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anyone can please explain how do fast bump and rebound work?!?

Discussion in 'ACC General Discussions' started by Luciano Micale, Mar 15, 2019.

  1. when i go over some curbs some cars bump and others not. How do fast bump and rebound work?!?I mean if i scroll on the right the indicator means that is faster or slower. thks in advance for your replyes

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  3. Max Doubt

    Max Doubt Racer

    Higher value = stiffer (harder to compress/less absorption, faster to rebound).
    Lower value = softer (easier to compress/more absorption, slower to rebound).
  4. Serge M

    Serge M Alien

    My understanding is that fast bump and rebound apply to sudden shock such as hitting a curb at speed. Normal bump and rebound work more in the general range such as car diving under breaks and turning corners.

    That’s just a generalisation though, could also be wrong :)
    Alistair McKinley likes this.
  5. nimp

    nimp Rookie

    Stiffer rebound leads to a slower extension of the spring. Softer rebound to a faster extension.

    I'll try to explain how the dampers affect handling in ACC as easy as I can. If you want to know how they technically work, there is plenty of good stuff out there.
    As far as setting them up in ACC is concerned, you can think of them as added springrate on each individual wheel, if and only if the spring on that wheel is undergoing compression or extension.
    The dampers used on the GT3 cars also distinguish between fast and slow damping, which comes into effect depending on how fast the force is put into/taken out of the spring.
    Generally speaking the slow damping is in effect for driver inputs (turning/braking/accelerating) and fast damping deals with bumps and curbs.

    Since different manouvers put forces predominantly onto a specific pair of springs, you can finetune handling in different phases of a turn depending on what springpair is doing the most work in the situation.
    For example turning into lesmo 1 at monza and applying brake at the same time means the front left spring goes into compression and the rear right spring goes into rebound. The other 2 springs are negligable in this situation, because the forces from turning and braking cancel out. In this case, if you are happy with your overall car balance, but the car starts understeering heavily in this situation, softening the front bump or stiffening the rear rebound can improve handling, without affection steady state balance.
    Another example would be improving bad traction out of slow corners in a front engine car and soft rear springs. Since the soft rear springs delay the weight transfer to the rear axle, but the tire generates more grip with more load on it, they make it difficult to put power down. Increasing the rear bump, can help shift the weight to the back quicker to improve acceleration without screwing over overall cornering balance.

    Now the fast damping deals with curbs and bumps. If your only concern is clearing curbs, running as soft fast bump as possible without the car bottoming out is beneficial. Reason being that this allows the spring to compress more, absorbing more energy and therefore pressing the tire harder into the surface. The fast rebound should be set slightly stiffer. This prevents the stored energy to be released to quickly and the car from "pogo sticking" of the ground.

    Usually there is an adjustable speed at which a damper switches from slow to fast damping. However blancpain apparently doesnt allow this adjustment, so this setting is fixed for us. While testing I found something pretty interesting. The transition speed is set really low in ACC. This means that the dampers switch to fast damping rather early. When it comes to handling it means that fast driver inputs, instead of being handled only by the slow damping value, also are affected by the fast damping. To go back to the lesmo 1 example. The front left compresses violently enough for the damper to go into fast bump quickly before it slows down and the slow bump takes over.

    Knowing this leads to 2 things. It's not possible to setup the fast damping only based around dealing with curbs and bumps (which is rather annoying). But it opens up possibilities to finetune the handling in transitional manouvers even more (which is nice to have).
  6. Fast and slow are descriptions of the rate of movement in the shock. Slow happens in transitions i.e. braking, cornering, and accelerating. Fast would be driving over a curb, (this is a very simple description) as far as the dynamics of dampers that is a study that can consume many hours.
  7. iVG

    iVG Alien

    You clearly don't understand how damping works. Nimp is right there.

    It would help if you have a spring/damper setup from a bicycle so you can visualise the forces. Dampers only slow the movement of the spring up and down. They cannot hold the weight of the car. During compression of the spring the bump value is taken into account which is why it's called Bump. Basically any compression of the spring will be counteracted by the damper to some extend. After compression the logical next step is extension of the said spring. So after the damper has slowed the compression, not it's time to decompress or extend the spring. This is where the Rebound value of the same damper comes in play. Technically you can have a very soft bump and very stiff rebound damper which will mean that the spring will compress freely and very quickly but then will be slowed down by the damper during extension. This would work the other way around too, a very stiff bump damper would not allow the spring to compress quickly but once that force goes away - it will almost immediately allow it to extend back. This is just hypothetical scenario so you can visualise that is bump and what is rebound.

    Now that we know that stiffer bump counteracts compression of the spring and rebound counteracts extension we can clearly see that the higher value of bump simply means stiffer bump (spring is counteracted on compression) and higher value of rebound means stiffer rebound (spring is counteracted on extension).

  8. Max Doubt

    Max Doubt Racer

    Ahh sure, thanks guys is this correct?

    Higher value = stiffer (harder to compress/less absorption, slower to rebound).
    Lower value = softer (easier to compress/more absorption, faster to rebound).
  9. Serge M

    Serge M Alien

    Not quiet as bump and rebound are seperate so high value would be resistant to compression and quick to rebound while low value would be easier to compress (softer) and slower rebound
  10. Max Doubt

    Max Doubt Racer

    Thanks Serge, I think thats the first thing I wrote when I confused the rebound aspect - as the others pointed out, a softer(lower) setting to rebound allows the overall decompression to occur faster as it refers to the force preventing the return movement (I think? :confused::rolleyes:).
    Easy to confuse the wording with the inverse relationships, as I did o_O:).

    A clearer summary may be;
    Higher value = stiffer (harder to compress/less absorption, so less readily able to both compress and rebound).
    Lower value = softer (easier to compress/more absorption, so more readily able to both compress and rebound).
    Last edited: Mar 16, 2019
  11. Serge M

    Serge M Alien

    Think of it all less in the sense of the shock itself but more in the sense of vertical wheel movement. If you run over something like a curb at a fast speed the wheel moves up quickly (bump) but then when go over that curb and back on to the tarmac that wheel needs to come back down quick enough to keep in contact with the surface (rebound).

    Say if you are going over a curb at 40km/h the wheel won’t get a sudden shock (relatively) up and down so soft settings will work as they will be fast enough to react but if you run over that same curb at 200km/h the wheel will need to react faster, especially on the rebound as it will need to come back down at a much faster rate to stay in contact with the track. If the fast bump in that scenario is too stiff the wheel will bounce up over the curb and lose contact, but if it’s not stiff enough at the other side of that curb it will be in the air instead of being pushed back down quickly and hitting the ground
  12. iVG

    iVG Alien

  13. Serge M

    Serge M Alien

    How so?
  14. iVG

    iVG Alien

    High value on bump would be resistant to compression and high value on rebound would be resistant to extension of the spring. The value purely means stiffness of the damper or how resistant it will be to movement. The only difference between bump and rebound is that hey counteract opposing forces.
    Freddie Seng, Aristotelis and Serge M like this.
  15. Serge M

    Serge M Alien

    Makes sense
  16. iVG

    iVG Alien

    Yeah I know it's simple just need to wrap your head around it. Once you do that it's simple.

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