Squirm under hard braking
08-05-2012, 06:50 PM
#1
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Squirm under hard braking
I'll try to just dive in. I haven't experienced this problem to this degree before, but as of right now hard braking zones are causing the rear end of my car to squirm under braking, which is ruining my turn in and obviously hurting laptimes.
Details on the car before I go further:
MY2002 AP1.
Eibach front sway
AP2 rear bar
KW clubsports
Js racing front camber adjusters
255 square under 17x9 wheels. Kumho Ecsta XS tires
Alignment: -2.8 square front and rear. 0 toe front, 1/8" rear.
Brakes: Carbotech XP10 front, Stoptech rear (street pads, I know - we will come back to this later.)
The car is lowered about 2 inches from stock. What I *THINK* is happening under hard braking is the decompression of the rear is causing dynamic toe out changes -> resulting in squirm and eventually ruining corner turn in and cornering speeds. I realize I could probably brake earlier and smoother and let the car settle before turning in, but that is not the point of this thread.
Could not having a BSK cause this effect? Generally when BSK is tossed around on S2ki its for cornering, whereas my problem is exhibited under the braking prior to cornering.
1/8" rear toe is plenty for the car, I initially thought to add more rear toe, but it is not a static toe problem, but dynamic. I am fairly certain something funky is going on in the rear when I get on the brakes.
Lastly I realize there is more frontward brake bias than the traditional XP10/XP8 combo. I have run this same setup on my older S2000, which is an AP2. Obviously that car will not suffer as badly from bumpsteer, and I never received the problem before, hence why I am a bit confused as to why this car is acting so skittish.
Any help is appreciated, for now I've ordered a BSK and will install once its here and re-post impressions.
Track being used for discussion sake is BW CW13. This is displayed in the following corners: Sunrise and Sunset, entry into cotton corners, entry into Phil Hill, and entry into sweeter. All turns that lead to long straights - as you can imagine this is somewhat hurting my Vmax and effecting my laptimes significantly or I wouldn't be here
As always, I appreciate your guys help.
Details on the car before I go further:
MY2002 AP1.
Eibach front sway
AP2 rear bar
KW clubsports
Js racing front camber adjusters
255 square under 17x9 wheels. Kumho Ecsta XS tires
Alignment: -2.8 square front and rear. 0 toe front, 1/8" rear.
Brakes: Carbotech XP10 front, Stoptech rear (street pads, I know - we will come back to this later.)
The car is lowered about 2 inches from stock. What I *THINK* is happening under hard braking is the decompression of the rear is causing dynamic toe out changes -> resulting in squirm and eventually ruining corner turn in and cornering speeds. I realize I could probably brake earlier and smoother and let the car settle before turning in, but that is not the point of this thread.
Could not having a BSK cause this effect? Generally when BSK is tossed around on S2ki its for cornering, whereas my problem is exhibited under the braking prior to cornering.
1/8" rear toe is plenty for the car, I initially thought to add more rear toe, but it is not a static toe problem, but dynamic. I am fairly certain something funky is going on in the rear when I get on the brakes.
Lastly I realize there is more frontward brake bias than the traditional XP10/XP8 combo. I have run this same setup on my older S2000, which is an AP2. Obviously that car will not suffer as badly from bumpsteer, and I never received the problem before, hence why I am a bit confused as to why this car is acting so skittish.
Any help is appreciated, for now I've ordered a BSK and will install once its here and re-post impressions.
Track being used for discussion sake is BW CW13. This is displayed in the following corners: Sunrise and Sunset, entry into cotton corners, entry into Phil Hill, and entry into sweeter. All turns that lead to long straights - as you can imagine this is somewhat hurting my Vmax and effecting my laptimes significantly or I wouldn't be here
As always, I appreciate your guys help.
08-05-2012, 10:38 PM
#2
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Basic stuff first IMO - tire pressures even? This is the R&C section, so it should be a given that youve checked it.
Also, when was the last time you got it aligned? Alignments slip over time just from driving, but especially from the strains of track driving.
Have you tracked this car before?
Also, when was the last time you got it aligned? Alignments slip over time just from driving, but especially from the strains of track driving.
Have you tracked this car before?
08-06-2012, 03:28 AM
#4
Front spring rates? Front compression setting? Does it happen when braking in a straight line? What is your rear rebound set at? Rear aero? Have you tried adjusting any of the adjustable stuff?
