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      07-28-2019, 11:20 AM   #1
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Braking instability on new track build - Solutions?

Hi everyone. So I've been dealing with braking instability/movement under braking on a new track car build.

For some background, I tracked the car last year with the same brake setup back when the car was stock - upgraded pads, fluids and R compounds - and the brake stability seemed fine (wasn't pushing as hard as I am now though with the new build). I was running stock calipers, rebuild with brass guides, stainless steel lines, front backing plates with ducting and PFC 06 pads in the front (older version of the 08's). Initially was running 06's in the Front and 08's in the rear - this was causing the rear to lockup too easily, likely due to the large amount of weight transfer forward with the soft stock suspension. BimmerWorld recommended I use the PFC Z-Rated (PFC's street compound) in the rear, this solved the issue and the brakes worked great. No fade throughout the session and good balance on the otherwise stock car.

Fast forward to this year and the car has been fully converted to a race/track build. Fully gutted, 8 point roll cage, all suspension arms and bushing replaced with heim joints and solid bushings. Custom KW motorsports suspension, 800F and 1150R spring rates. Brakes were left unchanged however, as the setup mentioned above performed very well on the stock car I didn't feel the need to upgrades brakes immediately.

The first day test day with the new build I left the same brake setup as the previous season, PFC 06 in the front and the Z-Rated in the rear. No good anymore, with the much stiffer suspension the amount of weight transfer has been greatly reduced from stock, and the rear now doing more braking the Z-Rated wasn't a high enough torque pad. Also first noticed the instability under braking at this point, but I figured this was due to the brake balance being wrong due to the pads.
So, next track day I switched out the rears for some PFC 08's in the rear, technically should be the same, or very similar overall to the 06's I was running up front. Improved the braking in terms of stopping distances compared to before, but still had the wiggle/movement under hard threshold braking.
So what now? I've read of some people running higher torque pads in the rear to help issues like this, BimmerWorld and Turner have done/recommended this. So I picked up a set of PFC 11's for the rear - higher torque pad than the 06/08's. Ran this setup yesterday and the issue still seems to persist, the car just isn't that stable under hard braking, it's consistently moving around, not so much that you need to put in large steering correction, but enough to affect your confidence to attack the braking zones. The car will often pull to one direction or the other and movement around at the back end.

I don't believe this is an alignment issue, current alignment specs are:
Front: -3.3* camber, 0 toe
Rear: -2.5* camber, ~1/16 of toe IN per side, just above 1/8in total
1/4in negative rake - measured from the jack points
Running 275 NT01's all around for reference

I've read quite a bit of other members experiencing similar feelings with stock brakes on the track. It seems like the main solution is a BBK, some say just the fronts will solve it, others say you need F+R. Any other suggestions? Anyone have similar experiences?

Last edited by tsk94; 07-28-2019 at 11:38 AM..
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      07-28-2019, 11:40 AM   #2
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Quote:
Originally Posted by tsk94 View Post
Hi everyone. So I've been dealing with braking instability/movement under braking on a new track car build.

For some background, I tracked the car last year with the same brake setup back when the car was stock - upgraded pads, fluids and R compounds - and the brake stability seemed fine (wasn't pushing as hard as I am now though with the new build). I was running stock calipers, rebuild with brass guides, stainless steel lines, front backing plates with ducting and PFC 06 pads in the front (older version of the 08's). Initially was running 06's in the Front and 08's in the rear - this was causing the rear to lockup too easily, likely due to the large amount of weight transfer forward with the soft stock suspension. BimmerWorld recommended I use the PFC Z-Rated (PFC's street compound) in the rear, this solved the issue and the brakes worked great. No fade throughout the session and good balance on the otherwise stock car.

Fast forward to this year and the car has been fully converted to a race/track build. Fully gutted, 8 point roll cage, all suspension arms and bushing replaced with heim joints and solid bushings. Custom KW motorsports suspension, 800F and 1150R spring rates. Brakes were left unchanged however, as the setup mentioned above performed very well on the stock car I didn't feel the need to upgrades brakes immediately.

