Rebound Development?

kidwoo · Sep 23, 2004

Tame Ape said:
Question: I posted this initially in 'To the bottom' thread.

Why has no one developed high-speed/low-speed/position sensitive/etc... rebound control?

It seems like it would be a no-brainer to prevent and OTB situations in kinetically charged moments. ie: drops to flat, missed trannies, uphill trannies...

If a bike stayed hunkered down for a split second longer when fully bottomed instead of rebounding immediatelly would a rider have more control over the bike in a clutch situation?

I think the problem with that would be how would a shock differentiate between a big drop that should rebound slowly and a big hit on a trail that should rebound normally to be ready for more obstacles?

Bulldog · Sep 23, 2004

I'm no expert so it was explained to me in laymans terms, but Darren said my PUSHed Vanilla RC has some sort of high/low rebound system.

kidwoo · Sep 23, 2004

Bulldog said:
I'm no expert so it was explained to me in laymans terms, but Darren said my PUSHed Vanilla RC has some sort of high/low rebound system.

So what are you waiting for? Go jump off of a roof onto your driveway and see what happens.

Repack · Sep 23, 2004

I think that valving/shim stacks already do that to some extent, at least on the more expensive forks. I know that my SHiver doesn't feel like it rebounds much faster at the very bottom of its travel vs. the mid-point. I think that the main problem is that once you've bottomed you fork and moved you center of gravity forward, your done. Plus, I thin that most of the rebound energy generated in otb situations comes from you body bouncing of a bottomed fork/shock- there is nothing left to absorb the impact, so you go over.
But, myslef and other 5e riders I know feel that the r damping of the shock is progressive. IMy friends and I have gotten into "trouble" while riding and it seems like the shock knows when to stay bottomed for just a fraction of a second. I have cased some pretty big jumps pretty badly and it feels like the shock is rebounding slower. I know 1 jump at Sunday River that would have sent me otb if I were on a Fox. But I am now thinking what I said earlier about bottoming a shock harshly could be responsible for this 5e trait. Slower compression => your inertia slows down more before bottoming=> no otb.

Oh well. My brain hurts.

zedro · Sep 23, 2004

Repack said:
I think that valving/shim stacks already do that to some extent, at least on the more expensive forks.

yeah, where ever you see the term 'shimmed rebound circuit', basically you are getting speed sensitive rebound damping. Also this goes with basic damping theory, where for compression you want high damping rates at low speed and low at high speed, and the opposite for rebound (check out any vehicle dynamics book and they'll show you a neat little graph.

But also, the spring rate (not damping rate, since linkage systems can seperate those characteristics) can also help alot. For example on my bike, the spring rate (or leverage) is so low between top out and sag that less rebound can be used without getting bucked (although it still can feel like crap). In fact, with a 8.75/2.75 shock at 9" of travel, i'm running a 400# spring (i'm 160lbs) and the bike will sag under its own weight but still give me 2.75-3.0" of sag when weighted. So having less relative spring force on the suspension is like having relatively more rebound damping force.

zedro · Sep 23, 2004

Tame Ape said:
Okay, I understand all that. So a position sensitive regressive/normal rebound curve already exists? On forks as well? My most recent OTB was 100% fork related...

the damper itself is not position sensitive, and it doesnt necessarily need to be since the coil force vs. travel is a constant, unlike in the compressive direction where the ground cary vary the input forces at any point in the travel. Since the coil force is always known and the sole factor in rebound, the rebound scheme can be simplified, hence single generic rebound adjustments can work very well if its well designed in the first place. Of course experienced suspension techs may want to further tune the rebound curve for specific terrain, but i think this would go beyond most peoples needs or tuning abilities.

Currently Bored · Sep 23, 2004

I hate being on the 'rebound'... (jk)

Currently Bored · Sep 23, 2004

I'm sorry Tame Ape and everyone else! *Covers face with jacket, and runs off*

Currently Bored · Sep 23, 2004

Tame Ape seems to be lacking a soh.. Or are my jokes that bad?/

zedro · Sep 23, 2004

Tame Ape said:
Do you think there is applicabilty for a rebound 'stall' function in the event of a massive and complete compression. Not so much a bumper...

not sure what you mean, but no anyways

a properly designed (and tuned) rebound damper will function properly no matter what the condition, since the spring rate is a constant variable. Basically you want a fast initial response to get the suspension back to where it should be (ie. low damping rate at low speed or initial extension) but then you want to avoid an 'overshoot' where the suspension is moving too quickly and has too much energy which cause instability and bucking (ie. high damping rate at high speed). Basically you want to hurry up to slow down.

