Brake straight line stabilty
 

Hello,

Vehicle is a 22brz, changes are brake fluid, pads (initial track days 2022 and 2023), camber bolts and rce sway bar (this year - 2024).

On the track days, during the end of long straight (110 to 120 mph max), during braking (about 1.1G), the rear end becomes too wobbly. I understand that with this level of braking, not much tire traction is available on the rear, but the feeling I have is that the rear is moving due to instability on the chassis, or going toe out during suspension droop or deflection on rubber.

Did anyone experience similar conditions? What are the main countermeasures? Add toe-in on rear (hide the effect)? Any suspension pick-up relocation (reduce toe-out)? Coilovers with stiffer springs (reduce droop)? Any other? Which one is more recommended?

Regards

What's your current alignment? As you alluded to, rear toe-in helps with stability under braking.
Which brake pads are you running? Same compound front/rear? Some folks recommend staggered compounds (less aggressive in the rear), but it's not universally accepted for the Twins.

Looks like you had an '18 BRZ before. Did you track it? Did you experience similar concerns?

I am on square setup for the pads, I did follow some folks info here for that, but later I was able to get data and learned that this moves the bias rearward a lot, with a new Zcrit of 0.6G. I do not feel that the rear is moving because of locking, seems like abs is working well in this case. Unfortunately I do not have the brake log file to check it, and I may move to a more front bias in the future to improve balance.

My current alignment is factory, i was not able to go on my company’s alignment machine due to the ramp approach angle and in the end left this way. Ran 2 seasons with stock suspension to better learn the car. To me seems to be the first weak link. Fot next track day I am going to get a pro alignment and may zero rear toe to test if this is a suspension design issue. Good direction or go to some toe-in and call it the day?

You would want toe in for stability in the rear. 1/32 to 1/16. Not sure what a "pro alignment" entails. But adding some sort of camber adjustment in the front helps tremendously. -2 degrees for the front is a good medium between street and track. Track only you would want about -3 to -4.

And remember that the factory alignment is a range of values, so the particular setting your car came with may not be great!

- Andrew

I've experienced the rear end dancing around under braking as well. Particularly noticeable at turn 1 at Laguna Seca. 2:21 in this video seems relevant:

https://youtu.be/MeJrWD43Grw?si=qHrDCDriVq1Mx80L&t=141

So the rear toes out under droop, which means it also toes out under braking. The first three potential solutions that I can think of:

A) Add additional rear toe-in, easiest and cheapest route

B) Increase front spring rate to reduce dive, which probably means revamping your entire suspension setup

C) Alter the rear anti-squat geometry with a product like the one in the video

I can't do C because it's illegal in STX and I'm not super keen upping the spring rates on my daily driver, so I've settled on option A. Factory spec is 1/6th degree of total rear toe in, I've basically doubled that to 1/3rd of a degree. It helps noticeably but doesn't solve the issue entirely, I might bump it up even more next alignment.

Not sure I see how this would help, and it might actually hurt instead, but you didn't go this route, so moot point.

The amount of weight transfer depends on the COG height and the rate of deceleration (and mass and wheelbase). If you reduce how much the front compresses, that keeps the COG higher, and in turn increases the weight transfer for a given deceleration. More weight transfer means more lift in the rear.

I've tracked this car on stock springs, 4.5kish front springs and 6k front springs. The higher the front spring rate, the more stable and level the car was under braking, no comparison. With factory suspension and 245 RE71Rs I was practically sawing at the wheel to keep it in a straight line.

Continuing your logic, competition cars running ultra high spring rates (3-5 hz) and slicks would stick their butts three feet in the air under braking. I can't say I've ever seen that, personally.

I don't doubt that things improve with higher front spring rate, my point was that the implied mechanism of front spring rate reducing rear toe-out doesn't make sense. It's possible you never meant to imply that, but if you did and there's an error in my logic, please point out at which step I go wrong.

I never said it would be a strong effect, only the direction of what happens doesn't fit what you seemed to be implying. A vanishingly small increase in rear height is still not a decrease, and certainly not three feet in the air.

What I suspect you're seeing is a front bump steer effect, not rear. Mcpherson struts are not linear in their camber curve, going flat and even reversing when far enough away from level position, and bump steer is closely related. With stiffer front springs, you're reducing how much of that curve is exercised, keeping it from going to where front bump steer starts doing weird things.