Tire pressure for changing handling characteristics - AutoX

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Nov 24, 2003
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Middle of Iowa
Looking for some expert advice ie. @CapriRacer

I am coming here because on the C5 forums, everybody is an expert, and the advise is split nearly 50/50...in opposite directions.

I run a 2004 C5 base Corvette manual with the Z51 suspension upgrade and enough power mods to dyno around 400 at the wheels. I run the stock tire size up front, and +20mm width in the back (reduced aspect ratio by 5 to keep stock RR). Stock pressure is 30/30. In bright sunny conditions I usually run 32/32 for AutoX.

I previously raced motorcycles (SV1000S), so my experience has been when I want to increase grip, I lower tire pressure. You do this until you get about a 10% rise in pressure on the track, and are not overheating the tires (getting snotty, or see heat discoloration). I would run between 2-4 psi higher in the front.

My last two runs in the Vette, I was using this logic I gained from motorcycles, and I'm not sure it holds for cars??? So that is my Question.

I experienced a loose rear end (oversteer) in damp and cool conditions, so I lowered my pressures front and rear to 30 psi. I still had a loose tail, so I lowered the rear to 28. I then experienced a lift-off oversteer snap spin. My first spin since my vary fist race in a RWD car. Fast forward to this last race, and it was colder yet...just above freezing. My first run was 30F/28R and I spun again - same lift off, snap oversteer. I lowered the rears down to 27 thinking it was lack of grip in the rear that was giving me the problem. I kept is slower the next several runs, but the rear still felt loose (lack of grip, not wallowing). First run of the afternoon I was still at 27 rear, and it had warmed up, and dang it, I spun again. My confidence was shot, and I basically pooched out my last 3 runs...ugh.

Ok, now I start doing more research, and I am finding papers that say to DECREASE oversteer, especially lift off oversteer, I should INCREASE rear tire pressure. Was I making the condition worse with my old knowledge? When I do forum research I find the camps split about 50/50.

There was no change in tires, alignment, etc. from when the car was performing well at 32/32 as compared to these colder temps. Yes, I realize my tires don't grip as well in the cold, but other folks on the same tires in the same temps using RWD cars were not experiencing the same problem I was. I really think my lowering of the rear pressures was making things worse.
 
Ok, now I start doing more research, and I am finding papers that say to DECREASE oversteer, especially lift off oversteer, I should INCREASE rear tire pressure. Was I making the condition worse with my old knowledge? When I do forum research I find the camps split about 50/50.
If posted opinions, if they are to be believed, are 50/50 on this question, then maybe the answer doesn't lie where you're looking?
Perhaps weight on the rear axle is more effective than tweaking the tire pressure?

Also, with all the tire specs out there, who's to say the set of tires you are using take to such tweaking?
 
Looking for some expert advice ie. @CapriRacer

I am coming here because on the C5 forums, everybody is an expert, and the advise is split nearly 50/50...in opposite directions.

I run a 2004 C5 base Corvette manual with the Z51 suspension upgrade and enough power mods to dyno around 400 at the wheels. I run the stock tire size up front, and +20mm width in the back (reduced aspect ratio by 5 to keep stock RR). Stock pressure is 30/30. In bright sunny conditions I usually run 32/32 for AutoX.

I previously raced motorcycles (SV1000S), so my experience has been when I want to increase grip, I lower tire pressure. You do this until you get about a 10% rise in pressure on the track, and are not overheating the tires (getting snotty, or see heat discoloration). I would run between 2-4 psi higher in the front.

My last two runs in the Vette, I was using this logic I gained from motorcycles, and I'm not sure it holds for cars??? So that is my Question.

