Tire Pressure for RS 6 9 spoke wheel?

[carbonLORD]

Hello all.

My RS 6 wheels cam in the other day from a Daytona in LA which upgraded to 19's at delivery and I just wanted to confirm the correct psi for my A6 4.2.

Suggestions? I have 37.5 front 36 rear currently. Too high, too low, stagared psi no good?

Please advise.

Thanks!

<img src=http://www.carbonLORD.com/42RS6.jpg>

[TheBrit]

I don't know off the top of my head, but I know the pressures listed on the filler flap on my RS 6 suggest the same pressure all round. The 19's are 40psi, so I guess you'll be looking in the 36-38psi region.

And I just wated to say that your car is looking awesome, cL.

[carbonLORD]

No responses on AW :confused: I appreciate it.

[jimmy94507]

From the door tag (same symbols as the 4.2):
lightly loaded 36 frnt and rear
fully loaded 42 frnt/rear
Someone on this board posted that when talking to a factory engineer, they advise +2-3 lbs for aggressive driving (Benman, did you post this?). I've been running mine at 40/39 and the wear looks pretty good. They were x-rotated at 5100 miles since there was some indication of the "sawtooth" pattern which was described in the 4.2 TSB. I will have them switched side-to-side after about 1500 miles.
Regards, Jim

[360M]

Originally posted by carbonLORD

Suggestions? I have 37.5 front 36 rear currently. Too high, too low, stagared psi no good?



Why would you want to go higher in the front? That will only enhance understeer.

[jimmy94507]

Why would you go softer? Additional air pressure stiffens the sidewall a bit. The factory engineer must have recommended additional air pressure to improve the perfomance for aggressive driving, not detract from it.
Jim

[360M]

Originally posted by jimmy94507
Why would you go softer? Additional air pressure stiffens the sidewall a bit. The factory engineer must have recommended additional air pressure to improve the perfomance for aggressive driving, not detract from it.
Jim

I'm not sure if you're responding to my post. If you are, my point is that lowering the pressures on your front tires will dial out some understeer, since you'll get more traction there. You can also accomplish this by raising the pressure of the rear tires. This is common practice when tuning cars on the track, where it's not uncommon to start out with cold pressures as low as 25 psi.

Of course for the street it's a different story, where a factory engineer is accounting for ride comfort, tire life, safety, etc. Personally, I try to keep the pressures dialed in for performance over longevity.

[jimmy94507]

Yes, 360M, I was responding to your post.
And, yes, I agree with this statement - "Of course for the street it's a different story, where a factory engineer is accounting for ride comfort, tire life, safety, etc. "
Hence, 36/36, Frnt/rear tire pressure on the RS 6 door tag is given to meet all of these "general" considerations. When the engineer stated, increase the tire pressure 2 - 3 lbs for aggressive driving, I translated this to mean it would increase the overall performance of the car. Perhaps I'll be able to give some feedback on tire pressure after the Nat'l at the end of the month?

Regards, Jim

[JP4]

Originally posted by 360M
Why would you want to go higher in the front? That will only enhance understeer.

On the contrary....higher in front will reduce understeer. I use about a 3 lb dif between front and rear for that reason.

[360M]

Originally posted by JP4
On the contrary....higher in front will reduce understeer. I use about a 3 lb dif between front and rear for that reason.

:confused:

Umm...do you mind explaining the theory behind that?

More front tire pressure = smaller contact patch = less grip on front = less steering = tendency to understeer.

Less rear tire pressure = more contact patch = more grip on tires that point the car straight = tendency to undertsteer.

Of course, DOT-R tires on the track are a completely different animal and you must tune according to tread temperatures.

[JP4]

Originally posted by 360M
:confused:

Umm...do you mind explaining the theory behind that?

More front tire pressure = smaller contact patch = less grip on front = less steering = tendency to understeer.

Less rear tire pressure = more contact patch = more grip on tires that point the car straight = tendency to undertsteer.

Of course, DOT-R tires on the track are a completely different animal and you must tune according to tread temperatures.

Well...360M we've managed to oversimplify the subject with our "blanket" statements. Truth is that we're both right depending on the "starting point". If tires are at their optimum pressure for a given set of conditions (ie. ambient air temp, track temp, tire temp, track surface type and condition (wet/dry), car weight and distribution, alignment settings etc etc) then indeed increasing pressure in the front tires would make them stiffer reducing their physical resilience which would reduce the mechanical grip coefficient. The tire would then also be running cooler then optimum which would reduce the chemical grip that comes from the rubber compound itself. Result of increasing pressure on front tires that STARTED at their optimum pressure = UNDERSTEER.

Reality is that the recommended pressures for most cars as stated by the manufacturer are optimum for everyday driving NOT high performance driving.

So...if we start under the assumption that the tire pressures are lower than optimum for high performance driving and most "street" cars fall under this category we find that a lack of front end grip (understeer) from less than optimum pressures causes the tires to OVER-heat reducing chemical grip. Mechanical grip is also reduced because the tire is rolling over more than normal which actually reduces the contact patch and allows the tire to slide too much. This type of understeer can be reduced by ADDING pressure to the front tires.

As for the contact patch size, since most street cars are heavier up front having tire pressures the same on tires of the same size on all four corners means that the contact patch on the rear will be smaller than the contact patch on the front. Having 2-3 pounds more pressure in the front will partially compensate for the weight difference front to back making the size of the contact patches closer to the same.

It would also help to pay attention to the pressures changes from a cold tire to a hot tire. If the pressure increase from cold to hot is less than 5 pounds and the car is understeering then the tires are probably overinflated. This would cause them to not heat up the compound enough making them mechanically to hard and riding up on the crowns reducing the contact patch too much.

If the pressure change is around 10 pounds or more then the understeer may be caused from the excessive heat making the compound greasy/slippery and allowing the tires to roll over to the sidewalls. In this case the understeer can be reduced by adding air.

A good method to see how far the tires are rolling over is to mark the tires with white shoe polish or with a grease pencil. Put the mark from the edge of the tread block over onto the side wall and when you come back in after some "spirited" driving see how much of your mark is left. It will be quite obvious if the tire is rolling over too much or not enough.

I hope this clarifies a somewhat complicated subject a bit.

JP4

[gmbh6]

I've always wonered about that.....thanks. :bang:

:cheers: