I have a question about the relation between tire pressure and traction. I have an AP2, and last year, for autoX and track event, I ran with a 3 psi difference between the fronts and rears, with the fronts at a higher pressure.

My understanding is that the AP2 comes from the factory with a tendency to understeer. It could be because of the suspension design or the size of the front tires. Granted, you can get the car to oversteer if necessary. I ran 225/255 combo last year with a gendron front bar and KW V3's, and the tire pressure differential seems to work well.

Since I'm going to run 255's all around this year, the question I have whether the tire pressure should be the same all around now. The assumption is that the increase in front grip will offset any understeer tendencies built into the car. So, tire pressure increase equals more traction assuming the pressure doesn't go too high?

Any thoughts or input?

 

From what I understand tire pressures are adjusted in order to affect the spring rate of the front or the rear end of the car. Lower pressure is equivalent to a softer spring rate for example.

edit: somehow I confused section width with pressures

 

First of all, you shouldn't use tire pressure to tune in/out oversteer/understeer. A tire's rubber compound has an ideal operating temperature, and ideal inflation pressure ranges for ideal contact patch shape and carcass deformation/shape. Pressures should be set to idealize those things. At your (our) level, this usually means very close to equal front & rear pressures; especially since you have 255's all around now. It also means not extremely low or extremely high pressures (ie, somewhere within the manufacturer's specified range is probably where you'll want to be).

Second, it's not a blanket general statement that "going up in pressure increases grip" or "going down in pressure increases grip" (no those aren't typos). Those statements are false.

A short lesson...
When you change pressure, you change the size of the contact patch. Lower pressure = bigger contact patch, higher pressure = smaller contact patch. This should be obvious and intuitive. Pressure also affects how much the tire carcass deforms as a section of tire rolls into and out of the bottom (contact) area of the tire: Lower pressure = more carcass deformation, higher pressure = less carcass deformation. The tire's carcass (near the contact area) deforming as it rolls and twists from vehicle forces is the main generator of heat in a tire. At the extreme, if you go too low with pressure, the tire is deflated so much that it's riding on the stiff sidewalls, and the outer edges of the contact patch (the parts near the sidewalls) get extremely hot, and the tire loses grip because the compound is way outside of its ideal temperature range. At the less extreme end of this direction, going too low in pressure simply results in enough carcass deformation to generate too much heat, and again the tire goes above it's ideal temperature and loses grip. Going up in pressure decreases the heat generated by carcass deformation. Go up way too high, and the tire will never even get "hot".

***Generally***, going down in pressure will increase grip; until the tire gets too hot from carcass deformation, or it starts to ride on the sidewalls and loses grip because all the download (and therefore friction) is located on small areas underneath the sidewalls (and then gets too hot). This is because lower pressure = more compliance from the tire and rubber tread blocks, allowing them to 'mold' to the tiny irregularities of the asphalt better.

That ^ kindof falls apart though as you get into racing tires and cars with significant aero. Kindof like how Newtonian kinematics breaks down under extreme conditions... Also, there's alot more going on here than what I posted but hopefully I got you thinking and pointed you in the right direction.

 

Thanks for the input. So, from reading your response, it sounds like I need to pretty much run equal tire pressure front and rear, making sure the pressure's within the range of the manufacturer's recommendation.

So, do you run the same tire pressure front and back since you run 255's all around?

 

I do.....I run NT-01 with a hot psi of 40.

I havent used a pyro to tell if that is where I should be or if my camber is right.......from the wear things look pretty even with -3 front and rear.

What pressure are others out there running their NT-01?????

 

i would say keep all tire pressure same, do the suspension setting first and fine tune tire pressure last!

 

I was running 39 and 36 hot last year with my NT01's. (225/255).

 

Your tires should have whatever pressure allows them to use the most grip. An easy (semi accurate) way to do this is by using chalk to mark the wear lines. Chalk starting on the tread running down the sidewall, then go run some laps. If the chalk rubs off on the sidewall, raise your pressures. If the chalk is still present on the tread, lower the pressures. When the chalk is off the tread but still on the sidewall, have about as much grip as you are going to get out of that setup.

If you use that method, when you get off the track and take your race tires off, check the wear on inside edges. This will tell you whether you have the right amount of camber/caster dialed in. If the inner sidewall has wear marks, decrease your camber. If the outside of your tire is worn more than the inside, increase it. Then go back out an repeat the chalk procedure. If you tune it this way, you should end up wearing (using) all of the tread across your tire, giving it the maximum grip.

Of course this is all just an estimation of exactly what S2Kretard was talking about. You are looking to evenly distribute temperature across your tire for maximum performance. It's generally safe to assume if you are wearing the tire evenly, you're close to achieving this goal. The only way to be sure though is to use a tire pyrometer and take inner/middle/outer temps immediately after coming off the track.

 

hm, well from my own experience (using new RE01R's) - i would drive to the lapping day on them at "cold" OEM pressure of 32psi (2 hrs drive all hwy!), do about 4 laps as they heat up and the car gets very squirrly, thus now up to about 42psi or so when i check each one (hot). i will then drop each of them to about 30 to 33psi or so, and the car sticks pretty decent from there on out. (rear will be sometimes the 30-31psi mark, while front somtimes 31-34psi, all hot). \

so basically, it shows running 'stock' pressures 'hot' (while lapping) works rather well. ill see about bumping the front more this season to test out the effects (maybe the rear a bit too). this is all on stock suspension btw, w/ spoon xbrace and rear tie bar, plus cusco front strut bar. doing a whiteline front sway this season too.

 

Yup. All of your factors point to close-to-equal pressures being ideal for you. Low hp/torque car, close to 50/50 weight distribution, and equal size tires/wheels... all of these things are reasons to have close to equal tire pressures. As for how low you can go with pressure... A pyrometer + good seat-of-the-pants feel would help you determine just how low you can go. The pyrometer will tell you what areas of the contact patch are hottest, and how the heat is distributed. Your butt, if sensitive, can tell you whether a certain pressure change is faster or slower. If you don't want to spend money on that, then my $.02 is that if your tires get greasy within ~2 laps of 10/10ths driving, you're too low. But that advice kindof depends on what tires you're on, so... not sure how much it's worth. Probably about 2 cents. With all the street tires I've driven, it's come down to the question of "how long do I need the tire to last before it gets greasy?". If it's a Time Attack lap, then I need a warmup lap and a flyer, that's it. If it's a practice session or an HPDE day, then you want them to last longer before getting greasy. With street tires and 10/10ths driving, it's not a question of 'if' they'll get greasy but 'when'.

Yea, for the Berk car at SuperLap we always went out on equal tire pressure all around. As you can see from my sig, the car tended to over-rotate a bit to start with, so the rears usually got a little hotter than the fronts by the end of each session. If you click my sig and watch that lap, you'll see how the tires got hotter and hotter over the course of the lap, and that made the car get more and more oversteery. The tires were getting greasier and greasier.... We went too low (28psi cold I think?) for that session, and that was the consequence. So there's a good example of going too low! ( ) It might've been a good thing for us to go a pound or three higher in the rear, as the rears were getting cooked before the fronts due to the oversteery nature of the car and the yahoo behind the steering wheel. But we only ran the car that one day and didn't get a chance to try it.

Keep in mind that with tires, there are almost no absolutes. Things often come down to each specific model of tire, what car they're on, car balance, and what you ate for dinner last night. Keep an open mind, pick up a book or two, and learn from your experiences