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Rear axle hop during track braking

GBoss7500

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Hi all,

Been struggling with rear axle hop under hard braking on track in my '12 Boss. Anything over ~0.8g's, especially if the pavement has some ripples. Getting some pretty violent vertical movements from the rear axle which of course de-stabilizes the whole car, needing release and re-apply of the brakes to get back under control. This is during straight line braking. The approach to turn 7 at Sonoma is pretty much guaranteed to happen once i start pushing the braking zone. Has also happened to me braking for turn 5 at Laguna and a few other places I can't recall. Otherwise the car handles and performs great on track. Rear end feels planted and stable on power.

I have the Maximum Motorsports road & track package, which includes MM springs (progressive rate, avg. 340 lb/in F, 300 lb/in R I believe), koni yellows, shortened bump stops, rear LCA's + relocation brackets, and UCA. Shock rebound setting on all corners is nearly full soft. 275/40/18 Nitto NT01 tires square. Stock brake calipers and rotors, pads are Raybestos ST-47 F, ST-43 R. This front/rear split was recommended to me by David Ray from his experience with Spec Iron.

I track it with traction & stability control fully off (hold button for 10 sec while on the brake). I'd expect that increasing the shock rebound setting in the rear would make the problem worse, so have not messed with that.

Anyone dealt with this issue before? Could it be caused by the stock ABS not being calibrated properly for the setup I have?

Thanks!
 

GBoss7500

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LCA’s are mounted to the upper of two hole positions on the LCA brackets, so I have no way of reducing anti-squat with current setup. Will measure angles of the LCA’s next. Other things to look for?
 

Norm Peterson

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I agree with Grant to the extent that anti-squat (actually, its evil twin) is involved.

But I think your UCA inclination is to blame despite what I'm guessing is about 2" of downward LCA axle side relocation. 2" of lowering (per MM site) puts the UCA at a pretty significant downhill inclination (going from axle to chassis) and I think you're looking at a little over 80% anti-squat and a side view swing arm length under 30".

I wouldn't think that the mu for ST43 pads would be too high (I found one source claiming 0.38) and throw the anti-lift out of whack that way.


Norm
 

Grant 302

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^Agreed. My initial guess what the wrong way. This arrangement roughly maintains the 'stock' LCA angle...might help to try the lower position. A picture might help confirm the angle.

While this angle 'works' (arguably) on a stock Boss, I think with higher rate springs some changes need to be made. It does make sense to me that the car puts down power well, but the possible fixes in rear geometry for the hop might tend to reduce that.

I think the first thing I'd try is stiffening the front damping. Or increasing the anti-dive geometry in the front by raising it or using tall ball joints. Or the lower LCA position. Just change one thing at a time.
 

GBoss7500

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Thanks guys,

My LCA's are damn near perfectly horizontal with car at static ride height. Compared with stock, I believe the rear is somewhere around 1.5" to 2.0" lower with the MM springs I have. I mis-spoke on the relocation brackets - they are FRPP not MM, and I'm currently using the upper of the two mounting holes. If I use the lower hole position, the LCA's would be going uphill from axle to chassis, which means more anti-squat, and should make my problem worse, right? Should my LCA angle actually be going downhill from axle to chassis?

Good idea on inspecting the condition of UCA pivots / bushings - will do that.

Been running rear rebound on the Koni's ~1/4 turn from full soft. Had always assumed more rebound damping would make the problem worse, but now questioning if that's the case - will at least try going firmer to see if it makes a difference.
 

67GTA

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Thanks guys,

My LCA's are damn near perfectly horizontal with car at static ride height. Compared with stock, I believe the rear is somewhere around 1.5" to 2.0" lower with the MM springs I have. I mis-spoke on the relocation brackets - they are FRPP not MM, and I'm currently using the upper of the two mounting holes. If I use the lower hole position, the LCA's would be going uphill from axle to chassis, which means more anti-squat, and should make my problem worse, right? Should my LCA angle actually be going downhill from axle to chassis?

Good idea on inspecting the condition of UCA pivots / bushings - will do that.

Been running rear rebound on the Koni's ~1/4 turn from full soft. Had always assumed more rebound damping would make the problem worse, but now questioning if that's the case - will at least try going firmer to see if it makes a difference.
Reviving an older thread here, as I’m experiencing this exact same issue and this thread was left without resolution - hoping that @GBoss7500 will give us an update on a fix. And perhaps our regular suspension guru’s will have some advice.

Ideally, I would have made one change at a time, but wasn’t feasible for this change as one of my Rival S tIres was slightly corded on the shoulder, forcing a planned switch to a new 19” APEX setup. I changed the rear UCA, as I had the older style Whiteline poly bushing unit and was fighting an unpredictable snap-oversteer mid to corner exit (suspected bind in the UCA). Went to their newer style Max-C bushing UCA. They don’t offer a UCA mount, so I decided on the J&M unit to pair it with. Steeda spherical bearing on the differential side. I also changed out the bump stops from the OE to the ones included with the KW Variant 3 kit. At the same time, I made a slight adjustment to the spring perches to raise ride height - about 1/4” on the perch, and backed off the rebound settings at all 4 corners since previous settings were near max hard.

