Stability under high-speed braking

[dparm]

If I'm doing some serious braking in a straight line down from 100 to 40 or so, the car feels a little bit squirrely. The car feels stable while driving at speeds over 100, it's just that as soon as I begin to brake it feels nervous. I've now noticed this at two different tracks, two different sets of tires, and on stock brakes or the StopTech kit.

Am I possibly getting a bit of ABS engagement and feeling the pulsations tugging at the individual wheels?

[silvergray545]

What's your set up?
Tires
Brakes
Suspension

[dparm]

Originally Posted by silvergray545

What's your set up?
Tires
Brakes
Suspension

Sorry, I should've mentioned that.

Tires are AD08R, 255/35R19 and 275/35R19 mounted to OEM ZCP wheels. Brakes are the StopTech ST40 kit up front, OEM in the rear, with StopTech Street Performance pads on both axles. Suspension is stock.

As I mentioned, I saw this while running the stock brakes and PSS tires in the same size, too.

[elim11]

Alignment has to do a lot with this. If you run aggressive track/race alignment then chances are your tire contact patch when accelerating and braking is very narrow.

[1MOREMOD]

Pretty common. As said above alignment is a balancing act. To get maximal cornering and acceleration with side load car is usually squirrely in straight line. Better tour corner set up worse it will get. I don't even pay attention anymore or utilize it to rotate car some on entry.

[silvergray545]

From what it sounds like, you may have a toe issue. I would start by checking the basics. Alignment, anything suspension related (control arms, trailing arms, bushings, etc.) and you may want to take a look at the subframe bushings. Those seem to be a common failure on cars with track miles.

[dparm]

Originally Posted by elim11

Alignment has to do a lot with this. If you run aggressive track/race alignment then chances are your tire contact patch when accelerating and braking is very narrow.

Good info, thanks. I suspected it might be alignment related -- the car is running factory alignment specs, nothing unusual there. My dad and I were talking and he wondered if it might be caster.

The car has 65k on the clock but hasn't been a track car up until recently. I only had a few events last year and some autocrosses. Prior to that, it was a DD.

[elim11]

Originally Posted by dparm

Good info, thanks. I suspected it might be alignment related -- the car is running factory alignment specs, nothing unusual there. My dad and I were talking and he wondered if it might be caster.

The car has 65k on the clock but hasn't been a track car up until recently. I only had a few events last year and some autocrosses. Prior to that, it was a DD.

I don't believe caster is something that is adjustable on these cars. At least thats what I've been told. When I first got an aggressive race alignment, the car was "dancing" all over the place, under braking, turn in, etc. Loose car = fast car but it was just too loose for my liking. I dialed it back a little which helped me build confidence. Now I'm pretty use to expecting the car be a little squirly under braking and like 1moremod said, use it to your advantage when applicable.

[swartzentruber]

Before jumping to the immediate conclusion that something is wrong with your car, have you considered the very real possibility that it's your braking technique, particularly if you are fairly new to tracking and have also seen it with various brakes, tires, and tracks? I recall definitely getting my car "squirrely" (though an R32, not M3) under threshold type braking when I was first learning to track, and still will see it in certain corners when my braking isn't smooth enough. It's relatively easy to get a car unsettled under hard braking if you aren't smooth enough getting on the brakes. When you are first starting to learn threshold braking, there's a tendency to stab at the brakes (I've done it myself plenty of times), and you really need to learn to roll onto the brakes very quickly. It might be worth finding an instructor with good braking technique to take your car out both to possibly drive and check the brakes themselves, as well as carefully observing your technique, and see if that's what is unsettling the car.

[okusa]

Definitely an alignment and/or ride height issue.

[dparm]

I don't know if anything is wrong with the car, that's why I sort of alluded to the idea I might need to improve my braking technique (am I engaging ABS).

Last night's track event was a tricky one because I had to manage a bunch of variables that changed:

1. Stickier tires (AD08R, previously had PSS)
2. More brakes (StopTech kit, previously had OEM)
3. New track (never been on it, and never been on a track with banked oval sections)

Points 1 and 2 mean I have to adjust my braking technique from what I was used to. It might take some time to perfect it.

Originally Posted by swartzentruber

Before jumping to the immediate conclusion that something is wrong with your car, have you considered the very real possibility that it's your braking technique, particularly if you are fairly new to tracking and have also seen it with various brakes, tires, and tracks? I recall definitely getting my car "squirrely" (though an R32, not M3) under threshold type braking when I was first learning to track, and still will see it in certain corners when my braking isn't smooth enough. It's relatively easy to get a car unsettled under hard braking if you aren't smooth enough getting on the brakes. When you are first starting to learn threshold braking, there's a tendency to stab at the brakes (I've done it myself plenty of times), and you really need to learn to roll onto the brakes very quickly. It might be worth finding an instructor with good braking technique to take your car out both to possibly drive and check the brakes themselves, as well as carefully observing your technique, and see if that's what is unsettling the car.

