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Discussion Starter #1
I am finally going to have the time to take the 12c to the track. What are you guys running for tire pressures on the track?

I am assuming some of you have experimented with lower pressures than the street specs.

I will bring my tire temp probes and may try some different pressures but welcome your experience.
 

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Manual says 2.0bar (28lbs) Front and 2.2bar (31lbs) Rear when tires are cold.
I suppose that if you are planning to take it to the track, one can safely reduce the tires pressures by 2-4lbs on each tire to compensate for the increased tires pressures subjected as a result. Tire pressures typically increase by 5-7lbs depend on the intensity of your driving style under track conditions.

On a somewhat related note when I took the car beyond 300km/h, I noticed the intermittent flashing of the tire pressure warning sign, has anyone experienced this?
 

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Discussion Starter #5
Manual says 2.0bar (28lbs) Front and 2.2bar (31lbs) Rear when tires are cold.
I suppose that if you are planning to take it to the track, one can safely reduce the tires pressures by 2-4lbs on each tire to compensate for the increased tires pressures subjected as a result. Tire pressures typically increase by 5-7lbs depend on the intensity of your driving style under track conditions.

On a somewhat related note when I took the car beyond 300km/h, I noticed the intermittent flashing of the tire pressure warning sign, has anyone experienced this?
This was kind of what I was thinking as a starting point (ie 5 lbs less than street)
 

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You're going to end up taking nearly 10lbs out by end of day. Running at THill, right? Alessandro et al will be there - they'll take good care of your tires.
 

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On a somewhat related note when I took the car beyond 300km/h, I noticed the intermittent flashing of the tire pressure warning sign, has anyone experienced this?

i got this as well coupled with warning beeps as i approach 300 km/h on a closed airport runway. it did it almost on every run.
 

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i got this as well coupled with warning beeps as i approach 300 km/h on a closed airport runway. it did it almost on every run.
This is probably an indication of how sensitive and effective the TPMS is feeding back the tire pressure info back into the ECUs real time. I would most likely reduce the tire pressures to 1.9bar Front and 2.1bar Rear when cold at the next opportune time if ever I decided to do another >300km/h joyride.
 

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This is probably an indication of how sensitive and effective the TPMS is feeding back the tire pressure info back into the ECUs real time. I would most likely reduce the tire pressures to 1.9bar Front and 2.1bar Rear when cold at the next opportune time if ever I decided to do another >300km/h joyride.

Better not. You should actually raise it to 2.5/2.6 when going this fast.



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McDc is correct you should increase tire pressure for high speeds.
 

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Discussion Starter #13
McDc is correct you should increase tire pressure for high speeds.
While I am an experienced racer I have never done Vmax runs. So educate me: why should tire pressures be increased? Definitely counter intuitive.
 

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While I am an experienced racer I have never done Vmax runs. So educate me: why should tire pressures be increased? Definitely counter intuitive.
The given payload of a tire is significantly reduced at high speeds. If I remember correctly some 4-5% per each 10 km/h above 270 km/h for a Y-rated tire. So you need to compensate for that with higher tire pressure.

Furthmore, higher tire pressure reduces the tire's ability to "walk" on the wheel and therefore gives enhanced stability and reduces the enormous amount of heat produced.

Also keep in mind, the rear tires will still have a slippage of around 10% (!) at 300+ km/h which causes additional stress on the rubber.

I'm pretty sure that with just 2 bar (29 psi), the tires would simply explode in a longer high speed/vmax run.

That said, I would recommend 3.0bar (43 psi) for a longer 270+ km/h run like on the german autobahn where you can run this fast for minutes if the time is right (mostly at night).

If you are going fast/vmax for a very short time only, like on a runway or most race tracks, I don't think there is need to rise tire pressure to such levels.

Some more information: http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=72



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While I am an experienced racer I have never done Vmax runs. So educate me: why should tire pressures be increased? Definitely counter intuitive.
Because of the weight they bear, pneumatic tires' sidewalls bulge and their treads flatten as they roll into contact with the road. This results in dimensional difference between the tire's "unloaded" radius (i.e., between the center of the axle and the top of the tire) and its "loaded" radius (between the center of the axle and the road). The engineer's call the difference between the two radii "deflection." Increasing vehicle speed will cause the tires to deflect quicker and increasing vehicle load will cause the tires to deflect farther (if tire pressure isn't increased).
 

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Discussion Starter #17
Because of the weight they bear, pneumatic tires' sidewalls bulge and their treads flatten as they roll into contact with the road. This results in dimensional difference between the tire's "unloaded" radius (i.e., between the center of the axle and the top of the tire) and its "loaded" radius (between the center of the axle and the road). The engineer's call the difference between the two radii "deflection." Increasing vehicle speed will cause the tires to deflect quicker and increasing vehicle load will cause the tires to deflect farther (if tire pressure isn't increased).
I get the physics part. That's exactly why tire pressures are reduced for the track, to increase the contact patch and therefore increase grip. This is effective to a point... until there is too much heat (or sidewalls are too soft). Hence taking tire temps at the track is really important.

From the earlier posts, it makes sense to increase pressure for sustained high speed runs to reduce the contact patch, and therefore reduce heat build-up.
 

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Someone told me recently that it was never officially called Sears Point - that just happens to be the immediate area. However it will always be Sears Point to me (or "that-track-where-I saw-a-classic-Jag-demolish-a-classic-mercedes-in-turn-11-during-a-historics-race")
 

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The given payload of a tire is significantly reduced at high speeds. If I remember correctly some 4-5% per each 10 km/h above 270 km/h for a Y-rated tire. So you need to compensate for that with higher tire pressure.

Furthmore, higher tire pressure reduces the tire's ability to "walk" on the wheel and therefore gives enhanced stability and reduces the enormous amount of heat produced.

Also keep in mind, the rear tires will still have a slippage of around 10% (!) at 300+ km/h which causes additional stress on the rubber.

I'm pretty sure that with just 2 bar (29 psi), the tires would simply explode in a longer high speed/vmax run.

That said, I would recommend 3.0bar (43 psi) for a longer 270+ km/h run like on the german autobahn where you can run this fast for minutes if the time is right (mostly at night).

If you are going fast/vmax for a very short time only, like on a runway or most race tracks, I don't think there is need to rise tire pressure to such levels.

Some more information: http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=72
Thanks both Belg and McDc for the informative responses.
That being said, 3.0bar appears to be very high. Can anyone chime in the maximum pressure these high performance low profile tires are design to absorb under extreme conditions?
 
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