AP2 bumpsteer, differences between AP1 and AP2 rear geometry
08-11-2018, 02:23 PM
#1
AP2 bumpsteer, differences between AP1 and AP2 rear geometry
I'm experiencing a good amount of bumpsteer on my AP2, which is really noticeable at high speeds. Because of this I have to back off a good bit on the last corner at AMP and am not comfortable keeping the foot on the floor. It is not nearly as planted as my buddy's E36 M3 (though I don't expect it to be). It's ~100mph right hander with a midcorner bump that upsets the car. You can see it here:
at the 12 minute mark where the rear steers itself.
My main question is: how much bumpsteer should I expect from the AP2? I understand it should be "better" than the AP1 due to revised toe link mounting points/knuckle geometry but is it completely gone, or is this expected?
The car is currently set up at -3 camber and 1/8" total toe-in in the rear, and is on 11kg/11kg Ohlins set at ~12.75" in the rear. The rear toe link is above horizontal at ride height meaning it will toe in under compression and toe out under rebound (so it'll steer itself).
Since this is a used car, I've been wondering whether someone put an AP1 rear end in the AP2, though I really doubt it. There's no evidence that the rear end has been in an accident, etc to warrant a PO to change it out. I didn't see any P/N on subframe/knuckle to easily figure out what year it is from. For peace of mind, how can I tell the difference visually between AP2 subframe and knuckle vs. AP1?
If I can confirm I have an AP2 rear end then it will clear up that question mark and I can go ahead and get some rigid subframe collars, maybe a rear subframe brace, to see if it improves the rear end stability at high speed.
Thanks!
Setup info:
Alignment: Front: -3.0 camber 0 toe ; Rear: -3.0 camber, 1/8" total toe in (~0.3 degree total)
Suspension: SBG spec Ohlins 11kg/11kg set at 13" F and 12.75" rear (fender to hub)
Tires: Dunlop Z2SS in the front 245/40/17 set to 34-38psi, Hankook RS4 in the rear 255/40/17 set to 30-34 psi, delta usually 3-4psi; mounted on 17x9+45 RPF1
Shock settings: 10 clicks from full stiff in front, 7 from full stiff in the rear
My main question is: how much bumpsteer should I expect from the AP2? I understand it should be "better" than the AP1 due to revised toe link mounting points/knuckle geometry but is it completely gone, or is this expected?
The car is currently set up at -3 camber and 1/8" total toe-in in the rear, and is on 11kg/11kg Ohlins set at ~12.75" in the rear. The rear toe link is above horizontal at ride height meaning it will toe in under compression and toe out under rebound (so it'll steer itself).
Since this is a used car, I've been wondering whether someone put an AP1 rear end in the AP2, though I really doubt it. There's no evidence that the rear end has been in an accident, etc to warrant a PO to change it out. I didn't see any P/N on subframe/knuckle to easily figure out what year it is from. For peace of mind, how can I tell the difference visually between AP2 subframe and knuckle vs. AP1?
If I can confirm I have an AP2 rear end then it will clear up that question mark and I can go ahead and get some rigid subframe collars, maybe a rear subframe brace, to see if it improves the rear end stability at high speed.
Thanks!
Setup info:
Alignment: Front: -3.0 camber 0 toe ; Rear: -3.0 camber, 1/8" total toe in (~0.3 degree total)
Suspension: SBG spec Ohlins 11kg/11kg set at 13" F and 12.75" rear (fender to hub)
Tires: Dunlop Z2SS in the front 245/40/17 set to 34-38psi, Hankook RS4 in the rear 255/40/17 set to 30-34 psi, delta usually 3-4psi; mounted on 17x9+45 RPF1
Shock settings: 10 clicks from full stiff in front, 7 from full stiff in the rear
08-12-2018, 10:40 AM
#2
Did some reading and it looks like one of the easier ways to tell is based on the rear upper control arms / bushings. The AP2 has different rear upper control arms with larger bushings that deflect less than those on the AP1. The rear upper control arm mounting points are different, I believe they are narrower (?) on the AP2 but I cannot find definitive measurements between AP1 and AP2.
Pics of differences between AP1 and AP2 arms are here:
/
The lower control arms and toe arm are the same between the years.
The knuckles have different part #s but I am not sure what is different between them.
Either way, looks like I have all AP2 stuff on my car as I thought. Alignment is to spec. The subframe bolts may have been a few lb-ft loose, but not far off spec. I torqued them to spec + 10% and we will see if that helps, I doubt it though. Checked the rest of the control arm torque specs and they are all fine, bushings look in fine condition as well.
My ride height is not so far out of range of what other people are running, meaning the bumpsteer I am experiencing is probably expected.
I think next on the list will be to install spoon rigid collars, re-align using degrees rather than toe to eliminate possible mis-calibration for " settings, and possibly a rear subframe brace if the rigid collars do not improve the bumpsteer to my liking.
