The Tuesday Tune Ep 21: Low and High-Speed Rebound Adjustments - Video

 Its more fun if someone else turns my knob, but I am willing to handle it my self for when that somone else has a headache.

 I'm assuming this is because a heavier spring rate (coil or higher pressure) provided more force in the rebound circuit to push against a fully closed HSR and produce a suitably fast rebound speed? Whereas a lighter rider with a lighter coil/lower PSI, the shock doesn't have the same spring force returning it through rebound, so fully closed HSR results in too low of a return speed.

Just thinking out loud here.

 Thanks for the kind words! You can find all the Tuesday Tune videos on our Pinkbike blog here - www.pinkbike.com/u/VorsprungSuspension/blog

 I tend to agree, but I'll explain a little further. Spring force itself is position sensitive by definition - the further into the travel you are, the harder it is trying to force extension, and the further it needs to move to either keep the wheel on the ground or maintain ride height. What LSR and HSR adjustments should allow for (but don't) is to substantially alter the shape of the damping curve, ie to allow you to choose from progressive, linear or digressive curves, which do affect how the bike behaves on bigger motions as well as smaller ones (pedalling etc). In reality though, current adjusters both don't allow for that, and have a huge overlap anyway, so our opinion is that the best approach is to simplify as much as possible. That is basically what this video is about.

 Rebound is driven by the spring, minus the resistance of the inertia of either the rider (if the wheel is on the ground) or of the wheel (if the ground suddenly disappears). In first case the rebound wil be slower than in the second case.

 Because the spring force increases deeper in the travel, especially on air shocks where the spring is progressive. So the deeper from the stroke the shock is recovering, the more spring force there is to dampen, and logically the rebound speed is higher also. So the LSR affects mainly small bumps and HSR everything else. Makes sense?

 @hirvi: That's not really the correct way to explain it. Low speed rebound is for controlling slow shaft speeds, and HSR for faster shaft speeds. Small bumps like braking bumps at high speed, are firmly planted in the HSR zone. You could be rolling through a foot tall smooth "BIG BUMP", and not be in the HSR at all.

 If net force on the shock during extension is just a result of coil spring force, then shaft velocity is dependent only on the coil spring "position".

Starting at bottom out, shaft velocity will start at 0 (due to change in direction) with maximum acceleration, then achieving peak rebound velocity with 0 acceleration then slowing down towards top out (as shown on the right hand graph Steve drew).

So with this stated, with a given damping setting, you can obtain shaft velocities giving the spring position. Things do get a bit more complicated when you utilize air springs and when you factor in external forces acting on the shock during extension. For example if you prejump a drop and then the shock starts extending towards peak rebound velocity and you hit a bump with your rear wheel while being in the air, it could go from peak rebound velocity to 0 in an instant, to negative (compression in this case) and back to zero and positive velocity towards top out, all before landing the drop.

 dp

 @thuren: Well, that's that case with compression side of things, but not so much with rebound. You don't nearly bottom out your suspension on a "big smooth bump", do you? So regarding suspension movement that propably counts as a small hit=small compression=controlled mainly by LSR..

 @hirvi: If you are rolling through a big smooth bump, like a big roller on a pumptrack, and your weight is solid on the pedals even on the backside of this "bump bump" the weight you are applying is countering the spring force on the return to level, making LSR the main contributor. I like to think of it this way.... Weight on the pedals with the tire on the ground = LSR, if you are fairly deep in the stroke and the tire leaves the ground = HSR

 @thuren: in theory that sounds good, in practice it's easy to reach velocities well into the HS portion of the curve without the wheel ever leaving the ground, especially if you're moving around a lot on the bike pumping the terrain. Same reason why HSR adjustment affects the bike's behaviour on the lip of jumps.

 @VorsprungSuspension: Makes sense! I guess I feel like weight on the pedals "in the top half of the stroke" would be mostly LSR. Totally makes sense deep in the stroke how the built up spring force overpowers weight. Love this stuff thanks for all of this.

