Your Ad here
Be sure to sign up for the weekly RV Travel Newsletter, published continuously every Saturday since 2001. Click here.
Huge RV parts & accessories store!
You have never seen so many RV parts and accessories in one place! And, Wow! Check out those low prices! Click to shop or browse!

Friday, March 16, 2018

How accurate is your TPMS?

I have seen a number of people express some concern about the accuracy of the pressure readings from their TPMS. You can read my previous responses HERE  and HERE

I have also pointed out that the primary purpose or "job" of a TPMS is to warn the driver of a pressure LOSS.

I am inclined to think that some are still concerned with their exact pressure reading. I also have to wonder how some people are comparing various pressure readings reported by their TPMS. If they have external sensors, the simple act of removing and replacing a sensor can allow some air to escape. While I doubt that the small loss of some air will result in a meaningful pressure differential it does add some uncertainty.

So the engineering DNA in me kicked in and I a devised a plan to test 12 sensors. These come from two different companies. One set of 6 external sensors is from Tire Traker and one set of 6 internal sensors is from Truck Systems Technology.

The question is: How do I make the test both fair and useful. I decided to eliminate as many variables as possible and get all 12 readings from the same air chamber at the same time and compare them all against my personal digital hand gauges that I have checked against an ISO certified laboratory gauge.
Note my hand gauge reads to 0.5 psi which is way more precise than anyone needs for checking tires in normal highway use.

Here is the test fixture I made.

It has 6 bolt in valves for mounting the external sensors plus a valve for me to use with my hand gauge. Also since safety is always of concern when dealing with a pressure vessel I added a pop-off valve.  One end of the fixture has a cap that can be removed so I can place the internal TPM sensors inside the 4" tube. It also has a regulated air supply to compensate for the very slow leak around the cap and an analog dial gauge that allows me to constantly, visually monitor the air pressure to ensure the use of my hand gauge does not result in a pressure change that isn't immediately compensated for by the regulated air supply.

Here are the results of my comparison test. The target pressure is 80.0 psi as reported by my handheld digital gauge.

Set A  1 reading of 78 psi,  5 readings of 79 psi
Set B  2 readings of 78, 2 readings of 79 and 2 readings of 80 psi

I also recorded the temperature.
Set A  4 readings of 66 F, one each of 64 and 68F
Set B  4 readings of 69 F  and 2 readings of 68

I do not consider any of the differences in the readings of pressure or temperature to be significant or meaningful for a TPMS.  I would consider the pressure readings from all 12 sensors to essentially be equivalent.
You can learn a bit more about what a "meaningful" measurement is HERE.

Tuesday, March 6, 2018

How would I set inflation on a smaller single axle trailer?

Got this question from a reader. 

Our Jayco Hummingbird came from the factory with P235/75R15SL tires. The TT GVWR is 3,750 lbs and the GAWR for the axle is 3,500 lbs. This is a single axle TT. The P-rated tires were like a pogo stick at max inflation.

We changed from the factory tires to Maxxis 8008's in ST225/75R15 size. The factory aluminum wheels are good for a max of 80 psig according to the stamp inside.

Also converted to metal valves stems for running our TPMS....because I'm an engineer who tends to overdo everything I touch .

The heaviest CAT scale weight has been 3,320 lbs on the axle and 3,780 lb GVWR. We've since removed a few items to stay within the 3,750 lb. GVWR.

I've always kept the tire pressure at the minimum sidewall stamp of 65 psig (Max load of 2,540 lbs at 65 psi cold). After reading some of your blogs and looking at the Maxxis load chart, if I assume an equal split weight on each wheel we would have a worse case of 1,660 lb load. Of course a perfectly balanced load isn't likely to ever happen. But even with adding 10% it would put us at 1,826 lbs per wheel. Maxxis says that for our particular tire 40 psig would give us 1,880 lbs capacity.

I can't say that I'm comfortable going all the way down to 40 psig, but I feel ok with 50 psig, even though this is grossly over-pressurized for the given load. I know that at 65 psig the TT rides like a log wagon and we recently bent a spindle on the axle without even knowing it, I wonder it the limited travel of the torsion axle combined with the tire pressure came into play because we were under the GAWR of the axle and never even felt anything out of the ordinary during the trip, of course we are pulling a 3,750 lb trailer with a '17 GMC 2500HD w/ Duramax so we don't feel much anyways.

