high speed rear stability

[dailym3]

The first track day I did with my new M3, I noticed how unstable the rear end felt under braking at the end of high speed straights, say from 100mph and up. The car feels very stable in low and medium speed corners, but even braking in a straight line I get the sense that the back end really wants to come around. I tried both the MDM setting and all systems on. As loose as the back end felt I didn't try disabling the nannies. Of course I've seen the video's showing the oversteering, tail happy nature of these cars, but I'm wondering what other impressions are and how best to keep everything in check so to speak

[CanAutM3]

I have felt it too, where the rear quets jittery under hard braking. I think it has to do with the rear suspension alignment change as it gets unloaded (more toe out and less camber).

[MaynardZed]

Yes I've felt it too on some of my earlier track days with the car. I've added quite a bit of rear toe in since then to the alignment and don't recall noticing it on my last few track days.

I think its just the nature of the soft stock springs causing this. A lot of weight it getting transferred forward un-springing the rear. I wonder if any of the aftermarket rear toe link kits would help with this too? I think Dinan and Fall Line have them out now.

[M3 Esq]

Originally Posted by MaynardZed

Yes I've felt it too on some of my earlier track days with the car. I've added quite a bit of rear toe in since then to the alignment and don't recall noticing it on my last few track days.

I think its just the nature of the soft stock springs causing this. A lot of weight it getting transferred forward un-springing the rear. I wonder if any of the aftermarket rear toe link kits would help with this too? I think Dinan and Fall Line have them out now.

I think you're right. The stock ride height plus soft springs are probably causing the car to nosedive and unload the rear. Lowering the car on a good set of coilovers with a stiffer spring should help.

[nicknaz]

OP are you just hammering down on the brakes or is there a track surface change in the braking zone?

On sunday, i felt like this was worse at the end of the session and i actually though the front tires were the ones really deforming and causing the car to squirm a bit. I lay the blame (at least in part) on PSS tires!

[dailym3]

No apparent track surface change. So I've tried both late braking pretty aggressively as well as earlier braking not as aggressively. The later braking does seem to exacerbate the problem as you would imagine.

Could certainly be the lightening of the rear end, although I've driven softer sprung cars and not had this problem. Could be tires too, although I don't think tire temp is playing a large role as I noticed it at the beginning and end of the day. I will say though that the tires were definitely going away at the end as it was extremely hot that day. Brakes too for that matter.

As far as stiffening the suspension, I tried the car in both sport and sport+, the latter being pretty stiff, and the problem was still there.

I saw the Dinan rear suspension mod and can run this by them too. I know they feel like that mod in particular produces great results, I'm just not sure if it will have a bearing on this issue. From their write up it looks like it may. http://www.dinancars.com/product/r28...ries&mid=1178/

So I was wondering about what part the differential may be playing here? We're downshifting as we are braking typically. Could the diff be unsettling the rear tire grip and giving this loose sensation? Of course I can test this by keeping the car in the same gear as I'm braking to see if that makes a difference.

[MasterC17]

Hmm, haven't really had an issue with this, and I've done some high speed straights. Back straight of WGI I was going from 150 (GPS) down to 77, NHMS was 120 down to about 60, and Thompson is 140 down to 35. But I also have a fair amount of mods done, so interesting whether or not that helps.

[IND-Distribution]

I have felt the same in our M4... very inconsistent behavior at the rear of the car under braking, even on a smooth surface. I've felt this with a few different pad compounds. The rear isn't so much consistently locking too early, as there seems to be some unwanted geometry change occurring under braking.

There are four rubber bushings left in the rear suspension arms- I would expect for them to yield under braking and cause toe change that isn't necessarily desirable. We've not yet replaced the bushings found at the rear of the car for spherical bearings on our own M4, but the parts are available.

One rubber bushing is eliminated by adding toe arms, and replacement spherical bearings are available for the other three. Actually, Fall Line and IND also make an adjustable arm for arm #5 in the diagram below, but I don't recommend it unless used by a technician that has a good understanding of anti squat geometry and how making adjustments there will impact it. We've got a spherical bearing for position 6 in that diagram also, which can be used instead of replacing the whole arm.

[Duder]

I just did the M performance school and I noticed this a lot. we were doing braking exercises for cornering and under super hard braking the back end lifts off a bit and all the weight goes forward. so if there's any angle in the front tires, that back end will start to move a bit. never did it enough to cause any problems for me though unless I came in way too hot to a corner and didn't brake early enough and had to turn while still braking really hard. but when braking was done at the right moment for the corner it was perfection. the car is really well balanced

[nicknaz]

Originally Posted by MaynardZed

Yes I've felt it too on some of my earlier track days with the car. I've added quite a bit of rear toe in since then to the alignment and don't recall noticing it on my last few track days.

Maybe that (add more rear toe in) is the quick fix answer to both high speed braking stability for corner entry and power on stability for corner exit?

I'm currently trolling our forums for alignment ideas and am thinking of trying 0 toe in the front, -1.5 degrees camber and 4mm total toe in the rear. I think you have more than rear toe in than that, right?

[IND-Distribution]

Originally Posted by nicknaz

Maybe that (add more rear toe in) is the quick fix answer to both high speed braking stability for corner entry and power on stability for corner exit?

It helps a touch. We've increased the rear toe on our car and it does feel better.

