That's one reason (among others) a stiffer front bar or stiffer front springs will help in this case...keeping the rear of the car down. You can drive around it, but it is one admitted drawback of a bigger rear bar. Can't go much stiffer with springs up front unless you go to coilovers, so a front bar is a good choice.

Ideally you'd have much stiffer springs all around, but you just can't do that if you want to stay on stock struts and/or you want decent ride quality. Compromises are tough.


- Andrew

 

Everyone has accepted the higher rear pressures, but you realize you can get the rear to rotate more with a lower rear (compared to front) pressure, right?

Tires are odd (trust me, former Goodyear race engineer intern), so different tires react different ways. Depending on the tire, running a bit less pressure allows the sidewall to roll a bit, making the rear squirm. When the tire finally gets to the point where it cant roll over anymore, it will "snap" to its limit. You get a rear that feels loose, and upon the limits of adhesion it will break loose.

This may or may not work depending on setup and driving style, but it is something to try...

 

I'm also not a big fan of higher rear pressure on 60/40 distributed car prone to understeer. Higher rear pressure reduces the rear limit of adhesion and makes the evo's rear slide and oversteer prematurely.

Tire pressure should vary depending on brand/suspension set up/ track temp/type of surface/ moon phase etc, so there is no golden numbers. The old sidewall chalking mated to a pyrometer should always dictate what should be ran, always.

 

What do you run at? Its been over a year since I have autocrossed, I think last time I ran 40/37 F/R and upped it to 38 for the last run. Its been a while though so I forget. That was with a stock suspension and stock A046s, by the way.

 

Andrew, the problem is even if you could go higher in front spring rates(with shocks valved to handle them) to keep the rear in check and on the ground you can't because the evo's motion ratio dictates stiffer rear springs regardless of the sway bar rates. So the only solution is stiffening the front bar.

 

the beauty of what you guys do is that you can find any desolate parking lot & do endless amount testing....

n

 

I can safely say that typically the front pressure always end up around two degrees higher than the rear, but again the pressure ran should be a results of the conditions and the set up.

Not to go to far off topic but a somewhat even spread of your temperature across the thread is an indicator that your not running too low or two high.
Higher temp in the middle(in relation to the sides) = pressure too high and vice versa

the minimum pressure is controlled by chalking the sidewall to make sure you are not rolling over the sidewalls.

 

I agree with the temp spread dictating tire pressure, if the camber is set properly. Looking at temp distribution (as I did regularly during Nascar practices with Goodyear) on a car that doesnt have the camber adjusted from the factory isn't going to tell much; it could be improper tire pressures or most likely a sign of wrong camber.

Not arguing, just discussing.

 

Both ways will reduce tripoding, and both have their own positives and negatives. The front bar has an impact on the balance of the car since it affects the wheel rates too.

I would probably increase the rear rate too if i could to balance it out, but if i couldn't i may still bump up just the front (or just the front bar like you said) if lifting was a big problem. Tuning the car around rear tire lift can be a problem, and more so when you're on stock struts, but you can get through it.

The fronts of EVOs roll a lot since the COG is so high up front compared to the roll center. Subaru guys were begging for the Whiteline roll center kit to come out for them first but Whiteline told everyone that it was much more necessary on EVOs.

I think you were correct here although you may not have been talking about rear tire lift specifically, just excessive front roll which can lead to lifting the rear wheel.


- Andrew

 

Yea, the front roll center is the big issue on the Evo. That imaginary point in space can actually be pushed under ground level by lowering the car too much, hence why lowered Evos tend to push like dump trucks...

Lowering the car lowers the CG, but it also lowers the RC in a much more drastic way, which increased the moment arm between them. Thats bad, for you non engineers, haha.

 

I set up the dealership I work for ex-world challenge bmw for scca road racing and the techniques I use for my auto-x cars are the same.

I find an empty parking lot, set up a skid pad and a slalom 1) Do tire temp and pressure baseline 2) camber curving 3) set up balance/feel with skidpad an a g meter 4) redo temp and pressure. The only thing I leave for the track is alignment fine tuning and wing blade height and angle.

 

I added a front underbrace and also the rear strut tower bar and v-brace from Cusco along with there 23mm 3-way adjustable rear sway bar and now have complete control over understeer. Played with tire pressure. Different tracks mean different settings. It's fun adjusting and seeing and feeling the difference over when it was stock

 

I'm not sure what the different settings are on your particular bar compared to stock and if you're running softer or stiffer because you haven't told us but I can tell you that the braces did little if any on controlling the evo's push.

 

Pretty cool! Ever auto-x with a Tom K in an STi in your region?

Do you use a particular method for tuning aero or is it based on feel/driver preference more?

That's one thing I know a little bit of physics for but never had a chance to play around with much on a real car. Not really sure where to start. Went from mech engineering for 3 years to aerospace engineer for 1 semester, then went right back to mech engineering


Nice BMW.


- Andrew

 

Andrew, I tune it mostly base on driver feel and lap time since you have no real way to test it after practice/qualification.

However, rear tire temp goes up considerably as you move the wing up and angle the blade more, so for what it's worth on colder days when the rear tires of the Bimmer is not heating up to my liking and the driver is yelling car lose all over the radio, I usually move the blade to roof height and and angle the blade as much as i can until the extra drag start lowering front straight speed and lap times.

Usually the added driver confidendence and the abilility to push harder usually net us a good half to a second per lap in wing adjustment.