Getting rid of the rear-end dance

[VictorH]

If you're on stock calipers and track your car you've probably experienced the rear-end dance under heavy threshold braking. You will come stomping along into your braking zone, clamp on the binders to slow things down and as soon as you do that the rear end goes into a non-rhythmic dance where the front is fine but the rear end of your car is moving around, sometimes enough that it feels uncomfortable, but generally doesn't change your line into the turn.

I was never sure why our cars did that so much (I've experienced it on other M3s too) and the complete absence of this problem in GT350, Mustang GT or most any P-car (don't own any of those others). When I upgraded to a PFC front kit only there was no change in this tendency, and I was still on stock rear calipers. However, this past weekend I had a bit of a revelation. No change to the car, alignment, tires or anything else. Only change was brake pads.

Was talking to Cobalt Friction guy Andie (great pads and super nice guy BTW), last year about wanting to try something other than PFC after I was accidentally shipped some PFC 12s (thought I was getting 11s and didn't notice it until months later) and absolutely hated them. At the same time was having some trouble getting my car to rotate with brake application when I needed it. So the recommendation was a rear pad ( for the stock rear caliper) with a bit more bite than the fronts. Many of you know this is not typical as you generally want to run the same compound all the way around, but in this case having more bite in back really helped the rear end a lot.

I'd say perhaps 80% better than before which to me is huge. This was an unintended benefit as I was wanting to have a bit more rear brake to get the car to turn, but turns out that having more rear brake action helps the whole car squat down and probably takes some load off the front which minimizes dive and keeps more weight on the rear tires. I'm only stating this as my experience, I'm not certain this is the best idea for every driver (it's probably not) but in my car helped with stability under heavy braking from high speeds by a lot.

[roastbeef]

Have you upgraded your front tension arms to monoballs? It helps quite a bit.

[VictorH]

Yes, I have long ago, if fact I'm on my second set, but didn't make a whit of difference in what's going on in the back-end. Did you have a different experience?
What i'm referring to is straight-line braking, not anything to do with initial turn-in.

[roastbeef]

yes, the monoballs helped settle the nervous rear under heavy straight braking, but monoballs with rear race pads really minimized it. i think there is a tiny bit of it that naturally occurs with that much weight transfer. there are other factors that may be present like tire tread squirm, toe, and track surface being level, but the major contributors are monoballs and too much bias due to lack of ideal pads on the rear brakes.

[M3 Number 86]

I haven't experienced this at all. If anything I've had to subconcsiously correct the wheel but not by much.

I've tried:

PFC08 all around
Carbotech 12/10 and 10/8
GLoc 12 with PFC08 rear
GLoc 12/10

Does alignment have anything to do with it? Maybe front to back spring rates? Do you have a pretty soft spring up front? Maybe it is nose diving and lifting the weight off the rear too much?

[VictorH]

Well it's all relative. Under 70-80 mph it's really a non-issue.
I'm talking about hauling the car down from 145 mph to 30-40 mph. When you're converting 100 mph of motion to heat, that's where you'll notice the rear of the car bouncing around a bit. Similarly 150 down to 100 mph at max threshold braking same deal. I'm not saying the car should feel like it's on rails but some cars just move around a bit more than others. The M3 is one that tends to move around a bit more. It's not really a defect or anything it's just a characteristic of the car, but in my case was improved by more rear brake bias.

[admranger]

Originally Posted by VictorH

Well it's all relative. Under 70-80 mph it's really a non-issue.
I'm talking about hauling the car down from 145 mph to 30-40 mph. When you're converting 100 mph of motion to heat, that's where you'll notice the rear of the car bouncing around a bit. Similarly 150 down to 100 mph at max threshold braking same deal. I'm not saying the car should feel like it's on rails but some cars just move around a bit more than others. The M3 is one that tends to move around a bit more. It's not really a defect or anything it's just a characteristic of the car, but in my case was improved by more rear brake bias.

I don't recall this at all at COTA (145 to 45mph on back straight). Stock brakes w/PFC-08's. I had a modest alignment with minimum spec rear toe.

I've had the back end squirm before under braking, but it was always on full or near full tread depth RA-1s (we only had black and white cars back then, no fancy 4k color cars like you kids have...).

Regardless of my experience or other's experiences, do what works for you since that's who is driving the car!

[gmx]

Alignment definitely affects it. Rear toe -8.5mm here:
youtu.be/sMBrFHK8JPg?t=60
I run +2.5mm now which is more on the conservative side but helps tremendously. PFC08 all round.
Same track, same corner, only a proper alignment helped:
youtu.be/yHu-ZZnSRJI?t=56

Rake, suspension setup I imagine also definitely play a role.

[dparm]

Alignment will affect stability under braking, definitely.

For what it's worth, even my Corvette with pretty substantial aero and the mag ride suffers from this. It's not terribly common but there were a few times at COTA (back straight) where I could even see it in the telemetry.

My best advice is to adjust your braking technique slightly. If the weight transfer to the front is too fast, the rear will feel very squirrely.