In summary, here’s the 4 changes made:

-275/35R18 to 305/30R19 Rival S tires
-changed UCA
-slightly raised rear ride height
-moved rebound setting to mid-range of the adjustment scale at all 4 corners

These changes fixed the original issue - instead of snap-oversteer, the car is now very predictable at the limit and can be very easily rotated. Also, can apply full throttle at apex and traction is very good.

Unfortunately, I now have a huge problem with wheel hop under hard braking. Bad enough that I had the car swap ends at the top end of a straight at Buttonwillow last weekend. Not cool. Since the changes, I have mostly driven on Big Willow, which only has one hard braking zone into 3, and axle-hop would happen if I was aggressive into the brake zone. I mostly avoided it by braking early and never had it upset the car to the point of causing a spin. With HPDE 3&4 sold out at Buttonwillow last weekend, it was rare to not be braking way early because of the train of cars on every section. Final session, last few minutes I finally had clear track. Came into Cotton Corners a bit hotter than any point during the day, but should have been no problem. Enough typing - this video is better than my description....

 
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Norm Peterson

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Reviving an older thread here, as I’m experiencing this exact same issue and this thread was left without resolution - hoping that @GBoss7500 will give us an update on a fix. And perhaps our regular suspension guru’s will have some advice.

Ideally, I would have made one change at a time, but wasn’t feasible for this change as one of my Rival S tIres was slightly corded on the shoulder, forcing a planned switch to a new 19” APEX setup. I changed rear the UCA, as I had the older style Whiteline poly bushing unit and was fighting an unpredictable snap-oversteer mid to corner exit (suspected bind in the UCA). Went to their newer style Max-C bushing UCA. They don’t offer a UCA mount, so I decided on the J&M unit to pair it with.
I'm guessing that you're using the lower hole in the J&M UCA mount.


Norm
 

racer47

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Rear axle brake hop is the negative side effect of anti-squat. In general terms, the more anti-squat the better the forward bite. But if you go too far, that anti squat that helps load the rear tires tires during acceleration also unloads the rear tires under braking.

Sometimes when the brakes are cold, there is no brake hop. But then as the fronts heat up, you lose front braking force, apply more brakes and then the extra rear brake load can make the axle hop.

Everyone focuses on the lower control arms. But you need to know the upper arm angle and mounting points to calculate anti-squat. To fix the problem you can reduce anti-squat percent, or go to more aggressive front pads, or less aggressive rear pads or if it really is just when the fronts are hot, add front brake cooling.

Purpose built race cars have the ability to finely adjust anti-squat to be able to find the perfect combination. We don't have that luxury. So I ended up using the BMR UCA mount with the 3 holes, https://www.bmrsuspension.com/?page=products&productid=965&superpro=0

Between that and the lower arm holes I was able to get the squat and ride height and roll steer that I wanted. I also split the pads with more aggressive on the front and one step less on the rear.
 

67GTA

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Do you have the same issue with your LCA angle as GBoss7500? What settings are you running for TC/AdvanceTrack? I couldn't really tell from the video but do you get a lot of brake dive?
Using the Whiteline LCAs (adjustable length) and the Whiteline LCA relocation brackets. The brackets only have one hole. At static ride height, the LCA is at a edit - 3* angle, not 7*- higher on the chassis end.

Running with AdvanceTrack fully off (10 second hold w/brake)

Not much brake dive - the car is relatively stiff with the KW V3s. I did go up a few clicks on bump setting in the front - now running at 10 clicks from soft, ~18 clicks available.
 
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67GTA

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Everyone focuses on the lower control arms. But you need to know the upper arm angle and mounting points to calculate anti-squat. To fix the problem you can reduce anti-squat percent, or go to more aggressive front pads, or less aggressive rear pads or if it really is just when the fronts are hot, add front brake cooling.

Purpose built race cars have the ability to finely adjust anti-squat to be able to find the perfect combination. We don't have that luxury. So I ended up using the BMR UCA mount with the 3 holes, https://www.bmrsuspension.com/?page=products&productid=965&superpro=0

Between that and the lower arm holes I was able to get the squat and ride height and roll steer that I wanted. I also split the pads with more aggressive on the front and one step less on the rear.
I’m running G-Loc R16 fronts and R12 rears, which was consistent before & after the UCA change. The J&M UCA mount has 2 holes. I’m using the top hole (closest to the chassis), as it is supposed to be the same location as the OE mount.
 