The first day test day with the new build I left the same brake setup as the previous season, PFC 06 in the front and the Z-Rated in the rear. No good anymore, with the much stiffer suspension the amount of weight transfer has been greatly reduced from stock, and the rear now doing more braking the Z-Rated wasn't a high enough torque pad. Also first noticed the instability under braking at this point, but I figured this was due to the brake balance being wrong due to the pads.
So, next track day I switched out the rears for some PFC 08's in the rear, technically should be the same, or very similar overall to the 06's I was running up front. Improved the braking in terms of stopping distances compared to before, but still had the wiggle/movement under hard threshold braking.
So what now? I've read of some people running higher torque pads in the rear to help issues like this, BimmerWorld and Turner have done/recommended this. So I picked up a set of PFC 11's for the rear - higher torque pad than the 06/08's. Ran this setup yesterday and the issue still seems to persist, the car just isn't that stable under hard braking, it's consistently moving around, not so much that you need to put in large steering correction, but enough to affect your confidence to attack the braking zones. The car will often pull to one direction or the other and movement around at the back end.

I don't believe this is an alignment issue, current alignment specs are:
Front: -3.3* camber, 0 toe
Rear: -2.5* camber, ~1/16 of toe IN per side, just above 1/8in total
1/4in negative rake - measured from the jack points
Running 275 NT01's all around for reference

I've read quite a bit of other members experiencing similar feelings with stock brakes on the track. It seems like the main solution is a BBK, some say just the fronts will solve it, others say you need F+R. Any other suggestions? Anyone have similar experiences?
Do you have video with data overlay that you're able to post? I am interested as to particular corners at particular tracks.
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      07-28-2019, 11:51 AM   #3
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Quote:
Originally Posted by ThunderMoose View Post
Do you have video with data overlay that you're able to post? I am interested as to particular corners at particular tracks.
Not yet, been mainly focusing on dialing in the setup of the car. The best way I can describe it is; any threshold, straight line braking will cause some instability to varying degrees.

Don't have data acquisition system in the car yet either, that's something I'm looking to get in the near future as well.
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      07-28-2019, 12:12 PM   #4
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Quote:
Originally Posted by tsk94 View Post
Not yet, been mainly focusing on dialing in the setup of the car. The best way I can describe it is; any threshold, straight line braking will cause some instability to varying degrees.

Don't have data acquisition system in the car yet either, that's something I'm looking to get in the near future as well.
Without data and video, its subjective speculation at best.

Your alignment and set up sounds pretty typical, so perhaps playing around a bit with compounds like you're doing may help.

I don't know your experience level, but I am assuming you're pretty seasoned basis your build.

if you were relatively new, I'd probably jump to technique. BigJae1976 helped me early on with moving from heavy initial spike to earlier softer on high speed corners. Also, many intermediate drivers tend to get off the brakes too soon and this can cause issues, especially when you need to trail brake to/or beyond apex. Turn 11 and 12 at VIR is a classic example where you must brake less but longer...braking begins at 2 board before 11 and doesn't end to just before apex of 12. But doubtful this applies to you as an advanced driver.

My advice is to get your data acquisition set up and look at your brake application. Also, you can look at individual wheel speeds to see how they are responding. You can compare with DSC on and off to see what the computer is suggesting compared to how you're doing it.

Good luck - keep us posted.
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      07-28-2019, 02:56 PM   #5
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Quote:
Originally Posted by ThunderMoose View Post
Without data and video, its subjective speculation at best.

Your alignment and set up sounds pretty typical, so perhaps playing around a bit with compounds like you're doing may help.

I don't know your experience level, but I am assuming you're pretty seasoned basis your build.

if you were relatively new, I'd probably jump to technique. BigJae1976 helped me early on with moving from heavy initial spike to earlier softer on high speed corners. Also, many intermediate drivers tend to get off the brakes too soon and this can cause issues, especially when you need to trail brake to/or beyond apex. Turn 11 and 12 at VIR is a classic example where you must brake less but longer...braking begins at 2 board before 11 and doesn't end to just before apex of 12. But doubtful this applies to you as an advanced driver.

