Pushy-Loose a tale of a poorly setup car (and how to fix it?)

Discussion in 'Road Racing Forum' started by captdistraction, Jan 11, 2018.

  1. Fabman

    Fabman Project: "Frankenstang"

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    And the PWC cars? Real tires, and bigger tires.
    I figured the same as you, bigger tires needed bigger springs but it just didn’t pan out. Felt like I was driving a board with no movement. The 500’s allow the suspension to move and work, and with a car this heavy that’s a good thing. Maybe I’ll end up with 550’s eventually.....but for now I’m going to stick with what’s working and fine tune that.


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  2. Fabman

    Fabman Project: "Frankenstang"

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    I had a spec mustang at my shop for a while with 600’s in it. The driver was complaining about a push they could never get out of it....


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  3. Fabman

    Fabman Project: "Frankenstang"

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    Spec mustang runs 400or 450 rear springs with no rear sway bar and the watts in the top hole....hows that for trying to balance out a pushing car? You can get away with that when your only making 300 whp. I make more than double that.....rear bite is a lot harder to come by.


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  4. Fabman

    Fabman Project: "Frankenstang"

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    [​IMG]



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  5. captdistraction

    captdistraction Not necessarily the Stig

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    Whew lads, can of worms!

    On the spring side of the discussion, car weight and tire weight can be driving factors. My car at one point with driver has a minimum weight of 3501 via classing sheet, so somewhat portly as reality is even more heavy.

    @Mad302 - That's what the shock gurus at maxcyspeed believe I should do. Their advice:
    "More rebound in the rear and less compression in the rear. Sounds like it’s shifting too much weight to the front under braking and not planting enough on exit. If I had my way I would probably drop your spring rates about 100lbs in the back. You might try dropping the compression first. If the car really responds to it. That would be a sign the spring rate was too high."

    @ArizonaBOSS - this has been mostly noticed on all the tire setups I've run as a common behavior of the car. Having driven a certain similar red car that had better turn-in characteristics I definitely feel its somewhere in the car's setup. More fresh tire up front (IE the move from 295 to sticker 315 up front) helped, but I feel it "masked" the issue more than anything. A good example is our qualify session from Bondo, after the best lap I wanted to try to follow your speed into some of those chicanes on the backside of the "main" half of the track. I ended up blowing right through a couple because there wasn't enough there for me to be dumb.

    @JAJ yes absolutely right on all points. damper tuning is something I want to get into as I've not moved my dampers at all. They work well, but I paid for fancy dials.

    @Big Black - some stuff to try, I need to get in on a test and tune day so I can try some of those setups. I'm more concerned about entry push than the loose exit.

    @The_Dead_Cow - definitely addressing those. I'm waiting on some parts, but I have some free weight to give up (remove strut bar, etc) and other items I can do now.

    @Grant 302 - the tire heat data came from my last sessions, so its new data I've not had a chance to act on yet. Front bar is full soft as I've not swept it in testing, the rear instant center/AS settings came from Kelly at BMR and I'm very happy with how those feel

    @2012-Boss - I've run both low and high rear roll centers. Ran more low settings with a lot more rear bar (eibach adjustable rear, 25?mm), and ultimately I like how the car feels more with a higher RC and less bar. I know its loose on exit, but before it was somewhat less progressive and loose by mid corner, and seemingly slightly more slow based on my driving style. However I'm able to go back and forth on those settings without too much work. Agree on the change mentality.

    So the smart thing to do is to build a change list to document, phase in testing, and make remarks on each change. I'll keep everyone informed, but I'll start from no-cost changes (turn knobs!) to the low cost ones (corner balance) before getting into buying my way through the problem.
     
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  6. Grant 302

    Grant 302 OPM Spent: $265,852 Moderator

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    Forgot to ask: What angle are you running on the A-arm between the pivot and center of the ball joint?
     
  7. 2012-Boss

    2012-Boss TMO Addict

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    Since you have already tried the watts link pivot point, I would start with shock tuning like maxcyspeed recommends. It may not get you where you want to be, but it will probably help you decide if a spring and bar change is the solution..