 Yes and no. In this particular case (DHX2 rebound) yes, that is basically correct, although there is some smoothing going on (notice that there isn't any force at zero speed even with LSR fully closed and the HSR anywhere but minimum) due to some small shims on the nose of the poppets in there, so there is a variable aperture bypass circuit for the LS circuit whether the poppet/rod valve itself has begun to open. In the case of the compression, those are two of the three circuits, so there's also the main piston valving that plays with it too, and at that point the mathematical breakdown becomes far more complex.

With regards to setting HS damping to maximum before adjusting LS damping, I'd only recommend that specifically in rebound, though in a similar way I'd recommend setting HSC before LSC where possible. Check out our previous videos on low and high speed adjusters, they cover quite a lot of what we're discussing here.

Edit: seems that you edited your original post so this reply won't make sense in context!

 @VorsprungSuspension: It still does. I just wanted to clean up my thought. Today i found out that editing pinkbike comments deletes the original if you don't copy paste. Thanks for the reply.

 Probably dumb question... Vivid R2C has starting/ending rebound. Which is the equivalent of HSR?
I'd say at bottom out there's more force, so the shock tries to extend faster so ending rebound=HSR...

 @Uuno: I´m pretty sure one would set the ending-stroke rebound quite slow so it wouldn´t buck you off after a tough landing or a g-out and set the starting Rebound quite fast so you would get Maximum traction over roots and braking bumps. At least that seems to make sense to me, but i never rode a vivid.

 @Samplidude: yes you could set it up this way to. However (AFAIK) RS went the opposite direction with rapid recovery in their forks, to avoid loosing travel over consecutive hits( end stroke rebound faster than beginning stroke rebound). But probably there isn't an overall "right" setup

 @Uuno: What RS call "ending stroke" is HSR.

 Actually, even with the HS valve under enough preload to hold it closed, they still behave in quite a linear manner as an unpreloaded shimmed damper does, as mentioned in one of the other comments - the shims on the nose of the poppets flex open from zero speed regardless of the HS preload. This is a very smart and robust element of the design of both the DB and X2 dampers.

There is a common misconception that LSR stabilises the chassis - in reality stability is more affected by the bigger motions where the HSR comes into play, though as mentioned, in this case the LSR and HSR adjusters have so much overlap that both external adjusters are actually affecting it. Does this have to be balanced against insufficient ability to track the ground? Absolutely - see last week's video.

 First things first - there is no "right and wrong" way to find your ideal setup. You are, at the very least, using the adjustments to tune the things they affect the most, which is definitely a good thing. The difficulty comes when opening up the LSR also opens up the HSR (or vice versa) and that then becomes a confounding factor. Here we are trying to explain what those confounding factors are so that you can understand and mitigate them.

 @VorsprungSuspension: Thanks for your reply. As you mentioned in your video, it did take me a few more plays of your video to "wrap my head" around this but I think I'm getting there now. However, this just brings up more questions:

My first take away from the video is that your method of tuning maximizes the linearity of the rebound dampening rather than making it more digressive. This leads to the question of what the advantages / disadantages of linear versus digressive rebound damping are and in which application each would work best. I believe linear would probably work best for my application while digressive would be more suited to coil shocks or air shocks with fewer volume spacers(?)

It seems to me that with your tuning method everyone would end up with either:
1) the HSR fully closed and LSR somewhere in the middle or closer to open OR
2) LSR fully open and HSR somewhere between fully closed and part way open.

Yet if you look at any bike or shock manufacturers' base or recommended tunes I don't think I've ever seen them recommending HSR fully closed or LSR fully open as a starting point. This again makes me wonder if there is an advantage to a somewhat digressive rebound setup for most applications.

 @Xorrox: your understanding of what I'm saying here is correct. I am not personally a big believer in digressive rebound damping for anything that's intended to be ridden primarily standing on the pedals. It's a long story but this is something I have studied in considerable detail, and basically what it comes down to is that low frequency inputs below a certain frequency can be actively damped by the rider up to a certain amplitude (the rider's free range of motion). However, these low frequencies can actually see high velocities too if the amplitude is high enough. Meanwhile, high frequency inputs (that are above the ~3Hz threshold that a rider can respond at) force the suspension to react. although if the amplitude is low, these frequencies are not actually generating particularly high velocities - as a result, they're controlled more by the low speed damping that we frequently associate with big slow movements. This is why people commonly associate low speed compression (and rebound) with small bump compliance - because it does affect that quite a bit.

