82transam

iTrader: (18)

Ok, I did some searching, I kinda firgured this would be readily available info, guess not. What I really want to know is what the theory behind changing tire pressures is. Like, If you want more understeer would you lower front pressure and raise rear or whatever. Does anyone know of any good sites describing this? I was trying to figure it out before and the logic escapes me. A good explination should help. TIA

peejay

Seems that raising the pressure increases lateral grip while decreasing acceleration and braking grip, and vice-versa.

At least, this has been true for pressures between 50 and 20psi on the stock size tires/rims. At some point increasing pressure decreases grip in all directions, and at another point decreasing pressure decreases grip in all directions. For the tires I've been running, 20psi is the lower limit, below that forward traction drops off as well as lateral. (Don't e'en *think* about lateral grip at 20psi ) And I am really leery of going beyond 50psi as well, I've been near tire explosions (one only 10 feet from where I was standing) and it is not something you forget easily.

ajazeller

The answer can get slightly complex.

But generally if you lower the tire pressure the tires sidewalls become "more flexable" and thus reduces scrub. Thus if you lower the front tire pressure the car should turn in easier (less understeer on corner entry). if you raise the rear pressures then you will increase oversteer on corner exit. However you can drastically affect wear, and not fully utilize available grip if you just use this method.

The best way to deturmine the correct pressures (on a track) is to take the temperature accross each tire. It should have the same temp accross the tire...If it doesnt you eather change pressures, toe, or camber depending on what temp readings your getting. But this is probably well above what you need to know....

On a road car, just set tire pressures as directed by the tire manufacurer. If autocrossing, learn about the shoe polish method. If road racing, buy some books and start reading.

84stock

iTrader: (5)

quit guessing with regard to such an important subject. As a 41 year old with 12 years in the tire industry I can give you the "right" answer with no dangerous bullshit. The most important thing to remember is that the tire is a medium to contain air which supports the vehicle. 95% of the vehicle is supported by the "air", not the the tire. Soooooo, when you underinflate, the tires have to compensate, and they have their limits!! A severely underinflated tire can remain intack as the air rushing by it help to compensate for the heat build-up. But once you stop. kabooooom! You can adjust pressure to a degree that isn't below the weight carrying capability of the tire (read the sidewall). This will help create or compensate for under/over steer. Some feel their tires stick better underiflated, that is because they get dangeroulsy overheated and the compound starts to sfoten with the heat. Most tires have at least 12 different compounds bonded together, this stress often lead to tire failure. The firestone 500 (old guy will remember this one) was famous for blowouts. The reason, salt penetration though the compounds permeated the steel belts causing the bonding of materials to fail (this product was recalled and sold in South America). The firestone SUV tire manufactured for Ford, here they used lighter plies of polyester to improve ride characteristics. But since non-entusiasts don't monitor tire pressure, the treis were often stressed past their limits. If you think this is bullshit, take a guage and check your tires, most of you will be undrinflated by at least 5psi. That's 15% below recommendation and causes the tire to stress. This is why your 150,000 km tires wear out in 80,000 km and you show up on my door bitching about it.

84stock

iTrader: (5)

sorry about the spelling error, didn't proof read, lol

peejay

80,000km on a tire? I've heard of such a concept but I've never experienced it.

My tires go from new to half worn in about 30,000km... at which point the tread is getting "hard", wear slows down drastically, and grip is pretty much gone since the rubber is more like a hard plastic.

The '96-99 Ford Tauruses came with tires that would last an easy 180,000km (yes, the conversion is correct, I really do mean 100,000 miles) with some tread remaining before the wear bars show. HOWEVER even though the tires wore like iron, they would grip like iron even when new. I rmember changing a lot of those tires with only 30k on them (miles, kilometers, either one) and tons of tread left, but the customers found them to be outright dangerous in the wet and in the snow. I forget what brand tire they were, General or Uniroyal, but they sucked heavily. The leasing companies loved 'em, since they'd never have to change the tires over their life of the car.

