Trailer tires vs Light truck tires, ratings and usage questions

[Justin00Stang]

I am build a flat deck special purpose trailer. Overall diameter and load carrying ability of the tire are of importance. I want to run the smallest diameter tire I can that will still hold the load that I need it to.

Previous trailer design used a 15" wheel and 29" ST radial tire rated at 2500lbs, though I never had a failure, the rating was marginal with a car loaded on the trailer for transport.

The new design will use a 16" wheel, I've found 30.5" diameter ST trailer tires and 31.5" diameter LT trailer tires, both rated at 3000lbs.

It would be much easier to build the trailer with a 30" tire. I have found several light truck tires made by Hankook and Good Year that would easily meet my weight and diameter requirements.

The bottom line, is there any reason that I should not use light truck tires on a trailer?

Thanks,
Justin

 

[Xwheelguy]

Trailer tires are made with stiffer sidewalls than vehicle tires. The stiffer sidewalls help prevent the trailer from swaying side to side as you travel down the road. If it were me, I'd stick with trailer tires.

Tim Flater
Senior Designer
Enkei America, Inc.

http://www.enkei.com

Some people are like slinkies....they don't really have a purpose, but they still bring a smile to your face when you push them down the stairs.

 

[cibachrome]

Contrary to popular belief, ST trailer tires are not that much more special than the same LT, except they allow you to put more air in it to get the load rating up. As you add air the cornering stiffness drops and the trailer sideslip (sway) actually gets higher (=badness).

There are a number of 16" and 17" sizes which meet your OD requirements. You can run them at lower pressure which will mean a better ride. This means that the battery, wheel bearings and other shake prone stuff on your car payload will be much happier. Considering the value of the cargo, I'd recommend the LT's. They are likely to be more fresh from the factory too. There are some P-metric tires than you could use, too but you said "LT". The P's will be a larger wheel but that's your call.

 

[Justin00Stang]

nkwheelguy, I had always heard that before.

cibachrome, Interesting, based on what you are saying the relationship between pressure and stiffness is actually opposite of what one would think.

The trailer ST, LT, and truck LT tires I have looked at all need 80-85psi to support roughly 3000lbs.

 

[CapriRacer]

I am going to second Cibachromes point that there isn't much difference between ST tires and LT tires. But I am going to disagree about increasing inflation pressure decreasing the cornering stiffness and therefore "sway". I think the term "cornering stiffness" has a different meaning relative to a tire's "force and moment" characteristics (After I have done a little research to confirm this, I'll post back.)

It seems to me that "sway" is more a function of the deflection of the tire - both laterally and vertically - and more inflation pressure results in a stiffer tire - in both directions - and therefore less "sway".

I really don't think using P metric tires is a good idea - especially on a space limited situation. For one, P metric tires need to be derated 10% when used in a trailer application. Secondly, the lower inflation pressures P metrics use would tend to sway more (See above).

Justin - a couple of tips:

1) Be sure to include plenty of reserve capacity when selecting tires. Most of the tire load tables assume "ideal" conditions. Trailers - as you well know - tend to be run over curbs - or worse - when turning corners. Heat is another factor that isn't well accounted for in the load tables. I'd recommend a minimum of a 15% reserve capacity.

2) Be careful when calculating weights. Many folks tend to underestimate what a trailer weighs - not to mention the load being carried. The final result - the fully loaded trailer - ought to be weighed - tire by tire, if possible - to verfiy the actual values. Plus this weighing should be "worst case" - everything you can possibly imagine being carried and the liquids full to the brim! I can't begin to tell you how many times I've had folks tell me they weighed the trailer, only to find out they weighed it new with no water in any of the tanks - white, grey, black or whatever. Water is surprisingly heavy! Fuel, too!

 

[cibachrome]

IMHO: Load normalized cornering stiffness is all that counts as the key force & moment property of a trailer tire. "sway" is axle sideslip and corresponds to a Bundorf cornering compliance, just like in a car or truck. The reciprocal of the tire's normailzed cornering stiffness is the trailer's sideslip gain in deg/g. High sideslip gain is bad; If its higher than the tow vehicle's rear cornering compliance, it looks really cool on a motorway exit: the trailer is hanging out beyond the puller.

DATA=measurements show these stiffnesses range from .140 to .230 at 1225 kg in the 380mm SLR constraint set. ST235/80R16s are .150 to .165 at 5.8 bar and 3.8 bar pressures (ie they will have higher sway). Yet a P275/60R20 at 2.4bar can be .230. LT285/70R17(Nissan Truck) =.180. In all cases, normalized cornering stiffness drops with increasing pressure. Yes the LTs are usually run at 5.85 bar. Car haulers are dual axle so there's plenty of tire stiffness. Go with the LTs.

BTW: You may be more familiar with the term "cornering coefficient". This is often used in discussions of tire properties related to understeer, yada, yada, yada. Cornering coefficient is FY at 1 degree slip at some load, normalized by that load. It is NOT the derivative of the FY function. Sometimes they are close, sometimes, no cigar. The weaker the tire in cornering, the poorer the correlation. Trailer/truck tires are generally weak for cornering because they have a lot of air in them to hold up the load. Air pressure drops normalized cornering coefficient/cornerng stiffness in the sizes offered in the marketplace and at their recommended pressures. This has been established as common fact in product liability Courtrooms all around the world.

 

[patprimmer]

Large trucks use very high tyre pressure.

I have travelled very many miles towing car, boat and box trailers and caravans with a wide variety of cars.

I have sometimes done this with heavy loads and/or at high speed.

I have used my old, but not quite worn out car tyres or new light truck (so called 6 ply) tyres. I typically run about 40 psi or more for best results re sway control.

My only data is trip times that are typically somewhat quicker than most not towing a trailer and the ratio of overtaking vs being overtaken numbers. I am rarely overtaken, even when towing.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[cibachrome]

Tooltip: It isn't uncommon to find tires which increase their normalized cornering stiffness when tested in the 26 to 35 psi pressure range. In fact it is often the goal of many development programs to determine just where the 'optimum' pressure is. This effect is very dependent on the tire's construction recipe, even within the same brand, size and usage code (summer, mud & snow, touring, etc.) However, over 35 psi, not gonna happen. This term addresses the trailer sway issue, which was the OP's issue.

Now, the actual cornering force will continue to increase with pressure but it will have to generate more sideslip to get the job done. Good for oval track racing, bad for road courses, especially those where the race car is in the trailer. Sideslip = sway, and will blowup when a jackknife progression develops.

The pressure vs. load dilemma is the bane of every truck design since people/operators may choose to put as much in a truck as will fit instead of what can be safely handled.
Just the facts,
MaM