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Rebound philosophy

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Hello,

Lately I have been pondering rebound damping.

I would like to open a technical discussion regarding the rebound damping curve for forks and shocks.

What do you look for in your rebound damping?

How do you know your Low medium and high speed rebound are correct?

obviously a fairly linear curve might be an obvious starting point, But it seems that two stage rebound stacks are most common.

Its hard for me to put into words what I look for In rebound damping.

From a riding standpoint, on a dirt bike 

I don't want it to kick or bounce from to little damping, I have noticed at the extreme a lack of rebound on a fork can actually make a rebound event feel like an additional impact event through the bars.

I don't want it to pack from too much rebound damping, Obviously I don't want the bike to ride in a packed down condition reducing travel for the next obstacle.  This also makes a bike feel harsh, riding deeper in the progression from the forks air chamber, and the shock in the firmer part of the linkages' action with less travel available.

From handling standpoint,

Too fast rebound up front may tend to make the bike run wide as the fork nearly instantly extends when letting off the front brake/getting on the throttle

Too slow rebound up front allows the bike to turn in a more settled fashion but gives away grip

Too fast rebound on the rear allows the rear of the bike to hike up and just feels awkward

Too slow rebound on the rear an, there is a lack of traction.

I had more to add but im out of time for now.

 

 

 

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6 hours ago, EnglertRacing said:

Too slow rebound on the rear an, there is a lack of traction.

For me, this is the stand out statement from your post.  Albeit it a little simplified.

In the early days of dyno testing, the single biggest improvement I made was to increase low speed rebound damping in the shock.  This made a massive improvement in traction on smoother surfaces. The bike now hooks up and drives when other bikes are breaking traction. The chassis has a much more planted feel, resulting in more consistent steering characteristics.

But this increase in low speed rebound damping creates two issues. 

1. Due to crosstalk of the rebound adjuster, it also increases low speed compression damping (well actually it increases overall compression damping, but has the greatest effect on LSC as a percentage), resulting in a reduction of initial comfort. I think this is the biggest reason people have been reluctant to screw their rebound adjusters in, in the past.  But if you can tune the shock to have an ideal compression curve with the rebound adjuster screwed in, then it is a win win situation.

2. Increasing the low speed rebound damping, increases the overall rebound damping.  Like you've mentioned in the quote above, too much overall damping can result in a reduction of traction, but you failed to mention that this happens in rough terrain over a series of multiple bumps.  So if you increase the LSR damping to improve traction on smoother surfaces, you need to make a corresponding reduction in HSR to prevent packing in bumpy terrain.  

Edited by dwb79

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9 hours ago, dwb79 said:

For me, this is the stand out statement from your post.  Albeit it a little simplified.

In the early days of dyno testing, the single biggest improvement I made was to increase low speed rebound damping in the shock.  This made a massive improvement in traction on smoother surfaces. The bike now hooks up and drives when other bikes are breaking traction.

Now this is exactly the opposite of what I've learnt. Less rebound at any speed allows the rear to follow the ground more closely, and prevents unloading the tire on even mild changes in surface level which means more grip.

Same thing on the asphalt, which is arguably the smoothest surface (if you don't count ice).

I like to run only just enough rebound to keep it all under control.

So, minimal very low speed (bleed hole/adjuster), enough low speed to control the chassis (brake and acceleration lift) and just enough high speed to control the rear from 'kicking' too much (spring). A slightly digressive curve.

Basically the same in the front too.

If the front rebound is too light, it makes the front feel 'busy' like it's trying to do too much all at once.

On the rear, usually the first thing I notice if it's too light is the rear kicking up off jumps.

You're right about putting it into words, this took Me ages to write. Hope it makes sense.

I'm interested in everyone elses opinion/experiences now too, especially when Myself and Dwb, seem to feel so differently.

Suspension seems to be such a personal thing, I feel for pro tuners trying to make everyone happy.

 

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15 hours ago, EnglertRacing said:

Lately I have been pondering rebound damping.

... obviously a fairly linear curve might be an obvious starting point, But it seems that two stage rebound stacks are most common.

 

A simple tapered shim stack produces a linear edge lift stiffness. On a shock the linear stack stiffness produces a digressive damping force curve. Linear coil springs on cone valves produce the same digressive curve shape.

