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-   -   Another First Auto-X & Tire pres questions (http://986forum.com/forums/showthread.php?t=40316)

jb92563 12-01-2012 09:33 PM

Another First Auto-X & Tire pres questions
 
Did my first Autocross recently, it was blast!.

The instuctor mentioned that the rear tires were not "Locking Up" and said I might want to adjust the tire pressure.

We did not get a chance to make adjustments so i never found out how.

Any guidlines on how to adjust tire pressures when they could use some tweaking to get better perfomance?

I was running 32psi front and 36psi rear on Hankook V12's with 215mm wide front and 265mm wide in the rear.

Seems my rears where breaking away a bit too early so I could not take full advantage of my manueverabilty and speed in the curves and had to slow up too much.

I guess I need a bigger contact patch in the rear to help it stay attached to the ground.....more or less psi?

Mrmaddbrad 12-02-2012 11:24 AM

Heres a quick, super general guide to tire pressures:

1. Less PSI means more grip.

2. More PSI means less tire roll.

3. Play with them to learn what works.

4. When it comes to boxsters, count your blessings when the rear breaks away before the front at an autocross. It seems they all understeer like crazy.

NOTE:
You're tire pressures may be fine. However; playing with them helps to learn your car. Different weather and driving conditions will always make your car behave differently. As a SCCA national champion once told me "Even if your tire pressures are right, if you're not driving your car properly it may seem that they're wrong."

Best of luck,
~Brad

jb92563 12-03-2012 12:19 PM

Thanks for the tips.

I guess I need more grip in the rear so lowering the pressure a couple PSI may help that.

Since the fronts are sticking well, my turn ins are very sure footed with the help of brake to load the front tires, but the rear tends to come unstuck as the direction change completes so perhaps I need to get on the gas sooner as well to load up the rears to keep them stuck down.

I think I'll start a notebook to track what I did and whether it improved things or not.

I'll drop the rears 1 psi to 35psi for next time out.

Next I need "Harry's Lap Timer" so I can video and analyze what I did that day.


I was hoping to get a lot more feedback but perhaps the racers are on other forums?
Is there a busier Racing tips forum somewhere or do the racers share no secrets in general?
Just trying to learn and am looking for more info on tuning tires.

Flavor 987S 12-03-2012 12:29 PM

Ray, how old are your tires?

san rensho 12-03-2012 12:36 PM

Quote:

Originally Posted by jb92563 (Post 317904)
Thanks for the tips.

I guess I need more grip in the rear so lowering the pressure a couple PSI may help that.

Since the fronts are sticking well, my turn ins are very sure footed with the help of brake to load the front tires, but the rear tends to come unstuck as the direction change completes so perhaps I need to get on the gas sooner as well to load up the rears to keep them stuck down.

I think I'll start a notebook to track what I did and whether it improved things or not.

I'll drop the rears 1 psi to 35psi for next time out.

Next I need "Harry's Lap Timer" so I can video and analyze what I did that day.


I was hoping to get a lot more feedback but perhaps the racers are on other forums?
Is there a busier Racing tips forum somewhere or do the racers share no secrets in general?
Just trying to learn and am looking for more info on tuning tires.

If you are on the brakes as you turn (trail braking) then that explains the rear coming out. Trail braking does that, which is one way to get around understeer.

stephen wilson 12-03-2012 01:44 PM

I've only done one autocross, but noticed that the quick transitions of slaloms make a normally understeering car oversteer. Sort of like the "scandinavian flick" in rally racing.

jb92563 12-03-2012 01:48 PM

Quote:

Originally Posted by Flavor 987S (Post 317909)
Ray, how old are your tires?

2 months, Hankook V12's

Granted its my daily driver but there is at least 60% or more tread left and I only did one other "event" the Performance Driving school a month before.

jb92563 12-03-2012 01:57 PM

Quote:

Originally Posted by san rensho (Post 317914)
If you are on the brakes as you turn (trail braking) then that explains the rear coming out. Trail braking does that, which is one way to get around understeer.

I brake before any turning takes place like we were taught in the PDS except where I end up coming in too hot every now and then and I definitely know my mistake at that point and the consequences of breaking in any part of the curve.

I mostly noticed my rear tires loosing grip a bit in a big sweeping curve while accelerating.
It was throttle steering through most of that part and the instructor felt the tires weren't "locking up" as well as they could.
To make matters worse the track was banked AWAY from the inside of the turn.

He mentioned adjusting pressure but did not say which way before he had to jump in his own car to grid for his laps.

san rensho 12-03-2012 02:28 PM

If its not trail braking oversteer,then I would lower the pressure in the rears to get more traction. On the track, I usually go 2-4 PSi lower in the rear than the fronts.