Having a progressive linkage (or spring rate) helps this and may allow for lower rebound settings so you can have that fast initial response (depending on damper design).

OGRipper · Sep 23, 2004

Zedro, glad to mentioned spring rate, and curious for your thoughts on this: Isn't the rebound rate inversely proportionate to the compression rate? Doesn't "shock rate" as commonly used refer to the compression curve? If so doesn't it follow that rebound will have the reverse rate curve, assuming a linear spring rate?

So for example, does a bike with a falling initial compression rate have a rising rebound rate? And does that make a bike more likely to buck a rider on hard hits?

The reason I ask is that my rig has a falling initial shock rate, and I feel it has a tendency to buck on hard hits unless I run more rebound damping than usual...am I on track here?

zedro · Sep 23, 2004

OGRipper said:
Zedro, glad to mentioned spring rate, and curious for your thoughts on this: Isn't the rebound rate inversely proportionate to the compression rate? Doesn't "shock rate" as commonly used refer to the compression curve? If so doesn't it follow that rebound will have the reverse rate curve, assuming a linear spring rate?

So for example, does a bike with a falling initial compression rate have a rising rebound rate? And does that make a bike more likely to buck a rider on hard hits?

The reason I ask is that my rig has a falling initial shock rate, and I feel it has a tendency to buck on hard hits unless I run more rebound damping than usual...am I on track here?

yeah that sounds about right, although offhand you'd think the rising rebound damping rate would be ideal, but i'm guessing the spring rate trumps that. And what i said is really a simplification, since everythings a balancing act between the damping rate, spring rate, the velocity and force (position). It's really a mind-f**k the more you think about it and how everything relates to each other.

What i would like to try is to get a linkage with a regressive to progressive spring/damping rate (or just damping rate rather), with the crossover point around (probably past) the sag point, so the suspension wants to stay in that region, ie. doesnt want to overshoot the sag point on rebound and wants to easily compress to the sag point from full extention. You could probably get some wicked high speed response for the small stuff without making the bike too unstable....not sure how that would affect pedalling tho.....ahhh my head...

OGRipper · Sep 23, 2004

Uh oh, if your head hurts you're either hung over or we're getting kind of serious here, and I know I'm about to get a headache too.

You bring up a good point, one I hadn't thought of, which is that the rebound damping might have a rising rate too. That makes sense but I pretty much assumed that the rebound damping was constant and that, combined with a rising rebound spring rate (a product of the falling initial compression rate), was giving me the bucking feeling. But as I think about it I can see how there are a few other things that have an impact and...yep...it hurts.

What you described sounds kind of like the concept behind the axle paths on the VPP and dw designs. I'm not sure about the spring rate curve on the dw bikes but my vp-free has a falling to rising comp rate. If the dw bikes have a rising to falling comp rate, the rebound rate (which should be the inverse I think) should have the characteristics you're talking about, at least to some extent...?

zedro · Sep 23, 2004

OGRipper said:
What you described sounds kind of like the concept behind the axle paths on the VPP and dw designs. I'm not sure about the spring rate curve on the dw bikes but my vp-free has a falling to rising comp rate. If the dw bikes have a rising to falling comp rate, the rebound rate (which should be the inverse I think) should have the characteristics you're talking about, at least to some extent...?

the VPP modelling i had showed a rising to falling rate, where you want a falling to rising. With a rising to falling, you'd want a progressive shock like the 5th.

And i'm thinking having both links activate the shock might be the key to getting a falling to rising rate...i was fooling around with the concept but couldn't get a decent physical layout, so i havent explored what kind of rates you can get.

shock · Sep 23, 2004

Ok, I want in on this, as this is a subject near and dear to my heart, but be forewarned, my responses will tend to be long winded and tedious (what's new?)

First virtually ANY shock or fork is velocity sensitive. I'm talking about the velocity of the shock stroke, not the bike. That is to say, the faster the shock is stroking (in either direction) the more damping force will be produced.

