Rear Bump Stops

Discussion in 'Suspension and Chassis' started by lakeside, Mar 20, 2018.

  1. stuntman

    stuntman TMO Advanced

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    Maybe i'm not understanding your question, or maybe it was poorly written.

    If you're referring to the differences in jacking and weight transfer left to right with a track-arm/PHB, and the difference in roll center migration R-L as well as the Engine Torque Reaction which passes into the suspension springs and produces a diagonal load change on the wheels (which makes it easier to lift the LF tire than the RF tire when accelerating out of corners), then that's another one of your tangents that detract from the static roll center location (which is at the center of the car at rest).

    Yes roll centers move, yes for a 3-link track bar suspension design the roll centers and jacking is different from right to left, I've already said this before, but you continue to segway into examples that fit your argument that are out of the scope of the topic at hand.

    I don't have a car on a lift, so where is the intersection point?

    So you want straight line grip, not rear grip when accelerating out of a corner. Ok, then soften your rear springs and add anti-squat.

    Until you actually do something to test to see if you are, you won't be learning anything.
     
  2. Grant 302

    Grant 302 OPM Spent: $587,402 Moderator

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    Nooooooooooo!!!!!!!!!! No. No. No. I gave up drag racing over a decade ago. :rolleyes: And that's not what I'd do if I still was. My car leaves the line just great. Not 'afraid' of anything at stoplights...

    LOL. I'll give you a pass, since you obviously don't know me well, despite both being members here since 2011... I test all sorts of ****. That's the main reason to try the squat setup. TEST AND LEARN!! :D
     
  3. Grant 302

    Grant 302 OPM Spent: $587,402 Moderator

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    Sure. I'm not a writer or a teacher. It's difficult at best to discuss technical matters when there is little agreement about what's needed to understand the dynamics.
     
  4. stuntman

    stuntman TMO Advanced

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    You seem to be all over the place. My comments are focused on helping you improve rear grip for accelerating out of a corner.

    As do I. Which comes back to my initial recommendation of putting a very little amount of effort into verifying if you're sitting on the bumpstop when accelerating out of a corner. From my experience i'm pretty sure you are, and if you are, not matter what you do to your setup, springs or geometry, you're not really going to improve your corner-exit rear grip.

    Oh trust me, I know ;)

    And this is exactly why I don't often post. Giving away free advice that helps others who don't want to listen, do something different than what they think is right, or go outside their comfort zone to improve their cars. I get paid to do this and actually make people faster. It's pretty much useless to try to be benevolent in helping fellow enthusiasts.
     
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  5. TMSBOSS

    TMSBOSS TMO Addict

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    Stuntman. Not so fast. LOL

    I do appreciate the efforts and the knowledge I am picking up. Thanks

    Your point on the bump stop on corner exit is huge....at least to me. If you are on the bump stops you can ignore all the impact all the other components/angles have on traction. You simply ain't using the other components.

    I would guess a lap or two with a go pro mounted in the suspension would have answered a lot of questions.
     
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  6. Grant 302

    Grant 302 OPM Spent: $587,402 Moderator

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    :D Fair enough!

    I certainly planned on investigating or removing the bump stop *after* I make the changes toward more squat. That probably wasn't clear either.
     
  7. lakeside

    lakeside TMO Intermediate

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    Well here's the seat of the pants impressions after VIR. Took the bump stops off completely at home before the drive from Charlotte to VIR. I run 315's on the street to fill up fenderwells and they do fine with the Ford Racing bumpstops. Without the bumpstops on the street, tires would rub pretty good on hard dips or bumps. So not running them on the street was not a great idea. After changing to my track wheels and tires (305's) the car felt great on the track, never felt like it was bottoming out anywhere, never heard any clunking.
    So my plan moving forward, leave them in on the street, take them out on the track.
     
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  8. stuntman

    stuntman TMO Advanced

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    Sounds like a waste of time. You won't get a valid read on the pro-squat change if you're already on the bumpstops.

    You gain a significant amount of travel removing the bumpstops, and when the axle does bottom out into the framerail it's noticeable but often does not upset the car much more than the stop itself, and you won't damage the frame even on a really hard it.

