|
Suspension | Chassis | Brakes -- Sponsored by 949 Racing Relating to suspension, chassis, and brakes. Sponsored by 949 Racing. |
![]() |
|
Thread Tools | Search this Thread |
![]() |
#1 |
Senior Member
Join Date: Jul 2020
Drives: 18 Brz (Sold) / 22 BRZ
Location: Michigan
Posts: 656
Thanks: 1,051
Thanked 442 Times in 258 Posts
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
|
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 |
![]() |
![]() |
![]() |
#2 |
Senior Member
Join Date: Apr 2020
Drives: 2018 BRZ
Location: Dayton, OH
Posts: 992
Thanks: 1,429
Thanked 841 Times in 482 Posts
Mentioned: 13 Post(s)
Tagged: 0 Thread(s)
|
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? |
![]() |
![]() |
The Following User Says Thank You to Ohio Enthusiast For This Useful Post: | strat61caster (07-04-2024) |
![]() |
#3 |
Senior Member
Join Date: Jul 2020
Drives: 18 Brz (Sold) / 22 BRZ
Location: Michigan
Posts: 656
Thanks: 1,051
Thanked 442 Times in 258 Posts
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
|
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? |
![]() |
![]() |
![]() |
#4 | |
Senior Member
Join Date: Dec 2013
Drives: 2014 Scion FRS (Asphalt)
Location: Redmond, WA
Posts: 559
Thanks: 39
Thanked 173 Times in 143 Posts
Mentioned: 6 Post(s)
Tagged: 2 Thread(s)
|
Quote:
__________________
AVO Stage 2 Turbo Build Thread
https://wheelwell.com/tyler-nguyen-1...014-scion-fr-s |
|
![]() |
![]() |
The Following User Says Thank You to tyler_win_photo For This Useful Post: | Stonehorsw (06-30-2024) |
![]() |
#5 |
And remember that the factory alignment is a range of values, so the particular setting your car came with may not be great!
- Andrew |
|
![]() |
![]() |
![]() |
#6 |
Member
Join Date: Sep 2018
Drives: 2014 FR-S Monogram
Location: Nor Cal
Posts: 57
Thanks: 52
Thanked 64 Times in 34 Posts
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
|
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:
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. |
![]() |
![]() |
![]() |
#7 | |
Member
Join Date: Feb 2023
Drives: 2017 GT86
Location: California
Posts: 65
Thanks: 13
Thanked 41 Times in 30 Posts
Mentioned: 2 Post(s)
Tagged: 0 Thread(s)
|
Quote:
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. |
|
![]() |
![]() |
![]() |
#8 | |
Member
Join Date: Sep 2018
Drives: 2014 FR-S Monogram
Location: Nor Cal
Posts: 57
Thanks: 52
Thanked 64 Times in 34 Posts
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
|
Quote:
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. |
|
![]() |
![]() |
The Following User Says Thank You to glhs386 For This Useful Post: | Ohio Enthusiast (07-12-2024) |
![]() |
#9 | ||
Member
Join Date: Feb 2023
Drives: 2017 GT86
Location: California
Posts: 65
Thanks: 13
Thanked 41 Times in 30 Posts
Mentioned: 2 Post(s)
Tagged: 0 Thread(s)
|
Quote:
Quote:
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. |
||
![]() |
![]() |
|
|
![]() |
Thread Tools | Search this Thread |
|
|
![]() |
||||
Thread | Thread Starter | Forum | Replies | Last Post |
FS: OEM Brake Rotors + Brake Lines + Clutch Line + Steering Rack Bushings | fc3s_johnny | Brakes, Suspension, Chassis | 11 | 10-05-2020 11:26 AM |
Help! Brake line leakage after steel braided line install. | ysu | Suspension | Chassis | Brakes -- Sponsored by 949 Racing | 26 | 02-09-2020 11:38 PM |
High horsepower, straight line car forums... | husker741 | Scion FR-S / Toyota 86 GT86 General Forum | 42 | 11-01-2013 01:50 PM |
Brake line degradation | so26 | Issues | Warranty | Recalls / TSB | 8 | 05-14-2013 09:47 PM |
SS Racing Brake Line | K1 Motor Works | Brakes, Suspension, Chassis | 5 | 06-29-2012 03:36 AM |