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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)

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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)

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Wednesday, February 15, 2017

Tow vehicle tire question

Dear Roger,

"Last week I purchased new E rated tires for my F150. Went with the BF Goodrich KO2 (275/65/18) and so far I'm very happy with them. These are larger than the tires that came from the factory (265/60/18). I have a quick question.

"The tire shop originally put the PSI at 35 based on the door sticker, but after reaching out to BF Goodrich they suggested 45 PSI for everyday driving. They didn't give any recommendations for tire pressure while pulling a trailer since there are so many variables like trailer size, weight and cargo. The tire shop suggested going to 50 pounds and then deflating back down after the trip. I'm just curious if others actually bump up their PSI while towing, and have you noticed a difference in doing this? Just trying to figure out if there is a benefit to doing this.

Some important information is missing, so let's identify our assumptions:

1. We are talking about the tow vehicle tires - the F150.

2. The original poster didn't identify the OE (original equipment) tires as being P type or LT type. This makes a major difference and providing that information up front will eliminate guesswork for those trying to help with the answer. With a 35 psi door jam sticker number, I suspect the OE tires are Passenger type and the replacement tires are LT type.

3. Since it is the air pressure and not the tire construction that supports the load on a tire, buying LR-E (80 psi) tires when you never run higher than 65 (LR-D) would be a waste of money. Also if you are changing from P type tires with a max inflation of 36 to 41 you need to confirm the wheels are rated for higher load and inflation of LT tires.

4. The door jam pressures are based on the car company making some estimates on how much and how often you have the vehicle empty or fully loaded. You might check the owners manual and see if they give an empty and loaded tire inflation suggestion. The inflation in the OE tires is what is needed to support the GAWR but few people run that heavy all the time with their pickup.

5. I suggest you get the F150 on some truck scales when empty and again when fully loaded with the trailer also fully loaded, and learn the real facts of the various axle loads under both conditions.

6. Knowing the real loads, you can use the tire Load & Inflation tables to learn the MINIMUM cold inflation pressure for the F150 for the two situations. I suggest you use a margin of +10% of the table inflation to establish your CIP (cold inflation pressure). See other posts on weight and inflation if you need more details.

7. Trailer tires are a completely different situation and should always be inflated to the tire sidewall inflation. You should also run no more than 85% of the trailer tire max load rating when on a scale. Cornering, sway and side wind loads have been shown to shift loads side to side by 10% or more. Also the Interply Shear on tires in trailer application needs to be considered as this is a major contributor to trailer tire belt separations.

Hope this info helps clarify.

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