Rear Bump Stops

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

  1. Grant 302

    Grant 302 OPM Spent: $587,402 Moderator

    Messages:
    6,493
    Likes Received:
    1,544
    Joined:
    Nov 23, 2011
    Location:
    In the OC
    This is generally true, but the way instant centers are 'calculated' aren't always correct. Panhard bar for example. No way it's just at the center of the bar. That's just a way to simplify things and is close enough for most.

    And again, maybe splitting hairs on definitions...but a reaction from anti-squat isn't weight transfer.
     
  2. stuntman

    stuntman TMO Advanced

    Messages:
    108
    Likes Received:
    30
    Joined:
    Jan 31, 2011
    Actually, yes.

    Not sure what "jerk" is, but reducing anti-squat (or adding pro squat) by lowering the instant center does reduce weight transfer just like lowering the roll center or softening the swaybar reduces weight transfer (but increases body roll) and thus keeps more vertical load on the inside tire which increases the tractive effort of the pair of tires .

    If the mounting points on the axle and chassis are equal distance from the centerline, then yes the RC is the center of the panhard bar at rest.

    As any car pitches and rolls, roll centers will migrate vertically and horizontally.
     
    Last edited: Apr 21, 2018
  3. Grant 302

    Grant 302 OPM Spent: $587,402 Moderator

    Messages:
    6,493
    Likes Received:
    1,544
    Joined:
    Nov 23, 2011
    Location:
    In the OC
    No, and no. It reduces the RATE of weight transfer.

    Jerk is the rate of change of acceleration. Basic physics.


    No, it's not actually. And yes, it migrates quite quickly with a panhard. Draw the free body diagram, and compare the differences in a right hand and left hand turn. The actual roll center isn't in the middle.
     
  4. stuntman

    stuntman TMO Advanced

    Messages:
    108
    Likes Received:
    30
    Joined:
    Jan 31, 2011
    Actually yes and yes.

    So you're telling me that you believe lowering the roll center or softening the sway bar has zero effect on total weight transfer? (In roll)

    I have and the RC can be in the middle of the chassis centerline. It depends on the mounting location and as I said before, drastically changed with pitch and roll.
     
    Last edited: Apr 21, 2018
  5. Grant 302

    Grant 302 OPM Spent: $587,402 Moderator

    Messages:
    6,493
    Likes Received:
    1,544
    Joined:
    Nov 23, 2011
    Location:
    In the OC
    I think you're confusing RATE of transfer and TRANSFER.

    Say two cars lift both inside tires to the same height in the same corner. One with shocks set firm and one soft. The firm one will do it at a faster rate (get there first) but the total transfer for both will be same 100%.

    No, but any calculated 'value' for the moment will be wrong.

    Yeah, 'it depends' :rolleyes:

    With a panhard, find me *any* arrangement where the instantaneous roll center is at the vehicle center line. Go.
     
  6. stuntman

    stuntman TMO Advanced

    Messages:
    108
    Likes Received:
    30
    Joined:
    Jan 31, 2011
    Dampers are a transient device. They don't have much of an effect steady state.

    Softening a swaybar or lowering a roll center does affect the rate of transfer if you keep the damping the same, and they also increase body roll. But a softer bar and lower Rc REDUCES weight transfer across a pair of tires, and puts more vertical load and increases the tractive effort of the inside tire (and increasing the total tractive effort of the pair of tires). Since you seem to be struggling with grasping this concept (and most do) I suggest buying and reading Milliken's book "Racecar Vehicle Dynamics".

    Instant Centers are not roll centers. You draw a line from the center of the contact patch of one tire to it's corresponding instant center, and the second for the other tire. Where those two lines intersect is the roll center. For a PHB, the RC is the height of the midpoint of the bar. So if the mounts are equal distance from the centerline of the car, the RC will be at the chassis centerline.
     
    Last edited: Apr 21, 2018
  7. Grant 302

    Grant 302 OPM Spent: $587,402 Moderator

    Messages:
    6,493
    Likes Received:
    1,544
    Joined:
    Nov 23, 2011
    Location:
    In the OC
    Agreed.
    Yes, but not sure why you're throwing the same damping qualifier in there.
    No. In absolute terms, it doesn't. That's only true in a relative sense compared to the other axle or outside of the limits of working suspension geometry. It's just slower to transfer. Still sounds like you're confusing the two.

    No, I'm only talking ROLL CENTER there. The actual point will be virtual or 'instant' if you can follow me. It's still not the midpoint of the bar. Sorry.
     
  8. stuntman

    stuntman TMO Advanced

    Messages:
    108
    Likes Received:
    30
    Joined:
    Jan 31, 2011
    Not really. You seem to still be confusing body roll with weight transfer.

    While changing the RC or swaybar stiffness does change the relative load transfer distribution of the front tires relative to the rear, let's simplify things by focusing on a single axle/end of the car.

