Rear Lateral Movement During Heavy Breaking

Discussion in 'Road Racing Forum' started by ScottyD_GT, Oct 15, 2018.

  1. ScottyD_GT

    ScottyD_GT TMO Intermediate

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    When I break heavily on track days, for example at the end of a long straight, my back end wiggles to the left and right (I.e oscillates laterally, but not periodically). Would like help diagnosing why and how to solve this.

    My initial thinking is this is caused by better brakes in the front than the rear... and the stopping force is so unbalanced and the rear so light/unloaded that the slightest steering wheel movement or uneven pavement rotates the back end. The tires catch and straighten the back end before it rotates too far in one direction.

    Anybody have other ideas or is this the cause? Would better rear brakes help?
     
  2. VoodooBoss

    VoodooBoss Rick Moderator

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    What aero do you have on your car?
     
  3. Grant 302

    Grant 302 OPM Spent: $587,402 Moderator

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    Sounds more like suspension geometry issues. Maybe spring rates too.

    What is your complete setup?
     
  4. ScottyD_GT

    ScottyD_GT TMO Intermediate

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    Stock ‘18 GT PP1 aero. So the PP1 front chin splitter and the rear wing. Side skirts I suppose. Not sure how much downforce these help with vs looks.
     
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  5. ScottyD_GT

    ScottyD_GT TMO Intermediate

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    ‘18 GT Perf Pack 1 setup with Ferodo DS1.11 pads in front. I put on square 19x11 wheels with 305/30/19 Nitto NT01 tires.

    Camber plates but stock PP1 suspension. Haven’t lowered the car yet, that’ll be in the off season.
     
  6. 11GT50

    11GT50 TMO Intermediate

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    I think you may be feeling rubber in the rear subframe & suspension flexing. I have a GT350 and it does the same thing braking from 140+. The car wont actually go anywhere but it does feel a little unsettling if you're not used to it.
     
  7. Grant 302

    Grant 302 OPM Spent: $587,402 Moderator

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    Added grip from 305 NT-01s will show limitations of the stock suspension components. I don't think just lowering is necessarily better in this regard.
     
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  8. ScottyD_GT

    ScottyD_GT TMO Intermediate

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    That makes sense, could be it. As @Grant 302 said, the r-comp tires could be revealing the faults in the suspension.
     
  9. steveespo

    steveespo Lord knows I'm a Voodoo Child Moderator

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    Front compression is to soft and rear rebound is also too soft. The whole spring/shock/bar setup is too soft for track duty. You are experiencing brake dive and unloading of the rear suspension. Not much you can do besides changing your braking style to longer/softer on the pedal vs threshold braking. Good news is Ford and aftermarket have some good parts that will improve both handling and body control. Ford Track Suspension package works well or you could contact Cortex or Vorshlag for advice on coil over setups. Not spending your money but no cheap fixes for heavy track duty, if you are quick then the stock stuff shows its weaknesses early.


    Sent from my iPhone using Tapatalk
     
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  10. ScottyD_GT

    ScottyD_GT TMO Intermediate

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    That makes sense too. With only 3 track days left this year, suspension work will be done this winter.

    Thanks for the input!
     
  11. Boone

    Boone TMO Advanced

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    I had this same issue on my '05GT. The issue was the rearmost front control arm bushing. The rubber bushing compress too much under load (cornering or hard braking) as you add grip, and the alignment of the control arms is compromised. Under braking, you feel it in the rear of the car because it is unloaded, but the issue is up front. I don't claim to know how similar the geometry is with your '18GT, but it's a good place to focus your solution.
     
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  12. CSL

    CSL TMO Advanced

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    Rear subframe bushings are big culprit here.
     
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  13. VoodooBoss

    VoodooBoss Rick Moderator

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    My 350 also did this. Very noticeable at Thunderhill on the back straight braking for T14 and at Sonoma braking for T7.
     
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  14. Bill Pemberton

    Bill Pemberton TMO Addict

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    Have seen this with various new cars and changes to their ABS systems. Many went from 3 channel to 4 channel years back and this experience seemed to crop up with some vehicles. Folks have had some success changing to different pads and others have worked on suspension. There have been comments by some that the changes to tire size could affect braking parameters with the computers, but most find this occurrence seems to be adjusted by changes in the above ( pads , shocks/springs, or even alignment). One other question, do you run with everything off, or are you in Advance Track Mode?

    I never noticed this issue when I had my GT 350 , but I was running super aggressive front pads and fairly light rear pads, in contrast. Not saying this is right, just an observed situation on my part. Also I run with everything off.

    I have noticed this, though , in running a few other guys cars, and it could be related to tires, pads, suspension, or alignment, so good to continue to ask questions and check with your friends who have similar setups.
     
  15. ScottyD_GT

    ScottyD_GT TMO Intermediate

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    I run in Track Mode. I don't completely turn AdvanceTrack/ESC off. If you didn't notice this in your GT350 with super aggressive front pads and light rears, this points me towards the suspension/IRS.
     
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  16. Bill Pemberton

    Bill Pemberton TMO Addict

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    I turn everything off , as I can say I have noticed somewhat unsettling ( to me ) issues when I left Advance Track on -- caused by hard braking , going over curbing or fast corners at turn in points. This was with my 2012 and 2013 Boss 302s and I was not used to nanny devices ( none in my 08 Viper ACR) so, at first, I thought there was something wrong with the car -- but those things do come on if you are aggressive or if pavement undulations fool the system ( just my humble opinion). As noted, though, in the GT 350 , everything was off------
     
    Last edited: Oct 17, 2018
  17. ScottyD_GT

    ScottyD_GT TMO Intermediate

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    Now that I think of it...I heard some odd noises from the rear diff and maybe something else this past weekend at Summit Point. I'll try to turning everything off next time. I didn't before more out of laziness than anything else.
     
  18. Bill Pemberton

    Bill Pemberton TMO Addict

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    Don't turn them all off unless completely comfortable with that, but with Stability Management and the ABS system kicking in from surface undulations, brake pressure , tire slip angles, g forces , track debris, etc. , there can be some simple answers one has to view, along with all the other comments , listed above.
     
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  19. ScottyD_GT

    ScottyD_GT TMO Intermediate

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    Does turning off AdvanceTrac turn off ABS as well?
     
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  20. Bill Pemberton

    Bill Pemberton TMO Addict

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    No , it just keeps the system from kicking in due to what it anticipates are somewhat drastic chassis/suspension changes. Nothing turns off ABS, except system failure.
     
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