Rear Bump Stops

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

  1. stuntman

    stuntman TMO Advanced

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    Ok, it'll vary depending on your tire size and gear ratios, but the point is the same. You'll be getting into them in a low speed, tight corner.

    Not very accurate and you're probably getting into the stops a lot more than you realize. Sitting on the stops when accelerating out of a corner does not have to be 'harsh', but it will increase the effective rate and hurt rear grip.

    Removing the axle mounted bump stops wouldn't hurt the konis. I'm pretty confident they wouldn't bottom out before the axle does into the framerail. But it's your car. Do what you want.

    Not sure what that means.

    From what point of reference? Upward angle going from front to back (front chassis mount lower than axle mount) or vice versa?
     
  2. Coz

    Coz TMO Beginner

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    Does the discussion (i.e., height) about bump stops change if one is using the softer Ford Performance MPN# M-5570-A bump stops? I replaced mine when I went with the Roush springs and Koni Yellows and they certainly felt softer, and slightly shorter, thanthe originals.
     
  3. Norm Peterson

    Norm Peterson Corner Barstool Sitter

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    Not convinced about this part. People had some durability problems with Konis, and one of the commonalities was a lowered car. Which is part of why the rear of my '08 was only lowered about 5/8" instead of the 1.3" or more that virtually all "lowering springs" drop the rear by. I think these pictures are with the spare, a floor jack, and a flat box of miscellaneous tool in the trunk. Sorry, but I don't have any pictures of the actual b/s clearance and it's too dark to go take one now. BMR's handling springs for a GT500 (220 lb/in) if it matters.

    On BMR GT500 handling springs (rear shimmed, web).jpg On BMR GT500 handling springs (rear as shimmed, web).jpg

    Norm
     
  4. stuntman

    stuntman TMO Advanced

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    It's not too difficult to measure the distance between the axle and framerail at ride height and the amount if shock travel.

    Just saying.
     
  5. lakeside

    lakeside TMO Intermediate

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    Hey Stuntman, if you're going to start introducing common sense here, I'm walking away from this conversationo_O
     
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  6. Norm Peterson

    Norm Peterson Corner Barstool Sitter

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    Common sense here for a car that's at most 20% track is that I accept a little b/s contact due to bumps and uneven pavement with passengers or a temporary trunk load over getting either axle-to-frame contact or shock bottoming. Inertial motions aren't big enough by themselves individually and probably not as the vector sum of any possible pair of lat & long g's. I'm not too concerned about the first 1/8" or so of b/s compression no matter what puts my car into it that far anyway. Bump stops do have a little 'give', those other situations don't (short of breaking something).

    I thought I had a picture of the clearance, but it doesn't seem to be handy. My yard is fast becoming a lake, so measuring/photo'ing it isn't happening today either.


    Norm
     
  7. stuntman

    stuntman TMO Advanced

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    Removing the bumpstop will get you around 2" of extra travel. Your ride quality will be significantly improved, especially when the car is weighed down with passengers and luggage since you won't be constantly sitting on them with an exponentially higher wheel rate.

    But it's not my car and I don't have to ride in it ;)
     
  8. Grant 302

    Grant 302 OPM Spent: $328,347 Moderator

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    @blacksheep-1

    Rob, what bumpstops were you using on AJ's car? Were they cut at all?
     
  9. lakeside

    lakeside TMO Intermediate

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    The whole reason for my question was the fact that while bleeding the rear brakes I noticed that there was only about an inch gap at static ride height. Thought there might be a little bit of a concensious, but........ I’ll probably remove them for the trip to VIR this weekend and report back with some clever in depth analysis.
     
  10. stuntman

    stuntman TMO Advanced

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    Yea, that's not much. Have a friend bounce the rear of the car and look at that 1" gap disappear with only 150-200lbs acting on it. Now imagine the forces of the weight of the car moving around, let alone pulling a couple vertical Gs from the elevation change at VIR.
     
  11. Grant 302

    Grant 302 OPM Spent: $328,347 Moderator

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    I think is matters what your particular setup is...especially springs and rear geometry.

    Suspension geometry really makes a difference comparing static changes (like a friend bouncing the rear) vs the dynamic load transfer through say a panhard and spring+shock. Simply put, there are 'jacking' reactions that somewhat prevent roll depending on the setup which you are ignoring.

    You were quick to dismiss the 350lb./in. rate I'm using, but even with the relatively soft 18mm bar, the first 1" of roll travel would be about 450 lb. *before* considering the panhard (in my setup) and current anti-squat from the outside LCA.