1-digressive.png.649d33540fea2a5e2f3d51f62854a8e1.png

Putting a crossover in the shim stack vents low speed damping. Venting low speed damping makes the damping force curve “look” more linear. Stock rebound shim stacks run a crossover to get a linear like rebound curve.

2-linear.png.180b08eaecf8c7a77d0ea96f694428a4.png

But, to get a linear curve you have to use the right crossover shim diameter. Too small of a crossover and you get a progressive damping force curve. Too large of a diameter and you are back to the digressive curve shape of a simple tapered shim stack.

Optimum rebound damping

Spring-mass-damper theory defines damping response in terms of zeta. Zeta values greater than one are overdamped. Zeta values less than one are underdamped and baby-buggy after a bump.

spacer.png

Shim ReStackor response calculations print out the zeta value across the range of suspension stroke depths. To get a consistent suspension “feel” zeta values need to be constant across the stroke depth range so short strokes around race sag and deep strokes near bottoming all produce the same zeta response value. Getting a flat zeta curve is simply a matter of hacking around on the rebound stack, changing the crossover configuration, to get a flat zeta curve.

Getting a constant zeta value requires something pretty close to a linear damping force curve. But the curve needs to be corrected for the progressive increase in link ratio through the stroke and the progressive increase in gas spring force for a fork. Shim ReStackor response calculations include those effects.

spacer.png

The example above needs less rebound damping at low speed to bring the zeta value down on short 5 inch strokes and more rebound damping at high speed to get 10 inch stroke depths back on target. As shown, the setup is going to pack on small bumps at get loose on deep strokes, i.e. kick.

The fix is put a crossover in the simple straight stack used in the example. The crossover will vent low speed damping to keep the 5 inch stroke from packing. And then, use a stiffer high speed stack above the crossover to get more rebound damping at the higher speeds on the 10 inch stroke.

Figuring out what is needed is just a matter of hacking around on the stack, adding or removing shims, until the zeta curve flattens out.

Edited by Clicked
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17 hours ago, dwb79 said:

For me, this is the stand out statement from your post.  Albeit it a little simplified.

In the early days of dyno testing, the single biggest improvement I made was to increase low speed rebound damping in the shock.  This made a massive improvement in traction on smoother surfaces. The bike now hooks up and drives when other bikes are breaking traction. The chassis has a much more planted feel, resulting in more consistent steering characteristics.

But this increase in low speed rebound damping creates two issues. 

1. Due to crosstalk of the rebound adjuster, it also increases low speed compression damping (well actually it increases overall compression damping, but has the greatest effect on LSC as a percentage), resulting in a reduction of initial comfort. I think this is the biggest reason people have been reluctant to screw their rebound adjusters in, in the past.  But if you can tune the shock to have an ideal compression curve with the rebound adjuster screwed in, then it is a win win situation.

2. Increasing the low speed rebound damping, increases the overall rebound damping.  Like you've mentioned in the quote above, too much overall damping can result in a reduction of traction, but you failed to mention that this happens in rough terrain over a series of multiple bumps.  So if you increase the LSR damping to improve traction on smoother surfaces, you need to make a corresponding reduction in HSR to prevent packing in bumpy terrain.  

What type of stack is this with?

do you mean only stiffening it with less bleed?

or are you stiffening the low speed portion of the stack on a two stage stack as well?

or are you closing bleed, and softening the low speed stack resulting in a more linear damper curve.

would it be possible for you to post some graphs elaborating on this?  - not asking for anything current or proprietary, even a doodle would be great....

 

7 hours ago, DEATH_INC. said:

Now this is exactly the opposite of what I've learnt. Less rebound at any speed allows the rear to follow the ground more closely, and prevents unloading the tire on even mild changes in surface level which means more grip.

I tend to see similar effects,

Less lsr/more bleed, always seems to have more traction up to the point that its causing chassis stability issues.

7 hours ago, DEATH_INC. said:

Same thing on the asphalt, which is arguably the smoothest surface (if you don't count ice).

I like to run only just enough rebound to keep it all under control.

So, minimal very low speed (bleed hole/adjuster), enough low speed to control the chassis (brake and acceleration lift) and just enough high speed to control the rear from 'kicking' too much (spring). A slightly digressive curve.