Pics of differences between AP1 and AP2 arms are here:
The lower control arms and toe arm are the same between the years.
The knuckles have different part #s but I am not sure what is different between them.
Either way, looks like I have all AP2 stuff on my car as I thought. Alignment is to spec. The subframe bolts may have been a few lb-ft loose, but not far off spec. I torqued them to spec + 10% and we will see if that helps, I doubt it though. Checked the rest of the control arm torque specs and they are all fine, bushings look in fine condition as well.
My ride height is not so far out of range of what other people are running, meaning the bumpsteer I am experiencing is probably expected.
I think next on the list will be to install spoon rigid collars, re-align using degrees rather than toe to eliminate possible mis-calibration for " settings, and possibly a rear subframe brace if the rigid collars do not improve the bumpsteer to my liking.
08-12-2018, 11:28 AM
#4
It does look quite low in the youtube cover picture for some reason. Here's another picture which I think is more representative.
08-12-2018, 02:10 PM
#6
how many clicks from full hard ?
tyre size, make & pressure are you running,any arbs upgraded?
it may help the readers
tyre size, make & pressure are you running,any arbs upgraded?
it may help the readers
08-12-2018, 04:19 PM
#7
Fender lip to center of hub
It's Ohlins SBG spec 11k/11k; I run 10 clicks from full stiff in the front and 7 in the rear. If I go any softer in the rear it's definitely worse. I haven't tried any stiffer in the rear because I already have issues with putting power down on corner exit but am thinking it may be a good next step to try.
Stock MY04 ARB all around. Handling is a little understeer biased on turn in but easily rotates on turn in w trail braking, mid corner balance favors neutral to slight oversteer, and corner exit balance favors oversteer.
It's on 255/40/17 Hankook RS4 in the back set at 30-34psi hot and Dunlop Z2SS 245/40/17 at 34-38psi hot in the front, I keep a Delta of 3-4 front to rear. For anyone who might be thinking it's because of mismatched tires, the car also did this when I was running Z2SS all around.
I'll update this info in the OP.
Appreciate the input. The car does look like it leans a lot on the high speed corners from the video. Maybe some stiffer spring rates all around would also help in addition to chassis bracing that I am planning. I will reclock the upper control arm bushings since I forgot to do that today just to confirm, and then probably do the rigid collars first unless others have better suggestions.
Stock MY04 ARB all around. Handling is a little understeer biased on turn in but easily rotates on turn in w trail braking, mid corner balance favors neutral to slight oversteer, and corner exit balance favors oversteer.
It's on 255/40/17 Hankook RS4 in the back set at 30-34psi hot and Dunlop Z2SS 245/40/17 at 34-38psi hot in the front, I keep a Delta of 3-4 front to rear. For anyone who might be thinking it's because of mismatched tires, the car also did this when I was running Z2SS all around.
I'll update this info in the OP.
Appreciate the input. The car does look like it leans a lot on the high speed corners from the video. Maybe some stiffer spring rates all around would also help in addition to chassis bracing that I am planning. I will reclock the upper control arm bushings since I forgot to do that today just to confirm, and then probably do the rigid collars first unless others have better suggestions.
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08-12-2018, 08:07 PM
#8
16 is fast and tricky, as the car gets light and the suspension unloads coming on to the short front straight. I made the mistake of being too far to the left coming over the bump once and it cost me a headlight/taillight/front bumper when I got in the marbles. You might try moving more to the left out of 15 and start your turn into 16 a little earlier so the car is more straight going over the bump.
I haven't heard of many that autocross or track the S2000 that haven't switched to a bigger front sway bar. It may be hard to believe at first, but a larger front sway bar will help to stabilize the rear suspension.
I haven't heard of many that autocross or track the S2000 that haven't switched to a bigger front sway bar. It may be hard to believe at first, but a larger front sway bar will help to stabilize the rear suspension.
08-13-2018, 01:00 AM
#9
Correct rear spring preload <maybe on /near bump stops><doubt this is overlooked>just an idea.
08-13-2018, 02:28 PM
#10
16 is fast and tricky, as the car gets light and the suspension unloads coming on to the short front straight. I made the mistake of being too far to the left coming over the bump once and it cost me a headlight/taillight/front bumper when I got in the marbles. You might try moving more to the left out of 15 and start your turn into 16 a little earlier so the car is more straight going over the bump.
I haven't heard of many that autocross or track the S2000 that haven't switched to a bigger front sway bar. It may be hard to believe at first, but a larger front sway bar will help to stabilize the rear suspension.
I haven't heard of many that autocross or track the S2000 that haven't switched to a bigger front sway bar. It may be hard to believe at first, but a larger front sway bar will help to stabilize the rear suspension.
