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Discussion Starter #1
I have always thought that suspension tuning with air pressure goes like this.Which ever end of the car you add air to reduces the grip on that end.Add air = reduce grip.Yet I have read on more than one occasion just the opposite.In fact in a recent MM&FF article about the CMC project car it plainly states "add air= add grip".This is crazy!
 

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I've contemplated over that as well. On my car, it seems the lower the air pressure, the more grip goes into the tire. Maybe it's just me.

However, I can see where the add air = more grip statement. If you're adding air to the tire, you're making the rubber in the tire "work" less(bending/deforming). Like a thin piece of metal when you bend it back and forth a bunch of times, it gets hot and finally breaks into two pieces.

If the tire gets too hot from "working", then I suspect it will get greasy/over heated. If you can make it work less by adding air, you've essentially reduced the temperature with the side benefit of increasing grip.

This is just me thinking out loud, i'm sure tire-guru can come in here and drop some more technical words to answer your question.

With my old V710 Kumho's, on an 80-ish degree day, you could run those suckers in the mid 20's psi and they'd grip great. Put them up to 35 and it felt like glass. The Azenis street tires I use now seem to not have any dependance on tire pressure. 25 to 35, I don't really notice much difference, but then again i'm an ameature at this.
 

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I think there's a range where you get maximum grip. Overinflate and you get the middle of the tire bulging, promoting less grip on the outsides. Underinflate and you get more grip on the outside, less on the center.
 

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^Yep, that's about it. You can lessen grip at either end of the car by going out of the tire's sweet spot in either direction, up or down. Depending on the conditions and the type of tire, it will make more sense to go one direction or the other, most times. E.G. when we were running the 245 V710 on the RX8 (8" rim) and wanted to get it to loosen up a little (rear shocks already at full "firm"), dropping pressures would do basically nothing, because of the sidewall construction of the V710 and the rim section. On the other hand, adding 2-3psi would accomplish exactly what we were looking for. You also end up, if going way one direction or the other, getting into problems with temps, particularly when going down in pressure, and wear. So, playing with pressures can be a good thing at an event, but if you are really sacrificing grip at one end to keep up with the other and are putting yourself in a position to have poor/abnormal wear, it isn't the "answer"...time to look at other parts of the set up.
 

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Discussion Starter #5
Yeah I'm not talking about major amounts of pressure one way or the other.But suspose that the mfg recomendation is 30 lbs.Do you add or subtract air to give that end of the car more grip?
 

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Yeah I'm not talking about major amounts of pressure one way or the other.But suspose that the mfg recomendation is 30 lbs.Do you add or subtract air to give that end of the car more grip?
I dunno.

Manufacturers have a whole bunch of reasons for recommending a certain air pressure be used...ride quality, gas mileage, not having tires explode ;)

None of which is about maximum grip. The only thing you can really do is experiment, see how it feels, check obvious things like roll over and temps, and quantify what each change does on the stopwatch...afaik. Maybe there's a better way, I just don't know what it is...
 

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I dunno.

Manufacturers have a whole bunch of reasons for recommending a certain air pressure be used...ride quality, gas mileage, not having tires explode ;)

None of which is about maximum grip...
"Recomended pressure" can mean a lot of things.

There' the maximum pressure printed on the sidewall. Lots of folks think this is the recomended pressure. It is not. This it the DOT-mandated maximum inflation pressure. In other words, do not exceed this pressure under any circumstances.

Now, if a manufacturer gives you a pressure recomendation for a street tire you can pretty much assume his priorities when figuring out that pressure are:
  1. Safety.
  2. Tread wear
  3. Performance (maybe)
For performance driving, I'd probably start at that pressure and work from there.

Now if we're talking about a race tire (or DOT-legal R-compound tire,) the manufacturer probably does have an inflation pressure he's going to recomend for the best performance. If you're lucky, he'll have several recomendations based on the car and its weight.

If you're lucky.

He almost certainly will have an optimum temperature range for the tire. that's the best thing you can use when tuning air pressures (and alignments.) You want to get your tire temps right, and you want them even across the durface of the tire, inside to outside (usually, but occasionally you'll run across a manufacturer who wants to see a different temperature profile across the contact patch -- the old BFG R1's were like this.)

So, go out and beg, borrow or steal a tire pyrometer and hit the track! Once you get the temp right, note the hot pressure. THAT's your target pressure.

(Unfortunately, that pressure is likely to change with the track, driving style, and weather, so don't take it as gospel -- keep that pyrometer handy!)
 
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