checking tire loads by tire temperature

[steve zannella]

Reading the following on an RV web site made me wonder if this could be a way to SWAG the ball park tire pressure short of being able to weight each corner of the RV and setting proper pressure.

Has anyone heard of this or know it to be true?



There is an easy way to arrive at an optimal pressure for each tire on your RV, truck, car, whatever. Get one of the laser thermometer guns. You can find them all over for less than $100. In THEORY, a tire that is perfectly inflated for the LOAD it is carrying, would show the same temperature across the face of the tread. (Inner tread ridge, middle tread ridge, outer tread ridge). In real life there is Camber, and a number of other factors that will affect the inner or outer temperatures. So in real life you are looking for an EVEN SPREAD of temperatures across the face of the tread.

You need to take the coach out on the highway and drive at normal speeds for say 10 miles. Get off at a ramp, and quickly pull over to a safe spot where you can take and record your temperatures. This must be done quickly as the temperatures will even out. For each tire you will record the temperature read on each of the 3 places on the tread.

A tire that has the correct pressure would have numbers of say 130 degrees Inner, 125 Middle, and 120 outer. The actual number values are not important, just the relationship between them. Notice in the above example, there is an even SPREAD across the face of the tread. A tire that is UNDER INFLATED would show say 145 degrees Inner, 130 Middle, and 140 Outer. In this case the Middle is too cool in relation to the Inner and Outer. A tire that is OVER INFLATED may show 130 degrees Inner, 130 Middle, and 120 Outer. Now the Middle is too warm in relation to the Inner and Outer.

In this way you can arrive at the optimal pressure for each tire. Just make sure you are not going outside the manufacturer's range.

In my case Goodyear says 95 lbs. This is the cold pressure for MAXIMUM load. Airstream says 70 lbs. (345 Motorhome) I have found that with our normal load, the fronts are 78 lbs., the duals are at 77 lbs., and the tags are at 72 lbs.

I hope this provides another idea that can help.

[Karl Welhart]

Steve,

This guy's theory comes from automotive racing.  I think in ideal conditions it could work very well for our motorhome applications.  However, IMHO there are too many outside factors that will make this theory hard to be accurate in practice.  For example, sun light, wind direction, inner/outer rears and so on will play a big part in the heat measurements on the tire face.  As this guy points out, the amount of time from when you stop until the time you take the temp reading will make a huge difference.

Anyway, I think the proven procedure of weighing each corner and using the tire pressure/load rating charts from the tire manufacturer are the best way to adjust air pressure.  I do (as some will disagree) adjust each tire for that load (i.e. LF=115, RF=105, RRs =95 and LRs 105) based on those real weight measurements.

[Gerald Farris]

Steve,
I do not think that this method is one that should be used on a coach because there are too many variables that can cause you to underinflate your tires, and under inflation is dangerous because it can cause a blowout. In my opinion, you should run your tires at maximum inflation for safety reasons until it is weighed at each corner, and then adjust your inflation pressure to the proper level by the tire manufacturer's inflation table. Also, I run the same pressure in all tires on an axle as recommended by the tire manufacturers by using the load on the heaviest loaded tire or dual to calculate the pressure.

Gerald