So if it were yours what pressure would you choose? I've been running at 65 psig and I think that's too much, 50 psig sounds good to me, but it's still too much pressure according to the weight charts.....


Here is the answer I gave him.

My approach
OE tires P235/75R15 are  rated for 2,280#@35 psi but on a trailer we need to De-Rate the load capacity so 2028/1.1 = 1844# load capacity.

Your measured axle load was 3,320#
If we assums a nominal 53/47 side to side split we get 1,760# for heavy end  and a 60/40 split gives 1,982# for the possible heavy end of the axle.

An ST225/75R15 LR-C is rated 2,150# at 50 psi. Since we are looking at a single axle trailer we can check the tables and find 40 psi is rated to support 1,880# and 45  psi can support 2,020#.

Since we always select the pressure needed that can support the heaviest end of an axle and we always inflate all tires on any one axle to the same inflation we could select  40  to 45 psi for our CIP.
I would set my TPMS Low Pressure warning level to 40 psi and my CIP to 45psi.

If this was a multi-axle trailer we would want to lower the special belt shear forces and run a higher inflation. Maybe 50 psi minimum

Thursday, March 1, 2018

Can I run my LR-E at 65 psi? or is this overloading the tire?

As trailer owners start applying the new Goodyear Endurance ST tire, many are discovering that for some sizes the Endurance tire is only available in a Load Range that is higher than their OE tires. Some are concerned about what inflation to run. I have even seen some claim that running a LR-E at LR-D inflation i.e not 80 but at 65 that the "tire will be overloaded, heat up and fail".

While I understand some of the confusion I do not agree with some of the concern or replies.

Tire load capacity is a function of the tire size and inflation level as long as you stay in the same "type" tire.  By "type" I mean P type or LT type of ST type or for large RVs "truck" type.

If you stay with the same type and use the same numeric "size" then the only thing left to change is the Load Range or "Ply Rating". While I do not like using Ply Rating as it is an old and discontinued nomenclature it may help for better understanding in this post for you to think of the old term.

Important Point. "It is the air pressure that supports the load NOT the Ply Rating." This statement is supported for every tire made by every tire company in the world through the use of Load & Inflation tables. These tables show a size and then for different levels of inflation the load capacity of that tire when inflated to that level. You will never see a tire shown where a LR-D at say 65psi can support 1,500# and for the same size the same tire when having a LR-E rating shown a higher load capacity at 65 psi. Not even just 1 pound more.

So a LR-E can support the same load at 50 psi as a LR-C or the same load at 65 psi as a LR-D at 65.

You will not be overloading the LR-E if you load it to the 65 psi rating shown for thet type & size tire and inflate it to 65 psi as you would for a LR-D. Since you are not overloading the LR-E tire it is not going to overheat at 65psi with the 65 psi load so the LR-E tire is not going to "overheat" at 65 psi any more than the LR-D will "overheat" if it is loaded to the 50 psi load rating and inflated to 50 psi.

When going to a higher "Ply Rating" you can then increase the CIP which increases the tire Load Capacity which means it will actually be running cooler because of the greater "Margin". The higher inflation will also lower the Interply Shear which may lead to longer tire life.

When making the change you do need to confirm the upper inflation level for the rim. The wheel manufacturer should provide that information. As an alternative the wheel will have a max load capacity stated. Looking at the OE tire size that comes on that wheel look for the inflation that corresponded to that load and I would consider that to be the wheel inflation rating.

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.

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.

Friday, February 9, 2018

Quick post on Belt Separation not a "Zipper" failure

Starting to catch up on posts on various RV forums after 10 days on a beach in Jamaica not thinking about tires BUT here we go.

Post about this tire that lost it's tread.

There was a reply about "zippering"

"Your tires look like they threw the whole tread off. I always assumed that the treads I see on the road are bad retreads from truck trailers, but it appears that this is not the only source of thrown treads.

Note that Michelin has documented a "zipper" condition on their motorhome tires. It consists of a circumferential crack on the sidewall just where it meets the tread. It could be the start of a delamination of the whole tread, which is what you seem to have.

You might look this up on the Michelin website. It might result in a warranty claim.
The only good way to protect against such a problem is by inspecting the tire regularly, which is not easy to do with an inside dual, especially since the zipper starts on part of the tire only and doesn't start on the full circumference. I wouldn't have any confidence at all that a TPMS would catch such a problem."

