How would I set inflation on a smaller single axle trailer?

Got this question from a reader. 

Our Jayco Hummingbird came from the factory with P235/75R15SL tires. The TT GVWR is 3,750 lbs and the GAWR for the axle is 3,500 lbs. This is a single axle TT. The P-rated tires were like a pogo stick at max inflation.

We changed from the factory tires to Maxxis 8008's in ST225/75R15 size. The factory aluminum wheels are good for a max of 80 psig according to the stamp inside.

Also converted to metal valves stems for running our TPMS....because I'm an engineer who tends to overdo everything I touch .

The heaviest CAT scale weight has been 3,320 lbs on the axle and 3,780 lb GVWR. We've since removed a few items to stay within the 3,750 lb. GVWR.

I've always kept the tire pressure at the minimum sidewall stamp of 65 psig (Max load of 2,540 lbs at 65 psi cold). After reading some of your blogs and looking at the Maxxis load chart, if I assume an equal split weight on each wheel we would have a worse case of 1,660 lb load. Of course a perfectly balanced load isn't likely to ever happen. But even with adding 10% it would put us at 1,826 lbs per wheel. Maxxis says that for our particular tire 40 psig would give us 1,880 lbs capacity.

I can't say that I'm comfortable going all the way down to 40 psig, but I feel ok with 50 psig, even though this is grossly over-pressurized for the given load. I know that at 65 psig the TT rides like a log wagon and we recently bent a spindle on the axle without even knowing it, I wonder it the limited travel of the torsion axle combined with the tire pressure came into play because we were under the GAWR of the axle and never even felt anything out of the ordinary during the trip, of course we are pulling a 3,750 lb trailer with a '17 GMC 2500HD w/ Duramax so we don't feel much anyways.

So if it were yours what pressure would you choose? I've been running at 65 psig and I think that's too much, 50 psig sounds good to me, but it's still too much pressure according to the weight charts.....

http://www.maxxis.com/trailer/trailer-tire-loadinflation-chart

Thanks!"
Here is the answer I gave him.

My approach
OE tires P235/75R15 are  rated for 2,280#@35 psi but on a trailer we need to De-Rate the load capacity so 2028/1.1 = 1844# load capacity.

Your measured axle load was 3,320#
If we assums a nominal 53/47 side to side split we get 1,760# for heavy end  and a 60/40 split gives 1,982# for the possible heavy end of the axle.

An ST225/75R15 LR-C is rated 2,150# at 50 psi. Since we are looking at a single axle trailer we can check the tables and find 40 psi is rated to support 1,880# and 45  psi can support 2,020#.

Since we always select the pressure needed that can support the heaviest end of an axle and we always inflate all tires on any one axle to the same inflation we could select  40  to 45 psi for our CIP.
I would set my TPMS Low Pressure warning level to 40 psi and my CIP to 45psi.


If this was a multi-axle trailer we would want to lower the special belt shear forces and run a higher inflation. Maybe 50 psi minimum

Can I run my LR-E at 65 psi? or is this overloading the tire?

As trailer owners start applying the new Goodyear Endurance ST tire, many are discovering that for some sizes the Endurance tire is only available in a Load Range that is higher than their OE tires. Some are concerned about what inflation to run. I have even seen some claim that running a LR-E at LR-D inflation i.e not 80 but at 65 that the "tire will be overloaded, heat up and fail".

While I understand some of the confusion I do not agree with some of the concern or replies.

Tire load capacity is a function of the tire size and inflation level as long as you stay in the same "type" tire.  By "type" I mean P type or LT type of ST type or for large RVs "truck" type.

If you stay with the same type and use the same numeric "size" then the only thing left to change is the Load Range or "Ply Rating". While I do not like using Ply Rating as it is an old and discontinued nomenclature it may help for better understanding in this post for you to think of the old term.

Important Point. "It is the air pressure that supports the load NOT the Ply Rating." This statement is supported for every tire made by every tire company in the world through the use of Load & Inflation tables. These tables show a size and then for different levels of inflation the load capacity of that tire when inflated to that level. You will never see a tire shown where a LR-D at say 65psi can support 1,500# and for the same size the same tire when having a LR-E rating shown a higher load capacity at 65 psi. Not even just 1 pound more.

So a LR-E can support the same load at 50 psi as a LR-C or the same load at 65 psi as a LR-D at 65.

You will not be overloading the LR-E if you load it to the 65 psi rating shown for thet type & size tire and inflate it to 65 psi as you would for a LR-D. Since you are not overloading the LR-E tire it is not going to overheat at 65psi with the 65 psi load so the LR-E tire is not going to "overheat" at 65 psi any more than the LR-D will "overheat" if it is loaded to the 50 psi load rating and inflated to 50 psi.

When going to a higher "Ply Rating" you can then increase the CIP which increases the tire Load Capacity which means it will actually be running cooler because of the greater "Margin". The higher inflation will also lower the Interply Shear which may lead to longer tire life.

When making the change you do need to confirm the upper inflation level for the rim. The wheel manufacturer should provide that information. As an alternative the wheel will have a max load capacity stated. Looking at the OE tire size that comes on that wheel look for the inflation that corresponded to that load and I would consider that to be the wheel inflation rating.