C mod tire pressures

[PDM075]

Ok ...so running in less than warm conditions on Sunday... dusty asphalt lot.... got me rethinking cold and hot tire pressures.


Fire away what you guys run and do you bleed off as tires heat up?(I run nitrogen in my tires to reduce the pressure buildup as much as possible in all my cars )

[-pru-]

For the Hoosier R25B FF rear (7.25") and FC front (7.0"), I run 16 psi rear / 14 psi front from cold to hot. That is, as the tires warm, bleed to maintain 16/14...

[CM/FFdriver]

Hmm... that is interesting that you run higher PSI in the rear.

What ever PSI I'm experimenting with between 10 and 20 PSI (20 PSI is what the road racer use) I did try 12 on a very cold day in Calif. HAHAHa it was in the 50's it worked great, lots of tire flex builds heat.

Now I change tire PSI based on tire temp, and so far based on that I adjust tire PSI after my first run seams to be working.

Ben

[lowside67]

We also run higher in the rear: typically +1.5 to +2.0psi higher. We run in general very low pressure.

-Mark

[Gary Godula]

Hoosier recommends 14f/16r as the minimum pressures for the FF Bias ply tires. I never run below these pressures. I also normally do not bleed my pressures down between runs. Check, yes. Bleed, no.

It is also important to verify your gauges. I certify my pressure gauges annually using the dead weight pressure stand in the instrumentation lab at my office.

[CM/FFdriver]

Hoosier recommends 14f/16r as the minimum pressures for the FF Bias ply tires. I never run below these pressures. I also normally do not bleed my pressures down between runs. Check, yes. Bleed, no.

It is also important to verify your gauges. I certify my pressure gauges annually using the dead weight pressure stand in the instrumentation lab at my office.

Hey Gary,

Very interesting I'm going to try this, we have an event on Sat. and other then making the PSI even from side to side I'm going to let them rise, good test mode tomorrow.

About my gauge though I do my best not to drop them at any event, and if I do drop the gauge, I'll check it with my back up gauge. We could send our gauges to you to double check, what do you think? LOL

Ben

[Gary Godula]

Hey Gary,

Very interesting I'm going to try this, we have an event on Sat. and other then making the PSI even from side to side I'm going to let them rise, good test mode tomorrow.

About my gauge though I do my best not to drop them at any event, and if I do drop the gauge, I'll check it with my back up gauge. We could send our gauges to you to double check, what do you think? LOL

Ben

Interesting concept.....you are the National Champion (congratulations again, BTW) and soundly crushed me in Lincoln, and you want to take my advice on tire pressures??? Something just doesn't seem right with this conversation.

[lowside67]

Phil can probably add more context but we did do some experimentation from around 10/12 to around 16/18. We found that in general we didn’t find the times to change much through most of the range but the feel of the tires definitely did. Both Phil and I preferred the lower pressures as they seem to provide more communication whether they are gripping or sliding. As the pressures went up, the likelihood to slide went up and I’m not good enough to want to have to manage that with my hamfisted inputs.

-Mark

[CM/FFdriver]

Interesting concept.....you are the National Champion (congratulations again, BTW) and soundly crushed me in Lincoln, and you want to take my advice on tire pressures??? Something just doesn't seem right with this conversation.

Gary,

I look at this way, you never know what you'll find out about your car. I've let the tire PSI rise at other events but never put a lot of though to it and then when I did check them for the last run they could be 5 lbs off (course dependent) but this weekend I will let the tire rise up and pay more attention to the feel and time, PSI, temp, wear. You never know.

Gary, I don't take anyone's opinion lightly, I listen, talk to, watch, read about and assess the out come. If I stop talking I'm contemplating your opinion. LOL

Mark, but whats the surface your running on?


Ben

[pleavens]

Mark, but whats the surface your running on?


Ben

Ben, thats the real question. My opinion is that the R25b works fine over a wide range of pressures assuming concrete and warm-to-hot ambient conditions. So as long as you're happy with the balance/responsiveness it would make sense to run somewhere in Hoosiers recommended range. Packwood, Pitt Meadows and the airport locations we regularly run, show a significant improvement (time wise) using very low pressures, even in hot conditions. Personally I don't like the wear patterns and the grain buildup that we get in exchange, but the data is what it is.

Phil

[pleavens]

Phil can probably add more context but we did do some experimentation from around 10/12 to around 16/18. We found that in general we didn’t find the times to change much through most of the range but the feel of the tires definitely did. Both Phil and I preferred the lower pressures as they seem to provide more communication whether they are gripping or sliding. As the pressures went up, the likelihood to slide went up and I’m not good enough to want to have to manage that with my hamfisted inputs.

-Mark

Yep, it's hard to say if the gains come from actual raw traction, or from making the drivers job easier. Just the realities of using a tire that is far from optimum.

[CM/FFdriver]

Ok Gary,

Tried the rising tire PSI thing and it didn't work it was 65 temp today and I will try in an 85 temp day and see the results.

Learned something big today though so it wasn't a total loss, rechecking and double the rechecking before the next event.

Ben

[captain_pants]

Thanks to all for sharing! I've been running higher pressures (17) and matching front to rear. I'll definitely try lowering the fronts more than the rears for our next event.

It sounds like this may have made the car more knife-edged than needed, which is exactly what I'm trying to improve. Always go back to basics...