My advice is to get your data acquisition set up and look at your brake application. Also, you can look at individual wheel speeds to see how they are responding. You can compare with DSC on and off to see what the computer is suggesting compared to how you're doing it.

Good luck - keep us posted.
Thanks for the input. I'm definitely not ruling out braking technique. I'm fairly experienced myself. The car was build by myself and my father. He's got lots of experience with 20+ years of high performance driving, 10+ years of wheel to wheel racing and multiple regional grassroots class championships - he's also experiencing the same thing when he drives the car, which is why I came here to see if others have had similar experiences.

I agree without data it is all subjective - that's for sure the next step to see what exactly is going on. The brakes themselves are working well overall, they don't fade and stop well, so unless necessary I don't want to drop 8k on a BBK setup, especially if the issue could still persist.

I also want to make it clear that this by no means makes the car undriveable, it's simply doesn't inspire maximum confidence when really pushing the car.

Last edited by tsk94; 07-28-2019 at 04:22 PM..
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      07-28-2019, 04:40 PM   #6
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First, I'd check the rear alignment settings. The E9X M3 rear axle reacts totally different than every previous generation. As the rear unloads under braking the rear tires toe out which could be the reason for the instability.

To check this, put the car on an alignment rack and pull down on the rear. It toes out. You can add some stability if you add some toe in at the rear but you're going to add more wear going straight line.

I could have this wrong...I don't think the M Lock diff locks up under decel. There's a long thread of different limited slip diffs but I'm thinking a clutch type 2 way diff can help this.

I believe I only had the wiggly rear end when I was using MDM but not sure. Definitely not confidence inspiring when you get on the brakes at high speeds.
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      07-28-2019, 04:58 PM   #7
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Quote:
Originally Posted by bigjae1976 View Post
First, I'd check the rear alignment settings. The E9X M3 rear axle reacts totally different than every previous generation. As the rear unloads under braking the rear tires toe out which could be the reason for the instability.

To check this, put the car on an alignment rack and pull down on the rear. It toes out. You can add some stability if you add some toe in at the rear but you're going to add more wear going straight line.

I could have this wrong...I don't think the M Lock diff locks up under decel. There's a long thread of different limited slip diffs but I'm thinking a clutch type 2 way diff can help this.

I believe I only had the wiggly rear end when I was using MDM but not sure. Definitely not confidence inspiring when you get on the brakes at high speeds.
Thanks. Couple of points to add. The alignment I posted above is the current alignment, it was checked after the last track day and is unchanged from what it was set to.

Car is driven with everything off, in fact, the EPIC tune deleted stability and traction control completely, and the computer modules have been removed.

The car also has a 2-way 8 clutch Drexler unit, 60% lock up on accel and decel.
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      07-28-2019, 05:42 PM   #8
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Quote:
Originally Posted by tsk94 View Post
Quote:
Originally Posted by bigjae1976 View Post
First, I'd check the rear alignment settings. The E9X M3 rear axle reacts totally different than every previous generation. As the rear unloads under braking the rear tires toe out which could be the reason for the instability.

To check this, put the car on an alignment rack and pull down on the rear. It toes out. You can add some stability if you add some toe in at the rear but you're going to add more wear going straight line.

I could have this wrong...I don't think the M Lock diff locks up under decel. There's a long thread of different limited slip diffs but I'm thinking a clutch type 2 way diff can help this.

I believe I only had the wiggly rear end when I was using MDM but not sure. Definitely not confidence inspiring when you get on the brakes at high speeds.
Thanks. Couple of points to add. The alignment I posted above is the current alignment, it was checked after the last track day and is unchanged from what it was set to.

Car is driven with everything off, in fact, the EPIC tune deleted stability and traction control completely, and the computer modules have been removed.

