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.
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.
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
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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