Tires: Special Trailer tires, ST vs LT Type Tires

Question
The 6000# RV trailer axles present a great problem when it comes time to replace the original equipment special trailer tires. Many will blatantly go against DOT recommendations and go with a lower load carrying capacity light truck tire. To be more specific, from the OEM ST235/80R16 LRE to the LT235/86R16 LRE. In the process they can give up more than 400# of load carrying capacity per tire. Can you justify this procedure? If not what is your recommendation(s).


Cal

Answer
Carl,

Let's see.

An ST235/80R16 Load Range E has a load carrying capacity of 3420# at 80 psi.  Plus it has a speed restriction of 65 mph unless the inflation pressure and / or the load is altered in specific ways.

An LT235/85R16 Load Range E 3042# single, 2778# dual at 80 psi and also has a speed restriction of 65 mph unless the inflation pressure and / or load is altered in specific ways - BUT - if the tire has a speed rating (and most of them do!), the speed rating supercedes the speed restriction of 65 mph.

The basic tire principle is that load carrying capacity is a function of the amount of air the tire has - both in terms of volume and pressure - the speed the tire is going, the ambient temperature, and the roughness of the road surface and a host of other factors.  People are most familiar with the load vs pressure thing - and perhaps the tire size thing.  But the speed and temperature things are less well known.  And the road surface thing?  Almost unknown and even experienced tire folks don't always know about that.

So trying to put this in perspective, LT tires are designed for motorized vehicles, where ST tires are not, so an LT tire used in an ST type application would gain a bit of load carrying capacity.

But let me examine just the speed restriction issue.

If I take the ST235/80R16, I can increase its speed restriction to 85 mph (the equivalent of an "N" speed rating), by increasing the inflation pressure by 10 psi and decreasing the load by 10%.  So if I take the load at 70 psi (3130#) and reduce it 10% (to 2817#) and use 80 psi (+10psi), I have less load carrying capacity at the same inflation pressure as an N speed rated LT tires.

- BUT -

Not only are most LT tires more than N speed rated, they are quite likely to be at least Q speed rated (2 steps up).  The net effect is that a speed rated LT tires is much more capable than an ST type tire.

But that comparison was for slightly different sized tires.

So just for a more complete picture, there are ST235/85R16 tires (notice the difference in aspect ratio).  If I do the same calculation, I get 3015# at 80 psi with an 85 mph speed restriction - about the same as an N speed rated LT tire.

Hope this helps.