The car also has a 2-way 8 clutch Drexler unit, 60% lock up on accel and decel.
Dang. Car sounds like a beast. I'd love to see this in person.

What are your corner weights?

Any way to drop the rear a bit more?

Do you run at any US tracks? If so, what kind of times?
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      07-28-2019, 06:21 PM   #9
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Quote:
Originally Posted by ThunderMoose View Post
Dang. Car sounds like a beast. I'd love to see this in person.

What are your corner weights?

Any way to drop the rear a bit more?

Do you run at any US tracks? If so, what kind of times?
I'll find the corner weights and get back to you.

The rear could probably be dropped 5mm, maybe 10.

Haven't run any US tracks yet, home track right now is Castrol Raceway in Leduc, Canada.
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      07-28-2019, 11:22 PM   #10
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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 -2.2ļ 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.)
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      07-29-2019, 08:05 AM   #11
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Quote:
Originally Posted by dogbone View Post
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 -2.2ļ 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.
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      07-29-2019, 09:08 AM   #12
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Cool. Glad to hear youíve already added toe. I think itís a reasonable and cost effective first step.

Since your track is bumpy, after you drive the car and feel the toe change, if it doesnít fix it, I would mess around with the suspension settings. Do some wild changes to the rebound and compression settingsó-stuff that seems completely overboard simply to see if you can get a different feel in the braking. I canít give you any advice on which way to go with the knobs. Just go crazy one direction and then another. Again, this is just to see if the suspension changes the braking feel. If it does, then you can start to work to dial it in.

Since you have remote reservoirs, donít completely ignore nitrogen pressure in the reservoirs. I have found that experimenting with nitrogen can tune the behavior of the dampers a bit. It is a bit subtle but was worth it for me.

And what is your sway bar situation? On my track car, I went with a stiffer sway bar in the front, but left the stock one in the rear. If you have stiff springs in the car, a really stiff sway bar may overly restrict individual damper movement.

Is your diff considered a 1.5-way or 2-way?

(if you know my thread, you know I tried stiffer springs and just could not get the car to handle the way I wanted. And Bryan Hise, who was with JRZ and is now with Olsen, used to lecture me all the time that a stiff race car is a myth. After all these years of driving and listening to other peoplesí experiences, I do believe that when you have a high quality damper on the car, that you should go with the softest springs you can get away with. But that takes some experimentation. Of course, Iím sure that the people at KW are knowledgeable. But something does not feel right in the car. Soó-you gotta experiment. If the alignment and suspension settings donít help and you donít want to spend the money on BBK yet, I would experiment with spring rates next.)

When do you hit the track next? Iím curious to hear how things go.
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      07-29-2019, 09:33 AM   #13
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Quote:
Originally Posted by dogbone View Post
Cool. Glad to hear youíve already added toe. I think itís a reasonable and cost effective first step.

Since your track is bumpy, after you drive the car and feel the toe change, if it doesnít fix it, I would mess around with the suspension settings. Do some wild changes to the rebound and compression settingsó-stuff that seems completely overboard simply to see if you can get a different feel in the braking. I canít give you any advice on which way to go with the knobs. Just go crazy one direction and then another. Again, this is just to see if the suspension changes the braking feel. If it does, then you can start to work to dial it in.

Since you have remote reservoirs, donít completely ignore nitrogen pressure in the reservoirs. I have found that experimenting with nitrogen can tune the behavior of the dampers a bit. It is a bit subtle but was worth it for me.

And what is your sway bar situation? On my track car, I went with a stiffer sway bar in the front, but left the stock one in the rear. If you have stiff springs in the car, a really stiff sway bar may overly restrict individual damper movement.

Is your diff considered a 1.5-way or 2-way?

(if you know my thread, you know I tried stiffer springs and just could not get the car to handle the way I wanted. And Bryan Hise, who was with JRZ and is now with Olsen, used to lecture me all the time that a stiff race car is a myth. After all these years of driving and listening to other peoplesí experiences, I do believe that when you have a high quality damper on the car, that you should go with the softest springs you can get away with. But that takes some experimentation. Of course, Iím sure that the people at KW are knowledgeable. But something does not feel right in the car. Soó-you gotta experiment. If the alignment and suspension settings donít help and you donít want to spend the money on BBK yet, I would experiment with spring rates next.)

