Well was my first time at the track today (Yay popped my cherry lol), and was trying to play around with tire pressure to improve my grip and behaviour of the car (understeer/oversteer). How do you guys calculate tire pressure, or is it all just trial and error? Also do you normally set different pressures for front and back or all the same all around? The tires that I'm running are the Falken 912's i know they're not the best track tires, I wanted to get the 615, but i'm going to wait till i get my good size rims to get those.

 

Check your pressures right after you come off the track. You'll probably want to run a little less than 40 lbs hot in most cases. Start with all of them at the same pressure and adjust from there (a little less for more grip). I don't know much about the 912s but the 615s get really slippery when hot so look to keep them under 38 hot.

 

Actually, less pressure equals less grip. For example, if you are experiencing understeer, you can increase the front pressure or reduce the rear pressure.

 

I think it's a little more complex than that.

First, you want to find the optimum hot pressure for your tires and your camber settings. You should use a probe-type tire pyrometer for that, comparing readings at inner, middle and outer spots on the tread. The middle should be roughly the average of the inner and outer.

Then you can use tire pressue to modify the front/rear balance of the car. The tire is part of the spring system. In general, stiffer springs give less grip, softer more grip. I lower my rear tire pressure to reduce oversteer.

 

To my understanding less pressure also means more grip. Look at all the Drag guys. The less tire pressure, the wider the contact patch of you tire becomes

 

more grip in a straight line. the pyrometer discussion above was spot on. You should also buy a book on suspension tuning since you're heading in that direction.

Here's a good article on tire pressures:
http://rotarynews.com/node/view/223

 

^^^ thanks for that link. I've been planning on buying the G-Force Engineering book for the first gen, Recommended?

 

Yes - buy the book.

Using my IT racecar as an example (it was built in part based on recomendatons from G Force Engineering book):

What I am looking for is the optimum hot pressure for my tires. It varies by brand. Kumho V710's like 36PSI hot while Hoosiers were better at 38-40PSI hot. It also varies by tire type within a brand - Kumho V700's liked more pressure than the V710's for example. The tire manufacturer or a qualified distributor can provide this data - for example the Tire Rack lists this for some competition tires. Ultimately a tire pyrometer is needed at some point to validate alignment a pressure settings. YOu are looking for even temps accross the tread face

I set my cold pressures 6-10PSI lower than the desired hot pressure. How much heat (pressure) is generated in a tire during a session depends on air temps (hot day vs cool day), track temp (sunny day vs cloudy day), the track I am at (more left than right hand turns for example) and how I have the car setup (loose vs tight). I keep notes for the various tracks.

On my car I want all tires to be at the same pressure by the end of the race.

 

that sometimes means you're starting out with different pressures in different tires, it helps to have a log.

 

i also should add, that the factory tire sticker on the car says 26psi for an SA and i think 28 for a gsl-se, which are actually about right

 

A tire pyrometer( the probe type) and how the car feels for the driver is where you start. Get a baseline for cold pressures from somebody who knows your tires and start there. Drive the car, try bleeding or increasing pressures to effect the feel. When you get a good feel and good temps across the tire with your hot pressures let the tires cool over night if you can, or at least a few hours out of the sun, and measure the pressures. That is your cold starting pressure when you go bsck to the track.

 

Correct for street driving, not racing or solo.

Racing tires in a racing environment is a different deal. Even street tires at a solo benefit from more tire pressure than what Mazda recommends.

Here is a really cheap but effective method of determing tire pressures in Solo II. Put chalk marks on the tire's shoulder in three different locations around each tire. The chalk mark should cover the tread and the sidewall just below the shoulder. After you take a lap look at the chalk. If all off the chalk is gone from the tread and the sidewall - add more air. If chalk marks are still on the tread take away air. Don't be supprised if your tires need 35-45 PSI to when you are done.

This is why an air tank and a good tire pressure gauge are one of the first things an autocrosser buys. Oh and chalk too.

 

Very true a log is a great idea. Some tracks like Gateway in St Louis place a premium on right side tires because all of the fastest turns are on the oval. So it is not uncommon to set right side pressures 2-3 psi lower than the left tires because they get worked more and gain more heat and pressure.

 

very interesting thing about the chalk idea and will most likely give that a try. Thank for all the help, any more "cheap" tricks like that would be awesome.

 

one of the unexpected challenges at the 25 hours of thunderhill, is starting the race at about 60degrees F and having the temp drop in the middle of the night into the 30's (there were some snow flurries).

so when the car comes in for tires you have to do a little math...

 

Yea that would really tax my brain cells!