While the reply was well intended it didn't in my opinion offer the correct  answer. A closer examination finds areas that were rubbed smooth.

This is usually an indication of long term separation probably after operating for hundreds or possibly thousands of miles with a small separation at the edge of the two steel belts.

 I commented that "IMO the picture indicates not just a "tread separation" but the top belt separated from the bottom belt. This is a long (thousands of miles) term progressive failure. Many times a close thorough inspection can discover the signs beforthe tire comes apart."
I also advised "RE "Zipper" comment.  A classic "Zipper" failure occurs not at the tread sidewall jusnction and certainly not in a textile body tire as the one seen in the picture but is a steel body tire that has been run severly underinflated. This results in steel fatigue in mid sidewall as seen in the picture in THIS post.


Friday, February 2, 2018

Not a "Blowout" but definitely was a Run Low Sidewall Flex failure. No TPMS was cause

Read the following on an RV trailer forum:
"BOOM it happened again, however, this time it was with the upgraded Maxxis tires, seems to me that a lot of these "trailer tires" are just plain garbage. 
The only positive thing I can say is that the Maxxis tires did not blow up like the TowMax tire did thus I suffered zero damage this time, call that a win-win.
All tires were checked prior to departure and filled to 80PSI, less than 20 miles to our destination (220-mile trip) I saw a bit of smoke coming from right side pulled over and the side wall gave out. 
Changed it with the spare Goodyear Marathon that camping world lovingly overcharged me for last year ($197.67 to be exact) and went on my way very slowly. 
Found a tire shop called Gattos in Palm Bay FL that had my size 235/80R16 and installed four new Goodyear Endurance tires (FYI tire shop was way less than camping world by $34 dollars per tire!)
Why 4 you might ask, well it just made sense to me kids, wife & dog in the truck better safe than sorry. Took the three good Maxxis home with me as they look fine, may use one and make a second spare tire. 
Let's hope the Goodyear Endurance tires are as good as they say at least they are made in the USA VS the Marathon that's made in China & had a really bad track record. 
Anyways just sharing my experience will be purchasing a TPMS system very soon before our next trip that's for sure. Any recommendations? Seems many offer a six-tire system all I need is a 4 tire system."

My reply

Sorry to see you had another tire failure but there is no such thing as a "failure-Proof" tire. All tires need to be properly inflated. ALL THE TIME. Not just at the start of a trip.
The pictures provide the evidence with that nice 360 flex that resulted in one side ending up with just the bead area and lower part of the sidewall. This piece of convincing evidence is seen in upper left part of the picture.

I hope you held onto the other 3 tires as there is no reason to expect them to fail as they apparently didn't lose air like the failed tire did.
In case you wonder about my diagnosis you might review THIS blog post where I was provided with a number of sharp pictures of the failed tire.

As covered in THIS post, 99+% of tire failures are due to one of two different causes. Your tire has clear and convincing evidence of failing due to Run Low Flex Failure of the sidewall.

I would strongly suggest a TPMS be installed so you could get advanced warning of air loss.

Note: I have even seen instances where the valve core sticks open and if the valve cap is not metal with an internal "O" ring the tire just takes longer to go flat as plastic caps are really, IMO only reliable at keeping dirt and small birds out of the valve core sealing ring. You might review THIS post on valve cores to understand why metal caps with 'O"rings or TPMS sensors are the only items I consider acceptable to use on the end of a valve stem.

Thursday, January 25, 2018

Another question on why Trailer tires fail more often than tires on motorized vehicles.

On a thread on an RV trailer forum Peter made this statement
"At highway speeds interply shear is not as much of an issue, as I understand things."
I posted this reply:
Speed itself is not the issue that causes Interply Shear in Radial tires. Two things happen to radials as they are driven.
One is the tread (and belts) must flatten out when the tire contacts the road. This results in the steel cords moving relative to each other. This is a shear force.
Second is that when the contact patch or "footprint" is forced to turn a corner there is some slipage between the direction of travel that aligns with the center of rotation and the actual direction of travel.

On motorized vehicles the front tires have a slip angle and the side forces are what actually results in the vehicle turning. But of you were to project the center of rotation toward the center of the driving radius you will find that those centers are close together This is due to the "Ackerman" designed into the front end alignment.
Yokohama has this nice graphic.