I experienced a loose rear end (oversteer) in damp and cool conditions, so I lowered my pressures front and rear to 30 psi. I still had a loose tail, so I lowered the rear to 28. I then experienced a lift-off oversteer snap spin. My first spin since my vary fist race in a RWD car. Fast forward to this last race, and it was colder yet...just above freezing. My first run was 30F/28R and I spun again - same lift off, snap oversteer. I lowered the rears down to 27 thinking it was lack of grip in the rear that was giving me the problem. I kept is slower the next several runs, but the rear still felt loose (lack of grip, not wallowing). First run of the afternoon I was still at 27 rear, and it had warmed up, and dang it, I spun again. My confidence was shot, and I basically pooched out my last 3 runs...ugh.

Ok, now I start doing more research, and I am finding papers that say to DECREASE oversteer, especially lift off oversteer, I should INCREASE rear tire pressure. Was I making the condition worse with my old knowledge? When I do forum research I find the camps split about 50/50.

There was no change in tires, alignment, etc. from when the car was performing well at 32/32 as compared to these colder temps. Yes, I realize my tires don't grip as well in the cold, but other folks on the same tires in the same temps using RWD cars were not experiencing the same problem I was. I really think my lowering of the rear pressures was making things worse.
Go back to the 32/32 that worked best for you. What are the door placard #s?
 
3 psi of tire pressure is not the difference between looping the car on-course and not looping the car.

Once you're in the tires comfortable operating envelope when it comes to pressure, nailing tire pressure is the last 0.3-0.5 second of performance.

The oldest truth of auto-x: Stop trying to fix the car to account for your driving. Other drivers aren't you and you aren't other drivers. It's a big leap to assume they weren't spinning because they had their tire pressures more figured out than you did.

In my region, I was a mid-pack, sometimes in the trophies, driver in my C5. I wasn't higher because I didn't have the right tire pressures, or because I didn't have the correct number of turns on my Koni's, or even because I had Koni's and not uber-expensive Penske shocks. It's because everyone who finished ahead of me was a faster driver.

I'm not trying to be harsh, but if you had to read about the difference between raising and lowering tire pressure and couldn't actually feel the difference in how the car behaved means it wasn't a big enough difference to matter.
 
Same model tires front and back, from the same factory, made around the same date? Seems like your rears just don't grip the same as the fronts in the wet? Also maybe the wider rears takes longer to heat up?
On colder days with my old Neon on older r-comps, I'd warm the fronts up on the launch with some tire spin, give away a couple tenths there and then have better grip for the whole run was the theory. Maybe get yours toasty at the start and see what happens?
Could be your driving style is to aggressive for the wet as well? and maybe your tires have not great wet break away characteristics? My BIL in Vancouver at a set of continentals for the wet days only, as they were much better in the wet, and not great in the dry compared to the top AutoX tires. He won their club championship that year doing that.
 
Your observation does seem to be the reverse of the expected relationship between pressure & traction.

Years ago when I was actively autocrossing (RX-7 twin turbo, Panoz Roadster), everyone followed a similar process as you describe. On same size tires, best grip was always lower pressures than the door placards recommend. So reduce pressures for more grip, until you get signs that pressures are too low. Signs include: inspect the tire sidewall corners after each run for evidence of scuffing, a squishy feel when driving, or front/rear traction imbalance. In the Panoz which weighed 2500 lbs, ideal pressures were about 25 psi on Kumho V700 tires.

You mention temperatures as a factor. Different tires respond differently to temperature changes. They all lose traction but some more than others. This could of course change the pressure/traction balance. Being at near freezing temps, all bets are off. High performance tires are meant to operate in warm conditions.
 
Looking for some expert advice ie. @CapriRacer

I am coming here because on the C5 forums, everybody is an expert, and the advise is split nearly 50/50...in opposite directions.

I run a 2004 C5 base Corvette manual with the Z51 suspension upgrade and enough power mods to dyno around 400 at the wheels. I run the stock tire size up front, and +20mm width in the back (reduced aspect ratio by 5 to keep stock RR). Stock pressure is 30/30. In bright sunny conditions I usually run 32/32 for AutoX.

