability for your tires, I would certainly consider using the "dual" load to establish the minimum inflation. I would further run +10% pressure to avoid day to day temperature and pressure variations. I would also consider a TPMS mandatory.
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