    I spent some time on the street trying to prove myself wrong between Sunday and yesterday. Tested a bunch of tight rights and lefts up to about 30 mph. Even in a cul-de-sac and goosing the throttle looping around, I couldn't get my rear to squat.

    .
     
  12. stuntman

    stuntman TMO Advanced

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    Dynamic load transfer or suspension geometry was not ignored. Simply put, if you can get someone bouncing on the rear of the car to compress the suspension that 1" of travel that he had before getting into the BS, then he would be into them heavily when dealing with the verical Gs due to the elevation changes at VIR.

    Ok, but what is your diagonal weight transfer when pulling 1G of cornering and say 0.5Gs of acceleration?

    Can you get your tires up to operating temp on the street, is the street cul-de-sac have a similar level of grip as the track, and are you putting power down in 2nd & 3rd gear like you would on track in a low speed hairpin? (That test probably does not tell you anything).
     
  13. Grant 302

    Grant 302 OPM Spent: $328,347 Moderator

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    So you say.
    Yet it would need someone pretty unrealistically heavy to do the same on my car.

    Not enough. That's why I want to change things for more squat.

    Yes.
    Better.
    More. Stabbing the throttle like I *wouldn't* do at the track. I said goose the throttle. I don't do that intentionally at the track.

    OK. :rolleyes:
     
  14. stuntman

    stuntman TMO Advanced

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    That sounds contradictory. Do you want better power-down grip?
     
  15. blacksheep-1

    blacksheep-1 TMO Addict

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    Yes, they went from tall to short, to nothing and back again, depending on the class. I think you are right on the money with a 350 LB rear springs and a 18 mm bar..(or even no bar) Don't forget, after you brake hard, then accelerate the whole car is acting like a fulcrum, so you're not just getting the...I don't know..the drag launch type of compression. Your front shocks are totally compressed, the cars weight is on the front tires, then you hammer it and all of that tries to go backwards. Different than starting with the car dead still.
     
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  16. Grant 302

    Grant 302 OPM Spent: $328,347 Moderator

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    That's the idea! Gonna take some relearning for driving style. I don't expect it to be a 'holy grail' sort of change. Something else to try and mostly to learn.

    Thanks, Rob. Different than using a LOT of anti-squat too...I appreciate the tips!
     
  17. stuntman

    stuntman TMO Advanced

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    If you want better power-down grip, reducing anti-squat, or softening the rear springs and bars reduces weight transfer.

    Body movement in roll/pitch/dive/squat is not weight transfer. A common misconception.
     
  18. TMSBOSS

    TMSBOSS TMO Addict

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    Let’s see if I understand weight transfer. It’s not the motion of mass per se. it’s the affect that motion May have. Like when a drag car fully transfers weight through motion and the motion/weight sets fully on the rear axle. Full weight transfer. But if the suspension “cushions” the movement and or deadens the movement, weight transfer may not occur.
    Am I close?
     
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  19. stuntman

    stuntman TMO Advanced

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    Close. Weight transfer always happens when a force acts on the CG. Softer dampers slow down the transfer of weight, that's why an old floppy cadillac floats all over the place and isn't responsive to inputs until the body takes a "set", and why stiffening dampers makes the car respond quicker, because weight is transferred into the tire quicker, which is the inverse of the slower body movement.

    It's all about roll resistance from geometry (or anti squat/lift/dive) and springs/bars and how the CG acts on the roll (or instant) centers.

    Lowering the instant center acts longitudinally like how lowering a roll center acts laterally (which is similar to softening a swaybar), by increasing body roll and reducing weight transfer by keeping more vertical load on the opposing tires.

    Another example, a stiffer swaybar (or higher rc) primarily transfers more weight across a pair of tires, increasing response and weight transfer, reducing the total tractive effort (grip) of that pair of tires, and reducing body roll.
     
    Last edited: Apr 21, 2018 at 12:58 PM
  20. Grant 302

    Grant 302 OPM Spent: $328,347 Moderator

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    No, not really. See the 'learning to squat' thread. Anti-squat essentially works with jerk not acceleration.

    Yes, generally agree, but technically no. Softening springs and bars *slows* the rate of transfer. Might sound like splitting hairs, but it's true.

    Yes, agree, it is not the only component. More of a symptom than a cause. This is why my car doesn't roll much but transfers weight laterally just fine. Increasing the diagonal transfer is my near term goal. Again, see the squat thread and see Mad Hatter's progress so far.
     

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