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High Speed Rebound, needless?

High Speed Rebound, needless?

High Speed Rebound, needless?

(OP)
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?

RE: High Speed Rebound, needless?

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.

RE: High Speed Rebound, needless?

(OP)
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.  

RE: High Speed Rebound, needless?

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.

RE: High Speed Rebound, needless?

(OP)
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.

RE: High Speed Rebound, needless?

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.

RE: High Speed Rebound, needless?

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

RE: High Speed Rebound, needless?

V^2 = 2*a*s?

RE: High Speed Rebound, needless?

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.

RE: High Speed Rebound, needless?


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.

RE: High Speed Rebound, needless?

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.

RE: High Speed Rebound, needless?

(OP)
>> 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.

RE: High Speed Rebound, needless?

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)

RE: High Speed Rebound, needless?

Quote:

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.

RE: High Speed Rebound, needless?

(OP)

Quote:

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

Correct, and this is accomplished through low speed rebound.

Quote:

>> 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.

RE: High Speed Rebound, needless?

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'.

RE: High Speed Rebound, needless?

(OP)
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.

RE: High Speed Rebound, needless?

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.

RE: High Speed Rebound, needless?

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.

RE: High Speed Rebound, needless?

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


Norm

RE: High Speed Rebound, needless?

(OP)
BTW, after speaking with Claude Rouelle the explanation that PTWizz gave earlier is what we came up with as well.

RE: High Speed Rebound, needless?

Bildon, with the information received from PT and Claude, what will your decision be WRT the dampers and why?

RE: High Speed Rebound, needless?

In many high speed/ high downforce style racecars with significant suspension travel (ie. more than 1" total) mechanical grip is hugely diminshed if the dampers do not have any high speed rebound force.  Looking purely at the contact patch hysteresis a tire sees, using only HS compression damping can cause very large accelerations in the rebound direction (and very large forces in the compression direction) and the tire load variation becomes worse.  There may not be potholes on most racing circuits (of course there are some tracks that pretty much do) but seemingly slight disturbances in the surface profile become very exagerrated when struck at 210mph.  Tire vibrations can be damped in both the rebound and compression directions, but using only HS compression (or only HS rebound) will result in force spikes larger than if you had a reasonable distribution of both...and thus less overall damping force in each direction.  A mass/spring/damper system vibrating would show a similar effect.

Excessive HS compression will also transmit larger forces from bumps to the sprung mass, which can upset the racecar as well as the driver.

I think that many open-wheel racecars depend wholly on "aero grip" and thus sprung mass control and tire alignment control are paramount.  HS rebound can be unnecessary, in this case.  As someone said, it is very application specific but HS rebound has more use in racing than just coming down off curbs.

RE: High Speed Rebound, needless?

(OP)

Quote:

but using only HS compression (or only HS rebound) will result in force spikes larger than if you had a reasonable distribution of both...and thus less overall damping force in each direction.

Thank you for that very good explanation.  Not many have managed to give me real reasons. Thanks!

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