Dialing in BFG r1s

[Swank911]

just got a set of BFG r1s for my 73 911E for auto crossing. Put them on yesterday with 28psi and took them to the course. What a difference. No more sidewall roll as with the 185/70s.

Question is, what is the best was to test/set up these tires, I notice a little bit of push in the front end, do I bring the the pressure up in the front tires, or the rear to MINIMIZE the understeer?

[BRAINIAC]

They aren't going to feel right until they are broken in and have a few heat cycles under their belt. They'll feel greasy and mushy for awhile. As for tire pressures, I'd raise the front pressure to minimize understeer. I tried it both ways on my 72 at autocross and much prefer the higher pressures.

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Tyson Schmidt
72 911 Cabriolet
92 C-2 Cabriolet

[BRAINIAC]

Oops, just realized you are running R-compound tires. They like lower pressures than street tires. Someone with more experience with race tires should offer advice.

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Tyson Schmidt
72 911 Cabriolet
92 C-2 Cabriolet

[scca_ita]

Run 26 in the front. At first they will feel spongy but they will heat up very fast. Run 3 to 5 lbs more in the rear depending on how much over steer you want. the more in the rear the more over steer.

[Clark Griswald]

Hate to be so dissagreeable, but.... Increasing pressure in the rear will decrease oversteer.

[BRAINIAC]

Clark, it's a different ballgame, I think, with R-compound tires compared to street tires. Judging by his user name, he probably knows what he's talking about.

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Tyson Schmidt
72 911 Cabriolet
92 C-2 Cabriolet

[rattlsnak]

If you want to do this correctly, you must use a tire temp guage. You want equal temps on the inside, middle and outside edges of the tires. If the temps are equal, but too low, than take some air out,etc. Once this is accomplished-by varying tire pressures- then deal with why the car is pushing or sliding by adjusting your suspension, and then repeating the tire temps again. It is not wise just to add/remove air on certain tires to correct for car setup problems.

[Swank911]

I know I need to finish setting up the car, but either way. The tires need to be at the correct pressure to contribute to the package.

[Clark Griswald]

Brainiac, I run DOT R compound tires on the track and I can assure you that increasing rear pressure will reduce oversteer, just like with street tires.

The reason for this is that "all else being equal" a reduction in tire pressure will increase the slip angle of the tire. More slip angle rear = More oversteer. More slip angle front = More understeer.

If you would like to see a good table that shows the anticipated effect of various tire pressure and suspension changes will have on your cars handling, check out the following very good ariticle. The table is about 1/4 the way down:

http://www.bmwclub.ca/ccrart3.htm

It also discusses and compares various DOT R compound tires on the market today, including the R1.

The article is written by some BMW club guy (boo hiss) but is chock full of information.

The good thing about tire pressure is that is cheap and easy to change. Try varying pressures, front and rear, on the track and observe the effect it has on the car's handling.

[Randy W]

I'm assuming these are G-Force R1s and not the previous incarnation of the R1s (no G Force printed on the sidewall). The prior R1s reacted completely opposite to all other R tires, because of their stiff sidewall - lowering the pressure tended to reduce sliding on that end - typical hot pressures for the old R1s were 23/25. The latest G Force (from last year) reacts like most other R tires - higher pressures than optimal tend to reduce sliding at that end - typical pressures for G Force tires are 27/30. Optimal tire pressures are car dependent on weight, suspension and alignment. Using a pyrometer correctly is best, but you can gain some idea by altering pressures at an autocross and track and noting the differences - you may find that lowering pressures induces a slightly mushy feel but drops lap times, or that raising pressures increases responsiveness, but that the car wants to slide more quickly. There are no hard and fast rules regarding tire pressure.

Randy W

[Swank911]

these are original R1s, bought right after heat cycled and stored

[lsolon]

Since were on this subject, what about pressure in a yoko y032?

[Randy W]

Reportedly, AO32Rs pressures are hot 24/28 on a 2200 lb race 911, higher (up to 28/31) on heavier street 911 used on the track.

Randy W

[kwikt 911]

Are you using 205/50's? If so try 30psi front and 34 psi rear.


Kevin
72 911T/E {RUNNING HOOSIER A-X TIRES NOW}