REDHOT2
REDHOT2
6/25/17 7:13 p.m.

I'm trying to fine tune the tire pressures on my car for open track use. My car is a 2004 VW R32. According to the Tire Rack Tire Tech,in order to reduce understeer, you could increase front tire pressure and/or reduce rear tire pressure. However, in the Grassroots Motorsports trackside companion booklet that was given to me at Track Night In America, the chassis tuning cheat sheet states just the opposite. Lower pressure in the front and higher in the back. My goal is to reduce understeer as much as possible.

Naturally I'm a bit confused. Could you give me your input on this topic.

Regards,

Tom

Pete Gossett
Pete Gossett MegaDork
6/25/17 8:16 p.m.

First step is getting your front temps set. To do this correctly you really need a pyrometer, however unless you're running camber plates you probably won't have enough camber to get reasonably even temps across the tire. If that's true you just need to adjust the front pressures until you get the best grip.

At that point you can start working on the back pressures. Yes, either increasing pressure, or lowering it, outside of the range of optimal grip can reduce traction at that end. Generally, the stiffer the sidewall(DOT tires or extreme street) the more likely increasing pressures will reduce the contact patch, and thus grip.

bentwrench
bentwrench Dork
6/25/17 8:35 p.m.

Test test test

If your pressures are already low or high you will only be able to change one.

You can adjust the sway bar at one end or the other also.

Tire temps should be a bigger concern for pressure choice.

Reducing tire grip with pressure may hurt you in other areas.

Also, Too tight can make it snap loose when the tire becomes overloaded.

Sway bar, shocks and alignment all need to be worked on to find your base line.

REDHOT2
REDHOT2 New Reader
6/27/17 9:39 a.m.

Thanks for the feedback. I do have a pyrometer and will be using it soon. I am running 35psi in the front and 25psi in the back currently. Front shocks are set to full soft and back shocks set to full hard. Front sway bar set to full soft and rear set to full hard. Front camber is set to 2 1/4 negative, 0 toe, and the rear camber is set to 1 1/2 negative, 0 toe. I think I have a good baseline now, the tire temps need to be a priority.

Thanks!

NEALSMO
NEALSMO UberDork
6/27/17 10:02 a.m.

The Tire rack recommendation is the opposite of what I've been taught and actually found to work. Understeer is lack of grip in the front, so higher pressures would reduce grip even more. I use to run about 4 psi higher in the rear of my GTI to induce oversteer.

jfryjfry
jfryjfry Reader
6/27/17 12:15 p.m.

Raising or lowering pressure depends on where you start.

If you have 5lbs of pressure in your fronts and start raising it, grip will get better until a certain point and then drop off. Same if you start at 80 psi and start dropping.

But changing the rears will/could affect the fronts as well.

It's all relative - and all black magic ;)

REDHOT2
REDHOT2 New Reader
6/27/17 6:01 p.m.

Thanks!

djhedges
djhedges
6/15/18 6:14 p.m.

The first car I tracked was an 04 R32.  It's still stock and occasionally sees the track over the race car.

My recollection from my last event was something like this.  1st session cold 36psi front/44 psi rear.  Less grip than usual in the front and too much lift off oversteer.  Dropped the pressures by ~2-5 psi.

Remaining sessions I kept  dropping the fronts down to around 39psi hot, maybe 40.  Rears were around 46 hot if I recall correctly.  I'm running Contential DW tires and the fronts were pretty done after the event.  Luckily it was only a single day.

The reason I run so much pressure in the rear is to intentionally reduce grip and help the car oversteer.  The race car is FWD and we run a staggered tire setup.  Skinner and less grippy tires in the rear.

Keep a close eye on the rears because you don't want to run more tire pressure than the recommended tire limit.  Next time I'd start cold at 33 front, 40 rear.  If you get a chance go to an autocross and go crazy with air pressure +-5psi between runs.

Also try running a bit (1/16"-1/8") of toe out in the front.

stylngle2003
stylngle2003 Reader
6/21/18 11:58 a.m.

I believe, based solely on what you posted, that your rear pressures are too low, but would have to see how much sidewall roll and tire temp variation you're seeing to know for sure.

 

alignment specs seem about like a great starting point.  may need more front negative camber depending on the pyro reading. the only other feedback I can give is with your front shocks/bar and rear shocks/bar at the maximum of one end of the spectrum, you don't really have anywhere to go but one direction on both.  

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