Tire presure revisited

[PShipman]

So ... as Curt would say ... this is definitely a slippery slope ...

So I learned that a set of Weltmeister bars both set to a mid bar setting definitely makes the S2 a lot looser in the rear than the stock bars (so I yesterday softened the rear by moving from 15mm to 7mm from the bar end)

But I am still a bit confused by differing opinions on tire psi (let's just talk in COLD psi for purposes of this discussion) ...

I ran the RAs at 31 front and rear (both 225/50 x 16) and never fiddled with it on Saturday ... and I got heard from a couple of people that I should drop the pressure in the rear a few lbs to increase grip ... BUT I heard from another person (who should know) to increase the psi in the rear by a few lbs to stiffent the side walls and increase grip.

Now I know that there is a continum where a psi of 0 lbs or a psi of 1000 lbs will be no grip extremes but where/how does psi affect grip in the middle 20 lbs to 40 lbs range??

[Jad]

You really need to talk HOT not cold. Each tire will warm differently based on the track (right side more heat turning left, etc). According to Toyo, your tires need to be about 38-40 psi hot for optimum grip. Once you have the car set their, check the pressure when the tires are cold the next day to get a rough idea of cold starting pressures. ~31 seems ok for a warm day, probably too low for a cold day.

As far as other adjustments, trial and error is the best way. Lower the rear 2 lbs and see what you think. Generally, lowering the end with less grip is the rule, and RA-1's are so stiff, sidewall flex is not an issue at reasonable pressures.

Have fun on the slope

[PShipman]

Jad ... thanks for the great answer ...

And ... how do koni ajustments affect the equation???

[Jad]

The koni's work the same as the sway bar, just to a lesser degree. Soften the end you want more grip in, or stiffen the other as the fronts are easier to adjust.

[PShipman]

Thanks again ... by the way, Jad, I really liked your in car video ... intersting to see your line and how you would really "stay out" much longer than I do ... gave useful "food for thought" for the next event!

[Jad]

Should be better video when I dial out the miserable push the car had. Not enough runs to get things dialed. First session it pushed too much so I changed the shocks for the second session since I was too lazy/busy to do the sway bars. Didn't work, so for timed runs I tried different tires, and that made it worse, so my line probably wasn't too good as I really was fighting and trying different things to overcame the inability to get the tail around

But, staying out is very important. I think about 80% of the drivers turn in too early. The problem I had, when I went to turn in, the car would not pitch, it just plowed to the outside. You should stay out, turn to the apex and hit full gas by the apex, not drift to the track out while lifting and gasing to get any rotation like in the video Ted posted of my driving.

[ttweed]

... by the way, Jad, I really liked your in car video

I didn't know what video you were talking about at first, Perry, but I think I found it on the "Photos" page--the "Video 2" on this page, maybe?

... intersting to see your line and how you would really "stay out" much longer than I do...

Yah, it looked like he was "staying out" all the way around some of those corners!

TT

[PShipman]

Yah, it looked like he was "staying out" all the way around some of those corners!

Speaking of staying out, Mr. Tweed, someone asked at the start line if your "rocket ship" should be disallowed ... until you roll down the windows!

[lrayner]

Jad, in general, would the adjustment approach you suggested for oversteer/understeer be the same approach to take to try to reduce rear wheel spin; that is, to think of it as the same as oversteer?

And, yes, I am fully expecting any reply to include a reference to ones tendency to stab at the throttle, as demonstrated on the corner working truck

[Jad]

That is correct (on both statements ). The wheel spin is caused from the car leaning and lifting the inside rear wheel. With little weight on it, it spins easily, so if you flatten the car, by stiffening the front and softening the rear, there is more weight on the inside tire and it will be less likely to spin.

Your other options are to have the car straighter when getting on the gas or roll onto it smoother and earlier to avoid the breakaway causing jab of the throttle.

[PShipman]

That is correct (on both statements ). The wheel spin is caused from the car leaning and lifting the inside rear wheel. With little weight on it, it spins easily, so if you flatten the car, by stiffening the front and softening the rear, there is more weight on the inside tire and it will be less likely to spin.

Thanks ... this is very useful info addition!

[ttweed]

Speaking of staying out, Mr. Tweed, someone asked at the start line if your "rocket ship" should be disallowed ... until you roll down the windows!

The side windows can be removed, by undoing about 20 fasteners per side, but they can't be "rolled down", unfortunately. Taking them out would certainly improve ventilation in the cabin, so I could turn the A/C off and gain a few more HP.

TT

[ttweed]

The wheel spin is caused from the car leaning and lifting the inside rear wheel.

Your car does not have a LSD, Jad?

TT

[Jad]

My recent cars have had LSD's, but my n/a 944's and 944S2 did not. The new 911 has LSD AND weight on the back AND push, so putting the power down is definitely not the issue thus far with the new car....

I think the LSD (or lack thereof) is why a 2.7 Boxster will beat an S or a Cayman at the AX, as 250+ HP is too much for 1 tire, but spec cars and S2's with 208 hp or much less, can do fine without them from my experience. They don't hurt, but I feel the gains are very minor if any in a low HP car at AX.

[ttweed]

The new 911 has LSD AND weight on the back AND push, so putting the power down is definitely not the issue thus far with the new car....

Oh, OK...I thought so, but I misunderstood your answer to Leigh as meaning you were having a problem with wheelspin in the new car.

My bad,
TT