[robbo mcs]

Some of this is alignment etc

However, electronic intervention is also happening. electronic brake force distribution is constantlly working, in conjunction with pitch and yaw sensors, abs, stability control etc etc. The systems are working to keep the car decelerating as hard as possible, but also keep it in a straight line. you will notice this behaviour is much less noticeable when you trail brake, ie brake hard and turn into a corner at the same time.

I am sure if it was possible to datalog brake force to each individual wheel you would see constant adjustments being made during these hard decelerations.

[CanAutM3]

Originally Posted by dailym3

As far as stiffening the suspension, I tried the car in both sport and sport+, the latter being pretty stiff, and the problem was still there.

Stiffening the dampers will not change how much the car dives under braking. That is why you haven't seen any impact.

When "stffening the suspension" is mentioned, folks imply steffening the spring rates. This will reduce the amount of dive. Lowering the car can also help by reducing weight transfer.

[MaX PL]

yeh felt this at Lime Rock this week down the straight into turn 1. going from about 120-125 to 40-50.
happened about 30% of the time.

[chillindrdude]

i felt the rear end get little floaty, side to side when braking hard from 120 to 40.

is this what you guys are talking about?

it never felt like it wanted to come around. but i was braking in a straight line.

[CanAutM3]

Originally Posted by chillindrdude

i felt the rear end get little floaty, side to side when braking hard from 120 to 40.

is this what you guys are talking about?

it never felt like it wanted to come around. but i was braking in a straight line.

It's not like the car wants to swap ends on you. It is just that the car gets jittery and darts left to right. You need to make steering corrections and can be a little unerving when making a late pass with another car in close proximity.

[MaynardZed]

Originally Posted by nicknaz

Maybe that (add more rear toe in) is the quick fix answer to both high speed braking stability for corner entry and power on stability for corner exit?

I'm currently trolling our forums for alignment ideas and am thinking of trying 0 toe in the front, -1.5 degrees camber and 4mm total toe in the rear. I think you have more than rear toe in than that, right?

Yes, I believe I'm about double that amount of toe-in for the rear. Felt like you could really toss it into the turns and it wouldn't over rotate, just stays in a nice light four wheel drift.

I liked neutral toe for the front for an overall setup. I'm currently toe out in the front which was a tad better for turn in on the track, but pretty bad for street driving.

http://f80.bimmerpost.com/forums/sho....php?t=1144712

[xander85]

Ive been having this issue for a while now, at every track day in braking zones over 100mph. I've noticed the people that dont have the issue tend to have modified suspension (springs/coilovers). Im thinking the soft factory suspension might be the culprit...

[dailym3]

Originally Posted by CanAutM3

It's not like the car wants to swap ends on you. It is just that the car gets jittery and darts left to right. You need to make steering corrections and can be a little unerving when making a late pass with another car in close proximity.

Yes, exactly. Extremely vigilant, minor steering corrections are needed and it is unnerving. Although I find that those minor steering corrections are needed even when braking in a straight line, and not passing anyone. Without any steering corrections I got the impression that the back could come around in either direction.

Interesting thought regarding the traction control system and maybe even in MDM mode the minor rear brake adjustments that are being made by the computer are making the rear end less stable side to side. It does kind of feel that way. And maybe the lightening of the rear due to the dive of the front under hard braking is triggering the computer intervention with the rear brakes? Maybe its all related? So maybe I need to try traction control off.

I'm still suspicious regarding the rear diff though too. Any thoughts on this?

[CanAutM3]

Originally Posted by dailym3

Yes, exactly. Extremely vigilant, minor steering corrections are needed and it is unnerving. Although I find that those minor steering corrections are needed even when braking in a straight line, and not passing anyone. Without any steering corrections I got the impression that the back could come around in either direction.

Interesting thought regarding the traction control system and maybe even in MDM mode the minor rear brake adjustments that are being made by the computer are making the rear end less stable side to side. It does kind of feel that way. And maybe the lightening of the rear due to the dive of the front under hard braking is triggering the computer intervention with the rear brakes? Maybe its all related? So maybe I need to try traction control off.

I'm still suspicious regarding the rear diff though too. Any thoughts on this?

I am talking about straight line braking. On a late pass, both cars are going straight and braking side by side. That is where it gets unerving, when you are suddenly pointing towards the car beside you .

My experience is with DSC off.

From what I have read, the diff goes mostly open under hard braking, so it shouldn't be a problem.

The fact that MaynardZed was able to alleviate the problem with a simple rear toe-in adjustment tends to indicate that it is related to alignment changes due to rear wheel travel (rear lift, not front compression).

[chillindrdude]

Originally Posted by CanAutM3

I am talking about straight line braking. On a late pass, both cars are going straight and braking side by side. That is where it gets unerving, when you are suddenly pointing towards the car beside you .

My experience is with DSC off.

From what I have read, the diff goes mostly open under hard braking, so it shouldn't be a problem.

The fact that MaynardZed was able to alleviate the problem with a simple rear toe-in adjustment tends to indicate that it is related to alignment changes due to rear wheel travel (rear lift, not front compression).

off topic, but do you think the differential runs cooler at the track all nannies off versus MDM or DSC modes activated?

[Jadski]

I'm curious if IND noticed any improvements of rear end stability with increased downforce with their rear wing?