Establishing the Correct Tyre Pressure

by Charlie Brown
Reprinted from The Standard (Capital Triumph Register)

Excepting those purists amongst us who insist on the "correct" look of the Redline, chances are the tyres on your TR aren't the same size or of the same handling characteristics as original equipment (to the latter, I say hoo-ray!). Over the years the numbers on sidewalls have changed, as have tread patterns and compounds. Maybe you've always tried to match the original size with something close, or perhaps you've intentionally "plus sized" your wheels or tires to improve handling. Whichever, there's a good possibility that those inflation pressures listed on your door jamb or in your manual are no longer appropriate to the modern rubber that carries your TR.

You need to establish a correct set of pressures for your car, tyres, and driving style. The variables include load (both weight and lateral force), tyre compound, sidewall stiffness, am-bient temperature, and surface conditions.

What you're trying to do is create the largest possible contact patch of tread on the road, for improved adhesion, while avoiding excessive sidewall flex that would degrade handling and accelerate tyre wear. Assuming your car's steering geometry is correct, front and rear, you achieve this optimum "footprint" by adjusting tyre pressures. And, as you know, over- or under- in-flation is a sure way to kill a tyre prematurely.

There's a little trick autocrossers use to establish opti-mum front and rear pressures during an event. Even though you don't push your tyres as hard (or do you?) the method for estab-lishing pressure is the same--chalk marks.

Three or four small chalk marks, equally spaced, around the outer corner of the tyre, just where the tread meets the side-wall, will provide you with a good indication of under- or over-inflation. A tyre pyrometer (fancy name for a thermometer), used to measure temperature across the full tread (the hotter the area, the more work it's doing) is more accurate, but a lot more expen-sive and complicated to use.

Here's the simple method. Get your hands on a plain stick of white chalkboard chalk. Make the aforementioned marks around the outer corners of all four tyres, extending about 1/2 in. onto both the tread and sidewall (if you use a silicone-based tyre prep, this may be a challenge). Drive your car in a normal man-ner, on a dry day. Take a test drive around the neighborhood. Make left and right turns, maybe go out for a bit of highway speed -- just make sure the tyres are fully warmed up. After some spirited driving check the marks.

If chalk is still apparent at the outer edge of a tread, that tyre is over inflated. If the marks have been scrubbed off the sidewall, you need some more pressure. You want the chalk mark to end just at the corner, about where the tread meets the sidewall. I realize some tyres have tread that wraps slightly around to the sidewall--some interpretation will be needed, but this method will get you real close. What you're looking for is chalk just scrubbed off the tread, but not off the sidewall.

To paraphrase, chalk is cheap. If your marks don't look right, adjust pressure by about 2 p.s.i., re-chalk and try again. Remember, front and rear pressures may be different. Pressures may vary with seasonal changes. Another thing, always try to use the same pressure gauge. I've seen 5 p.s.i. differences be-tween two gauges! Get a good one and stick with it. A 0-60 p.s.i. dial type is more accurate and easier to read then the pencil style.

Once you establish correct pressures for your car, save them in writing somewhere. Remember also that this exercise is just lost motion if you don't check the pressures at least weekly.

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