GBoss7500

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Hi everyone,

@67GTA, I believe I’ve solved the issue of the braking hop recently, so good timing on reviving this old thread. Thanks for the video post - looks very similar to my experience, glad you got away with it and had nothing to hit :)

In my case, seems it was an aftermarket single piece driveshaft causing the problem. Must have been some bind or something in the extreme angles under hard braking. My local shop was able to replicate the wheel hop on the street, even after making some other changes to the UCA and LCA’s. We then swapped in the OEM 2 piece DS w/ carrier bearing and the issue went away. Perhaps the pinion angle was wrong for that type of DS.

I’ve not tracked it since so can’t confirm 100% it’s solved, but I think I’m in a happy place with the setup. Now the entire suspension setup matches the Maximum Motorsports road and track package, with koni yellows, MM springs and bump stops, front bump steer kit, etc. I followed their advice on the rear axle too - MM spherical joint adjustable LCA’s, mounted in the upper holes of the relo brackets (parallel to ground). MM spherical panhard bar. Brand new OEM UCA.
 

67GTA

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Thank you @Coz, @Norm Peterson and @racer47 the quick responses. The consensus seems that I need to reduce anti-squat. Understood that reduction will reduce forward bite, which is ok because traction at throttle pick up is not an issue now with the bigger tires.

I don’t think I can adjust anything further on the UCA. I was considering the J&M LCA relocation brackets since they have 3 holes for adjustment. They list the holes as 2”, 2 3/4”, and 3 1/2”. I need to measure my current Whitelines and make sure they’re in the higher range of these adjustments.

Quick clarity check - reducing the downward angle of the LCA’s (from chassis to axle) to closer to level will reduce anti-squat?
 

GBoss7500

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And 67GTA, back to your situation I agree with others comments that excessive anti squat
Thank you @Coz, @Norm Peterson and @racer47 the quick responses. The consensus seems that I need to reduce anti-squat. Understood that reduction will reduce forward bite, which is ok because traction at throttle pick up is not an issue now with the bigger tires.

I don’t think I can adjust anything further on the UCA. I was considering the J&M LCA relocation brackets since they have 3 holes for adjustment. They list the holes as 2”, 2 3/4”, and 3 1/2”. I need to measure my current Whitelines and make sure they’re in the higher range of these adjustments.

Quick clarity check - reducing the downward angle of the LCA’s (from chassis to axle) to closer to level will reduce anti-squat?
Thank you @Coz, @Norm Peterson and @racer47 the quick responses. The consensus seems that I need to reduce anti-squat. Understood that reduction will reduce forward bite, which is ok because traction at throttle pick up is not an issue now with the bigger tires.

I don’t think I can adjust anything further on the UCA. I was considering the J&M LCA relocation brackets since they have 3 holes for adjustment. They list the holes as 2”, 2 3/4”, and 3 1/2”. I need to measure my current Whitelines and make sure they’re in the higher range of these adjustments.

Quick clarity check - reducing the downward angle of the LCA’s (from chassis to axle) to closer to level will reduce anti-squat?
I agree that looking at your anti-squat geometry carefully makes sense. Yes, making your LCA’s more flat versus being downhill from chassis to axle will reduce anti-squat. Consider if DS might be impacting things too. From other opinions I’ve read here, and from a phone call with Maximum Motorsports, I’ve concluded that messing with the UCA’s is more of a drag racing thing meant for extreme launch traction, and there is some inherent goodness in the stock bushing compliance for track day / road racing applications. Maybe it’s needed for more extreme setups than what I’m running to correct pinion angles, but it may be worth trying the OEM UCA?
 

67GTA

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Hi everyone,

@67GTA, I believe I’ve solved the issue of the braking hop recently, so good timing on reviving this old thread. Thanks for the video post - looks very similar to my experience, glad you got away with it and had nothing to hit :)

In my case, seems it was an aftermarket single piece driveshaft causing the problem. Must have been some bind or something in the extreme angles under hard braking. My local shop was able to replicate the wheel hop on the street, even after making some other changes to the UCA and LCA’s. We then swapped in the OEM 2 piece DS w/ carrier bearing and the issue went away. Perhaps the pinion angle was wrong for that type of DS.

I’ve not tracked it since so can’t confirm 100% it’s solved, but I think I’m in a happy place with the setup. Now the entire suspension setup matches the Maximum Motorsports road and track package, with koni yellows, MM springs and bump stops, front bump steer kit, etc. I followed their advice on the rear axle too - MM spherical joint adjustable LCA’s, mounted in the upper holes of the relo brackets (parallel to ground). MM spherical panhard bar. Brand new OEM UCA.
@GBoss7500, great to hear that you’ve narrowed the issue down. I must say I’m surprised by what you found - I can’t say I’ve heard of anyone going back to the 2 piece DS. What brand was the aftermarket? Just curious if it had the adapter plate for the rear flange or if it bolted directly up. I’m committed to the aftermarket single piece now - Tremec swap requires a different length and front coupler.
Glad to see you back on the site and hopefully you’ll be back on track soon to test your setup.
 

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