The opinion that a shimmed valve stack is more velocity sensitive is incorrect. In fact the shimmed valve stack was really developed to be less speed sensitive. A shimmed stack generally has a certain "blow off" rate, after which it opens to a relatively large floaw area, preventing an extreme increase in damping force as the shock speeds up.

In comparison, an orifice damping system is more speed sensitive. An orifice is basically just a constant size hole. The faster you try to push oil thru it, the more resistance, or damping force is created. This increase is typically at an exponential rate.

If I can try to explain this, it means that for example, if you double the shaft speed, the force would increase by 4 times.

With a shim stack if you double the shaft speed, maybe the force only increases by 1.5 times. Does this make sense?

So for compression damping, simple orifice designs are usually plagued by soft initial damping (because at low speed the oil can flow thru the hole easily). But at high speeds the compression damping increases too rapidly, causing a spike, when for example the wheel hits a large square edge bump that needs all the available travel NOW to absorb, but the oil can't floaw thru the hole fast enough, causing a hydraulic lock to occur.

This can also happen with shimmed valve stacks if the stack and/or maximum flow area doesn't allow enough oil to flow. But generally this is a matter of tuning. With an orifice, if you make a bigger hole, you get no damping at all at low speeds.

I have had a lot of succes in with a shimmed compression stack and an orifice rebound valving for that reason. Generally the highest rebound velocities occur after full compression. Why? Because when the shock is compressed fully, it has the full energy of the completely compressed spring to try to comtrol. For example, if the spring is 500lbs/inch, and there is a 3" stroke, the spring is trying to extend the shock with 1500 pounds of initial force. But if your going thru a rock garden and only compressing the shock 2" then there is 1000 pounds of force trying to extend the shock, so the rebound velocities are a bit slower.

So having a higher rebound force with shaft velocity is not as bad (and can be good), while having a higher compression damping force with velocity tends to be useful only in extreme bottoming situations, i.e. landing from a jump or drop, or maybe a water bar or big "g-out".

Ok, that's all for now, I gotta eat. But I'll be back for more......

zedro · Sep 23, 2004

shock said:
First virtually ANY shock or fork is velocity sensitive. ........

of course, but for convenience sake, saying "speed sensitive" would refer to the proper (generic) damping scheme (higher speed/lower rate yadda yadda) and not the opposite. Most people are aware that spiking is an obvious increase in damping rate vs. velocity (ok maybe subconsciously at least), but would never accuse their JrTs of being speed sensitive (maybe just sensitive to speed...)

shock · Sep 23, 2004

zedro said:
of course, but for convenience sake, saying "speed sensitive" would refer to the proper (generic) damping scheme (higher speed/lower rate yadda yadda) and not the opposite. Most people are aware that spiking is an obvious increase in damping rate vs. velocity (ok maybe subconsciously at least), but would never accuse their JrTs of being speed sensitive (maybe just sensitive to speed...)

Hmmm, I'm really not sure what you mean by that. It seems that you are implying that there are dampers that have a tendency to produce a lower damping force with increased shaft velocity, or speed. I have never seen a damper that would produce this response (on a dyno for example).

That is not to say that the rate of increase is less with increasing speed, but the forces at a higher shaft speed are usually higher, no matter what.

And be careful with the term "proper" damping scheme. For rebound damping, an increasing rate of damping force can be very benificial, and in some cases, this is true with compression damping as well.

But anyway, historically, speed sensitive refers to a damping force that increases with speed. Springs on the other hand, are impervious to the shaft speed as they are purely position sensitive.

As for jr t's, they are overly speed sensitive in compression, resulting in the "spike"

Just trying to define nomenclature in the interest of further discussion.

It is possible to build a shim stack that is relatively impervious to speed, sometimes this is called "regressive" valving", where a certain force is reached, and then a blowoff function prevents the force from increasing further. But even then, these are usually "accelleration sensitive" , meaning that if the rate of speed increases to quickly, a spike can still occur.

frorider · Sep 24, 2004

zedro is using 'speed sensitive' in the everyday sense that is commonly found in, say, Marzocchi literature. in other words, if you pick up the phone and talk to a marz. tech, they will tell you that a 'speed sensitive' shim stack damper is superior to the simple hole-in-a-shaft designs, for the reasons you've described above.