This image has been resized. Click this bar to view the full image. The original image is sized %1%2.

Multi axle trailers however have two axles and 4 tires, with no tire rotating around a centerline that points to the center of the turn radius.

Here is what happens to trailer tires.

These tires are forced to higher than normal "Slip Angle" through any turn, Not just the extreme tight turns when backing into a parking space.
Damage to tire structure is cumulative and while a small turn imparts less shear than t tight ture even small forces can do damage on a molecular level.
Here are some links to various articles on Interply Shear.

Duals on a large truck slip on the pavement when forced through tight turns and with inflations over 100 psi they do not deforn as much as trailer tires with lower levels of inflation.

Increased inflation will lower the interply shear. It will never lower it to the level seen on a motorized vehicle Only Passive Steer axles on trailers similar to wat is seen on the back end of cement trucks can lower the shear too but I don't see any RV trailer company offering that expensive alternative. Afterall they know you can't make a warranty claim on failed tires on the RV company as belt failures from Interply Shear is a long term proposition.

Friday, January 19, 2018

Snow chains in RV application

Most of us only deal with ice in our drinks or as used to get our beer cold but some folks may find themselves wanting? to drive in areas where roads get snow and ice covered and even in areas where the use of chains is required.
The question is; What does the RV owner do when it comes to using chains in the dual application?

If we choose to travel in such areas, it is important to have the correct type and size of "chains" for our application or we may end up doing damage to both our tires and the side of our coach.
There are a variety of types of chain set-ups and even some alternatives such as "cables". I cannot address the legality of the alternate styles in all areas other than to offer that if there is an advisory that chains must be used and you get stuck and do not have chains on the unit you may face some fines and other penalties so you need to confirm the details of the requirement in your area before spending your money or before traveling out and about on snow and ice covered roads.

If you do not have duals your choice is to be sure you select the equipment correct for your size tire.

Duals have a few options. There is going to be a difference in cost, weight and ease of install so you need to do some research.
There are "Triple Rail" as seen in this video. There are some "cable" type systems as seen here, and some newer designs as seen HERE .

I have not been involved in any direct comparisons so cannot offer any advice on relative performance. For that, you may need to do some additional investigation on the internet or even talk with some over the road drivers. Looking at the different videos it appears to me that some designs may be more durable than others.

I do know that driving on dry roads with chains can cause some serious tire damage so only you can properly evaluate what system would be best for your situation.

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.

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.

Friday, January 5, 2018

ST type tire max speed

Originally Posted by LI Pets 
The 65mph is not really correct it depends on load and air pressure.

They can go to 80mph.
The above is from an online forum.

As I have pointed out previously:

The load formula that populates the Load/Inflation table, used by the tire industry for ST type tires is based on the assumption that 65 mph would be the MAXIMUM operation speed. If the operation speed was to be between 65 and 75 then users were to use a 10 psi higher value for their measured load.

Example ST235/75R15 with 2,030# measured load needs to run 50 psi minimum cold not 40 psi cold.

If running 75mph to 85 the Load capacity number must be reduced by 10% in addition to the +10 psi adjustment.

Example ST235/75R15 would be rated to carry (2,030 x.9) or 1,827# at 50 psi.

I know this adjustment process seems backward but that is the way tire loading is calculated.

Now I am sure some will say that the new ST tires come with speed ratings faster than 65. IMO many of these ratings were applied just to avoid import taxes. I know of no magic rubber that somehow gives an ST235/75R15 LR-C the ability to support 2,340# at 87 mph with 50 psi in it while an identical sized LT235/75R15 LR-C is only able to support 1,985#.

Before you say "Ya but the tire companies probably made big improvements in the new ST tires", I would ask what makes you think the tire companies would not want to be able to offer better load capacity to their LT tires?

Load capability is basically the tire air volume x air pressure, with adjustments for speed and expected service. So if you have a P/U pulling a TT the "service" would be the same and the speed would be the same so how can the tire with "ST" on the sidewall carry more load at the same speed?


Now you are more than welcome to believe in magic or marketing claims but IMO using the load /infl tables without doing the adjustments will probably result in an increased likelihood of belt separation. So when you have a failure please do not post something here along the lines of  "I just had a blowout. I didn't hit anything and always check my air".  Tire failures usually occur because of cumulative internal structural damage from heat and time. The excess heat comes from the combination of speed/inflation/load.