Friday, July 1, 2022

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RV Tire Safety: RVer complains, “I have too much tire pressure gain”

Here is a post on an RV forum with a question about too much tire pressure gain:

I’ve often wondered why the tires on my travel trailer will gain about 10-12 psi on a trip, and the tires on my tow truck only gain about 4-5 psi. The tires on my trailer are Goodyear Endurance load range E, with a cold pressure rating of 80 psi. (The original Chinese tire bombs were load range D with a cold pressure rating of 65 psi. I had the same problem with them gaining pressure.) I usually start my trips with the pressure set at 75 psi instead of 80 psi because I’m nervous about the tires gaining so much pressure. On a typical travel day in the summer (in Texas), the tires will usually go from 75 psi to about 87 or 88 psi cruising down the highway at 65 mph. I have a pressure monitoring system, and I have a lot of faith in it. My trailer weighs 9,950 lbs., according to the CAT scale.

My response:

Tire pressure will increase, i.e., gain about 2% for each increase in temperature of 10° F. I have covered this in detail in a few posts on my blog. Here is one post on that topic.

Since your TT tires carry a much higher load relative to the tire size, they have to work harder than your truck tires. Inflation on trucks gives tires a 10% to 30% reserve load while the inflation specified for TT tries may give 0% reserve. The sad fact is that based on actual tire loading data, a majority of RV trailers actually have a negative reserve (i.e., they are overloaded).

What can confuse the issue is when you change the load range, you can run a different pressure. When you increase the tire load range, you only get an increase in load capacity when you also run higher inflation.

Without knowing the scale reading for each axle and the actual tire sizes, I can only give you generalizations. It is very unlikely that your 9950# is evenly split across all four tires, and probably one or more is supporting more than 2,487#.

You should NOT get nervous about pressure gain, as we tire engineers know the temperature will increase and also the pressure will increase. You can email me directly at tireman9@gmail.com and I will be happy to work directly with you to resolve your questions and concerns.

Some information that will help us:

1. Complete tire size and Load Range info for both TT and TV

2. Scale readings for all 4 axles (TV and TT)

3. Cold inflation for the TV.  You said 75 for the four TT tires

======

Well, I posted the above a couple of weeks ago but have not heard back. So, either the original poster has lost interest, or didn’t check back, or doesn’t want to discuss the problem.

If you are reading this blog post then you probably care about your tire inflation and understand the normal pressure increase. If not, my offer to help still stands, but please provide the requested information so we both don’t spend time going back and forth.

Have a tire question? Ask Roger on his new RV Tires Forum here. It’s hosted by RVtravel.com and moderated by Roger. He’ll be happy to help you.

Read more from Roger Marble on his blog at RVtiresafety.net or on RVtravel.com.

 ##RVT1053

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Gene Sannes
1 month ago

During my years of pulling an RV, some time ago I purchased a tire pressure monitor for the RV. In every case the trailer tire pressure went up from 10 to 18 pounds. In 70–80-degree temperatures 10–11-pound increase was the norm. On a few trips and driving through Nevada in August and the outside temperature was 112 degrees as shown on my dash indicator. The tire pressure was at 109 PSI. We did stop for a lunch and the pressure went down quickly. My RAM 3500 pickup pressure, set at 80 PSI, never went over 90 PSI while towing. Without the trailer behind me, the pickup’s pressure never went over 5-8 PSI above the pressure set for driving without the extra load

Wayne C
1 month ago

I’m disappointed that the person didn’t respond because I have noticed the same thing so I’ll give it a shot.

1. Tire size and load range
Truck (RAM 2500) LT265/70/17 LR E (Michelin LTX)
Fifthwheel ST 235/80R/16 LR E (Goodyear Endurance)

2. Scale Readings with fifthwheel attached.
Truck (GVWR 9600 lbs)
Front axle 4850 lbs
Rear axle 5100 lbs

Fifthwheel (GVWR 10500 lbs)
Front axle 4000 lbs
Rear Axle 3550 lbs

3. Cold Inflation
Truck
Front 60 psi (factory recommendation)
Rear 75 psi ( 5 psi above factory recommendation)

Fifthwheel
All positions 80 psi (max cold inflation pressure)

My truck tires gain about 5 psi the same time the trailer tires gain about 12 psi. I have relaxed about the pressure gain because I have read your posts informing me that it’s normal. What I wonder is why the trailer tires gain so much more pressure than the truck tires when the truck is loaded 350 lbs over GVWR and trailer is a couple thousand lbs under GVWR. Thanks for your help

Roger Marble
1 month ago
Reply to  Wayne C

The main reason for TT tires to gain more pressure than the LT tires is that they are forced to support more load relative to their size & inflation than LT tires are.
If you ever look at the Load & Inflation tables and find an LT and an ST type tire of identical physical dimensions you will see that the ST tire is “rated” to carry more load than the LT type tire is. The basic theory behind that increased capacity is that ST tires will be traveling slower because people should not be traveling as fast when towing as when just driving the car or truck. When ST tires were introduced they were limited to 65 MPH MAX in an effort to offset the damaging effects of higher loading than seen in LT tires.

Tire load capacity is basically a function of volume and pressure as seen here Load = K x (Air Volume x Air Pressure) with different type tires having different “K” factor. Tires in LT applications are required to support lower load as a percent of their volume and pressure so they do not have to “work”as hard so they do not generate as much heat.

Please note that the actual load calculation is much more complicated as the response to air pressure is not linear and different aspect tires i.e. 75 series vs 85 series etc have some different factors that are applied to the actual calculation.

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