    Stiffening the swaybar or raising the RC increases weight transfer on that end of the car, reducing grip of the pair of tires. I really don't want to beat an already dead horse more than I already have done, so here are some quotes to back that up:


    According to Dave Segal (author of Ch. 7 in RVD by Milliken):

    "Anti-roll bars change the distribution of the lateral load transfer between the front and rear tracks, and also reduce body roll angle and add to the one-wheel bump rate of the suspension."

    "Because of the nonlinearity of tire lateral forces versus load curves, transferring load from the inside to the outside tire on an axle invariably results in a decrease in lateral track force - when compared to the same axle with no load transfer."

    "Raising the rear roll center will make the rear take more roll couple and the rear will saturate sooner
    (less rear grip)"

    "Roll center heights determine the proportion of lateral load transfer that is passed through the suspension linkage. The rest of the load transfer is passed through the springs and anti-roll bars as the vehicle rolls."

    "Raising the roll centers gives an anti-roll effect and reduces body roll"

    "High roll centers lead to jacking and lateral wheel travel on bump; these are undesirable".


    According to Terry Satchell (author of Ch. 17)

    "If the roll center is above ground level the lateral force from the tire generates a moment about the instant center (IC). This moment pushes the wheel down and lifts the sprung mass; it is called jacking. Jacking up also increases weight transfer because it raises the vehicle center of gravity, thus further reducing cornering capability."

    Again according to Mr. Satchell:

    "For a beam-type axle suspension: The roll axis is found by determining the two lateral restraint points and connecting them with a line. Usually the geometry is viewed in the plan view to determine the forward and rearward lateral restraint centers of the roll axis. For example, the point where a Panhard bar crosses the centerline of the vehicle is a lateral restraint point."

    "The point where the instant-roll-axis of an axle suspension intersects the transverse-vertical-plane passing through the wheel centers is the location of the roll center"


    =THUS the height of the Panhard bar at the centerline of the car is the roll center.
     
    Last edited: Apr 22, 2018
  9. Grant 302

    Grant 302 OPM Spent: $587,402 Moderator

    Messages:
    6,493
    Likes Received:
    1,544
    Joined:
    Nov 23, 2011
    Location:
    In the OC
    What you quoted on transfer is correct. But you are not understanding that transfer is RELATIVE like I said earlier and as Segal says above. Seems to be the part you are missing when you say:

    and,

    I think you understand now, what I'm saying, but still want to say I'm wrong? OK.
     
  10. stuntman

    stuntman TMO Advanced

    Messages:
    108
    Likes Received:
    30
    Joined:
    Jan 31, 2011
    I understood all along, you seem to be slowly grasping the concept.

    Now that you admit anti-roll bars increase the relative load transfer, you must now understand that increasing the stiffness on one end of the car, increases lateral load transfer and reduces grip on that end of the car.

    Yes you were wrong by saying that sway bars dont change weight transfer, and only affect the RATE of transfer.

    Are we on the same page now?
     
  11. Grant 302

    Grant 302 OPM Spent: $587,402 Moderator

    Messages:
    6,493
    Likes Received:
    1,544
    Joined:
    Nov 23, 2011
    Location:
    In the OC
    "thus" lol...that makes it conclusive. ;)

    Sure, you can quote that from all the texts you like, but that mid point is NOT the center which the body ACTUALLY rolls in the axle vertical plane.

    The convention is a simplification of two arcs. One for the panhard and the other from the body side connection to the CG. If you draw the diagram with both arcs, you can see that it is IMPOSSIBLE for the body to rotate about the conventional RC point.
     
  12. Grant 302

    Grant 302 OPM Spent: $587,402 Moderator

    Messages:
    6,493
    Likes Received:
    1,544
    Joined:
    Nov 23, 2011
    Location:
    In the OC
    Sure, this is why you don't understand why my GT is not sitting on the stops in a turn on the throttle. You quote jacking effects and insist on the RC location, but it seems you can ignore what you want. I don't think I can make you see the same page.
     
  13. stuntman

    stuntman TMO Advanced

    Messages:
    108
    Likes Received:
    30
    Joined:
    Jan 31, 2011
    If not, how about this as conclusive for the roll center height to be at the centerline of the PHB from Mr. Satchell:

    "Three-link & Track Bar suspension. In this design the side view is evaluated the same as all of the four link suspensions. The intersection of the projection of the lower arms and the projection of the upper arm for the side view IC. The torque reactions are taken by the single upper arm and the pair of lower arms similar to the other four bar link designs. Except the loads in the single upper arm are higher for the same inputs.

    The roll axis determination is made by finding the lateral restraint points. We know the track bar provides lateral restraint so the point where its centerline (pretty much the centerline of the car in an S197) crosses the centerline of the car is one of them. The other restraint point is determined from the lower control arms. The single upper arm cannot provide any restraint to lateral loads so it is not used in this determination. If the lower arms converge in the plan view, the intersection point is projected into the side view and now a line can be drawn between that point and the track bar point to produce the roll axis. Again the roll center is where the roll axis intersects a vertical plane through the wheel center.