08-06-2012, 04:32 AM
#5
Former Moderator
The first suggestion I'd make is to examine your braking technique. The rear squirm usually starts when the ABS in the rear kicks in. I had severe rear squirm under braking problems--so much so I was afraid I was going to spin during straight line braking on a bumpy track. I realized I was literally jumping on the brakes at the end of the straight. I found myself lifting my heel off the floor so I could hammer the brake pedal using my thigh muscles. I was forfeiting all technique and just letting the ABS handle the braking. By hitting the brakes hard the rear immediately went into ABS.
I had to fight to change my technique and smoothly transition from throttle to brake--not slow but smooth. By forcing myself to keep my heel in contact with the floor I could actually modulate the brakes and it solved the problem. If you want to run quick on a bumpy, high speed track you've got to smoothly transition from throttle to brake because on bumps the car will stay in ABS and continue to squirm.
If a software change doesn't help then here's my hardware suggestions: Our AP1 rear suspension bushings are old and the additional compliance might allow more dynamic toe out than your old AP2. Replacing the suspension bushings may solve the problem. A wing will definitely help and I would have suggested a less aggressive rear brake pad but you're already doing that. For your next alignment make sure they report what settings the car came in with to verify your settings didn't slip and if they didn't I'd try the stock rear toe-in of 0.25 inch (assuming you didn't replace the rear bushings--which is a total pain in the ass). I know a 1/4 inch of toe is a lot but your old bushings are probably flexing quite a bit.
I had to fight to change my technique and smoothly transition from throttle to brake--not slow but smooth. By forcing myself to keep my heel in contact with the floor I could actually modulate the brakes and it solved the problem. If you want to run quick on a bumpy, high speed track you've got to smoothly transition from throttle to brake because on bumps the car will stay in ABS and continue to squirm.
If a software change doesn't help then here's my hardware suggestions: Our AP1 rear suspension bushings are old and the additional compliance might allow more dynamic toe out than your old AP2. Replacing the suspension bushings may solve the problem. A wing will definitely help and I would have suggested a less aggressive rear brake pad but you're already doing that. For your next alignment make sure they report what settings the car came in with to verify your settings didn't slip and if they didn't I'd try the stock rear toe-in of 0.25 inch (assuming you didn't replace the rear bushings--which is a total pain in the ass). I know a 1/4 inch of toe is a lot but your old bushings are probably flexing quite a bit.
08-06-2012, 04:44 AM
#6
I still don't get the "less aggressive rear pads" approach. I've always run the same front/rear on the Z, the AP1, and now the FD, never with any stability issues in the braking zone (which is what I usually hear as the justification for "staggered" pads). ABS hasn't been functional on the AP1 for a few years now. Always locks the fronts first, on StarSpecs, on R-comps, and on winter tires. In the dry and in the wet (and snow!).
I'm definitely against lower-spec rear pads if they're street pads.
Rear brakes are in many cases non-vented (true for S2k) and have smaller rotors, pads, and calipers, and get less cooling airflow. In some cases rear pads can run hotter than fronts! Unless there's hard data showing that rear S2k pads run significantly cooler than fronts, there's no good reason to run higher-temp track compounds up front but lower-temp STREET compounds in back.
And if the rears *do* run cooler than the fronts, why stagger brake compounds with lower-spec rears which will burden the fronts even more?
I wouldn't run street pads in back except in a pinch, even if I did want to further bias braking to the front. I'd go with XP10/XP8 or something like that. But again, I've *never* had issues running the same pads front/rear, never seen the need to stagger front/rear compounds.
It is worth asking: Is the car squirmy on cold brakes, or does it get squirmier after a few braking zones? Possible that the rear pads are overheating asymmetrically?
I'm definitely against lower-spec rear pads if they're street pads.
Rear brakes are in many cases non-vented (true for S2k) and have smaller rotors, pads, and calipers, and get less cooling airflow. In some cases rear pads can run hotter than fronts! Unless there's hard data showing that rear S2k pads run significantly cooler than fronts, there's no good reason to run higher-temp track compounds up front but lower-temp STREET compounds in back.
And if the rears *do* run cooler than the fronts, why stagger brake compounds with lower-spec rears which will burden the fronts even more?
I wouldn't run street pads in back except in a pinch, even if I did want to further bias braking to the front. I'd go with XP10/XP8 or something like that. But again, I've *never* had issues running the same pads front/rear, never seen the need to stagger front/rear compounds.
