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Wednesday, May 25, 2016

Question on Interply Shear on trailer tires vs tow vehicle tires

A question from a reader of a post on a trailer forum

"Tireman, your concerns over shear puzzle me. G614s are LT tires as are the tires on my 2500 pickup. Only difference is G rated vs E rated. The truck mfg recommends 60 front 75 rear with 80 psi max on sidewall. Why is there no concern about shear on the front tires of my truck. It seems to me the frequency of shear forces is much greater on the truck than the trailer."

Ya, I understand your confusion
 But the reality is that when computer analysis is used to look at the internal structural loading, the fact that tow vehicle tires are all operating at very low "slip angle" (difference between travel direction and angle the tires are pointed to) is significantly lower than for tires on a trailer.

The reason for this is that the center line of of tire rotation for the tow vehicle tires points to the center of the radius while the trailer tires, especially on tandem axle trailers, does not.

This translates into a higher slip angle which means higher internal structural twisting forces on the belts. The computer model suggests 24% higher on the TT tires than TV tires even if all tires were the same with identical vertical load and inflation.

TV front tires have "Ackermann" alignment designed into the suspension but TT have no allowance other than bending of tires, springs, spring mounts and bushings but the forces to bend the springs etc have to go through the belts of the tires.

This is a MAJOR reason for travel trailer tire life to be much shorter than motorhome or tow vehicle tire life.

Hope this helps folks understand a bit of what makes tire engineering a challenge.

Editor: Here is an earlier article Roger wrote about "interply shear," if you want more information. 

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  1. So, I read the whole article and then you don't tell me what the life of the average travel trailer tire is!

  2. Based on posts on various forums it appears the "Average" life of a ST type tire in trailer application is 36 months or maybe 150 hours of operation, which ever comes first. Of course we may differ on what constitutes "average" usage.

  3. I wanted to thank you for this great read!! I definitely enjoying every little bit of it I have you bookmarked to check out new stuff you post. Richardson towing


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