[Tame Ape]

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.

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.

Yup, no reason come down on us like a librarian. My speculating was derived from, but not dependant upon, that thread. There is no reason to bring up Damn True hudder:.

Edit: Also, your buddy got bucked because the bike bottomed and then presumably bounced back up in the same manner as a HT would have. I assumed that the rebound, if controlled, could have mitigated a potiential ass jacking...

 

[MikeD]

Faster impact equals slower rebound and conversely slower impact would mean faster rebound.

Doesn't that sound like a recipe for Bender du jour? Slow impact-drop or jump, add fast rebound=catapult.

Seems like you'd want fast rebound from the fast hits...as you fly over stuff at high speed, you suspension works fast to follow the terrain and keep from packing up...then when you take a big, slow impact (landing), it's all syrupy to keep you from launching into orbit on the rebound.

No?

MD

 

[zedro]

Seems like you'd want fast rebound from the fast hits...as you fly over stuff at high speed, you suspension works fast to follow the terrain and keep from packing up...then when you take a big, slow impact (landing), it's all syrupy to keep you from launching into orbit on the rebound.

No?

MD

problem is, the damping on rebound will react the same regardless if you bottomed out from riding over a boulder or taking a drop; the problem is riding over the boulder lets the suspension extend on the other side of the boulder keeping the bike fairly stable, while on a drop the bike has no choice but to lift, helping the ejection scenario. Maybe if there was an inertia valve on the rebound circuit that slowed rebound when the bike is being accelerated upward, and unregulated during level or downward acceleration....

ok well i'm off to the patent office....

 

[MikeD]

I was riding this weekend with a blown fork damper, and as I went off a drop, the fork compressed...as I went over the edge, it extended at hypersonic undamped speed without any ground to press against. Felt like someone had reached up and grabbed my tire and yanked me down as the bike nose-dove and I flew OTB. Fun, fun. I kept imagining this zombie hand had reached up out of the earth and grabbed my wheel.

 

[Tame Ape]

problem is, the damping on rebound will react the same regardless if you bottomed out from riding over a boulder or taking a drop; the problem is riding over the boulder lets the suspension extend on the other side of the boulder keeping the bike fairly stable, while on a drop the bike has no choice but to lift, helping the ejection scenario. Maybe if there was an inertia valve on the rebound circuit that slowed rebound when the bike is being accelerated upward, and unregulated during level or downward acceleration....

ok well i'm off to the patent office....

well durrrrr to you Mr.Scientist, I'm going to build a bottom out rebound stutter dampner and have it activated by shaft location. I will put it in a third tube in a Romic or on a piggy backed, piggy back on a Fox.

 

[zedro]

I kept imagining this zombie hand had reached up out of the earth and grabbed my wheel.

perhaps it wasent a blown damper that caused the OTB, but rather the hits of LSD....

 

[MikeD]

No, no, that was an aftereffect of what might have been a mild concussion...new helmet, too, dammit...

 

[Kornphlake]

Yup, no reason come down on us like a librarian. My speculating was derived from, but not dependant upon, that thread. There is no reason to bring up Damn True hudder:.

Edit: Also, your buddy got bucked because the bike bottomed and then presumably bounced back up in the same manner as a HT would have. I assumed that the rebound, if controlled, could have mitigated a potiential ass jacking...

Sorry guys, I didn't mean to be offensive, I thought the rolleyes smiley expressed that. Damn True has come down on myself and others alot lately for participating in technical discussions as if we actually know what we're talking about (without knowing who any of us are or what experience we have I don't know how he could make that judgement.) My statement was really a jab at DT not at any of you.

The point I really wanted to make was that a botched landing can send you over the bars independant of the damper's performance. It's nice to blame equipment though and certainly a misbehaving damper can make a problem go from bad to worse. I'm not fully convinced that a properly behaving damper can make a problem that's already at worse be upgraded to bad though.

If you ride over a jump and your suspension is working perfectly the bike will not follow over the back side of the jump, but the forward inertia of the bike and your body will cause the bike to continue off the jump into the air. I am not certain that the effect is the same on a harsh bottom out but I have a feeling that there is some relation. Think of it this way, even a stone will bounce off water if thrown hard enough at the right angle.

 

[shock]

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

Ok, where was I? Damn, you guys sure talk a lot while I'm gone. Does any body actually work during the day? Is all this discussion on company time? Is it tax deductable? Ok, I digress....

No, most rebound (and compression) circuits are of the "deflected disc" or shim stack type. This is to avoid the hydraulic lock and spiking that can occur with orifice type circuits. In addition shim stacks give better control over low speed damping, i.e. pedaling.