I previously raced motorcycles (SV1000S), so my experience has been when I want to increase grip, I lower tire pressure. You do this until you get about a 10% rise in pressure on the track, and are not overheating the tires (getting snotty, or see heat discoloration). I would run between 2-4 psi higher in the front.

My last two runs in the Vette, I was using this logic I gained from motorcycles, and I'm not sure it holds for cars??? So that is my Question.

I experienced a loose rear end (oversteer) in damp and cool conditions, so I lowered my pressures front and rear to 30 psi. I still had a loose tail, so I lowered the rear to 28. I then experienced a lift-off oversteer snap spin. My first spin since my vary fist race in a RWD car. Fast forward to this last race, and it was colder yet...just above freezing. My first run was 30F/28R and I spun again - same lift off, snap oversteer. I lowered the rears down to 27 thinking it was lack of grip in the rear that was giving me the problem. I kept is slower the next several runs, but the rear still felt loose (lack of grip, not wallowing). First run of the afternoon I was still at 27 rear, and it had warmed up, and dang it, I spun again. My confidence was shot, and I basically pooched out my last 3 runs...ugh.

Ok, now I start doing more research, and I am finding papers that say to DECREASE oversteer, especially lift off oversteer, I should INCREASE rear tire pressure. Was I making the condition worse with my old knowledge? When I do forum research I find the camps split about 50/50.

There was no change in tires, alignment, etc. from when the car was performing well at 32/32 as compared to these colder temps. Yes, I realize my tires don't grip as well in the cold, but other folks on the same tires in the same temps using RWD cars were not experiencing the same problem I was. I really think my lowering of the rear pressures was making things worse.
Have you tried chalking your tires?
Outdoor temps are going to have an impact on the rubber your using.
 
Pressure will affect tire temperature. It also affects the size of the contact patch. It also affects how the car responds to bumps and weight transfer. Tires act as suspension as they flex, but tires have a lot more spring than they do damping. If the tires are flexing a lot, the suspension won't move as much as quickly and the overall suspension system may be underdamped.

I'm sure pro race teams have complex models for this stuff. Rules of thumb that work on one vehicle for one driver may not work for another. I would spend less time trying to understand the physics and focus on experimenting to find what feels right. I would record tire temperature, measured with an infrared thermometer immediately after a run, to see if grip is more strongly correlated to pressure or temperature. This could inform you how much adjustment you should be making based on ambient temperature.
 
What is the weight split between the front and the rear? That is, how much weight does each axle carry? It the car is nose heavy, then increase pressure in the front tires. Also, as you'll be the only person in the car, so increase left side inflation pressures by 1 lbs.
 
Immediately after a run get tread Temps with a temp gun in at least 3 places across the tire. Hotter in the center, drop a few psi, Hotter on the outside add a few.I worked with a friend running on a paved oval, lots of negative camber on the rf and about a quarter inch of toe out
 
I don’t know anything about racing or proper tire pressure for track.

When I was in the police academy , we had high speed vehicle training. The instructor set up two identical crown Vics. One had 25 PSI in the tires and the other had max pressure , 45 PSI. The car with 45 PSI handled drastically better. It was faster and cornered much much better. The car with 25 PSI was squishy in the corners and had slow less responsive steering.

The point of the exercise was to show the new recruits how important vehicle and tire maintenance were and how they impacted the vehicles handling.
 
First, vehicle dynamics and a tire's impact on that is not my forte. In the business, we call that force and moment.

You've received a number of good suggestions.

The only thing that hasn't been mentioned is that lowering the inflation pressure also reduces the speed the tire reacts to input. what might be happening is that lowering the pressure is causing the rear to not react fast enough, then it overshoots, resulting in oversteer.

As has been suggested, try raising the pressure. The other thing I can suggest is to work on smoothness. Slow steering inputs.
 
I don’t know anything about racing or proper tire pressure for track.