Basically the same in the front too.

If the front rebound is too light, it makes the front feel 'busy' like it's trying to do too much all at once.

On the rear, usually the first thing I notice if it's too light is the rear kicking up off jumps.

You're right about putting it into words, this took Me ages to write. Hope it makes sense.

I'm interested in everyone elses opinion/experiences now too, especially when Myself and Dwb, seem to feel so differently.

Suspension seems to be such a personal thing, I feel for pro tuners trying to make everyone happy.

 

 

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12 hours ago, DEATH_INC. said:

Less rebound at any speed allows the rear to follow the ground more closely, and prevents unloading the tire on even mild changes in surface level which means more grip.

 

31 minutes ago, EnglertRacing said:

Less lsr/more bleed, always seems to have more traction up to the point that its causing chassis stability issues.

How are you guys changing LSR only and not changing HSR?

If you are just unscrewing the clicker, or pulling a face shim, you are changing the entire damping curve.  Therefore its hard to determine if the improvement in traction is coming from a reduction in LSR or HSR or both.

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22 minutes ago, dwb79 said:

 

How are you guys changing LSR only and not changing HSR?

If you are just unscrewing the clicker, or pulling a face shim, you are changing the entire damping curve.  Therefore its hard to determine if the improvement in traction is coming from a reduction in LSR or HSR or both.

 

Your adjuster/bleed can only flow so much, once it exceeds that your stack takes over. So you can alter these to get what you want, then shim to bring the rest back in to line.

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But anythjng you do to say low speed has a big effect on high as well 

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Crossover diameter, thickness, and position.

If you soften the portion below the the cross over you have to stiffen the stack above the cross over to retain the high speed damping and if you stiffen the stack below the cross you have to soften the stack above to keep the high speed where it was.

Edited by EnglertRacing
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17 hours ago, DEATH_INC. said:

Now this is exactly the opposite of what I've learnt. Less rebound at any speed allows the rear to follow the ground more closely, and prevents unloading the tire on even mild changes in surface level which means more grip.

Same thing on the asphalt, which is arguably the smoothest surface (if you don't count ice).

I like to run only just enough rebound to keep it all under control.

So, minimal very low speed (bleed hole/adjuster), enough low speed to control the chassis (brake and acceleration lift) and just enough high speed to control the rear from 'kicking' too much (spring). A slightly digressive curve.

Basically the same in the front too.

If the front rebound is too light, it makes the front feel 'busy' like it's trying to do too much all at once.

On the rear, usually the first thing I notice if it's too light is the rear kicking up off jumps.

You're right about putting it into words, this took Me ages to write. Hope it makes sense.

I'm interested in everyone elses opinion/experiences now too, especially when Myself and Dwb, seem to feel so differently.

Suspension seems to be such a personal thing, I feel for pro tuners trying to make everyone happy.

 

Your point about the difficulty of putting things into words that makes understandable sense to the reader just might be the most important one ever made here on TT.  Especially regarding suspension performance/action!  Very insightful!

Edited by Piney Woods
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17 hours ago, DEATH_INC. said:

Now this is exactly the opposite of what I've learnt. Less rebound at any speed allows the rear to follow the ground more closely, and prevents unloading the tire on even mild changes in surface level which means more grip.

Same thing on the asphalt, which is arguably the smoothest surface (if you don't count ice).

I like to run only just enough rebound to keep it all under control.

So, minimal very low speed (bleed hole/adjuster), enough low speed to control the chassis (brake and acceleration lift) and just enough high speed to control the rear from 'kicking' too much (spring). A slightly digressive curve.

Basically the same in the front too.

If the front rebound is too light, it makes the front feel 'busy' like it's trying to do too much all at once.

On the rear, usually the first thing I notice if it's too light is the rear kicking up off jumps.

You're right about putting it into words, this took Me ages to write. Hope it makes sense.

I'm interested in everyone elses opinion/experiences now too, especially when Myself and Dwb, seem to feel so differently.

Suspension seems to be such a personal thing, I feel for pro tuners trying to make everyone happy.