    Or get better fitting street tires ;)

    Glad to hear your car was working well.
     
  9. Grant 302

    Grant 302 OPM Spent: $587,402 Moderator

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    Sounds like a reasonable plan. I might have to do the same with the upcoming changes.

    And that's your opinion. Seems like a waste of time to get a GoPro under there when I have pictures that tell me otherwise. The next time I get under the car will be to make the geometry changes I mentioned.
     
  10. stuntman

    stuntman TMO Advanced

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    The pics that I've seen tell me otherwise based off of what I know it actually takes to bottom out the bumpstop. But that's the great thing, no one is forcing me to help you, or forcing you to listen. You're more than welcome to have flawed test procedures, not learn from the results, and continuing to throw darts in the dark. We live in a great country.

    The real waste of time is the people trying to help others out, especially those who are arrogant and stubborn enough to not think critically about good advice. Good luck in your testing.
     
  11. kcbrown

    kcbrown TMO Intermediate

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    It’s not a waste of time on a public forum such as this, because even if you don’t convince the person you’re talking to, you might end up helping someone else in a similar situation who is watching the goings on. And even better, the message persists, so someone can see what you said long after you said it.

    I have a related question: do you believe it would be beneficial to handling to remove the bump stops from a stock (191 lb/in rear springs) setup? What would that do to the steady state cornering balance, if anything?
     
  12. Busdriver

    Busdriver TMO Intermediate

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    The issue with hitting the bump stop (as I understand it anyways) is that the wheel rate on that corner goes way up when that happens. Which changes the roll stiffness ratio front to rear and subsequent balance of weight transfer. If you're not actually touching the bump stop, then pulling it out would do nothing.
     
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  13. stuntman

    stuntman TMO Advanced

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    Correct, if you don't hit them, removing them will not do anything. The stock bumpstops are fine for stock springs and ride height. As you lower the car and increase grip, the reduction of travel becomes a big issue, especially over bumps or vertical loading from a banked corner or elevation change.

    Hitting the bumpstops has more issues than just increasing roll stiffness, it reduces vertical compliance and shocks the tire, reducing grip.
     
  14. Grant 302

    Grant 302 OPM Spent: $587,402 Moderator

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    One thing to understand is that there is a huge difference between touching the bumpstop and 'hitting' or sitting on it. Squeeze, press or step on one and you can see how much the force is needed to compress to different levels. It's not a binary issue or like an on-off switch.
     
  15. Grant 302

    Grant 302 OPM Spent: $587,402 Moderator

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    That's a great question, I believe directed at @stuntman ...
     
  16. stuntman

    stuntman TMO Advanced

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    I already responded to stock spring rates/ride heights.

    Its interesting to note that some OEM cars are designed to engage the bumpstops in cornering to add more rate to adjust the handling. Some BMW's engage the front bumpstops to help induce entry understeer purely in terms of braking/cornering forces, not from hitting bumps.
     
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  17. 759Rider

    759Rider TMO Beginner

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    Back in the day, bumpstops were just there to prevent metal to metal contact of parts.

    Now they are an integral part of the suspension system. Meaning the OEMs design them with to a specific height, shape, and density to achieve a desired ride goal.

    Removing, drilling out, or cutting them may have good or bad consequences depending on the particular scenario.

    I recently learned a bunch about bumpstops during a suspension upgrade on our Suburban. And I still don't claim to know much. More complex than I had ever imagined.
     
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  18. Grant 302

    Grant 302 OPM Spent: $587,402 Moderator

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    I see now. Your answer to kcbrown was tucked in between the responses to Busdriver.

    And I'm surprised you say they're fine. It's not hard to get into them at some tracks with elevation changes...even on the stock tires.
     
  19. stuntman

    stuntman TMO Advanced

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    Fine yes, but removing them is 'more' important for lowered cars and running better tires. You can still likely benefit from cutting them down or removing them on a stock spring/tired car, but it's not going to be as crucial.
     
  20. lakeside

    lakeside TMO Intermediate

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    My simple findings: Removing them for Track use was great. Removing them for Street use sucked. Good news is they only take a minute to remove and install at the track for guys like me that don't trailer their car.
     
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