When I was in the police academy , we had high speed vehicle training. The instructor set up two identical crown Vics. One had 25 PSI in the tires and the other had max pressure , 45 PSI. The car with 45 PSI handled drastically better. It was faster and cornered much much better. The car with 25 PSI was squishy in the corners and had slow less responsive steering.

The point of the exercise was to show the new recruits how important vehicle and tire maintenance were and how they impacted the vehicles handling.
On track or racing, people start with low pressure. I start BMW on track around 26-28psi. After 2-3 lsps, I am around 40psi.

What you referring are street conditions. Even in high speed chases, you will have moments where tire can shed off some temperature.

I think good suggestion here was to check tire temperature. Several things are constant on race track: people checking tire pressure and temperature, people checking brake temperature, and people talking brake pad compounds.
 
@DriveHard Another thing you can try is early braking instead of late braking. Then you get on the gas earlier and steer with the throttle (power oversteer) through the corner. The difference is weight distribution. This will get more weight on the rear wheels, which will improve traction compared to late braking which shifts weight to the front. See how that affects things.
 
Looking for some expert advice ie. @CapriRacer

I am coming here because on the C5 forums, everybody is an expert, and the advise is split nearly 50/50...in opposite directions.

I run a 2004 C5 base Corvette manual with the Z51 suspension upgrade and enough power mods to dyno around 400 at the wheels. I run the stock tire size up front, and +20mm width in the back (reduced aspect ratio by 5 to keep stock RR). Stock pressure is 30/30. In bright sunny conditions I usually run 32/32 for AutoX.

I previously raced motorcycles (SV1000S), so my experience has been when I want to increase grip, I lower tire pressure. You do this until you get about a 10% rise in pressure on the track, and are not overheating the tires (getting snotty, or see heat discoloration). I would run between 2-4 psi higher in the front.

My last two runs in the Vette, I was using this logic I gained from motorcycles, and I'm not sure it holds for cars??? So that is my Question.

I experienced a loose rear end (oversteer) in damp and cool conditions, so I lowered my pressures front and rear to 30 psi. I still had a loose tail, so I lowered the rear to 28. I then experienced a lift-off oversteer snap spin. My first spin since my vary fist race in a RWD car. Fast forward to this last race, and it was colder yet...just above freezing. My first run was 30F/28R and I spun again - same lift off, snap oversteer. I lowered the rears down to 27 thinking it was lack of grip in the rear that was giving me the problem. I kept is slower the next several runs, but the rear still felt loose (lack of grip, not wallowing). First run of the afternoon I was still at 27 rear, and it had warmed up, and dang it, I spun again. My confidence was shot, and I basically pooched out my last 3 runs...ugh.

Ok, now I start doing more research, and I am finding papers that say to DECREASE oversteer, especially lift off oversteer, I should INCREASE rear tire pressure. Was I making the condition worse with my old knowledge? When I do forum research I find the camps split about 50/50.

There was no change in tires, alignment, etc. from when the car was performing well at 32/32 as compared to these colder temps. Yes, I realize my tires don't grip as well in the cold, but other folks on the same tires in the same temps using RWD cars were not experiencing the same problem I was. I really think my lowering of the rear pressures was making things worse.

@DriveHard , simply, yes. As you can see above, there are contributing factors, but essentially the lower inflation becomes more “compliant,” eg., will “give” more easily. For instance, my FWD cars were prone to understeer, so I would shoot for a 2-3 psi higher front pressure than rear, to bias the front to hold its line with slightly more authority. Ideally I wanted both to start breaking together, for how I was driving at the time.
 
You're absolutely right with the general rule of thumb with lowering tire pressures for *more* grip; but that's almost only applicable in the dry. During wet conditions you want to raise the pressure to keep the tire pressure higher to stop any tire squirm and to let the treads channel out any water. Start with a higher PSI and work your way down until you find a sweet spot.
 
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