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Author Topic: Conflicting tire advice  (Read 7769 times)
500KEG8
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« on: October 18, 2013, 03:15:55 PM »

I am relatively new to autocrossing, so I'm always researching and asking for advice about this or that. Most of the advice that I've received has been extremely helpful, except when it comes to setting tire pressure, everyone seems to have their own "theories" as to how to go.

This last event was a good case in point; nearly everyone that I asked about setting the optimum tire pressure, esp. in the wet, seemed to all have conflicting points of view as to what the best tire pressure should be. But nearly everyone that I had talked with, including skilled veterans, all said that lowering tire pressure in wet conditions was the way to go.

Some quick research on Tire Rack shed some interesting light on this:

"TIRE PRESSURES IN THE RAIN

For both autocross and road racing, increase tire pressures 6-10 psi from what you would normally run in dry conditions. Hydroplaning occurs when a wedge of water develops between the tire and road surface. This wedge can actually lift the tire off the road and eliminate traction. Increasing the pressure rounds the profile of the tire by decreasing the deflection of the tire. This results in a smaller contact patch - narrower and shorter. It also helps keep the grooves in the tread open so they can channel the water out from under the tire."

Link to website: www.tirerack.com/tires/tiretech/techpage.jsp?techid=58

I'm still going to try different settings to see what yields the fastest times for me in the mean time, but I wanted to get some good input as to what everyone else thinks.

Thanks!
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catapultkid
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« Reply #1 on: October 18, 2013, 06:46:35 PM »

Ok. Quick thing to remember.

Autox in the dry for a street tire = UP the tire pressure to 10-15 psi (tire depending)
Autox in the wet for a street tire = UP the tire pressure to 6 or so psi (tire depending)

Therefore. From your AUTOX setting you will LOWER the tire pressure. From your STREET setting you will UP the tire pressure, just not as much.

That is how I see it.

David Ogburn's son can comment further, as his dad and I had been talking about his experience in the wet recently.
« Last Edit: October 18, 2013, 06:48:59 PM by catapultkid » Logged

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whiteryder
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« Reply #2 on: October 18, 2013, 07:10:28 PM »


I got more conflicting advice for you, just to keep things interesting.  Grin

Factory recommended pressures for my car are 37 front, 44 rear.
The tires say on the sidewall, "max pressure 50 lbs".
So no matter what advice I get, I'm probably *not* going to increase my rear tires by 10-15 psi!

In fact what I do, dry or wet, is reduce pressures to increase contact patch size.
YMMV.  That's what works for me.

At the track it's more important because at high speeds for 20-30 minute sessions
my tires heat up enough to increase pressure by 10+ lbs anyway.  So I *have* to
make sure I bleed the tires before I start the day, and keep checking & bleeding
throughout the day so they don't get bloated and reduce my grip.

In autocross you're only driving 60 seconds at a time, but if the pavement is hot
that still contributes. 

You can start with any rule of thumb you like (gotta start somewhere), but basically
you have to experiment with your tires on your car and find what gets you the best
performance.  Practice days are great for this.  Once you find a good set of pressures
that lets you optimally control the car, then on race day you just keep checking &
bleeding to make sure they stay at that pressure on your runs.

I also had someone else drive my car and figure out what pressures worked for him.
That was a bit lower than what I'd been running, so now I run at something in between.

Besides it's really cool to check your tires between runs.  Makes you look like you know
what you're doing.  Grin

Oh yeah - chalking.  Put a little chalk on your tires in a radial direction - they have some
in the trailer - and after your run see how much chalk wore off.  That gives you an idea
if your tires are squashing too much from low pressure.  (Some tires also have little arrows
to help you measure the same thing.  But you don't need glasses to use chalk.)


« Last Edit: October 18, 2013, 07:15:19 PM by whiteryder » Logged

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JCHVMSC
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« Reply #3 on: October 18, 2013, 09:41:12 PM »

You can also measure the temperatures across the tire and get some indication where the tire is seeing the most heat - more useful if you have adjustable suspension.

