Oversteer Under Hard Braking
 

A little background before I atempt to try and explain my issues.. I do have track and auto x experience with my other car (S2000) but I have not done either in my FRS yet I just daily it. However I have noticed the car under HARD braking highway speeds the back of the car gets very leight feeling and attempts to start wanting oversteer as well as the brakes locking. I was driving home from out of town with my wife and I was forced to get on the brakes hard at highway speed.. And I was shocked at how the car got so out of sorts. Is this normal with these cars? My car is stock with the all seasons... I've had problem after problem with the car throughout my ownership so it would not surprise me if the Alighnment was screwed up too. Or it could just be my driving style road surfaces whatever.

Anyone else notice this?

Thanks,

yes, getting hard on the brakes in a corner at speed will have the back end wanting to come around on you

If you are having a big problem street driving and the braking is unstable, check you alignment and make sure your toe is not totally out of wack.

Too much toe out will make rear end very unstable under braking.

You didn't specify whether you're in a straight line or not. As 7th mentioned, hard braking in a turn can certainly induce oversteer in nearly any car, but in a dead straight line it should be stable. If your alignment is screwy then straight line braking may be affected, or if the road is crowned/tramlined then the uneven surface may be affecting the rear unweighted tires more dramatically than the fronts with all the braking weight on them.

It's also possible your S2k had an extremely stable (understeer biased) alignment.

Sorry Yeah braking in a straight line that's very important to this topic. I was thinking it would be an alignment issue too just making sure this wasn't a common issue.its happened over and over on the street so it sure points to that.

Personally I feel that on a standard FRS/GT86 the front spring rates were too soft resulting in far to much weight transfer to the front under braking which overly unloaded the rear. IMO the front bushes are quite soft which exacerbate things.

Add me to the alignment camp.

I had my alignment adjusted after changing my camber on the fronts and I can jam the brakes and the car stops in a dead straight line with my fingers resting on the steering wheel for guidance.

Weight transfer won't cause stability issues or brake locking in a car with ABS and stability control. If your ABS dash light stays off then the wheels cannot lock under braking.

In olden days weight transfer would cause the rear brakes to lock easily, with disturbing effects on handling. This was a serious issue when front disc brakes were first introduced due to the high line pressures needed for discs as compared to the rear drums. All manner of rear brake line pressure limiters were employed until ABS solved the issue.

If your car gets squirrelly under hard braking that most likely means the front wheels are going toe out so you need to get them aligned. A four wheel alignment is required regardless so on the off chance that the rear toe is also wrong that will also be corrected (it is very difficult to tell which end is out just from your symptoms as front toe errors can induce rear steer effects). Camber is non-adjustable without special bolts.

Also, unless you have had the alignment checked since new it is possible the rear camber is incorrect. Uneven camber can induce steering under braking. Mine was out from the factory and required camber bolts to correct. Subaru supplies proper camber bolts if needed for this suspension design. This car uses basic Impreza suspension which is very good and tends to retain its alignment provided it was correct to begin with. Apparently, rear subframe installation tolerances are quite tight and if not installed perfectly from the factory then rear alignment will need correcting. However, it is unlikely that your brake steer effects are from the rear suspension even though it may feel like they are. If you felt the steering wheel trying to move on its own then the front toe is going out of spec as the front suspension compresses and the static toe in needs to be increased a bit.

I'd strongly disagree and point out that you don't need to be locked up to have instability in the rear, less weight on the rears results in less grip in the rear and on standard springs there's enormous weight shift forward.

That and after putting in decent coil overs with equal spring rates front and rear and changing the alignment from toe-in at the front to toe out it's now far more stable under heavy braking. What the OP is talking about was one of my biggest criticisms of the standard suspension.

springs don't affect weight shift.

I tend to agree with the input here. I made an appointment for tomorrow at the dealer. I'd like to know at least the current alignment specs and hopefully the car will be out of spec with the recommended. It really was surprising and concerning to me at how loose the car got under hard braking. I just don't get on this forum as much as I should so I wasn't sure if this has been a common reported issue. From this it sounds like it is. I'll post up what I find tomorrow.

weight transfer as an absolute value and it's visual representation are two different things


putting stiffer springs in the front will still have you transfer the same amount of weight to the front... only it will happen much quicker, but without the dive..


the lack of dive is what keeps your brain happy, but the reality is that nothing's really changed from a weight perspective, just a transient response.. which gives you confidence so you feel faster.

I haven't really had this issue at the track, even when threshold braking repeatedly. Only two times I can think of where I felt any instability under braking:

- First session of the day when a tire was wet and locked up at 100+ mph. Scary. Triggered a constant ABS light which had to be reset.

- Entering a specific corner at the track at a certain angle. There are slight undulations in the pavement, and while I didn't feel any rotation in the chassis when braking, I could feel slight movement through the steering wheel.

Other than those two instances, my car has felt rock solid under hard braking. I would guess maybe your brakes were cold at the time or perhaps the pavement was uneven. If it happens constantly though, I would definitely get the alignment checked out.

Weight transfer is a function of suspension geometry and braking force, not spring rates. The percentage of weight transfer under any given brake force is unaffected by spring rate but total braking force certainly is. Stiffer front springs increase the total braking potential of the vehicle on any given surface which in turn may cause more weight transfer due to higher forces involved (weight transfer is actually vehicle momentum acting through the CG and the suspension arm orientation, you can fit very soft springs and have reverse weight transfer by lowering the center of gravity below the pivot points of the suspension arms, i.e. anti dive can actually be arranged so that the front of the car rises under braking and reverse roll can be induced by similar tricks. Nobody does this in part because such vehicles would be very hard to drive). Your feeling that softer front springs result in more weight transfer to the front is the reverse of reality. Softer front springs force the front wheels to do more braking than the rear wheels reducing total braking force (well, the precise effect varies but in general this is true).

However, when braking in a straight line the vehicle must remain stable unless a wheel locks up because no yaw moment is created when braking in a straight line. Only uneven grip left to right can induce yaw and if the brakes are working correctly no yaw can be induced with an ABS equipped car. Indeed, yaw forces induced by braking one wheel is how stability control works and there are yaw sensors usually in the trunk area for this reason.

Braking hard enough in a corner to create instability is ill advised and has nothing to do with spring rates. In any event, stability control, traction control and ABS will prevent instability under braking even in a corner unless the suspension is not aligned correctly.