Contour Enthusiasts Group Archives Forums General Autocross and Open Track What tire pressure are you all running at?

With my KDW's i am running 40 fronts and 36-38 rears with an 00 SVT. Someone at the track had a sophisticated scale (drive up on 4 big metal plates) and my car with 1/2 tank of gas and no spare or anyone in it weighed 2900lbs almost to the tee. The only ****ty part is that our weight distribution is terrible. 64/36 Yikes!!! Only car that was worse was a Saturn. Best was an M-coupe with nearly a perfect score 49/51. Again what is your tire pressure?

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2000 SVT green
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I have a lowly GL with the 2.5 and MTX. I have tried running with "factory" pressure, and with max pressure for my tires (35 front and rear). It *appears* that I run better with higher pressure.

my $.02....
What else have you people tried?

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95 GL MTX 2.5L
mild enhancements

I'm not vastly experienced at autox so take this with a grain of salt. I'm running the stock KDWs also. First time out I ran 35psi all around. Found out later that really wasn't high enough compared with what the experienced guys were running. On that day the car had a full tank of gas and was really pretty neutral. Had some understeer but that was pretty easily controlled.

Next time out I upped the pressure to 44psi all around and thought I'd help the car by running the fuel down as low as possible. The thing pushed like a pig!

This week I'm going back to the full tank of fuel and probably 46psi in the front and 38-40psi in the back. We'll see what that does for the push.



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Chris
2000 SVT

With the cars' tendency to understeer - push like a pig, at 44psi ATWA you'll actually want to do the opposite of what you just stated...

You'll want to increase air pressure in the rear (not much from 44 though) and/or decrease air pressure in the front. You want the front tires to bite some more and the back tires to 'slide'. The other way around the best off you'll be is the same as you were, but most likely worse...

Try it at 36-39 front and 43-46 rear - see what that gets you.

derrick

Ran 48psi front and 42 psi rear today, with a full tank of fuel. Car was much more neutral with less of tendancy to push. Next week I'm going to try an even bigger split.

Here's the theory behind my pressures. A tire's pressure works along the same lines as a shock absorber. A firmer shock at the front would allow less weight transfer. A firmer tire does essentially the same thing. When the Contour is pushing it's because the front tires are being asked to re-vector too much inertia (ie. too much weight is being transfered to the front). In a car with already poor weight distribution you need to keep as much of the weight to the rear as you can. This is also why I chose to run with a full tank of fuel. Picture a Porsche 911. They are tailhappy (loose) because of the inertia all that weight in the back carries when you try to transition it into a corner. Sure lowering tire pressure works when you're looking for straight line traction on the drag strip or in the snow with a rear drive car. It allows more weight transfer! But when the problem is too much weight tranfer (braking and turning on the autocross course) more pressure (to a point) will help the car turn.



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Chris
2000 SVT

I ran with a lot less pressure then any of you just four or five pounds over stock, I also ran with 3/4 tank of gas, I guess the fuel is great for balance and weight dist, had its fare share of push but overall I thought the car was very chuckable, very netural and I was impressed.Loved it and it did quite well for a bone stock ride.

> A tire's pressure works along the same
> lines as a shock absorber. A firmer
> shock at the front would allow
> less weight transfer.

Actually, a tire works like a spring. The air-pressure in the tire is a somewhat adjustable spring rate, but the rate is so high in comparision to a regular suspension spring that its basically irrelevant for our discussion (> 1,500 lb inch).

Shocks do not affect the total weight transfer, they only affect the rate that the weight is transferred. Let's say you have a gas pressure shock that requires 50 lbs to get it to compress, once that 50 lbs is reached, shock will compress all the way. Addiing and additional 10 lbs will only increase the rate of compression.

> When the Contour is pushing it's
> because the front tires are being
> asked to re-vector too much inertia
> (ie. too much weight is being
> transfered to the front).

In essence the tire is overloaded. More weight on a tire gives it more traction, up to the point that it can't take any more. Air pressures affect a tires traction capacity to the extent that the tire's contact patch is optimized. It is better to error on the side of too much pressure because the capacity fall-off is shallower on the high side.

> But when the problem is too much
> weight tranfer (braking and turning
> on the autocross course) more
> pressure (to a point) will help
> the car turn.

High tire pressures give a tire a better turn in feel (firmer shocks have this affect also.) It can reduce corner entry understeer but mid-corner understeer is most affected by chassis balance and corner exit understeer is most affected by a driver's right foot.



All the above nit-picky stuff aside, Chris is probably going in the right direction.

In general I would mark the side of the front tires with shoe polish/chalk to check for rollover. Make sure that you are using all of the tread and not rolling the tire onto the sidewall. Look for scuffing to the tops of the treadwear indicator marks.

Pressures greater than what's needed to maximize the contact patch are used to reduce traction to some degree and balance the vehicle. Once I had the front tire pressures figured out, I would use rear tire air pressures to balance the car.


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John Coffey
johncof@veriomail.com

John, you're absolutely right. Well said.

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Chris
2000 SVT

NASCAR calculates tire spring rates at about 50 lb inch per 1 psi. That means if you are running 48 psi in your tire you have an effective tire spring rate of 2,400 lb inch.



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John Coffey
johncof@veriomail.com

Anyone ever thought about using helium in there tires to make the car lighter? I know it sound retared but i had to ask.

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-Andy Watson
'99 Silver Frost SVT
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Tint, Pullies and Chip!
Soon Hopefully.