When do you hit the track next? Iím curious to hear how things go.
Ya I think experimenting with the damper settings are the next step if alignment changes doesnít help the situation. Also good point regarding the nitrogen pressure - thatís also something to play around with haha.

Sway bar situation - currently running the Hotchkis front and rear sway bars, rear is currently on the softest settings. Front bar we are playing around with to tune the handling balance.

The diff is considered a 2-way, equal locking on accel and decel. The diff is from limitedslip.de - the OP of the big diff shootout thread.

Next track day is on 11th - Iíll keep the thread updated.
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      07-29-2019, 11:12 AM   #14
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Quote:
Originally Posted by tsk94 View Post
The diff is considered a 2-way, equal locking on accel and decel. The diff is from limitedslip.de - the OP of the big diff shootout thread.

Next track day is on 11th - Iíll keep the thread updated.
Florian (driftflo) is a super knowledgable guy. Glad to hear you have one of his diffs. A 2-way should lock quite easily on deceleration. The two rear wheels being locked like that should really bring a lot of stability to the braking. The fact that it's not is confusing to me......something is up.

In the end, it may simply be the brakes, but if brakes are just not in the budget, then you're destined to experiment with the other aspects of the car.

For me, the stock brakes were AWFUL on track right from the beginning. When they heated up, the steering wheel would shake like crazy and the car would wiggle. The more I drove them the worse it got. (I didn't have race pads.) AP's went on the front with race pads, and all my problems went away. Then, when I put on the rears a few months later, the car hunkered down that last 10% during braking and braking has never been an issue since.
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      07-29-2019, 11:41 AM   #15
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Not sure if I'm on the right lines here, but are you talking about steering shimmy under very heavy braking? I removed about 85% of it just by changing to mono ball FTAB from bimmerworld. I think the stock bushings deflect a lot. I also run stock brakes, with SRF, PFC-11's all around and RE-71R's. When tires are in their nice operating temp range I get very little lockup even with my foot through the floor.
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      07-29-2019, 11:50 AM   #16
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Quote:
Originally Posted by tsk94 View Post
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.
This is a bigger deal than you might think, and probably contributing significantly to the issues you're feeling. Braking happens at the highest speeds so aero is super important. Taking weight out of the car relatively increases the aerodynamic contribution to the tire contact patch (car weighs less, downforce stays the same). The GT4 front lip is plenty big enough to have some aero effect. If it's feasible to remove, try a session without it.

I've had issues with unusual pad wear in floating calipers like the stock M3, you end up with a pad that's worn unevenly and end up with a reduced surface area that's actually contacting the rotor. Have you tried swapping front pads to a brand new set? If that causes the balance to change significantly, this could be the cause. A front BBK really helps reduce this since the caliper can't twist like the floating stock setup.

How did you determine your initial damper settings? Did KW send some recommendations/dynos? I think you're on the higher end of spring rates without aero, but if the plan is to add some later I wouldn't bother with changing anything. If nothing above helps with braking stability, try reducing rear low speed rebound and adding front low speed compression. It will have other effects on handling, of course, but it will be a good starting point to tell you if the dampers are the root cause.
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      07-29-2019, 11:52 AM   #17
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      07-29-2019, 11:53 AM   #18
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Quote:
Originally Posted by dogbone View Post
Florian (driftflo) is a super knowledgable guy. Glad to hear you have one of his diffs. A 2-way should lock quite easily on deceleration. The two rear wheels being locked like that should really bring a lot of stability to the braking. The fact that it's not is confusing to me......something is up.

In the end, it may simply be the brakes, but if brakes are just not in the budget, then you're destined to experiment with the other aspects of the car.