until reading Shock's post, it hadn't occurred to me that strictly speaking, maybe the bike industry is using 'speed sensitive' in the the wrong way. but it's established usage, so i vote for zedro's usage.

shock · Sep 24, 2004

frorider said:
zedro is using 'speed sensitive' in the everyday sense that is commonly found in, say, Marzocchi literature. in other words, if you pick up the phone and talk to a marz. tech, they will tell you that a 'speed sensitive' shim stack damper is superior to the simple hole-in-a-shaft designs, for the reasons you've described above.

until reading Shock's post, it hadn't occurred to me that strictly speaking, maybe the bike industry is using 'speed sensitive' in the the wrong way. but it's established usage, so i vote for zedro's usage.

Well if we're going to use "marketing speak" to define technical terms, we'll be lost forever in "fasterlighterstrongertricker" and" new and improved", but I suppose it's a losing battle, like when I tried to get people to stop saying "brake jack" to describe everything that happens when you hit the brakes.

But (and I have never looked at marz's website), what, for the sake of argument is the marketing term "speed sensetive" referring to?

Whatever, a spring is a position sensitive device (force changes relative to deflection) and a damper is a speed sensitive device (force changes relative to speed). It's really just the physical nature of those devices.

But that's not my point anyway, just trying to lay groundwork for future discussion, because it starts to get complicated from here.....

Tomorrow, rising/falling rates and the effects on damping, and later, if anyone has the patience, how we address the problems that caused this topic to be posted in the first place.

More later......

frorider · Sep 24, 2004

sorry, i can't help laughing at the irony here. people like zedro and me tend to correct those who call a kona-type frame a '4 bar' when it's really a single pivot chainstay w/ a linkage-activated shock.

so rest assured, you don't have to use marketing speak.

in a perfect world, the hole in a shaft designs would be called "overly speed sensitive" or something like that, and shim stack designs would be called "speed stable" or something that sounds positive.

but for whatever reason it didn't turn out that way. i suspect that because 'position sensitive damping' is a good thing, then naturally enough people also talked about 'speed sensitive' as a good thing. rather than 'speed insensitive' or 'speed-neutral' or 'speed stable'.

okay enough about this.

zedro · Sep 24, 2004

shock said:
Hmmm, I'm really not sure what you mean by that. It seems that you are implying that there are dampers that have a tendency to produce a lower damping force with increased shaft velocity, or speed. I have never seen a damper that would produce this response (on a dyno for example).

no no no...i said damping 'rate', not damping force. The generic 'proper' damping scheme has higher damping rates in the low frequency area where the natural frequency is located, and lower damping rates in the higher frequency area....this is what i mean by speed sensitive in the relevant (desirable) context, not the generic one.

Jm_ · Sep 24, 2004

frorider said:
people like zedro and me tend to correct those who call a kona-type frame a '4 bar' when it's really a single pivot chainstay w/ a linkage-activated shock .

Ah, but it is a 4-bar. It has 4-parts, hence, 4-bar. Mainframe, seatstay, chainstay, and linkage.

Specialized's marketing department would have you believe that only they make a "4-bar" bike.

Specialized makes a horst link or "FSR" type bike, but you can easily call nearly any linkage bike a 4-bar, because it needs to have those 4 components to function. Even a giant DH bike has 4 parts, two linkage parts, swingarm and mainframe. You can jumble them up all you want, but the term "4-bar" was coined to describe linkage bikes. Specialized jumped on it and put out propaganda to the effect that "only we make a 4-bar bike, and everyone else is lying"....

zedro · Sep 24, 2004

Quote:
Originally Posted by frorider
people like zedro and me tend to correct those who call a kona-type frame a '4 bar' when it's really a single pivot chainstay w/ a linkage-activated shock .

naw, i correct those that say FSR = 'true 4 bar'....uhhg

but i prefer the single pivot/link activated dealy

frorider · Sep 24, 2004

Jm_ said:
Ah, but it is a 4-bar. It has 4-parts, hence, 4-bar. Mainframe, seatstay, chainstay, and linkage.

Specialized's marketing department would have you believe that only they make a "4-bar" bike.