My tires gain about 4 psi the first run, a little less the second and even less by #3. From then on they don't do much more

John
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« Reply #4 on: October 19, 2013, 04:59:59 AM »

I ran on my "R" compounds last Sunday. On a dry day, I'll run 46# front and around 42 in the rear. It varies. Last week I ran around 42 in the front, and 38 in the rear. The tires actually heated up a little bit. It stuck better than I thought it would, and was the first time I ever ran them in the rain. The tires are Nitto NT-01's. Factory recommended pressures are 31# front and rear.
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Mugamini
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« Reply #5 on: October 19, 2013, 06:16:50 AM »

I have a small comment on this issue.. Most Factory recommendation are for either Street or Track as in long run times.. no Short stents at the autox. This is where you get excessive heat build up and can get to tire to proper Operating Temps. Which in most cases isn't achievable at Autocross events. Operational Temp isn't just the outside but also the inside of the tire. The colder the outside air at the even will mean the tire inside the tire is colder and more dense. The rule of thumb I've used once I found the pressure my tires like to run right and not roll over is the benchmark I want to keep. For my Mini using 205/50/15 front and back I ran 42 on the front and 46 on the rear. If I wasn't checking and adjusting pressure on each run. I would Drop 2 lb of air for every 5 degrees the outside temp was below 77 Degrees. This would get the tires at the ideal pressures normally by the third run. Also check the Factory recommendations to see if the word Cold is next to the max pressure. which is normally the case to allow for heat build up.
 
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ccann26
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« Reply #6 on: October 19, 2013, 09:07:53 AM »

  I wanted to know the same thing. I have read that increasing tire pressure not beyond what is recommended by the manufacturer actually increases grip. I wanted to try this in the rain with the corvette at the last event. I had Nitto Nt-05 tires running 24 pounds in the rear and 31 pounds in the front. I ran close for the first three runs and felt I had no more time out on the course. I upped the tire pressure to 34 pounds in the rear and 44 pounds in the front for the last 2 runs :     62.429+1     61.402    61.618    60.392    60.622

I'm usually pretty consistent and I was surprised to see such a dramatic change in times despite what most people thought would happen I managed to get better times with more pressure.

Running higher pressure can give you the feeling of a snap break away but the grip up to that point is more consistent due to the tires construction. The feeling of a slower breakaway I believe is created by low pressures due to the contact patches ability to deform and "Roll around" this can create a better contact patch for acceleration but at the expense of cornering capability.

Just my observation  :  )

Chris.C
« Last Edit: October 19, 2013, 09:09:36 AM by ccann26 » Logged

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« Reply #7 on: October 19, 2013, 09:29:53 AM »

I'm usually pretty consistent and I was surprised to see such a dramatic change in times despite what most people thought would happen I managed to get better times with more pressure.

Was that on street tires?
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« Reply #8 on: October 19, 2013, 10:30:43 AM »


 At the rain event yes. But I have noticed better control/wear and steering response with the Hoosiers also.
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gunny
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« Reply #9 on: October 19, 2013, 01:37:26 PM »

I've been telling you guys to run higher pressures for 3 years now! ((vette drivers)
John Harmon tried it last year and saw instant gains and faster times.

Gunny
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500KEG8
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« Reply #10 on: October 24, 2013, 09:01:44 AM »

Thanks guys! Your input is definitely something to consider as I continue to do my research. I love the feedback I get with this group! I look forward to trying different things in the mean time, and will be happy to share results that help me to improve my times.
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« Reply #11 on: October 24, 2013, 10:13:37 AM »

I've been telling you guys to run higher pressures for 3 years now! ((vette drivers)
John Harmon tried it last year and saw instant gains and faster times.

That's interesting, since you found that lower pressures worked better when you drove the Porsche.

I think I know less about this topic than I thought I did.   Tongue
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« Reply #12 on: October 24, 2013, 12:37:09 PM »

yea, mid engine is a different feel.
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JCHVMSC
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« Reply #13 on: October 24, 2013, 06:32:16 PM »

One big variable in this area is car setup, a car with stock camber might like different pressures that if it had -3 degrees - one of the reasons for a tire temperature gauge

John
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sjfehr
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« Reply #14 on: November 01, 2013, 03:46:08 AM »

I believe Tirerack's advice is related to hydroplaning.  Higher tire pressures are going to reduce the size of the contact patch and reduce the risk of hydroplaning, which is probably THE most dangerous risk of driving in the wet.  For autocross, though, we tend to discount the risk of hydroplaning and instead try to maximize just plain ole' ordinary wet grip.  For that, we still want a nice wide contact patch.  Since grip is lower in the wet, weight transfer is lower, too, which means we can run at a lower pressure wet than we could dry.  Same thing in really cold weather.
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« Reply #15 on: November 01, 2013, 07:21:45 AM »

I believe Tirerack's advice is related to hydroplaning.  Higher tire pressures are going to reduce the size of the contact patch and reduce the risk of hydroplaning, which is probably THE most dangerous risk of driving in the wet.  For autocross, though, we tend to discount the risk of hydroplaning and instead try to maximize just plain ole' ordinary wet grip.  For that, we still want a nice wide contact patch.  Since grip is lower in the wet, weight transfer is lower, too, which means we can run at a lower pressure wet than we could dry.  Same thing in really cold weather.

I agree. I have talked to tire engineers who agree with this as well.
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