Rear axle hop during track braking

Discussion in 'Road Racing Forum' started by GBoss7500, Oct 27, 2019.

  1. GBoss7500

    GBoss7500 TMO Intermediate

    Messages:
    13
    Likes Received:
    4
    Joined:
    Sep 20, 2017
    Location:
    Berkeley CA
    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!
     
  2. Grant 302

    Grant 302 OPM Spent: $665,833 Moderator

    Messages:
    7,083
    Likes Received:
    1,938
    Joined:
    Nov 23, 2011
    Location:
    In the OC
    Where position are you using on the LCA brackets? Sounds like it might be too much anti-squat geometry causing the problem.
     
    Norm Peterson likes this.
  3. GBoss7500

    GBoss7500 TMO Intermediate

    Messages:
    13
    Likes Received:
    4
    Joined:
    Sep 20, 2017
    Location:
    Berkeley CA
    Thanks. Not sure - will check.
     
  4. GBoss7500

    GBoss7500 TMO Intermediate

    Messages:
    13
    Likes Received:
    4
    Joined:
    Sep 20, 2017
    Location:
    Berkeley CA
    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?
     
  5. Coz

    Coz TMO Race

    Messages:
    487
    Likes Received:
    364
    Joined:
    Mar 4, 2018
    Location:
    Philly Metro Area
    I've always heard that you shouldn't run the Konis on full soft. That they should be at least 1/4 turn towards firm. Try that and see if it helps.
     
  6. Mad Hatter

    Mad Hatter Gotta go Faster

    Messages:
    2,766
    Likes Received:
    1,276
    Joined:
    May 6, 2013
    Location:
    Santiago, Chile
    Check out your UCA to see if the bushing is worn or broken?
     
  7. Norm Peterson

    Norm Peterson Corner Barstool Sitter

    Messages:
    436
    Likes Received:
    371
    Joined:
    Oct 22, 2017
    Location:
    a few miles east of Philly
    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 likes this.
  8. Grant 302

    Grant 302 OPM Spent: $665,833 Moderator

    Messages:
    7,083
    Likes Received:
    1,938
    Joined:
    Nov 23, 2011
    Location:
    In the OC
    ^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.
     
  9. GBoss7500

    GBoss7500 TMO Intermediate

    Messages:
    13
    Likes Received:
    4
    Joined:
    Sep 20, 2017
    Location:
    Berkeley CA
    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.
     

Share This Page