How to determine tire pressure on non stock tires - Smart Car Forums
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#1 (permalink) Old 05-31-2011, 10:02 PM
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How to determine tire pressure on non stock tires

I have heard an easy way to tell if you have too much or too little pressure in your tires, is to draw lines accross the tires with chalk and roll your car down the driveway. Then look at the line. No chalk on the edges of each tire, underinflated. No chalk in the middle, overinflated. Try to get the line to wear even. Outside edges erased of both front tires, too much toe in. Inside edges, too much toe out. Steal your kids sidewalk chalk and try it. Beats waiting for the tire to wear out to discover you needed more or less air. Besides sidewalk chalk is dangerous to leave lying around. Ran over one with the lawnmower and broke my toe once.
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#2 (permalink) Old 06-01-2011, 06:52 AM
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Originally Posted by Stretchmobile View Post
I have heard an easy way to tell if you have too much or too little pressure in your tires, is to draw lines accross the tires with chalk and roll your car down the driveway. Then look at the line. No chalk on the edges of each tire, underinflated. No chalk in the middle, overinflated. Try to get the line to wear even. Outside edges erased of both front tires, too much toe in. Inside edges, too much toe out. Steal your kids sidewalk chalk and try it. Beats waiting for the tire to wear out to discover you needed more or less air. Besides sidewalk chalk is dangerous to leave lying around. Ran over one with the lawnmower and broke my toe once.
You HAVE heard of a tire pressure gauge, right !!
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#3 (permalink) Old 06-01-2011, 07:44 AM
 
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Stretch is trying to do it right. Just measuring inflation pressure won't show if the tire is making proper contact all the way across the tread. Over inflated will result in more of the tread in the middle doing the work and under inflated will be result in the edges being worn first. This is especially true for those of us that don't leave the stock tires and wheels on. There are a couple other ways to also check this that I use because my "kid" lives 1,100 miles away and has two "kids" of her own, but they don't often bring chalk for me to play with.

First I go to a big parking lot a few hours after a light rain. I look for low spots that have a little water standing and then make a gradual turn and coast through them. What that does is make a wet area on the tire (not the whole thing) and then there will be a series of four wet marks as I coast to a stop. The first mark will usually be a full width because the tires just left the puddle, but subsequent ones will have less and less water left and the footprints show clearly what area of the tire(s) have more contact pressure per square inch. Well that was how I did it in the 70's. But that didn't take into account the fact that the tires change shape with speed or how the alignment will impact what the tire is subject too, but it's a good start.

Today I drive about 25 miles, take out my hand held, infrared, battery powered, temperature gauge, shoot the tire across the edges and center and see if it's within 2-3 degrees of the same temperature, that way I know I'm close enough for my average driving. We're playing with handgrenades here, not horseshoes, unless on the track.
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#4 (permalink) Old 06-01-2011, 05:50 PM
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Pressure gauge works very well when you know what pressure to put in the tires. Stock tires should be inflated as stated on the vehicle plate. If you change tires to other than stock, everything is different. The same size tire on different widths will need different pressures to wear evenly and get the best handling. Front tires require different pressure from the rear because of weight bias. We can use 15, 16 and 17" diameter wheels, with widths from 4.5" to 8.5" in 1/2" increments. Too many tire sizes to list. We can stagger tire sizes. This can help you zero in the right pressure for the combination you are using. Once you know the pressure that works you can use the gauge to make sure it stays there.
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