Oh man, a 25 hour race sounds like a blast! I did a 2hr enduro last year and loved it. The strategy and prep for one of those events is really cool. Wish we had more of them around here.

 

Well with respect to tire pressures - once the optimum pressures are found don't forget that pressures can still be used to tune your car. As a rule of thumb, the following applies:

If you are loose - add air to the front and/or take air away from the rear
If you are tight - add air to the rear and/or take air away from the front

The theory being applied is that tire pressure is the eqivalent of spring rate. In keeping with this theory, the higher the air pressure the greater the spring rate of the tire. You can see this applied every sunday in NASCAR, IRL, F1, etc. Think about the number of times that you have heard a driver ask for a half pound of air pressure in or out of a tire to effect a change in the way his car is driving.

Taken further if you where actually changing springs and sway bars to correct for basic handling condition you would do the following:

If you are loose - add spring/sway bar rate to the front and/or reduce spring/sway bar rate from the rear

If you are tight - add spring/sway bar rate to the rear and/or reduce spring/sway bar rate at the front.

I would recommend that you make only one change at a tire - adjust the front only or adjust the rear only - and then evaluate the change.

Adjustments like these may need to be made because the course you are running that day - for example you need the car to rotate more so you loosen it up. Or you may need to make adjustments for the surface - asphalt vs concrete.

If you are having to make really radical air pressure settings to get a car to work - like 50 PSI in the front vs 35 PSI in the rear - this could indicate an alignment or suspense change that you need to make to correct the car. As always talk to other people at the event and compare notes. Pay attention to what the fast drivers are doing and don't be afraid to copy them.

Hope this helps.

 

Cool thanks! Just to clarify, loose means oversteering, and tight would be understeering? Is that what you mean by those terms? My next even there isn't till August, so I have a bit of time to learn some more and hopefully drop my times by a good 10 seconds I hope.

 

Mustanghammer: one of us is confused, and I'm afraid it might be me.

It was always my understanding that higher pressure would result in more stick. For a simple example, if I have too much oversteer I can either add pressure to the rear tires or reduce the pressure in the front tires.

I believe this is also in line with the information I read (possibly on the SCCA website) regarding how to make suspension adjustments to improve handling.

 

What folks are saying about spring rate and logs is spot on but try not to obsess over pressures and setup while you're learning. The focus should really be on the driver (at least at this stage) rather than the equipment. Just get them in the right operating range and focus on being smooth with your inputs (steering, brake, throttle) and precise with the line. Developing the right habits from day 1 is more important than having the perfect setup.

 

Each tire has an optimum pressure for maximum grip. It's far more complicated than a single number, since on a given track each tire gets worked differently. Sometimes you need to increase pressure for more grip, sometimes you need to decrease for more grip. In any case, the other guys are right - you want to set it somewhere reasonable and adjust accordingly.

It's been said, and it's not being silly - that you should adjust the end of the car that isn't working, rather than the one that is. So if you have oversteer beyond what you desire, adjust the rear pressures (or other setup parameters) to get more grip out of the rear. The "wrong" thing to do is to adjust the front - where the only way you'll rid your oversteer is by finding less grip in front. If you don't know whether to increase or lower the pressure, at least you're working on the right place. It may take a couple of tries to determine if going up or going down in pressure is what increases the grip.

Dave

 

2 hours is no fun! if you get a flat tire you're done. in the 25 we had time to pull the frame rail out with a tow strap and a couple of trucks.

check out www.k2rd.com

 

13-hour at VIR => two tranny swaps.

 

To a certain extent you are correct - an underflated tire will not hold a good contact patch and will result in a lack of traction. I am working from the stand point that you have optimized tire pressures for the car - IE chalk is wearing off the tire correctly or you have even tire temps. Now you are trying to adjust the car to compensate for an issue caused by the track surface and or course design. So your tire pressure changes will be small. 2-4 psi from optimum.

Increasing and decreasing tire pressures has an effect of the "spring rate" of the tire. I am simply applying the same logic that would involve spring or sway bar rate changes on the suspension. For example, when I want my car to turn in better (like when I run a Solo II) I increase rear spring rate (same as adding tire pressure) and take away roll resistance in the front by backing off the sway bar (same as reducing front tire pressure).

The opposite is true when I go racing - I want the car to be tighter (push) because the speeds are higher. So for racing I decrease rear spring rates and tighten up the front sway bar.

Any who....this is what I have always used. As you can see from the responses there are allot of ways to skin this cat. This is what makes Solo II and Road Racing so much fun and so hard to write a how-to article for.

 

Yes - Loose is Oversteer and Tight is Understeer. Sorry, I must have had a NECK-CAR race on my mind.