 

I'm the same I can't stand slow rebound I find any packing harsh and the bike unstable after multiple hits. The rear I have wider window of what I can put up with, for the front the window is very narrow. Yes so personal I have a mate who runs his bike low and slow and he loves it I can't ride it. I also find needle shapes to play a role here i.e a blunt KYB needle vs a tapered wp/sachs. 

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20 hours ago, DEATH_INC. said:

Now this is exactly the opposite of what I've learnt. Less rebound at any speed allows the rear to follow the ground more closely, and prevents unloading the tire on even mild changes in surface level which means more grip.

Ok, the comments about packing from slowing down the rebound, makes it appear that you guys have missed my point.  So let me try and explain in greater detail.

While the separation of low and high suspension velocities are subjective, I think we can all agree that low speed rebound velocities are going to be after small displacement movements.  High speed rebound velocities are going to be generated after high displacement when stored energy in the spring is high. Agree?

My earlier comments are focused around low speed rebound damping.  So think smooth ground, small bumps or chassis pitching. In these examples, the shock wont use much travel, so will remain around ride height and only experiencing low velocities and therefore only produce low speed damping.

We'll save the discussion about rocks, rough ground or braking & acceleration bumps for a little later.

My internal dialogue asked the question: what provides traction? I'm sure Clicked can (and hopefully will) break this down with a more scientific answer, but my simplified version (which might be completely wrong) goes like this... It's the spring pushing against the weight of the chassis that provides the downforce into the tyre, resulting in the knob penetrating the ground. Obviously the more downward force, the more the knob can penetrate the ground (or downward force of a road tyre on the bitumen) and the more traction we get. 

A bike with too little rebound damping (low speed damping because we are at/around ride height without much velocity), causes the chassis to raise.  As the chassis lifts up, there is nothing pushing the tyre into the ground.  The Trax shocks can be great in the chop, but they are the worst for finding traction when the rear partially unloads during chassis pitching movements.

If we run a little more rebound damping, so that we can hold the chassis stable, there is more chassis weight for the spring to push against and therefore it can push that knob into the ground more effectively. 

So once again, I'm not talking big bumps and high velocities.  That's where packing can occur, but that's another conversation.  I'm trying to tune traction at low velocities with increased low speed damping.  

I might have the physics behind the "why" wrong, but testing different "measured" damping forces has proven time and time again that more than average (more than most guys run) low speed rebound damping improves traction around ride height.  Make sense?

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2 hours ago, dwb79 said:

If we run a little more rebound damping, so that we can hold the chassis stable, there is more chassis weight for the spring to push against and therefore it can push that knob into the ground more effectively.   

  Make sense?

IMO it would make sense if you were adding compression low speed damping to control the chassis. Usual rebound configuration have a small cross over shim and a bleed hole, that procure light low speed rebound damping.

Some tuners designed a comp/reb low speed separator to increase compression and keep a light rebound.

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What if rebound packed, is that bad?

Packing rebound on the shock lowers the rear ride height, puts more weight on the rear wheel, which increases traction and drive out of the corners. Holding the rear end down is nice when drifting fire roads. Works, but only on a smooth surface.

A NASCAR cheat used to be heavily pack rebound to lower the car below the prerace checked ride height spec. Dynamic packing lowers wind drag and the car CG to improve cornering.

Packing rebound has some features and those features have been race proven to work – until you hit a bump. Dirt bikes are continuously on a bump or recovering from the last one - imo.

Chassis ride control

Shocks have to produce stiff low speed damping to keep the chassis from drifting around. Separate shock circuits allow that tuning to be stiff low speed compression, stiff rebound or both. 

Stiff low speed rebound has some features created by lowering the ride height, but those features cause the suspension to ride stiff even over stupid little ½ inch bumps. That shifts my focus toward stiff low speed compression damping and the idea of riding “high in the stroke” due to low speed compression damping jacking the suspension above ride height.

Controlling the chassis motions at ride height requires stiff low speed damping. That forces a choice between rebound packing or compression jacking. You have to choose one, I’m all in on compression jacking at the moment.

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Regarding the upward movement of the chassis not creating traction......this discounts the theory of equal and opposite reaction. The spring force release that extends the rear is also pushing the tyre into the ground. If the chassis is rising independant of extension of the wheel, we have a serious problem.

