High Speed Rebound, needless? 1

[apex944]

In an automotive racing application where would you find a use for 'high speed' rebound? Other than suspension failure I'm not sure I understand where 'high speed' rebound would come into play and therefore dont understand the need for 'high speed' rebound settings. Sales gimmick? Are 3 way dampers therefore sufficient?

 

[GregLocock]

I don't know. Given the low shock absorber velocities seen in most forms of circuit racing,compared with road cars, I guess you'd have to quantify what you mean by 'high speed'.

I would think that the behaviour after running over the kerb at the apex is probably the most relevant manouevre for the circuit cars, which is definitely a race winning tuning feature in some series.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[apex944]

Exactly. Running over the curb is the reason for high speed bump, however the rebound from this impact is not high speed. Therefore I'm starting to think there is no need for a 4-way damper.

 

[GregLocock]

Are you saying there is no advantage in getting the wheel back onto the track, or that this is what you do anyway with the low speed setting?



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[apex944]

Of course you want the wheel to return to the track ASAP. However as it returns it will not be utlizing 'high speed' rebound damping and will be damped by low speed rebound.

What I'm really asking is under what conditions can the rebound speed possibly be higher than what the spring rate will allow? Perhaps I'm missing that scenario.

 

[GregLocock]

Interesting way to look at it, what sort of velocity is that?

crudely, say 1g into jounce, say u/s is 10% of body weight, so wheel accelerates down at 11g, so in say 100mm v^2=2*9.81*.1,

v~1400 mm/s



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[JOJOMO]

Greg,

I don't really understand your calculations. Not to say they are wrong, I just don't follow. Could you elaborate a little?

John

 

[NormPeterson]

V^2 = 2*a*s?

 

[GregLocock]

yes, but it looks like I screwed up

a=11g (overestimate)

s=.1

v^2=2*a*s=2*11*9.81*.1

v~ 4500 mm/s

much better. now, that is an overestimate, but does at least get us into the range of shock absorber velocities seen on rough roads, and running over a kerb and off again is very like some rough road events.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Fabrico]

You need high speed rebound when there is a very quick maneuver, a wheel is off the ground, or a significant bump has upset the sprung mass to a point where its velocity is enough to send it skyward.

It is always slightly more effective to keep wheels in contact with the ground than it is to try and stop the upward movement of sprung mass with unsprung mass. A rebound rate that is too slow will be good at neither, and will more often have the wheel rebounding in the air.

 

[GregLocock]

I still got it wrong. If the wheel is 1 g into jounce, then the spring force is twice the body weight. If the unsprung mass is 20% of the body weight (a more likely estimate of the ratio for a racer) then the initial downward acceleration of the wheel is 10g

However, that won't be for long, so it is an overestimate.

v^2=2*a*s=2*10*9.81*.1

same answer as before, obviously.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[apex944]

>> You need high speed rebound when there is a very quick maneuver

How does a "quick maneuver" cause a greater acceleration than say the wheel falling into a pot hole?

I still dont see the need.

Or are you saying that "high speed" rebound velocities are nothing near "high speed" bump velocities? Perhaps this is where I'm making an incorrect assuimption.

 

[3rdwheel]

If you are refering to the 4 way adjustable dampers the low speed and high speed adjustments are to isolate adjustment of body motions -ride, roll and pitch (low speed) and track bumps (high velocity)

 

[Fabrico]

Or are you saying that "high speed" rebound velocities are nothing near "high speed" bump velocities? Perhaps this is where I'm making an incorrect assuimption.

That's more in the right direction, "high speed" is a relative term. Ultimate speed and load between bump and rebound are very different. As far as your original question, I would leave bump out of it for now.

Shock absorbers have several jobs to do in bump and rebound. In rebound, the most common job is reducing body roll. But if while making a turn the inside wheel goes into a pot hole, its rate of extension will significantly increase. That's because it also has a high priority job of helping the wheel stay in contact with the ground.

It needs to be pointed out that this can all vary quite a bit according to application.

 

[apex944]

>> In rebound, the most common job is reducing body roll.

Correct, and this is accomplished through low speed rebound.

>> But if while making a turn the inside wheel goes into a pot hole, its rate of extension will significantly increase.

Correct, and I'd think you would want no resistence until the tire reached the ground, where bump damping would regain control.

I suppose HS rebound may just be another name for that release point. A number of the best race shock manufacturers make 4 way shocks. However none of them go into any real detail (that I've found) about high speed rebound.

I'm going to ask Claude Rouelle this when I see him next week. And I'll report back.

 

[PTwizz]

There is no hard and fast line between 'low speed' and 'high speed' damping. From a damper engineers point of view, that line is where high speed adjustment (shims, valve etc.) becomes dominant over low speed (bleed) adjustment.
High speed rebound will, as suggested above, control wheel descent, particularly in recovery from kerbs etc. Although it may seem attractive to use very digressive rebound damping, excessive hub velocity will cause large contact patch load variation on 'landing'.

 

[apex944]

Thanks PT that does actually clear things up a bit for me.

>> excessive hub velocity

What creates "excessive" hub velocity in rebound?
I can't see the rebound velocity exceeding the spring's ability to uncoil from block height.

 

[PTwizz]

Bildon,

'Excessive' hub velocity is that which, combined with the unsprung mass, tyre stifness and tyre damping, causes 'unacceptable' acceleration and resultant contact pacth load variation. In simple terms, the harder you throw the wheel at the ground, the harder it hits and tries to bounce back.
What one defines as 'unacceptable' patch load variation is, of course, subjective.

 

[gt6racer2]

Hi, To add some comment, the thread is correct in that bump velocities are higher than rebound, given that the force in rebound is limited to the spring. You can calc the maximum rebound velocity easily based on geometry,spring rate/free length, travel, and approximate damper constant.
High speed rebound damping is most commonly used to control unsprung mass vibration, since this region of the damping curve has little impact on normal body motions, and hence allows control without impacting driver comfort. It is achieved by limiting the maximum flow area available by using small holes as the flow area - giving a rapid increase in damping at a defined velocity. Typically, in a road race application, the damping is so high anyway that unsprung mass control is not a big issue, and hence the high speed damping is not critical - and the damper valve will not use a high speed orifice restriction - using big ports instead of small holes. The only use for high speed rebound in racing would be off-road, where you need to slow the crash of the suspension into the rebound travel limiter.

 

[NormPeterson]

I wonder if Formula 1 is revisiting this in light of the ruling that outlawed unsprung damping masses?


Norm