RV Tire Safety
with RV tire expert Roger Marble
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 when driven: One, 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, when the contact patch or “footprint” is forced to turn a corner there is some slippage 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 result 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:
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 a tight turn, 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 deform 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 what 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. After all, 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.
Read more from Roger Marble on his blog at RVtiresafety.net.