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
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
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.....
Here is the answer I gave him.
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.
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