Will Higher Ply Rated Tires Stop Trailer Sway

Question:

OUR 2015 FOREST RIVER WILDWOOD 27RLSS HAD ST205/75D15 / ST225/75D15 AND ST205/75R15 / ST225/75415 WHAT ACTUAL PLY ARE THESE TIRES? WE DID NOT THINK IT WAS ENOUGH AND ARE GETTING SWAY IN THE TRAILER WHEN TOWING. SO BOUGHT ALL NEW TIRES EXCEPT SPARE FOR THE TRAILER THESE NEW TIRES ARE 10 PLY RADIAL TIRES. SHOULD THAT HELP OUR SWAY PROBLEM. WE ALREADY HAVE A REESE HITCH AND A SWAY BAR

asked by: Judith A

Helpful Expert Reply:

The ply rating of a given tire size is going to depend on the load range of the tire. Tire # AM3S638 for example is a load range C so it has a ply rating of 6. A lower load range in the same size would have a lower ply rating. By comparison an ST225/75R15 tire in load range E, # AM10303, has a ply rating of 10. Bias ply tires will have a stiffer sidewall compared to their radial counterparts.

The tires are likely not going to help much with sway. If you are using a single friction sway control bar such as # 83660 then it will not be enough to handle sway on your 2015 Forest River Wildwood 27RLSS for a couple of reasons. First, the dry base weight of that trailer is over 6,000 pounds. A single friction sway control bar becomes less useful above 6,000 pounds. And the length of the trailer is over 32 feet. Friction bars are good for trailers up to 25 feet but after that you would need 2 of them for adequate sway control.

Other factors that can increase the chance of sway include too little or too much tongue weight. You will want to shoot for 10 to 15 percent of the gross trailer weight. If the trailer is not being towed level then that can add to the problem. If the weight distribution system is not rated properly for the tongue weight or is not set up properly then that can also be a factor.

To choose a weight distribution system you will need to go by the total tongue weight of the set up. This is calculated by taking the tongue weight of the trailer, when loaded and ready to tow, and adding to that the weight of anything loaded behind the rear axle of the tow vehicle. I have included some links on weight distribution and determining tongue weight for you.

expert reply by: Michael H