cruiser

This weekend we had another trackday (hotlapping). I let my tires at stock tyre pressure and dropped my best time by nearly 2seconds. I get much better feedback from the car and I think the snap-on oversteer is more progressive now (I need to replace the rear suspension bushings). There was some sidewall flex and it was showing on the tires. Maybe I overinflated them the last time (had them around 38psi front and 35psi rear).

Just an observation I thought I'd post.

Time drop can also be partially attributed to the fact that I'm still getting used to the car and am getting better at knowing it (only have it for like 2000miles now).

RussTypeS

iTrader: (3)

were those pressures hot or cold? do you check your pressures AFTER each session? i have seen pressures go up as much as 10 degrees (running pure nitrogen!) on hoosiers on hot days (120+ track temp). street tires wont go up that much but your also not running nitrogen. be sure to check them as soon as you get out of the car after each session.

DamonB

Tire pressure will rise once they get hot no matter what you put in the tire. All gases expand when they are heated; even nitrogen. The idea behind nitrogen is that there is no moisture in it so the pressure climb is more consistent. A good dry source of compressed air is every bit as good.

82streetracer

iTrader: (1)

at a track, tire flex is the worst thing to have.

stay with the higher pressure at the track

when I brought my FB to a track day the first time with its stock 13x5s and 165/80/r13s i was folding the tires really bad and the car was all over the place. I got black flaged like 3 times for too much smoke on turns.

this nice autoX guy asked me what my tire pressure was and I told him the recommended 25 psi all around.

hea said to bump it up to 35-40 so I did, and what a difference, first lap around i dont think I waivered a Inch from the best line. Huge improvement.

I was getting thumbs up from the corner workers the rest of the day.

DamonB

That's got to be some sort of record! Never heard of that one!

cruiser

Actually I liked some flex. FD felt more tame and I found things starting to happen more progressively. Also, catching slides was much easier. I will increase tire pressure next time a bit and see what happens. I agree when you know your car fully that no flex is better.

John Magnuson

My FD seems best balanced with 32psi cold... just like the factory says...

DamonB

That's the trade. Higher pressures to an extent have more grip but are less forgiving, especially if your talking about the rear sliding under power. The car is much "tighter" at higher pressures, but you have less time to correct when things get out of hand.

I typically end up with the fronts 3-5 pounds higher than the rears. It makes the front point quicker and resist sidewall flex while allowing the rears to be a little easier to drive.

RussTypeS

iTrader: (3)

maybe i misuderstand you, but i never said nitrogen didnt expand with heat, in fact i stated an example of when it did. the main reason to use nitrogen is so the pressure does not rise as much. the expansion rate vs heat of nitrogen is roughly half that of normal compressed air.

also tire pressure is really not the right way to tune how a car feels. sure you can use it to fine tune your setup, but first you should tune your pressures (and camber) with tire temps in order to find the pressure (and camber) where the tires make the most grip. from there leave one end the same (front if the car understeers, rear if the car oversteers) and adjust the other end until your happy with the balance.

ideally you want to change the balance of the car with sway bars and spring rates. this way the car feels the way you want it to and makes the most of the tires. if you want a different response from the car try changing the width of the wheel, aspect ratio of the tire, or toe.

DamonB

It's just semantics. The pressure will rise no matter what is in the tire so weather it rises 5 psi or 10 psi you're cold pressures still have to take that into account.


Well once you show up to the track with your shocks, springs and bars on the car along with your alignment setup the only thing you have left is air pressure. That's easily done on the fly and is pretty much all you can do between sessions to correct for things like the surface temp changing. It's also far easier to give a different tire pressure a try rather than backwards engineer the entire racecar there at the track. Sure it won't be optimized as if you wrenched the whole danged car, but it's far better than doing nothing. Even if you have a "perfect" setup it won't be perfect all day.

ttb

anybody else do this? sounds like it makes a lot of sense.

cruiser

Yes, I always use more tire pressure on the front, even on the street.

rynberg

Not me. My 95 PEP doesn't have that gargantuan 93 R1 rear anti-roll bar () and feels more balanced with even tire pressures. But then again, I'm not exactly kicking everyone's *** at the auto-x, so I could be completely full of $!#$ too....

Mld>7

The reason it was feeling more progressive is because the tire was flexing and bending. This means more of the driving errors you make (like not being smooth) will lessen and make you smoother because of it. Don't take this as a personal attack because its not. A few driving schools in Europe use this tactic with there cars and under inflate the tire to train the driver to get comfortable with the car by giving them an exaggerated slower motion simulation of sliding or when the tires start to brake away. You had faster lap times because, like you said exactly, you were more comfortable in the car. Therefore you could drive it faster with the feeling that you were still in control. This doesn't equate to being faster at lower tire pressure. As it was mentioned earlier higher tire pressures with a confident driver will be faster. Not to mention the fact that with the lower pressure you give you uneven tire ware and can over heat the tire ruining a perfectly good set. As you get used to your car raise the psi slowly every time you go out, you will start knocking seconds off your best time.

DamonB

Whatever works for you. If even pressures make YOU faster you'd be silly to do otherwise.

Next time out leave your rear pressures the same and go up two pounds on the front; see what you think. Now go kick butt

cruiser

Mld7, no hard feelings

I know that the problem lies in my head, feet and hands atm and not in the tires pressure. But I figured I'd give it a try and it worked for me - I could push her a bit more and it showed on the laptime. Also, I left the gearbox alone, as with this huge-*** stock steering wheel I can't really do successfull opposite locks (big ones) as my legs are in the way - so I was running in 3rd gear all the time For next season I will be getting an upgraded IC, ATE Super Blue brake fluid, new tires, Momo Millenium steering wheel (the small one - 320mm) and maybe springs&shocks and some widefoot gas pedal. I'm looking to drop my time be another 2.5sec and beating those Imprezzas and M3
Just starting to use also the 2nd gear should bring another 0.5-1sec I guess...

Oh, and did I mention I will try higher tires pressure?