Quote:
Originally Posted by dogbone
Just some random thoughts and opinions. Not sure if anything here can help you:
I have a fully built E90---gutted, caged, full aero, OS Giken 1.5-way diff, BBK, JRZ, race slicks, etc. I always run -2.2º camber in the rear. My rear tire wear has always been perfectly even and my rear grip has always felt great. I run 1/8" toe-in on each side in the rear----total 1/4" toe-in in the rear. This has resulted in very stable braking for many years on this platform. And I am quite hard on the brakes since I am supercharged, like to brake late and use high grip slicks.
Since you are having problems, I would recommend you give this a try. The extra toe-in should give a bit more stability in braking, and slightly less camber should put more tire surface on the track. It's a low commitment change. If the alignment doesn't help by itself, you also might want to look at your suspension settings.
What was the theory for going with 1150lbs springs in the rear? In my opinion, such a stiff spring is not necessary in a gutted/light car and makes it edgy and unfriendly on berms. (Others would argue with me.) And most of your weight loss came from the rear of the car. My gutted E90 weighs 3212 lbs (no driver no fuel). I'm at 900lbs springs in the rear. I've experimented with several rates in the rear over the years. 1000lbs was more jittery/edgy than I desired. If you had stock suspension on the car before the race build, you have seriously upped your spring rates while significantly lightening the car---sort of a double whammy.
I don't know your build. Do you have aero?
For what it's worth, the owner of Cobalt Friction brake pads has always recommended a higher torque brake pad in the front and a lower torque pad in the rear on both my streetable M3 and my track dedicated M3. This advice has always worked out for me.
In the end, I believe the weakest point on an E9x M3 is the OEM single piston brake calipers. With a fully built car, I believe you can justify the cost of a decent BBK just from the fact that BBK pads are much larger, thicker and last longer. My friend with OEM calipers and Endless pads was burning up pads in less than two days......yikes. I'm a fan of AP Racing. Incredible brakes. I own two sets on two cars---the older CP5555 kit on my street M3 and the current Essex Radi-Cal kit on the track car. I was at a NASA event getting a check ride a week ago in my street M3 with the older AP kit. The instructor was like "woah woah!" as we went into the corner on a late brake. The car slowed 100% in control. After we got through the corner completely in control, he was exclaimed, "Nice!!!" hahahaha Can't beat a good set of brakes. 
(Of course, I've been arm-chair-quarterbacking here, but I'm just trying to put ideas in your head.) |
Hi Dogbone, thanks for the reply.
I'm well aware of your car, your build thread was referenced quite a few times while this car was being build
With regards to the alignment, we've added in a bit more toe like you've suggested to the rear, right around 1/8" per side now. We'll see how this helps the stability. Haven't reduced the camber yet, if the toe on it's on doesn't make enough difference then the next change will be slightly less camber.
For spring rates. The rear is a coilover setup, with 570lb springs, which is equivalent to a 1150lb divorced setup - I reference this number as it's easy to comprehend for most people as the divorced setup is the norm. The suspension itself is custom KW Competition 2-way Remote setup made specifically for this car. The spring rates (front and rear) were chosen based on KW's experience setting up similar E9X Race platforms over in Europe. They actually recommended both slightly stiffer front AND rear springs - but I opted to go a bit softer due to bumpy nature of tracks in my area. With that being said, the suspension is surprisingly 'soft' feeling and compliant given the stiff sounding stiff rates - and so far we've been very very impressed by its performance. For reference, the previous car I drove (my dad's E36 M3 Race car) ran Ground Control Advanced Design suspension with 500F 600R springs (yes, motion ratios are slightly difference but not by much), and this suspension was far more jarring, rough and stiff feeling then the current KW setup - and the rear spring rate in the E92 is almost double! The main point I'm trying to make is, the rear spring rate never has felt too stiff. Would a softer spring work better though - maybe?
The car currently doesn't have aero, just a front GT4 style splitter (mainly for the brake ducting application). Current 'home' track is tight and technical, pretty low speed, so aero doesn't play much of a role. In the future the car will have full aero though.
With regards to the brake pads. I've tried the higher torque in the front relative to the rear, then eventually made my way to what I have now (higher torque in the rear compared to the front) to try and fix the issue. Basically just been experimenting with different pad combos to see if there was any difference.
I definitely understand the benefits of a good BBK setup. The E36 ran PFC's all around and they performed beautifully. Other than the stability issues, the stock brakes have been working quite well. They stop well, don't fade and pad life seems to be about 10-15 on the current setup (probably thanks to the cooling) at a track that's known to be tough on brakes. A BBK is definitely on the list for a future upgrade though - just whether or not it's before or after other things at this point, like aero, weight reduction - will depend on what comes about from this current situation.