And, as I think Zedro correctly stated somewhere below (below? bwhahaahaha), hydrauulic lock would never occur at the instant you are referring to, as the transition between compression and rebound stroke results in zero velocity, where orifice damping produces essentially zero force.

Orifice damping stalls out at high speeds, where the oil just can't get thru the hole. Think three stooges going thru a door at the sam time...(whoop, whoop, whoop, nyuck, nyuck, nyuck) If your too young for that reference, do your research dammit!!

Besides, you don't want it to stall, you just want the rebound circuit to control the extension of the shock against the stored energy of the spring, which of course is a maximum at full compression....

 

[shock]

And sorry if you confused me with anyone from push industries. While (from reading threads about them) they seem to be doing a great job, and fulfilling a needed service, I can't claim any credit or any association for what they are doing...

To clarify, I'm Brian from Therapy Components (therapycomponents.com) aka Brake Therapy, Shock Therapy.

My background in shock design goes back to the 70's as a development rider for motocross shocks, and into the 80's and 90's as a shock designer for many auto (on and off) road racing teams/companies, and into the 90's and this damn century for mountain bikes....

 

[shock]

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.

And stop it with that "correct damping" and "mess up high frequency response" bs dammmit, this is supposed to be a hypothetical, theoretical, engineeering related thread where anything is possible....

 

[shock]

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.

This is somewhat true, when dealing with rebound, position sensitivity could be useful in this, case, because, basically, if the shock is at full compression, you know that the spring has a lot of stored energy that it's about to release, and controlling that release is the basis for why this thread started in the first place....

 

[shock]

I really don’t 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 that’s 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?

Well, lack of rebound damping might not be the only problem, but it is probably the majority..That fully compressed spring has a lot of stored energy... much more than your body weight....

Adding more compression damping is maybe a partial remedy, only in that it will keep the shock from compressing fully, but then you're not getting full travel, and in the situation where you're not going to bottom out, this is a serious detriment, i.e. a rock garden, where you need that back wheel to get up and over the rocks NOW.

As far as the bigger shaft giving more oil displacement and more damping, that is only true of damping circuits between the main shock and the reservoir. Even then it doesn't necessarily give you more damping, only more flow to control, which lets you control it in a finer increment....

But most compression damping occurs when the oil flows from one side of the piston to the other. In this case a larger shaft actually reduces the total oil flow, which by the way, is the shim stack, so there is definetely a trade off there.........

 

[shock]

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.

Right , you don't want a stall to happen, BUT the shaft velocitry increases VERY rapidly after full compression. Remember, at zero velocity, ther is zero rebound damping force, but at the same time, the stored energy of the spring is at its highest, now is the time to control the release of that energy, not after the shock has extended halfway and already imparted that energy to the rider, which is then very difficult to dissapate (at least until you hit the ground).

B the time the shock approaches topout, the damage is already done. At this point, the spring has very little energy to contribute to otb, that's already a foregone conclusion...

 

[shock]

Doesn't that sound like a recipe for Bender du jour? Slow impact-drop or jump, add fast rebound=catapult.

Seems like you'd want fast rebound from the fast hits...as you fly over stuff at high speed, you suspension works fast to follow the terrain and keep from packing up...then when you take a big, slow impact (landing), it's all syrupy to keep you from launching into orbit on the rebound.

No?

MD

Damn, I like this late night Friday posting, all you youngun's are out whorin and boozin, leavin me to get drunk at home and get a word in edgewise...

That is the conundrum, fast rebound when you need the suspension to react quickly to allow the tire to follow the ground and allow full suspension for the next impact, and slow rebound for those "oops"
moments where you need it to save your ass...

It's like, how you can be two places at once and not anywhere at all (ok sorry for the REALLY obscure reference, twenty bucks for whoever shows up at our interbike booth with the answer)

 

[zedro]

ok, we cant further discuss this until someone starts pulling out 2nd order differential equations and frequency force charts...

 

[shock]

problem is, the damping on rebound will react the same regardless if you bottomed out from riding over a boulder or taking a drop; the problem is riding over the boulder lets the suspension extend on the other side of the boulder keeping the bike fairly stable, while on a drop the bike has no choice but to lift, helping the ejection scenario. Maybe if there was an inertia valve on the rebound circuit that slowed rebound when the bike is being accelerated upward, and unregulated during level or downward acceleration....

ok well i'm off to the patent office....

Ok, now we're getting somewhere, hey Zedro, you're not as dumb as I look...

The "inertia valve" solution has some potential remedies for the problem. But before you spend a few thousand dollars on patent attorny bills, check out existing patents on this....