[Neil_Roberts]

My DB-6 has 3 IR tire temp sensors aimed at one front tire, and 3 on one rear tire. After considerable experimentation and analysis, I settled on 11.5 psi front / 13.5 psi rear on low-grip asphalt, bleeding down to those pressures before every run. The sensor data showed that the pressures were slightly low for the first half of a run, and spot-on for the second half. Knowing that, I would sometimes start at 12 / 14 if it suited the course design better.

Here is more detail:

http://thinkfastengineering.com/2012/08/solo-heat/

[alangbaker]

My DB-6 has 3 IR tire temp sensors aimed at one front tire, and 3 on one rear tire. After considerable experimentation and analysis, I settled on 11.5 psi front / 13.5 psi rear on low-grip asphalt, bleeding down to those pressures before every run. The sensor data showed that the pressures were slightly low for the first half of a run, and spot-on for the second half. Knowing that, I would sometimes start at 12 / 14 if it suited the course design better.

Here is more detail:

http://thinkfastengineering.com/2012/08/solo-heat/

I just don't understand why you would bleed down after each run...

Is it because in autocross you're never really getting to a stable maximum heat and therefore maximum pressure in the tires?

[TedV]

Depends how many runs ( seconds and minutes ) it takes for the tires to stabilize on temps and pressure. You might need to bleed or maybe not. Cloud cover comes rolling in or event is delayed for course cleanup and you may need to add air

[CM/FFdriver]

Depends how many runs ( seconds and minutes ) it takes for the tires to stabilize on temps and pressure. You might need to bleed or maybe not. Cloud cover comes rolling in or event is delayed for course cleanup and you may need to add air

This is what happen this last weekend Cloud cover came in and the tire PSI went down so it do anything for me, Checking this weekend should be 90+ degrees so we will see.

Thank you Neil good info.

alangbaker ; TedV is right I've had events were my tire PSI went up by 5 lbs in a 65 sec run that was an interesting day. Yes that was basic air out of my home air compressor.

Ben

[captain_pants]

Posting to say thanks to everyone. My car was MUCH easier to drive at 12/14 on very slippery asphalt this weekend. Not sure where I got the recommendation for higher pressures, but it's been stricken from my notes...

[Unbalanced Engineering]

Hey Neil,

After looking at pressures, temps etc, I tried lower front pressures, and we started rolling the tire off the rim. So careful everyone about how low you can go with your rim / bead seat etc.

Was this with the harder RR compound you ran a lot or the softer 25B?

Thanks!
Jason

My DB-6 has 3 IR tire temp sensors aimed at one front tire, and 3 on one rear tire. After considerable experimentation and analysis, I settled on 11.5 psi front / 13.5 psi rear on low-grip asphalt, bleeding down to those pressures before every run. The sensor data showed that the pressures were slightly low for the first half of a run, and spot-on for the second half. Knowing that, I would sometimes start at 12 / 14 if it suited the course design better.

Here is more detail:

http://thinkfastengineering.com/2012/08/solo-heat/

[TedV]

12psi on one guage may read 8psi on another. Always best to check calibration.

Do your rims have a safety bead? Not much surface area to the seating area of the outer rim half? I've had problems busting the bead off a rim at 0psi and some of the spun aluminum rims the tires fall off the bead Before all the air is out when pulling the valve core.

[CM/FFdriver]

Hey Neil,

After looking at pressures, temps etc, I tried lower front pressures, and we started rolling the tire off the rim. So careful everyone about how low you can go with your rim / bead seat etc.

Was this with the harder RR compound you ran a lot or the softer 25B?

Thanks!
Jason

Your right Jason, to low of tire PSI may roll the tire on the rim but if it feels good/works for them who's going to argue that.

Just changing your tire PSI up or down is going to change other things and what people are feeling is a bit better rake or the affective spring rate, who knows.

12psi on one guage may read 8psi on another. Always best to check calibration.

Do your rims have a safety bead? Not much surface area to the seating area of the outer rim half? I've had problems busting the bead off a rim at 0psi and some of the spun aluminum rims the tires fall off the bead Before all the air is out when pulling the valve core.

TedV , I think I've said this before. Have a new back up and check it back and forth, one less thing on my mine.

Ben

[Unbalanced Engineering]

Yeah, they have a safety bead, its a BITCH to get the rears mounted...

I was only pointing out that the harder RR compounds that Neil often autocrossed on may not have generated enough lateral force to roll the tire off at 12 psi. I'd have kept going down, if the tire would have stayed on....

Like you Ben, I have 2 gages that agree. Once one of them goes off I get another.

Thanks,
Jason

[Neil_Roberts]

Sorry about the delayed reply. The pressures that I ran were on new R25Bs. I have never unseated a bead, using Panasport or Keizer wheels. In the course of experimentation, I went down as low as 9.5 psi front.

Bleeding down the pressures before every run is a result of everything heating up as the day progresses, including the air and pavement temps.

I run dry compressed air in my tires. I added a dessicant filter to my home compressor to capture the moisture, and my track air can has some dessicant beads in it just in case I have to refill it somewhere else.

[Unbalanced Engineering]

No sweat. Interesting then as this was a set of Panasports.... Will try the experiment again with the Keizer's for the new car.

Thanks for clarifying Neil.

Jason

[CM/FFdriver]

Well I just had my gauge go bad over the last 2 events and I didn't know it, what I thought was 16 lbs of pressure was in reality 20 lbs, it was a gauge I really like so I may find out how to fix it.

The funny thing about is the car still work pretty good from both events just a different noise coming from the tires on the event with the 20 lbs of pressure.

Ben