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FK8 Brake swaying/wobble

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10
Hi all,

I've had an odd sensation in my FK8 a couple of times now. Basically when braking at moderate to heavy pressure from high speed the car feels like it goes into a bit of a sway/ wobble from side to side. Almost as if the back wants to slide out but then corrects itself. Is this normal? It made me clench my bum cheeks both times it did this.

Car is 2018. 11K miles. Original discs and pads. Conti's on the front (bit worn but ok) MPS4s on rear. (brand new tread).

I will add that it done it in the same location both times, which is a duel carriageway, slightly rough surface, slight curve to the road. The car brakes fine at lower/ normal speed.

My previous car was a modified 208GTI which i actually had to do a full on emergency break in once and even in that extreme, the car kept a very straight line with just a tiny bit of sway on the skid so this has taken me by surprise seeing as the FK8 is a much better car with better brakes.

So did the road conditions cause the wobble and the electric gizmos in the car were just kicking in and correcting/ keeping me straight.

Or

Something is dodgy with my brakes/ wheels/ electrics that needs urgent looking at?
 
so it's only happened at this one location? It doesn't happen under hard braking anywhere else?

If so, the road camber can defo be a cause
 
Normal I'd say. The rear end on mine is quite loose under high speed braking.

My theory is that it pulls so much g under braking, that the rear tyres are probably barely touching the ground. Throw a cambered road in and it'll move around a little.
 
Normal I'd say. The rear end on mine is quite loose under high speed braking.

My theory is that it pulls so much g under braking, that the rear tyres are probably barely touching the ground. Throw a cambered road in and it'll move around a little.
I would defo agree with that, it has happened to me a number of times on rapid braking.

People who have followed me on track say the rear end "dances" and it certainly feels a little loose.

Twice I have done emergency braking on the road and both time the rear slew from side to side, quite scary.


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The same used to happen on my FN2 CTR until I fitted quite an aggressive LSD. The LSD in the FK8 feels relatively tame and smooth, probably doesn't help keep the front wheels' speeds the same under braking.
 
Ok cool. Thanks for the feedback guys. It's very reassuring hearing that others have also experienced this.
 
out of curiosity, whats your thoughts on the reason; lack of rear brake or suspension? FK2 does the same and i'm swaying towards rear brakes
 
out of curiosity, whats your thoughts on the reason; lack of rear brake or suspension? FK2 does the same and i'm swaying towards rear brakes

I'm no expert at all but feels like a weight transfer /aero issue. The nose dips a lot under braking, lot of weight over front axle, rear is light so perhaps initially lacking the downforce on the rear aero. The front brakes are substantially stronger than the rears, I have locked mine once or twice, so could be a brake balance issue too.
 
Okay, I've just got pretty geeky with this and calculated the weight transfer between front and rear. Based on a 61/39 static weight balance, the centre of gravity being 40% of the total height, a friction coefficient of 1.15 for the standard tyres, here's the load split under maximum braking:

Front: 88%
Rear: 12%

Obviously there are other factors like aero, but it goes some way to explain why your rears don't do much of the braking! IMO the only cure would be a more aggressive LSD at the front, to keep wheel speeds the same side to side. That, or less grippy tyres and a few bags of sand in the boot...
 
Sure it's not to do with stability control? If this wasn't a modern car you could be sideways in a hedge or something
 
got thinking about this again this morning. could it be suspension bushes - hard braking causing them to move around on the front end?
not that i'm dismissing your LSD thought, i just dont have any real experience with them
 
I have so far only had this on roads, on my 8 trackdays so far this year it has not been an issue I remember, so better surface on track and the brakes would be fully warmed up compared to poor surface/camber with cool/cold brakes.

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got thinking about this again this morning. could it be suspension bushes - hard braking causing them to move around on the front end?
not that i'm dismissing your LSD thought, i just dont have any real experience with them
I can see where you're coming from. In practice it'd be road surface imperfections or cambers making bushes on one side flex more than the other.

The effect of those imperfections compressing the wheels and changing the toe/camber slightly, giving one tyre a bit more grip for a few milliseconds, and briefly pulling the weight of the car to one side would (IMO) have much greater effect on the car than the suspension bushes moving a couple of millimetres.
 
I have so far only had this on roads, on my 8 trackdays so far this year it has not been an issue I remember, so better surface on track and the brakes would be fully warmed up compared to poor surface/camber with cool/cold brakes.

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On a track the surface is grippier but almost always braking hard in a straight line, very few roads are smooth, flat and straight.
 
It's most likely undulations on the road surface which is made worse by aggressive geometry, such as when you're pulled around on a motorway in the lorry ridges.
 
Could be brake bias. We've just changed the wife's mini which did the exact same thing. Fine at moderate speed but once you get up to 90+ leptons and came on the brakes (not hard, just normal) it would feel a little like the thing wanted to swap ends. Car went in for its first service a couple of months ago and they noted that the rear brake pads were at 4mm and the fronts were 7mm... This setup/bias was how it left the factory and I thought explained a lot about what I'd felt.
 
Eddie did you notice this on my car today? One squiffy moment but otherwise felt good.
It looked planted at all times Matt, in fairness I don't think we did any really hard stops so it would not show itself.

Mine behaved itself but I had the benefit of following you so my braking points were easy

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