Friday, December 8, 2023


RV Tire Safety: New truck with old trailer – sway (stability) issues

with RV tire expert Roger Marble

I was asked by the moderator of an RV trailer forum to jump in on the topic by one of the members. Here is my response. I thought the info might be of interest to others.

My prime area of expertise is on tires, but I have also done extensive long-distance towing with a C3500 (454 gas) dually with an 8′ slide-in camper in the bed and pulling a 26′ enclosed trailer with a Camaro race car, tools, etc., inside. I have towed from Ohio to Florida and California, and even made a trip on the Blue Ridge Parkway while towing the race car trailer – so I think I understand the handling situation of a loaded truck and trailer.

Some observations on info in some of the posts in this thread:

“P”-type tires on the TV should be an indication that the TV should probably be limited to single axle “pop-up” or small boat trailers. It is important to remember that P-type tires must be “de-rated” in load capacity by 10% as they are not intended for truck or trailer application. Multi-axle trailers almost always indicate the need for a real truck (not just something that looks like one). This would mean LT-type tires and 3/4- or 1-ton vehicle rating.

HERE is a good post on how to match a truck and trailer. It is much better to do a little work and learn the ratings of both your truck and trailer than relying on the salesperson telling you that you can pull a 5-ton trailer with a Ford Ranger just so he can make the sale.

P- versus LT-type tires. No, they do not make tires “between” these two types. If there is no P or LT before the numbers in the size, then it is what is known as a “Euro-Metric”-type tire, and with an inflation of 36, it is definitely a P-type tire.

The P and LT and ST designations are found in tires designed and intended for highways in America. The inflation level molded on the tire sidewall is the giveaway:

• 35 or 36 is standard passenger-type tire.
• 50, 65 or 80 or higher is an LT-type tire, or a trailer-only ST type.
• In Europe, they use a “C” after the size to identify “Commercial” use, i.e., LT. Don’t confuse this with load range C, which is the U.S. light truck load lange (old ply rating).
• Tires marked “XL” or “Extra Load” are passenger-type tires (maximum inflation 42 or 51 on some) that aren’t quite LR-C light truck tires. This type was originally intended for “station wagons,” which have been replaced by SUVs, but are many times used on light-duty trucks as a cost-saving measure. (Passenger tires are really LR-B or “standard” load except for the XL passenger tires, but none say LR-B.)

Vehicle “sway” is a function of the tire inflation and the suspension design and vehicle loading. When inflated to the sidewall pressure there can be different sidewall stiffness but the difference should not be significant within the same type tire and same load range.

Weights. Empty weights are really of little value in determining what will run down the highway safely. Fully loaded and hooked up individual axle loads are what is needed. With trailers (and motorhomes) it is even better to know the scale weight on each individual tire position when the vehicles have been loaded to their heaviest, as many RVs have been found to be 500 to 1,000 pounds out of balance side to side. No, you don’t have to get on a scale every trip, but you should have at least one set of readings when fully loaded.

Stability. Increased tire inflation will almost always result in improved stability.

New tire “squirm.” When you apply any new tire to any vehicle you may notice for the initial 100 to 200 miles less stability than on the tires you just took off. This happens for a few different reasons:

1. All tires have a “release agent” that allows them to be removed from the curing mold. This is a slippery substance that will wear off in a few miles.

2. When you buy new tires you are not making a clean, apples-to-apples comparison. You, as the driver, are comparing a worn older tire with a new full-tread tire. Even if the tires were identical in construction and rubber compound you will get crisper and more solid handling with a worn (less tread) tire than a new (more tread) tire. In car racing, our tires may only have 2/32 or 3/32 tread depth while new tires come with 8/32 to 12/32 or more tread depth. In some classes of racing that require the use of “street” tires, new tires have most of their tread “shaved” off with the express purpose of improving handling.

Getting scale weights. While you may not want to make a special trip to a truck scale or gravel pit or feed supply to get your individual tire loads, you can at least stop at a truck stop on the Interstate on your next trip to get a reference number for each axle.

HERE is a worksheet to allow you to calculate individual position weights.

HERE is a YouTube video on how to weigh on a truck scale.

Trailer tire inflation. To lower the interply shear (force trying to create a tread separation) you should run AS A MINIMUM the inflation required to support 110% of the MEASURED load on the most loaded tire. Better yet, run the tire sidewall inflation number. You can learn more from THIS Google search, or go to my blog and read more than you ever really want to know about the tire mechanics involved in the tire construction.

Hope this helps.


Read more from Roger Marble on his blog at



Roger Marble
Roger Marble
Retired Tire Design and Forensic Engineer w/50+ years of experience. Currently has Class-C RV. Previous Truck Camper, Winny Brave, Class-C & 23'TT. Also towed race car w/ 23' open trailer and in 26' Closed trailer. While racing he set lap records at 6 different tracks racing from Lime Rock CT to Riverside CA and Daytona to Mosport Canada. Gives RV and Genealogy Seminars for FMCA across the USA. Taught vehicle handling to local Police Depts



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Burky (@guest_55237)
4 years ago

When my wife and I first started pulling trailer back in the eighties I made two huge mistakes that could have cost us and our two young daughter’s lives due to sway. We had bought an old 17 foot single axle trailer that needed new tires. Being rather poor and ignorant I put on some used small truck (think Ford ranger size- 14 inch??) radial tires that I found on “Swap and Shop”. I also built a rack out the back off the frame and on our first trip put a bunch of stuff on it. We took off pulling it with a full size Ford station wagon rigged with an equaliser hitch system. On the very first hill we started down, the trailer started to sway at about 70kph (40mph). In seconds it was so bad it was jerking us from shoulder to shoulder with tires screeching in protest. If anybody had been coming up the hill we may have had a head on. I was able to finally get it under control and stopped. Realising it must have had something to do with the tires and weight distribution, we took everything off the rack and put it inside the trailer up at the front to add weight to the hitch, and gingerly drove back into town and put new bias ply trailer tires on. We carried on and despite being very nervous had a good holiday. When we got home I moved the axle back as far to the rear as the tires in the wheel well would allow to add weight to the hitch and never had another problem. Lesson: soft side wall tires and not enough weight on the hitch are recipes for sway induced disaster.

Impavid (@guest_54865)
4 years ago

Did I miss it or is there no mention of mixing bias ply and radial tires. In many jurisdictions if you have bias and radial on a vehicle, tow or trailer, you’ll be shut down. They just don’t track the same.

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