[Richbot]

Take video of your hands.

[Cloud9blue]

Contrary to what everyone says here, I believe your issue is brake bias, assuming you are running the same pads front and back. OEM brake distribution tends to be rear heavy, factoring high grip (your AD08R has a lot more grip than factory tires, especially when warmed up) -> large amount of weight transfer to the front axle you see on the circuit, the rear end tends to get a bit loose.

Run more aggressive pads on the front axle. That completely solved the problem for me and quite a few others as well.

[dparm]

Originally Posted by Cloud9blue

Contrary to what everyone says here, I believe your issue is brake bias, assuming you are running the same pads front and back. OEM brake distribution tends to be rear heavy, factoring high grip (your AD08R has a lot more grip than factory tires, especially when warmed up) -> large amount of weight transfer to the front axle you see on the circuit, the rear end tends to get a bit loose.

Run more aggressive pads on the front axle. That completely solved the problem for me and quite a few others as well.

Yep, StopTech pads front and rear. I'm going to feel them out a bit more on Monday at Autobahn CC and see if I want to step up to PFC08s.

[nholmes]

Originally Posted by Cloud9blue

Run more aggressive pads on the front axle. That completely solved the problem for me and quite a few others as well.

I'll have to try this. My car also dances but I just chalked it up to not having any aero to keep the car planted at speed or my shitty tires.

[swartzentruber]

Originally Posted by Cloud9blue

Run more aggressive pads on the front axle. That completely solved the problem for me and quite a few others as well.

I didn't even notice the Stoptech Street Performance mentioned earlier in the thread. Those are fine for rears, but if you are running R-compound type tires, you should REALLY be running a track pad in the front. Your current setup is just asking for trouble. I'd agree that it's probably a brake bias issue combined with inexperienced technique. There are plenty of good relatively inexpensive ST-40 track pads, and one of the main BBK selling points is easy pad changes.

[admranger]

How new were the tires? I've had this happen with newish RA-1s as the tread blocks will squirm a bit. Not sure if the Yokes have the same issue.

Also, at 65k miles, I wonder how beat up your stock suspension bushings are?

[TheYoungConnoisseur]

Rear brakes are basically for stability and steering. Needs more rear brake of suspension is sprung and damped correctly

[M3 Esq]

Can you describe what you mean by squirrelly? Does it feel like the rear end wanders?

[bigjae1976]

I think it's an inherent issue with the E9X M3 with the LSD, suspension design and the OE brakes. Put a rear BBK on and the rear end will stay put. You really MUST not unsettle the car under braking or you'll get the wagging rear end.

You can also try a compound with more bite on the rear axle.

The rear suspension design toes out under braking and the diff, although pretty good, isn't fast enough to keep the rear end perfectly straight. Those are 2 things working against you.

I had an issue at a track where there's a bad bump in a heavy braking zone and the rear end would wag. After installing a rear BBK, never felt it again.

[dparm]

Originally Posted by M3 Esq

Can you describe what you mean by squirrelly? Does it feel like the rear end wanders?

Yes, a bit. It's not enough to get the DSC light to flash, but it is enough to feel through the steering wheel and require some extra concentration.

It really does sound like a technique thing. If I am being too abrupt on the brakes, the weight would be shifting forward too fast which will make the rear feel light all of a sudden (especially if I had just been accelerating, shifting weight to the rear).

I'll be at Autobahn CC's full circuit on Monday which has two big braking zones. I'll probably have an instructor ride along with me to provide some guidance in that.


We did receive instruction and went through drills on braking + trail braking at Bondurant, but the Corvettes had so much goddamn grip I have to wonder if that masked some mistakes I was making. It's also a newer, lighter car and just behaves differently than the M3.

[jphughan]

I never had this issue on my M3. I ran 1/8" toe-in per wheel in the rear. I agree that track pads rather than StopTech in the rear is worth considering if you're running R-comps, but I don't really see why a more aggressive FRONT pad will help the rear stay put since that would unload the rear even more -- but if that seems to work, then ok I guess. If you still get squirm, it could be your diff (squirming under braking is a common sign on the Porsche side that the weak LSD has turned into an open diff), but try simpler fixes first.