Rebound control has to play second fiddle to compression control and is best viewed as a reaction rather than an "independant" action. Given that the amount of rebound that occurs will depend entirely on how much compression occurs. If something compresses a long way, it will naturally have to rebound a long way.

Bikes that are too soft on compression will naturally react with large rebound movements and bikes that are too hard will fail to absorb the impact, creating a deflection. This deflection or "failure to absorb" is often mistaken as a violent rebound reaction. Deflections don't have to be sideways, they can also be and more often are, vertical.

Rebound will alter its velocity during wheel extension and passes through a number of stages. When the force of an impact (compression phase) achieves equalibrium with the combined resistance of spring, damping, friction, a momentary cessation of movement occurs. The wheel will now begin to accelerate from a dead stop along its path back to full extension. Acceleration will build and then reduce as the stored energy in the spring dissipates......stop, low speed, high speed, lower speed, stop. If the wheel is off the ground it will be the top out mechanism that halts momentum.

A great deal of potential rebound issues will be eliminated simply by getting your compression dialled first. If you still have a rebound issue after your compression is perfect, at least you will now be in a position to correctly assess exactly how big your rebound problem is.....or isn't.

What we should be seeking now is the best blend on traction and control. A freely extending wheel can reduce traction if it is not adequately controlled.....too much control and we limit the tyre's ability to trace the ground....also reducing traction. Whether you choose to utilise adjusting circuits or shimstacks (single or multistage) is up to you.

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An open rebound bleed on the shock feels nice when you're cruising along, and standing up.  Then, when you REALLY juice the throttle over bumpy terrain, it feels like wild uncontrolled shit and you have to chase the rear end of the bike around.  Torque and weight transfer are the big components here.  I've ended up running settings on the rear end that feel horrendously stiff and slow compared to a conventional offroad setup, but they let me drop the hammer.  The instant rebound response is very necessary for chassis control while under load.  And frankly, it behaves better and more level under gnarly deep braking chop too. 

I don't know why there's this convention to leave the rebound clicker super wide open then rely on a bunch of stiffer stacks to hopefully save you after the rebound motion has already begun.  Seems silly.  A super active rear suspension only works well if you never see bumps that are more than about half the height of your available wheel travel.  Beyond that height, the thinking needs to be about very high energy inputs that need to be absorbed, not complied to.

Edited by GHILL28
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4 hours ago, GHILL28 said:

An open rebound bleed on the shock feels nice when you're cruising along, and standing up.  Then, when you REALLY juice the throttle over bumpy terrain, it feels like wild uncontrolled shit and you have to chase the rear end of the bike around.  Torque and weight transfer are the big components here.  I've ended up running settings on the rear end that feel horrendously stiff and slow compared to a conventional offroad setup, but they let me drop the hammer.  The instant rebound response is very necessary for chassis control while under load.  And frankly, it behaves better and more level under gnarly deep braking chop too. 

I don't know why there's this convention to leave the rebound clicker super wide open then rely on a bunch of stiffer stacks to hopefully save you after the rebound motion has already begun.  Seems silly.  A super active rear suspension only works well if you never see bumps that are more than about half the height of your available wheel travel.  Beyond that, the thinking needs to be about very high energy inputs that need to be absorbed, no complied to.

Hard to make an assessment of this without knowing how much bleed vs how much shim force we are talking about. Generally we are talking about a very small amount of oil that is passing through the adjusting circuit and this flow is "greatly" affected by velocity. Whilst the adjusting ciruit is always open and always flowing....that flow is limited by its own dimensions. If your shimstack configuration doesn't comply well with your demands then you will always be in trouble when the transition from "bleed to stack" occurs. If you think about the adjuster as a timing mechanism it really helps with set up. The more closed the adjuster is, the faster the shimstack comes into play and the more oil is passing through it.....but not much more. The wider the adjuster circuit, then the transition is slightly later. Your shimstack design will also determine how smoothly this transition occurs. This is perhaps where I would look first given your feedback.

Cross sectional area

2mm bleed = 3.142 square mm  (small to normal)

3mm bleed = 7.069 square mm (quite large)

50mm shock body 1963.75 square mm    - 2mm bleed - 16mm shock shaft = 1759.52 square mm        ( - 3mm bleed 1755.593 square mm)

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