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Rear axle hop during track braking

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Grant 302

basic and well known psychic
You had me second guessing this - finally got a chance to remeasure this tonight. I can say with certainty that the ones I removed are 3” on center.

View attachment 57099

Awesome. I can’t remember what day it is...or what I measured 7 years ago. My apologies for my error there and the confusion. Thanks for the correction.
 
I just experienced my first episode with violent rear axle hop under hard braking with my 2011 Mustang GT. There has been a lot of talk about anti squat effecting brake hop. I reduced my anti squat so I could get better acceleration off the corners. reducing anti squat reduces the rate of weight transfer to the rear wheels under acceleration. On track, higher values of anti squat can break the rear loose as the weight is transferred too quickly to the rear wheels. I used a 2011 Mustang anti squat calculator published by BMR for my calculations. You reduce anti squat by having the UCA front bolted the bracket hole closest to the chassis and by having the LCA's from horizontal to slightly upward sloping. Using the BMR calculator I have 27% anti squat. My LCA's are upward sloped but almost horizontal at rest. You can't get much less unless you have downward sloping LCA's. I had Whiteline rear LCA brackets and switched to BMR brackets with the rear of the control arm bolted in the highest hole. With the change I was shocked at how much harder I could come off the corners, the down side being I now have wheel hop under hard braking problem. I broke the cardinal rule of dialing in a race set up by making two changes at the same time. I also upgraded to 15" rotors and 6 piston S550 Brembo's in the front and 13" GT500 rotors at the rear. I'm running Hawk HP plus pads front and rear. Reading some of the prior posts I realized that the wheel hop problem may be due to using the same brake compound front and rear. It wasn't a problem before. I think what may be happening in my case is that the really better front braking even with stiffer front shock settings is unloading the rear more than before. I think I may be getting wheel lock up especially with the larger rear rotor. I run with the DSC off. I've never been able to find out if the ABS is turned off when the DSC is turned off. The Ford info does not mention ABS with the DSC turned off. Assuming the ABS is enabled I would expect the ABS to limit the hop when cycling. With the new brakes it feels as if the ABS is working but I don't get any pedal vibration feedback like I have in other cars when in ABS. Maybe Ford cancels it out. In any event, before I change my anti squat I am going to install some much less aggressive rear brake pads to see if that solves the problem. I'll follow up after next event with new rear pads.
 
1,161
2,116
Exp. Type
Time Attack
Exp. Level
5-10 Years
SoCal
I just experienced my first episode with violent rear axle hop under hard braking with my 2011 Mustang GT. There has been a lot of talk about anti squat effecting brake hop. I reduced my anti squat so I could get better acceleration off the corners. reducing anti squat reduces the rate of weight transfer to the rear wheels under acceleration. On track, higher values of anti squat can break the rear loose as the weight is transferred too quickly to the rear wheels. I used a 2011 Mustang anti squat calculator published by BMR for my calculations. You reduce anti squat by having the UCA front bolted the bracket hole closest to the chassis and by having the LCA's from horizontal to slightly upward sloping. Using the BMR calculator I have 27% anti squat. My LCA's are upward sloped but almost horizontal at rest. You can't get much less unless you have downward sloping LCA's. I had Whiteline rear LCA brackets and switched to BMR brackets with the rear of the control arm bolted in the highest hole. With the change I was shocked at how much harder I could come off the corners, the down side being I now have wheel hop under hard braking problem. I broke the cardinal rule of dialing in a race set up by making two changes at the same time. I also upgraded to 15" rotors and 6 piston S550 Brembo's in the front and 13" GT500 rotors at the rear. I'm running Hawk HP plus pads front and rear. Reading some of the prior posts I realized that the wheel hop problem may be due to using the same brake compound front and rear. It wasn't a problem before. I think what may be happening in my case is that the really better front braking even with stiffer front shock settings is unloading the rear more than before. I think I may be getting wheel lock up especially with the larger rear rotor. I run with the DSC off. I've never been able to find out if the ABS is turned off when the DSC is turned off. The Ford info does not mention ABS with the DSC turned off. Assuming the ABS is enabled I would expect the ABS to limit the hop when cycling. With the new brakes it feels as if the ABS is working but I don't get any pedal vibration feedback like I have in other cars when in ABS. Maybe Ford cancels it out. In any event, before I change my anti squat I am going to install some much less aggressive rear brake pads to see if that solves the problem. I'll follow up after next event with new rear pads.
What struts/springs are you running? The bigger front brakes will induce more front dive on braking, which unloads the rear. The stickier the tire, the more noticeable it becomes.
 

Mad Hatter

Gotta go Faster
5,237
4,227
Santiago, Chile
The Hawk HP+ pads are quite grabby. As mentioned above, you have the 15" brembos that add way more front dive and your rears are getting light because of that. Not a big fan of the 13" rear brake idea. Having the HP+ in the rear would not help in any case.
 
What struts/springs are you running? The bigger front brakes will induce more front dive on braking, which unloads the rear. The stickier the tire, the more noticeable it becomes.
I'm running Ford Racing "K" springs front and rear and Koni sport shocks. The rear rear Konis were one full turn on rebound damping. I'll put in more next event to see if rear was under rebound controlled negatively effecting brake hop.
 

Mad Hatter

Gotta go Faster
5,237
4,227
Santiago, Chile
If I remember right, the K springs lowered the car quite a bit past the ideal point for handling. The P springs were more track focused.
 
If I remember right, the K springs lowered the car quite a bit past the ideal point for handling. The P springs were more track focused.
I' m using Ford K springs but I've corrected lowered suspension. I have Steeda front lower control arms with extended ball joints along with a bump steer correction kit. There is no real data I could find on difference in rear spring rates between P and K springs. Also lowering difference between them is maybe 1/ 4 to 1/2 inch. Car seems to work fine with K springs
 
I' m using Ford K springs but I've corrected lowered suspension. I have Steeda front lower control arms with extended ball joints along with a bump steer correction kit. There is no real data I could find on difference in rear spring rates between P and K springs. Also lowering difference between them is maybe 1/ 4 to 1/2 inch. Car seems to work fine with K springs
Vetracer, I suspect your wheel hop problems were caused by the change to your rear brake setup. The bigger rear rotor and increased distance between the calipers and axle centerline gave your rear brakes a bigger lever arm, so braking torque to the rear axle has increased and your overall brake balance has shifted rearwards. Too much rear brake balance can easily cause wheel hop on solid axle cars. This can be made even worse if you are now running a square tire setup but the factory braking system was tuned for staggered. Suggest going back to the stock rear brake setup and using split pad compounds (lower friction on rear) to see if that solves your problem. 27% anti-squat is not overly aggressive and I don't think would be causing rear wheel hop on its own.
 

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