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Thursday, February 23, 2017

Q&A on 5th wheel trailer and tow vehicle, and an announcement

 Dear Roger,
"I pull a fifth wheel trailer. When I am towing, I inflate all tires on the truck and fifth wheel to their max load pressure. The truck tires are load E, Max psi is 80. The fifth wheel tires are load G, full steel belted with max pressure 110psi. Weighing the rig at Cat scales shows me that the weights on the axles are not heavy enough to warrant using the maximum pressures. I would like to inflate the tires so that the psi matches the actual load. Is there a way to calculate this? I have scoured the internet and cannot seem to find any tables that would give me a clear answer. Thank you for your advice."

My answer:
Hi. There is much information in this blog that can answer your question and provide good background. But I'll provide some info here so you can get on the road with proper inflation. You can then review the info on the blog as time permits.

First we need to separate the tow vehicle from the trailer as the tires are different and the forces on the tires are also different.

Trailer.  Due to the fact that trailer tires are basically being dragged around corners rather than steered, there are forces called "Interply Shear" that place much higher forces on the belts of the radial tires when in trailer application as opposed to when on a motorized vehicle such as a motorhome or tow vehicle. You did not offer the actual measured loads on the trailer tires or the tire size so I can only offer general guides.  Be sure the load capacity of the tires at the inflation you are running is at least 10% greater than the measured load with 15% better. Were the LR-G tires the OE size and load range or did you upgrade to larger or higher?

Tow vehicle:
If you are running the OE size and load range your owners manual should give recommended minimum inflation levels for a loaded vehicle. Many pickup trucks offer "full load and light load" condition inflation recommendations. I recently (Feb. 15) did a post on tow vehicle tire inflation. I suggest you review it and let me know if you still have questions.

You also asked about load and inflation tables. Here are 5 posts on that topic.

* * *

Now, some of you may feel that a more detailed reply would have been in order, and I agree. However, it's very difficult to provide a more specific answer without more information. This brings me to the "Announcement" portion of this post.

With almost 260 posts in this blog, I've covered most features and topics but after reviewing the posts some people still have questions. So I'll do my best to try and provide specific answers. There are a few specifics that I will need in order to provide a useful reply.

With thousands of possible tire/vehicle combinations there is no way I can know the specifics of them all. So I will need your help. When you send a question please provide as many of the following details as possible:

•Tire size, including the letters in front of the numbers if any.
•Load range or the tire max inflation.
•Measured load on each tire position or each axle. If scale weights are not available then GAWR from the tire sticker/placard
•Specific application, i.e., trailer, motorhome, car, pickup
•Tire DOT if the question is about tire age or country of origin

Send your questions to me, Roger, at Tireman9(at)

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