Instability during high speed braking

[Flying Low]

A number of posters have mentioned instability at the track during high speed braking. And of course, the rearend likes to step out pretty easy during hard acceleration. I ran across this from Dinan:

Dinan’s high performance Racing Rear Suspension Link Kit adds precision and predictability at the limit.

The stock rear toe link is built with rubber bushings and a curved forged alloy rod which can easily flex. During hard acceleration this flex allows the rear wheels to have excessive toe change under hard acceleration and braking. All of this flex in the stock toe link causes the car to be unstable.

Dinan® stops the flex by replacing the rubber bushings with Teflon coated machined steel rod ends and the OEM curved bar with a straight billet aluminum rod, thus adding stability. The Dinan rod-ends incorporate left and right hand thread, which allows quick and accurate toe adjustment that is tough to achieve using the stock system. Like the rear toe links, the stock front trailing links also incorporate rubber bushings. Flex in these bushings allow the rear wheels to behave sloppily under heavy loads. Dinan adds precision by replacing these links with billet aluminum rods and Teflon coated machined steel rod ends. These front links are not adjustable, because there is no need to doing so.

Benefits of the Dinan® Racing Rear Suspension Link Kit:

+ Replaces the stock rear toe link & rubber bushings with a precision machine black-anodized aluminum tube and racing-quality rod ends.
+ Replaces the stock forward lower trailing link & rubber bushings with a precision machine black-anodized aluminum tube and racing-quality rod ends..
+ Dramatically improves vehicle stability during hard cornering.
+ Improves response by eliminating the deflection of the rubber bushings.
+ Rod-ends are Teflon-lines for quiet operation.
+ Rod-ends incorporate left and right hand thread, which allows quick and accurate toe adjustment by simply turning the aluminum tube. This eliminates the need to struggle with the factory eccentric adjustment.
+ Only rear toe links are adjustable. Trailing links are not adjustable.
+ RACING ONLY PRODUCT

Does anyone have experience with this kit? I'm thinking seriously about doing this. The tracks that I go to usually limit me to about 120-130 MPH and I haven't yet experienced the braking instability but I'm still learning the car and not pushing as fast as the car can go.

[brian99997]

For a grand, its not the worst money spent even to have easily adjustable rear toe. Braking from 130+ to anything in 2nd gear is unstable but not uncontrollable to me. I haven't ever had the car actually step out, but I do have to counter steer under braking at times.

Looking for anyone with experience here as well especially on the street since the track benefits are clear.

[Gearhead999s]

I do not find that my my M4 moves around very much under high speed braking.I see 147 mph at the quickest point at Mosport but I do find the abs is intervening a lot more than I would expect while trail braking.

[heckyeahbro69]

I don't think the issue lies in the rear end completely. I think it gets on the nose too much under hard braking and that causes the instability. I'm going to play around with front damping at pbir next week and see if that makes any difference. Also, maybe limiting up travel in the rear would help keep weight off the nose during braking If I had to guess though, it might make it worse as the weight of the car is trying to pull up the rear wheels...so maybe thats not the answer. If dinans toe link kit helps keep the rear wheels from deflecting and allows them to keep their set toe angle, maybe that part of it. Someone buy it and find out. Anyone have the fall line kit?

[JNoSol]

Fall-Line is more reasonably priced and race proven as well. Worth looking into and iND is the vendor.

[Cloud9blue]

Originally Posted by heckyeahbro69

I don't think the issue lies in the rear end completely. I think it gets on the nose too much under hard braking and that causes the instability. I'm going to play around with front damping at pbir next week and see if that makes any difference. Also, maybe limiting up travel in the rear would help keep weight off the nose during braking If I had to guess though, it might make it worse as the weight of the car is trying to pull up the rear wheels...so maybe thats not the answer. If dinans toe link kit helps keep the rear wheels from deflecting and allows them to keep their set toe angle, maybe that part of it. Someone buy it and find out. Anyone have the fall line kit?

I agree. Some sort of instability is inevitable at high G braking with a 50:50 weight distribution car. You pretty much end up with a 80F:20R weight distribution under heavy 0.7-0.8G braking. The rear end of our cars actually does very little in these scenario.

Any slight difference in the traction between the front left and right tires will cause a rotational force on the chassis.

And remember, the front control arm has rubber bushings on the subframe side, so it will deform a bit under those stress.

Of course, Dinan just want to sell your some parts that really isn't the solution to your problem. Personally, I would just deal with it and give corrective steering input in those scenario. If it really bothers you, you could try replacing the front bushing with monoball bearings instead.

[M3 Esq]

If you haven't experienced it why change anything? I'd only do the mod if you are having the issue. If not, there doesn't seem to be any reason. Save your money for something else. :

[M3 Number 86]

Maybe ppl aren't braking smoothly and upsetting the car way too much?

[CanAutM3]

Originally Posted by socal TW

Maybe ppl aren't braking smoothly and upsetting the car way too much?

Nope,

The stock F8X chassis does get squirmy under high speed hard braking. My experience tends to indicate that the chassis is overly sensitive to little undulations in the pavement. One of the little flaws of the stock chassis setup.