Topless 12-03-2012 03:23 PM

Hi Ray, glad you had fun out there.

Don't sweat tire pressures too much at this point. A downhill, off camber sweeper is a very tricky corner to do right and requires getting the front and rear tires to "set" with an even slip angle and very smooth throttle input to maintain corner radius, slip angle, and good corner exit. 90% of problems with off camber corners is just going in too hot with the car unsettled. Next time let your instructor drive the car for a lap to really feel the car balance if you think there is a problem.

After you do this for a while you will find that grip levels and car balance often change throughout the day as track surface temps change. Adapting your driving style to meet changing conditions is just part of the game.

For AX I generally start at 32psi cold and recheck and adjust immediately after each session so I end up with 36 hot pressure going into timed runs.

thstone 12-05-2012 12:38 PM

As Brad noted above, everything is a tradeoff and the trick is to find the sweet spot for your specific car, tires, and level of driving skill.

Its not that we don't want to share our "secrets" (hah, I wish I had some!), its more that we don't want to give you advice that might not help or advice that is all over the place. A lot of "racing" forums will be stacked with guys who have 911's and what works for a 911 won't nesscessarily work for a mid-engined Boxster. And even here in the 986 world, what works best for me (full spec suspension, lowered ride height, lots of negative camber, r-compound tires, and 50+ track days driving experience) might not work for you.

With that being said, in general, you probably want to start by lowering the rear tire pressure to get more rear grip. While doing this, you'll want to monitor how far the tires (front and rear) roll over in the corners to make sure that you're not going too far onto the sidewall. There are little diamond/triangle markers on many tires that you can use as a reference to gauge this. If you get too much tire roll, then increase the fronts and rears by 1-2 psi and test again.

Also, you'll want to find what tire pressure results in the most grip. Some tires grip more with lower pressures (around 30psi hot) and other tires grip better with higher pressure (mid-to-upper 30's). You'll also have to determine (most through experience) how much pressure buildup you get between cold and hot (dependent on your driving style, the track, the exact tire, and the weather conditions). With all of that being said, I agree with Topless' advice that a goal of 36 psi hot (front and rear) for your Hankook V12's is probably a good goal (start with fronts and rears at 32psi cold).

As you can see, you are trying to control several variables (grip, tire roll, pressure gain from cold to hot, etc) with one input (starting tire pressure) so its not as simple as "just set the tires at XX psi" kind of answer.

Hope this gets you heading in the right direction.

Eric523 12-05-2012 03:59 PM

Quote:

Originally Posted by Topless (Post 317990)
Don't sweat tire pressures too much at this point. A downhill, off camber sweeper is a very tricky corner to do right and requires getting the front and rear tires to "set" with an even slip angle and very smooth throttle input to maintain corner radius, slip angle, and good corner exit. 90% of problems with off camber corners is just going in too hot with the car unsettled. Next time let your instructor drive the car for a lap to really feel the car balance if you think there is a problem.

After you do this for a while you will find that grip levels and car balance often change throughout the day as track surface temps change. Adapting your driving style to meet changing conditions is just part of the game.

For AX I generally start at 32psi cold and recheck and adjust immediately after each session so I end up with 36 hot pressure going into timed runs.

I agree. An off camber corner with the typical poor road surface of an AX circuit is difficult to set-up for. Adapting driving style for this particular corner is likely the best answer if the car felt OK everywhere else. The recommendation of 36psi hot on street tires is pretty spot on as well.

thstone 12-10-2012 11:24 AM

Quote:

Originally Posted by jb92563 (Post 317904)
... my turn ins are very sure footed with the help of brake to load the front tires, but the rear tends to come unstuck as the direction change completes so perhaps I need to get on the gas sooner as well to load up the rears to keep them stuck down.

Braking too hard and too late can be detrimental by putting too much weight on the front and causing the car to be unbalanced in the turn. As you note, turn in is great but then the rear slides around. We've all experienced this condition at one time or another.

Next time maybe try to keep the car more balanced by braking smoothly before turn entry and then getting back on the throttle smoothly as soon as possible (by end of turn entry or at the start of the middle of the turn) to keep the front and rear balanced. This technique will get weight back onto the rear tires as early as possible and also allow you to throttle steer through the middle and exit of the turn which will reduce steering wheel input, tire slip angle, and understeer which all serve to slow you down.

Last, remember not to pinch the turn entry and unwind the wheel as soon as possible on turn exit. The tendency of new track drivers is to try to overdrive the car which tends to result in a lot of understeer. Unwind the wheel and let that beast free!


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