    More rear rebound and less front bump will each help entry understeer and exit oversteer.

    I have to pull out a couple of books this evening to full understand decreasing rear bump.

    The thing about shock changes is that a single change affects both entry and exit.
     
    Last edited: Jan 12, 2018
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  8. JAJ

    JAJ TMO Advanced

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    If anyone wants to dig deeper into how shocks and handling interact, you'll find this book interesting: https://www.amazon.com/Vehicle-Dynamics-Damping-First-Revised/dp/147724736X

    The author is Jan Zuijdijk, and the first letter of his last name is the "Z" in JRZ. I think he's retired now, but his book he takes the reader through the invention and development of the double-adjustable gas pressurized shock (which he started working on when he was an engineer at Koni) and he goes into the theory and practice of suspension tuning around these components. One of his key points is that every driver and every track has different optimal shock settings. Smooth fast tracks with long straights and fast sweepers will deliver faster times with stiffer shocks, where bendy tracks with bumpy pavement will gain time with softer settings.

    Basically, once you invest in adjustable shocks, you're committed to adjusting them. They are NOT set-and-forget items. The benefit is that you'll go quite a bit faster.
     
    Last edited: Jan 12, 2018
  9. 2012-Boss

    2012-Boss TMO Addict

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    EAC61F25-B445-4FFB-8F93-818A48FCB056.jpeg

    hmmm.... less rear compression looks to increase corner entry understeer and corner exit oversteer.

    One thing to understand about the table is only the listed shocks are moving in the direction of travel indicated.

    So, one corner exit the assumption is the outside rear is already fully loaded and increasing rear bump will have no effect on that tire. But, increasing bump will transfer weight more quickly to the inside rear.
     
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  10. 2012-Boss

    2012-Boss TMO Addict

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    and thanks for the book recommendation. I got a used copy for $15.
     
  11. Fabman

    Fabman Project: "Frankenstang"

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    I just picked up a copy myself...
     
  12. Albino500

    Albino500 TMO Intermediate

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    Chris - do you have the OEM bushings in your front lower control arms?
     
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  13. captdistraction

    captdistraction Not necessarily the Stig

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    Poly bushings in the stock arm, Howe thread in tall ball joints


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  14. Albino500

    Albino500 TMO Intermediate

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    and you have the extended outer tie rod ends or an adjustable bump steer kit?
     
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  15. captdistraction

    captdistraction Not necessarily the Stig

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    I removed a bumpsteer kit that was failing but made no appreciable difference in bumpsteer characteristics or turn in


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  16. Grant 302

    Grant 302 OPM Spent: $265,852 Moderator

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    Are you certain about that? Did you check/measure it?

    What's your static angle of the A-arm?

    My guess is that you'd be getting a fair amount of toe IN under compression. Depending on your A arm, opposite on droop. Could explain some of the issues and variability.
     
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  17. captdistraction

    captdistraction Not necessarily the Stig

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    before with the bumpsteer kit and measured setup, it still had the same entry push characteristics. However after a while I had issues with the steeda bumpsteer kit failing, so I went back to a stock style setup for fear of a catastrophic failure. I've been shy to replace that kit because it seems every aftermarket kit has had some form of demonstrated failure. I could indeed be getting toe-in under compression, but I can say with some confidence that the entry push existed beforehand
     
  18. Albino500

    Albino500 TMO Intermediate

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    Sir...please come out and step away from the vehicle.......
     
  19. ArizonaBOSS

    ArizonaBOSS Because racecar. Moderator

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    FWIW I am running OEM-style tie rod ends instead of a BS kit and can't tell a difference compared to when a BS kit was installed. Maybe I can't drive well enough to tell a difference but I can always change back later when I learn :)
     
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  20. captdistraction

    captdistraction Not necessarily the Stig

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    I am trying to avoid throwing parts at the problem just to throw parts at things.

    Given my statement of the push existed both with and without a bumpsteer adjustment I’m not sure what difference further adjustments in that particular category would make.


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