A question on "Bump" and "Rebound" settings.....

[Hawk]

Do the "Bump" and "Rebound" settings actually work on GP Bikes?

I tested them at the extremes of the settings and hardly felt any difference in the handling of the bike at all. Test bike was on the Verase 500cc bike.
Now surely these settings, in co-operation with the other suspension settings, that are supposed to help keep the tyres on the track surface at all times over, undulations, bumps, acceleration and braking phases, these settings should be vital to be set correctly otherwise you would surely risk some scary moments on track?

[Hawk]

Hey..... Have I actually found a subject that has foxed you physics guys?? Hehe

C'mon someone must know something about the workings of the bump and rebound settings to comment on whether they seem to work in GP Bikes or not?

Any takers? Max?.... C21?... Neoraptor?..... anyone?

[HornetMaX]

Hehe, suspensions setup is more an art than a science, because it involves an awful lot of feedback from the rider to the engineers and we all know that talking to an engineer is not easy   (assuming the rider can express what he wants, and this too is not given).

Anyway, GPB settings do work and you can feel the difference, even on a very smooth track.

The basics, just to be sure we're all aligned.Bump and rebound are  damping settings, in fact they are nothing more than frictions within the damper (obtained pushing oil through small holes, oversimplifying). That's totally different compared to the spring setting, which is a stiffness.

A stiffness tells you how much the spring (hence the fork) will compress under a given force. E.g. a 20 N/mm spring will compress by 1mm when under 20N, 2mm under 40N and so on.

A damping tells you how fast the damper (hence the fork) will compress. E.g. 20N/mm spring + damper under 20N will always end up compressed by 1mm, no matter the value of the damping. However, if damping is low, you'll reach the 1mm compression quickly while if it's high you'll ned more time to reach 1mm.

So small bump value ==> the fork will compress quickly, high bump value ==> the fork will compress slowly. Same for rebound, replacing "compress" with "extend".
Example a fork with high bump but low rebound will compress slowly (passing over a bump or under front braking, for example) but will extend quickly (passing over a pothole or when opening the throttle).

On many modern bikes you not only have separate settings for bump (compression) and rebound (extension), but you also have separate settings for low/high suspension speeds (technically, low/high flow rates). So you have 4 settings: bump low/high speed, rebound low/high speed. For example you can have a low damping for bump at low speeds (speed here is not bike's speed, it's the fork's speed) but a high damping at high speeds.

In motogp (and probaly sbk too) it's even more complex than that, so complex in fact that there are so many settings that a software tells you what to change and how, depending on what you want to achieve.

From here on, you're on your own

MaX.

[C21]

Test it on the 125cc 

To be honest i did not played /changed the settings yet that much...
Spring adjustment is definetly working , you can check that in the replay.....ahh.....asking for bump and rebound....

Example a fork with high bump but low rebound will compress slowly (passing over a bump or under front braking, for example) but will extend quickly (passing over a pothole or when opening the throttle).

in the worst scenario: at constant speed the fork will stuck deeper and deeper because of the low rebound if you´re driving upon a rough surface...imho 

[Hawk]

Hi Max.

This is very interesting information!

So on a ruff/bumpy track, if I set the springs pretty stiff, the bump high(to absord the bumps better), and the rebound high(to extend fast after hitting bumps), I should have better grip and handling? Do I have this correct?

Edit: Or is it with low rebound for this setting?

[Stout Johnson]

in the worst scenario: at constant speed the fork will stuck deeper and deeper because of the low rebound if you´re driving upon a rough surface...imho 

YES! that is sth you have to be aware of ... not many think of that

[HornetMaX]

So on a ruff/bumpy track, if I set the springs pretty stiff, the bump high(to absord the bumps better), and the rebound high(to extend fast after hitting bumps), I should have better grip and handling? Do I have this correct?

Hmm, no

High bump ==> the damper will respond (i.e. compress) more slowly to a quick road bump than with low bump: the height variation will hence be "transmitted" more to the chassis (than in the case with a low bump setting).

In general a softer spring and a low bump and rebound will allow your wheel to "follow" the track bumps better. The ride is more comfortable (that's important for road bikes).
The problem of course is that if the setup is too soft, the fork may bottom out (reach its maximum), especially under hard braking (which is bad, leading to poor braking performance).

For the rear, it is more or less the same, minus the braking constraint. As a general rule, a softer (low bump/rebound) rear will give you better traction.

Globally however, a harder bike will be more precise while a softer one will be less.

Overall, it's a big trade-off: precision (better if hard), road following (better if soft), avoiding bottoming out (better if hard) etc.

Spring stiffness will also alter the riding height (and the natural pitch of the bike).
Bump/rebound settings also alters the bike behavior entering/exiting a corner (over/understeer, even if using understeer to describe the bike widening the trajectory when opening the throttle out of a corner is a bit of an abuse).

MaX.

[Stout Johnson]

if I set the springs pretty stiff, the bump high(to absord the bumps better)

this is incorrect - in general low bump damping and rather soft springs should absorb surface irregularities better - but it should NOT be too soft, otherwise the effect is reversed as the spring will just hit bottom and therefore have no absorbing capability left... so why don't riders not run with soft settings all the way? --> because rather soft suspension, esp. in the fork, makes the turning feel slow and irresponsive - and that is sth you want to avoid when running a race bike so riders in general seek a rather hard suspension setting but have to find a compromise on rough surface....

PS: the master was faster

[HornetMaX]

PS: the master was faster

Just because the springs below my (mechanical) keyboard keys are stiffer than yours

MaX.

[Hawk]

@Max/Stout: Hehe.... I'm determined to get my head around this, so in layman's terms:

• Rebound: Low value setting = Faster extenstion of the suspension after hitting a bump
                  High value Setting = Slower extention of the suspension after hitting a bump

• Bump: Low value setting = Faster compression of the suspension after hitting a bump
             High value setting = Slower compression of the suspension after hitting a bump

Thanks for bearing with me guys. Hehe

[Stout Johnson]

PS: the master was faster

Just because the springs below my (mechanical) keyboard keys are stiffer than yours

MaX.

@ MaX: LOL! 

@Hawk: yes

[Hawk]

PS: the master was faster

Just because the springs below my (mechanical) keyboard keys are stiffer than yours

MaX.

@ MaX: LOL! 

@Hawk: yes

Hooray!!

Thanks guys, I can get my head around it thinking of it in that way.

Now I'll go put my head on ice. Hehe

[HornetMaX]

• Rebound: Low value setting = Faster extenstion of the suspension after hitting a bump
                  High value Setting = Slower extention of the suspension after hitting a bump

• Bump: Low value setting = Faster compression of the suspension after hitting a bump
             High value setting = Slower compression of the suspension after hitting a bump

Nicely put Hawk_UK, I may even re-use your wording somewhere !

MaX.

[Hawk]

Thanks, Max. Please do.

[Stout Johnson]

MaX, I think we should have these threads "featured" somewhere - they might be of interest to many - especially maybe guys new to motorcycle sims. Maybe you could feature these kind of threads under section "Setups"? (or some kind of "physics discussions")