[bigjae1976]

I had this issue until I installed a rear BBK.

My theory is this... The rear wheels toe in under squat. So the opposite occurs under braking. The rear end lifts and the rear wheels toe out which likely causes the stability issue.

I don't know my exact specs but I was close to OEM. I suspect adding a little bit of toe in will help but increase tire wear.

Adding a rear axle BBK eliminated the rear end wiggle. Not sure I can explain why?

[admranger]

Originally Posted by bigjae1976

I had this issue until I installed a rear BBK.

My theory is this... The rear wheels toe in under squat. So the opposite occurs under braking. The rear end lifts and the rear wheels toe out which likely causes the stability issue.

I don't know my exact specs but I was close to OEM. I suspect adding a little bit of toe in will help but increase tire wear.

Adding a rear axle BBK eliminated the rear end wiggle. Not sure I can explain why?

A rear BBK has the same effect as adding a higher torque rear pad; the rear squats more (or rises less perhaps) resulting in less toe-out.

Or it’s becasue they look so good. I’m not 100% sure which it is.

[admranger]

Originally Posted by dparm

Alignment will affect stability under braking, definitely.

For what it's worth, even my Corvette with pretty substantial aero and the mag ride suffers from this. It's not terribly common but there were a few times at COTA (back straight) where I could even see it in the telemetry.

My best advice is to adjust your braking technique slightly. If the weight transfer to the front is too fast, the rear will feel very squirrely.

Agreed. Technique can indeed be an issue.

[M3 Number 86]

Originally Posted by VictorH

Well it's all relative. Under 70-80 mph it's really a non-issue.
I'm talking about hauling the car down from 145 mph to 30-40 mph. When you're converting 100 mph of motion to heat, that's where you'll notice the rear of the car bouncing around a bit. Similarly 150 down to 100 mph at max threshold braking same deal. I'm not saying the car should feel like it's on rails but some cars just move around a bit more than others. The M3 is one that tends to move around a bit more. It's not really a defect or anything it's just a characteristic of the car, but in my case was improved by more rear brake bias.

Yes I'm approaching some turns at 140 and 125 as well.

While I haven't yet when I spoke to zechausen he told me to try a higher torque rear pad. I've always run less aggressives in the rear so I don't have to swap them as much lol

[SYT_Shadow]

Originally Posted by M3 Number 86

Yes I'm approaching some turns at 140 and 125 as well.

While I haven't yet when I spoke to zechausen he told me to try a higher torque rear pad. I've always run less aggressives in the rear so I don't have to swap them as much lol

no bueno

I run the same pads front and rear

I've felt the shimmy, I attribute it to braking too suddenly and steering the car slightly once it is under full braking

[dparm]

Originally Posted by M3 Number 86

Yes I'm approaching some turns at 140 and 125 as well.

While I haven't yet when I spoke to zechausen he told me to try a higher torque rear pad. I've always run less aggressives in the rear so I don't have to swap them as much lol

Sage advice. Try running the same pads front and back.

[M3 Number 86]

Originally Posted by dparm

Sage advice. Try running the same pads front and back.

I'll try next but endurance pads in the rear never run out. Maybe in a year lol

[dparm]

Except that you might not even be getting enough heat into those pads for them to be working. Endurance pads like sustained high temps.

[kyippee]

I am pondering PMU 999 pads up front and PMU HC+ in rear for that reason. Been running 999 all around and the wear on the rears is so slowwww

[VictorH]

Well my problem is largely fixed so I'm not worried about what else to try. I've run my neighbor's GT350 and my car back-to-back and they have different braking dynamics at the same speeds, same section of the track and same driver. If it were the driver's braking technique only, I'd expect the GT350 to behave the same as my M3.

I don't disagree that some of the suggestions posted here may be helpful, I suspect adding more rear rebound damping might help too. I didn't intend this to be a "I need to fix it issue" as I wrote about the fix that worked for me. Might be useful for someone else and others might want to try the other suggestions here. As was mentioned above, whatever works for you as the driver is best and each of us and our cars are different.

[bigjae1976]

Turner used to recommend Pagid yellows on the front axle and then blacks on the rear. The pagid blacks are a higher torque pads and have a lot more bite. I'd guess it's for this reason.

The pain was going through a rear pad faster than the front pad. That caught me unprepared one weekend.

[roastbeef]

Originally Posted by bigjae1976

Turner used to recommend Pagid yellows on the front axle and then blacks on the rear. The pagid blacks are a higher torque pads and have a lot more bite. I'd guess it's for this reason.

The pain was going through a rear pad faster than the front pad. That caught me unprepared one weekend.

everyone else recommends the opposite about the rear pads being more aggressive than the front pads...

[M3 Number 86]

Originally Posted by dparm

Except that you might not even be getting enough heat into those pads for them to be working. Endurance pads like sustained high temps.

Most tracks here have lots of braking. I feel like they would be. My rear caliper boots are pretty torn up lol

Maybe rear bbk for me soon.