Tire Advice

StirCrazy wrote:


Snip..

I can't agree with you there, I have been looking at st vs lt tires for 4 years now trying to decide which would be better for my 5th wheel. yes, there are different tread patterns, but the one common Theam of them is the lack of aggressive outside lugs which bite in and resist side movement like LT tires have. this enables ST tires to handle more lateral movment without increased resistance.

plus, another plus for the st tire is they are generally made of different rubber compounds that make them stronger than the equivalent LT tire based on a simular volume of air. Another side benefit of these is they generaly have a lower rolling resistance which equates to a fraction better fuel milage.

the other thing I have found also is to switch to a LT tire the closest thing in size to what I now have (235/80R16 ST) is a 235/85R16 . by switching to this I lose 500lbs (aprox) of capacity per tire.

Part of the biggest issue is dealers generaly put the cheapest tires on the rv's they can get so the weight of the rv is just covered by the tires and sometimes it is less as they assume the hitch weight as not being a load on the tires in some cases. for instance, my 5th wheel came with 10 ply loadmax tires so what I have found the best thing to do would be to just stay the same size in a st tire but go up to a 12ply or even a 14 ply if your rims can handle the higher air pressure requirements. I know a lot of people go to 14 ply then just run them at 90 which is probably fine, but they will run hotter than if they were at 110. When they design LT tires, they have less stiff sidewall and softer compounds, so they handle impacts from road hazards better (ie. potholes) and don't transfer it to the vehicle as much. This is great for a truck, but the downside is less carrying capacity for the same size of tires and more flexible side walls, and better tread grip. When combined with the more agressive tread patterens (usually the addition of shoulder lugs and deeper tread and such) this causes them to grip harder in tight turns which increases the lateral stresses on the belt layers.

the main issue is instead of looking at quality ST tires we are usually talking about switching from China bombs to LT, which ya, that is probably an improvement, but we should really be looking at actually weighing our setups (because we all know how accurate that tally plate is ;) ) and going with a quality ST tire that has the extra carrying capacity to cover our rigs and some breathing room

Hi Stircrazy,

I agree with much of your post; the lack of extra reserve capacity in the tire in a tandem or triple axle trailer setup is an issue direct from some of the RV manufacturers.

And I agree to upgrade a load range to gain more capacity, but not airing up the tire to the max sidewall cold pressure does not give you the full benefit of the upgrade. If you are going to upgrade, you need the extra pressure to gain the benefit to help ward off interply shear.

I may learn something new in this next area, so please explain your thinking. I "thought" I understood you are saying the LT tires with a more aggressive outer tread do not allow the LT tire to slip as much as an ST tire in a turn. Did I paraphrase that correctly? If so, help me understand how you came to that conclusion. Here are my thoughts on why I cannot connect the dots on the tread pattern to hard surface road slip in a dry setting.

I believe we both agree on the friction of the tire tread determines tire slip on the pavement between the tire and the road.

Friction is the main part of the slip equation, and there are only a few ways to change friction on a hard surface road to the tire. Again, I am talking hard surface road, not loose gravel, snow or dirt, mud, etc.

Here is my take based on the friction formulas. To change the friction between the hard surface road and the tire tread contacting the road, these two main variables must change.

1. The weight of the camper pressing down on the road
2. The coefficient of friction between the road surface and the tire tread

There is no variable for tread pattern or contact patch area of the tire in the friction force formulas.

Please help me see your point where the tread pattern changes friction between the tire and the road surface.

I agree with wet, slimy roads, tire rubber compounds, temperature, and the makeup of the road surface. etc. are factors in changing the friction coefficient. Just not sure how the tread pattern on a clean and dry road surface changes the fiction.

On your quest to sort out ST to LT on your camper, yes, sizing is an issue in some cases in finding LT tires rated in the right capacity. I went through the same issue when I had my first set of ST tire failures. I was on 15" ST225/75R15's load range D on a 10,000# loaded camper. After I sorted out what made the ST tires fail, I had 2 choices, upsize to Load range E in ST or jump to 16" and go LT load range E. I ended up going 16" LT load range E. I had to deal with vertical clearance between the top of the tire and the bottom of the camper clearance. I had to modify the suspension setup to get this extra clearance, I could do the work myself, and I'm glad I did. In my case, tire width was not the issue, but vertical clearance. If you want more on this, I did posts over the years on RV.net of each issue and how to overcome it; let me know, and I'll dig them up. And, I went with a highway tread pattern, not AT or other. I did not see any need other than the highway tread pattern in an "on-road" trailer setting.

John