Specialized makes a horst link or "FSR" type bike, but you can easily call nearly any linkage bike a 4-bar, because it needs to have those 4 components to function. Even a giant DH bike has 4 parts, two linkage parts, swingarm and mainframe. You can jumble them up all you want, but the term "4-bar" was coined to describe linkage bikes. Specialized jumped on it and put out propaganda to the effect that "only we make a 4-bar bike, and everyone else is lying"....

actually it wasn't just SPecialized, there was a period when 'true' 4 bar was used by many to refer to ellsworth and turner as well. i.e. 'true 4 bar' became shorthand for 'horst-link 4 bar'. another example of how language evolves and devolves. anyway we don't need to bore each other with this when we all know it all anyway. did you honestly think i didn't know how to count to 4??

Jm_ · Sep 24, 2004

frorider said:
did you honestly think i didn't know how to count to 4??

no, but specialized would prefer if you count to 4 in their way

zedro · Sep 24, 2004

Tame Ape said:
Sweet! A highly useful and articulate answer (not that Zedro's weren't).

Is this 'orefice'/hole system a standard item in most rebound circuits? What you described is basically what I was wondering about. In the event of a massive compression, the resulting hydraulic lock would generate a stalled response from the rebound circuit. It actually seems fairly simple to implement too, a Marzocchi SSV damping circuit transposed into the rebound guts...

edit: thought Shock was from Push not Shock Therapy

you wouldnt get a stalled response because the shaft speeds starts from zero (ie no damping force) and accelerates from there when rebounding. What it could do (just like compression spiking) is mess up the high frequency response.

Alloy · Sep 24, 2004

Tame Ape said:
Question: I posted this initially in 'To the bottom' thread.

Why has no one developed high-speed/low-speed/position sensitive/etc... rebound control?

It seems like it would be a no-brainer to prevent and OTB situations in kinetically charged moments. ie: drops to flat, missed trannies, uphill trannies...

If a bike stayed hunkered down for a split second longer when fully bottomed instead of rebounding immediatelly would a rider have more control over the bike in a clutch situation?

I really dont think the rebound is the problem. Your spring alone doesn't have the energy to fire you over the handle bars. Its the whip like motion throwing you off. Your rear goes to the bottom first then as the fork compresses the rear is rebounding using your own momemtum to throw you off. The solution is to add more compression damping to slow the rear down. And thats just what shocks are doing. Look at the DHX, the bigger shaft will give you more oil displacement for a better compression damping.

Does that make any sense?

zedro · Sep 24, 2004

Tame Ape said:
Hmm, you are correct, but a damper reacting specific to the position of the shaft could be included. ie when the shaft is in/near the bottom-out position in an emergency the 'stall' circuit can be activated to allow for rebound circuit spiking. That is possible right? I don't know much (anything) about how much real estate this stuff takes up in a shock body.

you dont want a 'stall' to happen because the energy is still stored in the spring and has to be released at some point anyways; what you want is a smooth and controlled release of the spring energy, with little resistance in the initial acceleration (low velocity) and relatively higher resistance when the shaft reaches higher velocity (around the mid-stroke), ie: velocity progressive rebound damping. Getting bucked otb after a full compression is because the rebound damping isnt doing enough work as it approaches topout.

Kornphlake · Sep 24, 2004

Where's damn true to call you Net-Spurts and E-speculators. This is a technical discussion among E-ngineers after all. :rolleyes:

I still maintain that in the case of the picture that started this all, the guy got bucked not because of his suspension but because of where and how he landed. When he hit the ground the bike wanted to stop, the rider however due to the nasty laws of physics decided to moving with or without the bike.

I think a hardtail rider would have had the same type of crash, the damper may have amplified a problem but it certainly wasn't the cause.

zedro · Sep 24, 2004

Kornphlake said:
Where's damn true to call you Net-Spurts and E-speculators. This is a technical discussion among E-ngineers after all.

I still maintain that in the case of the picture that started this all, the guy got bucked not because of his suspension but because of where and how he landed. When he hit the ground the bike wanted to stop, the rider however due to the nasty laws of physics decided to moving with or without the bike.

no need to be insulting. Anyways...

i agree that its rarely 100% equipment mal-tuning, but if you get bounced foward, thats the spring returning the energy of a drop or whatever; the other half of it is bad rider positioning/bike orientation. If ever you've set your rebound too lightly, you can get bucked from just normal trail riding, although not necessarily catapulted otb.

didnt know we were speculating about a specific situation, was talking generalities.

Rebound Development?

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