84stock

iTrader: (5)

the original Michelin were like that. Lasted forever but gripped on nothing!

rototiller1

goto a nascar tuning site, may nascar pc game even tell how to setup changes to make for tire pressures

84stock

iTrader: (5)

I you want a "safe" way to feel dramatic cahnges in handling with tire pressure changes, go rent a go-kart (one of the fast ones) for 30 minutes. You'll have quite the experience.

ajazeller

The trouble with a lot of people in the automotive industry is they assume that everyone else knows NOTHING. This is not only insulting but in most cases incorrect.

The guys question wasnt "what will happen if I under inflate the tires", it was along the lines of "how do I adjust my cars handling by changing pressurs" In which case my answer was correct.

Jeff20B

iTrader: (13)

My bike's front tire exploded once while filling it up at a gas station. That compressed air fills those little tires fast! Anyway, I was squating to get low enough to the tire to fill it, and the blast knocked me on my butt. I was deaf for a minute or two after.

The tire was previously filled with that fix-a-flat stuff and it luckily ruptured facing away from me. Heh, it sprayed that milky stuff all over the gas station's full service stall. Good thing there wasn't a car there at the time.

nickj

You shouldn't try to change the over/understeer ratio with tire pressures. A small difference won't change things much, and a large difference risks underinflating the fronts. You want to run all four tires at optimal pressure. Change the over/understeer ratio with springs and sway bars.

mar3

iTrader: (8)

Decreasing tire pressure in the rear will increase oversteer, contrary to intuitive belief...

A difference of two pounds of air in the rear killed my neutral handling at MSR last year. With 1.5 hr "rest" periods between track intervals, I can safely say they were at cold tire air pressures. I started at 35 all the way around and went to 37 in the rear because I thought I was good up front but still over-steering a bit...I shoulda gone with just one, the front started plowing like a drunkard in the second session.

My tire pressures are adjusted on all 3 vehicles monthly and the Aerostar every other month...I hate the thing, but I'll never turn down a free vehicle...

Rx7carl

All bullshit aside. Controlling handling with tire pressure is a crutch you should only use in an emergency. The NASCAR boys do it cause its the only adjustment they can make on a pit stop without losing time. Even a track bar or wedge adjustment costs valuable time. Think of a tire as a spring, cause thats really what it is. So your would use tire pressure as increasing or decreasing spring rate for handling purposes mostly. I say mostly cause the other thing to consider is this. It takes tires a few laps to come up to operating temp and build enough heat to raise the cold pressure up to the hot operating temp target. So for a short stint, when you get tires, you get them with a higher pressure so they get to the proper pressure quicker. Doesnt matter if they would get too high in 20 laps if your only running them for 5.


Now, lets talk about getting those pressures right. A pyrometer is a must. And not one of those infrared ones. Those are great for alot of things, but a probe type is best for tires. But if that all you have, use it. If your running neg front camber, expect the inside to be a few degress hotter than the rest of the tire. The center and outside should be equal. The rears should be equal all the way across as much as is possible. Center too hot? Drop the pressure. Outside/inside too hot? Add some pressure. If going low buck, use the shoe polish method mentioned earlier.

Bolox

sorry to be picky.. but if you ment 100,000 MIles.. thats closer to 160,000 Km (or 160,934Km)

silly imperial units

-Cheers

nickj

It ought to be intuitive that decreasing rear tire pressures increases oversteer, because the rears will have more "give" and run at higher slip angles. Same goes for decreasing front pressures to control oversteer.

Many more variables than tire pressure affect handling -- track temp, fuel load, surface conditions to name a few. Two pounds of tire pressure isn't going to make that much difference, not on a stock RX-7. We played with tire pressures a lot in our SS days, and in the 35-40 psi range a few pounds one way or the other wasn't critical.

Pyrometers are great, but they can't beat a stopwatch.

82transam

iTrader: (18)

Thanks for all the responses guys. I know the dangers of under/over inflation as well as the fact that unless is a highly modified car, being set up for a specific set of conditions adjusting the tire pressure a few psi either way is pointless. I merely wanted to know what the theory behind it was. thanks.

mar3

iTrader: (8)

Intuitive means it makes common sense w/o knowng why...your in-depth explanation rules out the intuitive factor, you consciously understand why...