    The lower arms may run parallel to each other and the centerline of the car (LIKE THEY DO IN A MUSTANG). If this is the case then they intersect at infinity. The roll axis would run through the track bar (PHB) point parallel to the lower arms in the side view. The roll center would be where the roll axis crosses a vertical line through the wheel center (AT THE CENTERLINE OF THE CAR)."

    Boom ;)


    By definition:

    "The roll center establishes the force coupling point between the unsprung and sprung masses. When a car corners, the centrifugal force at the Cg is reacted by the tires. The lateral force at the CG can be translated to the roll center if the appropriate force and moment are shown. The higher the roll center, the higher the rolling moment (and vice versa). You will also notice that with higher roll centers the lateral force acting at the roll center is higher off the ground. This lateral force x the distance to the ground can be called the nonrolling overturning moment. So roll center heights are trading off the relative effects of the rolling and nonrolling moments."

    In addition:

    "For an axle-suspension, the roll center height is a theoretical point where a lateral force can be applied without a rolling moment being generated on the sprung mass." (Not the point where the body rotates around)

    You can try to derail the conversation as much as you want about the definition of roll centers and how the unsprung mass does not roll about the centerline of the car in a 3-link/track bar suspensions despite the mathematical RC being pretty much at the centerline for an S197 and Nascar cup cars, and anything with a long PHB (vs some dirt track cars that are very short and mounted near the rear end housing).

    Let's go back to just talking about the effects of swaybars and roll centers and their effect on weight transfer (the main point):

    Softening your rear bar will reduce weight transfer across the rear tires.

    In addition:

    "Increasing rear anti-squat raises the CG and increases weight transfer to the rear wheels on acceleration."

    I've ran your spring rates and various rear bars, and I would put money down that you are. Until you put a video camera on your bumpstop or show me some potentiometer data, or do anything to verify that you're not; I'll go with my experience over your skepticism. ;)
     
    Last edited: Apr 23, 2018
  14. stuntman

    stuntman TMO Advanced

    Messages:
    108
    Likes Received:
    30
    Joined:
    Jan 31, 2011
    What tires are you running? Why do you need more rear grip for power-down?

    If you want the car to wheelie off of a corner like a Cup car, you could soften your rear springs, soften your rear bar (reduce load transfer across the rear tires, increasing total rear grip), stiffen your front bar, lower your PHB mounting point to lower the rear roll center (further transfer less load across the rear tires), add anti-squat to increase longitudinal load transfer, and now you're lifting inside tires off the ground (unless you're already sitting on the bumpstop).

    Or...Look to see if you're sitting on the bumpstops, try removing them, see what effect that plays, and you may find you don't need to go too far down that rabbit hole to improve rear grip. ;)
     
  15. DocB

    DocB TMO Advanced

    Messages:
    113
    Likes Received:
    32
    Joined:
    Sep 17, 2013
    Great summation post stuntman.
    But please leave me SOME grip for the front tires!
     
    stuntman likes this.
  16. Grant 302

    Grant 302 OPM Spent: $587,402 Moderator

    Messages:
    6,493
    Likes Received:
    1,544
    Joined:
    Nov 23, 2011
    Location:
    In the OC
    stuntman edits.png
    wow. WTF with the ninja edits? How many times do you need to update this post in 24 hours? :D

    You're finally making some sense. ;) But we're just right back where we started. :D
     
  17. Grant 302

    Grant 302 OPM Spent: $587,402 Moderator

    Messages:
    6,493
    Likes Received:
    1,544
    Joined:
    Nov 23, 2011
    Location:
    In the OC
    That's nice and all conventional thinking. Then you can certain explain how much our cars cant over laterally and differently left vs. right? Get that noodle going. It's more complicated than what you read.

    Read that part about jacking again, and draw a free body diagram of the panhard, and then include it in one with the all the rear components in steady state. Do that for both right and left turns. You'll *begin* to understand. I'll save the 'boom' for later. ;)
     
  18. Busdriver

    Busdriver TMO Intermediate

    Messages:
    47
    Likes Received:
    29
    Joined:
    Oct 14, 2017
    Location:
    Tucson, AZ
    What are you guys actually arguing about?
     
    Grant 302 likes this.
  19. Grant 302

    Grant 302 OPM Spent: $587,402 Moderator

    Messages:
    6,493
    Likes Received:
    1,544
    Joined:
    Nov 23, 2011
    Location:
    In the OC
    Uh, they're *not* parallel in a stock Mustang. Go measure!

    Perhaps it's not clear: I'm looking for more FORWARD grip. As-is the car TURNS great. I want to give up some turning ability for more exit/acceleration.

    Based on photos of my car sitting and track photos turning, I still don't see how you think my car is sitting on the bumpstop. The more I look at, the less I think it's an issue with the current setup.
     
  20. Grant 302

    Grant 302 OPM Spent: $587,402 Moderator

    Messages:
    6,493
    Likes Received:
    1,544
    Joined:
    Nov 23, 2011
    Location:
    In the OC
    Exactly. ;) :D
     

Share This Page