For me, the stock brakes were AWFUL on track right from the beginning. When they heated up, the steering wheel would shake like crazy and the car would wiggle. The more I drove them the worse it got. (I didn't have race pads.) AP's went on the front with race pads, and all my problems went away. Then, when I put on the rears a few months later, the car hunkered down that last 10% during braking and braking has never been an issue since.
Ya, Florian was great to work with and very knowledgeable. I figured the same, a 2way diff (mine locks 60%) should bring extra stability under braking. So it is a bit confusing.

My dad and I figure it may just be the nature of stock brakes being pushed quite hard. There may be only so much that can be done setup wise to compensate. Iíll play around with a few things to see if it makes much difference. The biggest concern we both have is, if we were to buy a BBK and the issue still persists that would be very disappointing - although that may just mean itís more setup related then brakes themselves. Quite a few people have mentioned the more hunkered down feeing the BBK provides, them the other question is do I do front and rear or just front to start?
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      07-29-2019, 11:56 AM   #19
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Quote:
Originally Posted by Montaver View Post
Not sure if I'm on the right lines here, but are you talking about steering shimmy under very heavy braking? I removed about 85% of it just by changing to mono ball FTAB from bimmerworld. I think the stock bushings deflect a lot. I also run stock brakes, with SRF, PFC-11's all around and RE-71R's. When tires are in their nice operating temp range I get very little lockup even with my foot through the floor.
No, not so much steering shimmy. More so the car just moving around under braking - pulling to one side, light feeling at the rear - general Ďinstabilityí. Itís not terrible but you definitely feel it moving around on the brakes when pushing hard.

All stock bushings and suspension arms have been replaced. Running SPL Race arms up front.
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      07-29-2019, 12:03 PM   #20
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Quote:
Originally Posted by fsmtnbiker View Post
This is a bigger deal than you might think, and probably contributing significantly to the issues you're feeling. Braking happens at the highest speeds so aero is super important. Taking weight out of the car relatively increases the aerodynamic contribution to the tire contact patch (car weighs less, downforce stays the same). The GT4 front lip is plenty big enough to have some aero effect. If it's feasible to remove, try a session without it.

I've had issues with unusual pad wear in floating calipers like the stock M3, you end up with a pad that's worn unevenly and end up with a reduced surface area that's actually contacting the rotor. Have you tried swapping front pads to a brand new set? If that causes the balance to change significantly, this could be the cause. A front BBK really helps reduce this since the caliper can't twist like the floating stock setup.

How did you determine your initial damper settings? Did KW send some recommendations/dynos? I think you're on the higher end of spring rates without aero, but if the plan is to add some later I wouldn't bother with changing anything. If nothing above helps with braking stability, try reducing rear low speed rebound and adding front low speed compression. It will have other effects on handling, of course, but it will be a good starting point to tell you if the dampers are the root cause.
Thanks for your input. You make some interesting points.

Firstly, I havenít tried swapping the pads out for a new set, they seem to be wearing fairly evenly, but I will take them out to take a look to see whatís happening.

Unfortunately the splitter isnít easily removable - it also provides the brake ducking so removing it to try a few sessions without it isnít that simple.

Yes, KW did provide both dynos and starting damper settings. The springs rates that were chosen were determined with the intention on running aero on the car, so currently without full aero this could be playing a part too.
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      07-29-2019, 12:25 PM   #21
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How is the handling of the car other than the braking instability?
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      07-29-2019, 01:42 PM   #22
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Quote:
Originally Posted by fsmtnbiker View Post
How is the handling of the car other than the braking instability?
Hmm, I would say itís pretty good currently. Turn in is sharp and understeer is minimal at worst, even in slow speed corners. Under power the car feels planted and stable. Rolling speed into a turn, no brake with little to no throttle, the car also feels quite stable.

The only thing Iíve noticed is the car does not like trail braking at all, the rear wants to come around. And this was prominent even with the lower torque rear pads and the current higher torque pads. This is surprising to me given the diff in the car and how much lock up under coast it has (60%). I donít know if this is related to the braking instability Iím having, or a separate setup issue on its own.
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