It is worth asking: Is the car squirmy on cold brakes, or does it get squirmier after a few braking zones? Possible that the rear pads are overheating asymmetrically?
08-06-2012, 10:43 AM
#7
I'm inclined to think it's a front toe-out issue. Springs tend to settle a bit and that will cause the front to lower resulting in more static toe-out. Just do a quick front toe check to verify. If you do have some toe-out, adjust the front end up until the toe is back to zero.
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08-06-2012, 11:57 AM
#8
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Basic stuff first IMO - tire pressures even? This is the R&C section, so it should be a given that youve checked it.
Also, when was the last time you got it aligned? Alignments slip over time just from driving, but especially from the strains of track driving.
Have you tracked this car before?
Also, when was the last time you got it aligned? Alignments slip over time just from driving, but especially from the strains of track driving.
Have you tracked this car before?
It ONLY happens braking in a straight line at high speed straights. Rear rebound is set to 7. Car has no aero, completely stock AP1 body with an Amuse front bumper. I have tried adjusting the adjustable stuff.
I'm inclined to think it's a front toe-out issue. Springs tend to settle a bit and that will cause the front to lower resulting in more static toe-out. Just do a quick front toe check to verify. If you do have some toe-out, adjust the front end up until the toe is back to zero.
I got a recommendation saying that something is loose. That is at least an easy check. If anyone else has anything to add please do, every bit is helpful.
08-06-2012, 12:18 PM
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The first suggestion I'd make is to examine your braking technique. The rear squirm usually starts when the ABS in the rear kicks in. I had severe rear squirm under braking problems--so much so I was afraid I was going to spin during straight line braking on a bumpy track. I realized I was literally jumping on the brakes at the end of the straight. I found myself lifting my heel off the floor so I could hammer the brake pedal using my thigh muscles. I was forfeiting all technique and just letting the ABS handle the braking. By hitting the brakes hard the rear immediately went into ABS.
I had to fight to change my technique and smoothly transition from throttle to brake--not slow but smooth. By forcing myself to keep my heel in contact with the floor I could actually modulate the brakes and it solved the problem. If you want to run quick on a bumpy, high speed track you've got to smoothly transition from throttle to brake because on bumps the car will stay in ABS and continue to squirm.
If a software change doesn't help then here's my hardware suggestions: Our AP1 rear suspension bushings are old and the additional compliance might allow more dynamic toe out than your old AP2. Replacing the suspension bushings may solve the problem. A wing will definitely help and I would have suggested a less aggressive rear brake pad but you're already doing that. For your next alignment make sure they report what settings the car came in with to verify your settings didn't slip and if they didn't I'd try the stock rear toe-in of 0.25 inch (assuming you didn't replace the rear bushings--which is a total pain in the ass). I know a 1/4 inch of toe is a lot but your old bushings are probably flexing quite a bit.
I had to fight to change my technique and smoothly transition from throttle to brake--not slow but smooth. By forcing myself to keep my heel in contact with the floor I could actually modulate the brakes and it solved the problem. If you want to run quick on a bumpy, high speed track you've got to smoothly transition from throttle to brake because on bumps the car will stay in ABS and continue to squirm.
If a software change doesn't help then here's my hardware suggestions: Our AP1 rear suspension bushings are old and the additional compliance might allow more dynamic toe out than your old AP2. Replacing the suspension bushings may solve the problem. A wing will definitely help and I would have suggested a less aggressive rear brake pad but you're already doing that. For your next alignment make sure they report what settings the car came in with to verify your settings didn't slip and if they didn't I'd try the stock rear toe-in of 0.25 inch (assuming you didn't replace the rear bushings--which is a total pain in the ass). I know a 1/4 inch of toe is a lot but your old bushings are probably flexing quite a bit.
Surprisingly you aren't the first one to recommend 1/4 rear toe. I may settle for 3/16th's if I can't figure out what the problem is.
I realize braking tecnique is going to account for a good bit of it. I am not braking for the life of me, however, nor flexing my thigh muscles just to have ABS do the work. In fact I don't even think ABS is engaging, FWIW. If all else fails I will have to change my technique obviously, but I am fairly certain that there is a certain issue I'm overlooking. I will look into replacing the bushings as well, but the car is a 60K mile prior garage queen. They are still 10 year old bushings, however.
Sadly, I will likely not ever run a wing. Not the intent for the car.