My initial involvement in this was with the Citroen rally/raid team in the early '90's. Fantastic cars that raced in the paris/dakar et/al rallies. Lots of horsepower, lots of suspension travel (about 3 feet) and primitive dampers.

They approached our company because they had a problem....after landing from a high speed jump, the back of the car would kick up AND PASS THE FRONT of the car in mid air!!!!!!!!! Bad enough, but when the car burst into flames (they carried like 100 gallons of gas) and killed the driver, they decided there had to be a better way...

Now, rider/driver positioning had nothing to do with this, you were a passenger.... the only choice was to slow down and lose the race, surely a fate worse than death... (or not, the driver never answered that question)

The first result was to tailor the rebound damping to deal with the stored energy of the spring, and the resultant HUGE shaft velocities, over 100 feet per second of shock shaft velocity in rebound, no f kidding!!. More than any data aqcuisition or shock dyno could measure. And they still can't, so when you hear me rant against data acquisition as telling the truth, there is some historical precedant.....

But we eventualy explored an "inertia valve" (I hate that generic term (like brake jack)), as a means to provide a different rebound damping response for different conditions. And it was very succesfull. Those cars went on to dominate Paris/Dakar, Paris/Peking, and other races of that ilk, and is still in use today.......

Ok, enough for now, digest and debate (once she leaves and the hangover wears off...)

 

[zedro]

well, as far as developping an inertia-mo-bob, by the time i could get something out, we'd have that magnetic damping dealy-o's with accelerometers linked to processors for damping units...

 

[shock]

ok, we cant further discuss this until someone starts pulling out 2nd order differential equations and frequency force charts...

Yeah right, forget that crap, I just figured out how to use someones quote in a post.....

It would take me all night to produce graphs to post, and I got drinkin to do!!!

And aren't you home a little early? Struck out again?!?!?!?

 

[kidwoo]

ok, we cant further discuss this until someone starts pulling out 2nd order differential equations and frequency force charts...

If you've got time to figure out instant centers of other people's suspension designs, I don't know why a lilttle d^2x/dt^2 should slow you down.

Me? I'm going to go figure out my ethanol resistance coefficient in relative terms of an asymptote that approaches dawn signifying an early ass bike ride at its tangent.

 

[shock]

well, as far as developping an inertia-mo-bob, by the time i could get something out, we'd have that magnetic damping dealy-o's with accelerometers linked to processors for damping units...

Yeah, and then they'd react too slow, because by the time the response was initiated, the initial event would be old news, now leave me alone I'm tryin to watch speedvision with blurred eyesite......

 

[Alloy]

Well, lack of rebound damping might not be the only problem, but it is probably the majority..That fully compressed spring has a lot of stored energy... much more than your body weight....

Adding more compression damping is maybe a partial remedy, only in that it will keep the shock from compressing fully, but then you're not getting full travel, and in the situation where you're not going to bottom out, this is a serious detriment, i.e. a rock garden, where you need that back wheel to get up and over the rocks NOW.

As far as the bigger shaft giving more oil displacement and more damping, that is only true of damping circuits between the main shock and the reservoir. Even then it doesn't necessarily give you more damping, only more flow to control, which lets you control it in a finer increment....

But most compression damping occurs when the oil flows from one side of the piston to the other. In this case a larger shaft actually reduces the total oil flow, which by the way, is the shim stack, so there is definetely a trade off there.........

When I say add more compression damping I’m not saying add so much that it stops the travel, just slow it down some. Maybe give yourself enough time to get your balance or get over what you just cased. Personally I would like a shock that would give you more compression damping in the later part of the stroke. As your example of getting over a rock garden, that’s right you do need that travel now and it would be pretty bad when your at the end of your travel and you shock cant rebound fast enough to keep up. As it would if you increase rebound damping at the end of the stroke.

On the issue about rebound being the cause of getting thrown over the bars. Personally I think that is crap. I bottom my shock pretty frequently. Its only when I really bottom out really hard that I’ll get bucked. Its not like the spring can be compressed more than the shock stroke therefore I don’t think the spring energy alone is the cause.

As for larger shock shaft, of course there’s a medium where its with in the right size so that you can still achieve enough oil flow through the shim stack as well as get a little more flow going into the reservoir. Also it may have something to do with “boost valve” Which I think works like a Curnutt, more oil displacement which pushes on the air spring harder, maybe giving a greater air volume adjustment? Which intern equals more pressure and due to the curnutt design more compression damping as the shock goes through its stroke. And if it is simular to a curnutt then it wouldn't have a shim stack just a pressure plate which may be easier to work around a larger shaft.

Dont take my word for it, I've never seen the internals of a dhx. Just latley I've been trying to figure it out cause I may buy one and I want to know how it works before I spend my money.