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Tire pressure?

brygx

New member
Location
Seattle
I'm running 17" w/ PSS at 35 psi. Getting conflicting information on what to do on the track:

40 front, 35 rear, to have more front grip: https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=58

35 front, 40 rear, basically the opposite, to loosen up the rear: http://www.golfmk7.com/forums/showthread.php?t=10471

Go down to 29 or 30 psi all around: http://www.golfmk6.com/forums/showthread.php?t=22225

Go up to 40+ psi all around: http://forums.fourtitude.com/showthread.php?1543553-Tire-Pressure-MKIV

In other words, different threads recommend exactly opposite settings. So. What tire pressure should I be running on the track, and why? GTI doorjamb says 35 psi cold.
 

joesmkv

Passed Driver's Ed
Location
East Bay
How does the car behave for how you drive?

For PSS I would start them closer to 28-30psi cold, and see where you end up. With those tires I would probably want them to be anywhere from 38-40 hot, maybe a few psi either way for the rear depending on the rest of the suspension set up.
 

SuperSkyline89

Das Schiesse
Location
Brampton, Ontario
Ride
2008 CW GTI
The problem with recommending tire pressures is that it depends on so many factors that there's no such thing as the best one. Even with the same car if you drive one way a pressure that works for someone else might not work for you.

When I ran PSS I found them to be too soft so I ran high pressures. At autocross I was right up at 50psi because just going down to 40 made my car understeer like a pig. You don't need as high of a pressure at the track because you aren't putting the car through such sudden direction changes but I'd start at 40psi hot and tune from there.

Use some chalk to mark lines on the sidewall and see how much of it wears off after some hot laps. It'll show you how much the tire is rolling onto the sidewall. If it's rolling onto the sidewall then add air
 

brygx

New member
Location
Seattle
When I was running 35 psi cold (didn't measure hot), the car understeers like a pig and the tires felt like they were soft and rolling over onto the sidewall.

I installed a H&R 24mm rear sway on stiff, disabled traction control, and added air to the front tires only (was in the ~41-42 psi range after a cooldown lap and back to the lot). Went to the track yesterday with that setup:

* On long sweeps, it used to just understeer. This time the tires were still squealing but I was able to hold the turn -- and the rear felt like I was going to slide out if I pushed it more.

* On a sharp turn that comes immediately after a hard braking point, I lost the rear twice and had to save it. I've taken that turn 20+ times and had never lost it until yesterday.

It was definitely scary/thrilling as I've been used to understeer all my life, even on the track. It seems like increasing front psi did indeed reduce understeer. I guess most people would consider this closer to neutral?

Should I bother trying the other combinations listed (like increase rear only) or is this the way to go? If not, why are there so many conflicting recommendations?
 

SuperSkyline89

Das Schiesse
Location
Brampton, Ontario
Ride
2008 CW GTI
It's hard to say what adding more air to the back will do because it depends where you are on the balance of grip. If your pressures are still on the lower end then adding more will give you more grip because it'll support the sidewall better. At some point you'll cross over to reducing grip because of over-inflation. The only way to find that point is with testing though

That's the problem with air pressures. It depends on so many factors from what tire you run, to how you like to drive, to how the rest of your suspension is set up. People will throw numbers at you but they should only be treated as recommendations. Even if they have the same parts you can influence a cars balance in many different ways so depending on how you've set up those other parts you might need different pressures

As far as general setup goes the more you can get these cars to oversteer the quicker they'll go. This page has a really simple overview of all the ways you can go about trying to do that

http://speed.academy/how-to-fix-understeer-or-oversteer-suspension-handling-setup-tuning/

For my own car I have a pretty tail happy set up at the moment. I'm running -2 degrees of camber all around simply for tire fitment. My solid rear sway bar is set to soft and my hollow front to medium, that completely transformed the turn in response but didn't cause any oversteer. I used to have toe out all around but this year went with neutral front and rear toe out along with a corner balance. That made it very tail happy, at autocross I'm full on drifting and at the track the rear is just on the verge of sliding. I can kick the rear out by transferring weight to the front and rein it back in with the throttle. I run 32psi hot on my Hankook R-S3's but they have a very stiff sidewall that lets me run that low.

That's just my current tune though. I'm going to play with sway bar settings, weight reduction, and springs over the next year so I don't know how that'll affect my settings
 
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