What does this mean? It means that, counterintuitively, the rider can't move fast enough to absorb some stuff that is affected largely by LOW SPEED damping, while we can stabilise a certain amount of motion that is occurring even at velocities well into the "high speed" region, provided that its effective frequency (calculable using FFTs) is low enough and that the amplitude doesn't exceed the rider's free range of motion. That's why we invariably end up with rebound damping coefficients that are much lower than heavy vehicles with a larger rigid sprung mass. The proof? Set your Inline in the open mode, and ride over a short rough section of trail whilst seated (carefully!), which effectively connects your body much more rigidly to the frame, and see how long it takes before you're genuinely terrified of getting sent OTB or the rear wheel is bouncing way up in the air. Try it again with your climb switch closed (much more digressive damping curve, heavier damping overall in both directions) and you'll see how that benefits what is essentially a vehicle with a more rigid sprung mass.

My opinion on HSR and LSR adjustments in general is that given that every one of them currently on the market offers you some degree of digression in the damping curve no matter how you set it up (at most it'll be fairly close to linear as you noted), and that they have such enormous overlap, is that they're a marketing effort above all else. They don't make performance inherently superior (in fact I'd argue they do the opposite), they don't make setup simpler (they definitely do the opposite there), and they don't even necessarily give you a wider range of adjustment than could be achieved with a single adjuster (even if that were really desirable), especially if you're going to allow for different valving configurations.

 @VorsprungSuspension: thanks Steve, a lot of content here as opposed to most of the articles here on PB. Keep it coming!

 @VorsprungSuspension: Very true...you're the man!

 @VorsprungSuspension: Thanks so much for this detailed explanation! As @Happymtbfr said, this is some of the most useful and thought provoking info I've found on any Mtb site in a long time. Cheers!

 @VorsprungSuspension: a single adjuster like an Öhlinscoil?

 @enduro29erHack: Yep - in the twin tube context that'd be the best example. The rebound damping in those is much more linear than in an X2 or DB damper.

 @VorsprungSuspension: brilliant!

 Not all adjusters are very effective, and the Mission Control adjusters in the pre-Charger era Boxxers were pretty ineffective in that regard due to a combination of adjuster geometry, insane oil displacement and poorly thought-out valving.

 Correct on all counts. The Vivid does have an effective compression adjuster as well, so if you do find that you're lacking compression damping because the low speed rebound is more open, it's usually not more than one click of additional compression to reach a similar level.

 The simple answer there is that if you feel like it's bucking, add more damping. As for how well rebound circuits deal with an air shock - depends on their specific characteristic. More spring force will always be able to generate a higher rebound velocity at a given damper setting, that is true - it's all part of the endless compromises involved in suspension setup

 Forks can be set up in a similar manner typically - they have less weight on them but they're more lightly sprung and damped too. Sounds like your biggest issue though is that you simply need to find a heavier spring.

 Yep, it will do that to a large degree in the CCDB Air versions and the IL version (not so on the CCDB coil CS or the Float X2 or DHX2) because the DB Air has no rebound shim stack on the main piston, so all the flow has to go through the LS and HS adjusters - but the LS circuit is fully closed off when you shut the CS.

 Exactly.... There is a reason why there NEEDS to be a good range of rebound control available. A light weight beginner rider will generally need slower rebound, but a very advanced rider at the same weight may have a problem getting the rebound fast enough. I have my rebound on my Lyrik/Monarch+ combo full open. It's perfect and plenty fast, but if I was not 200lbs(115psi in the Lyrik) I could not get my rebound as fast as I would like.

 @betsie it doesn't sound like what you're doing is all that different to be honest. Setting the high speed first dictates the range of low speed damping available, but on most of the twin tube dampers out there (CCDB coil being an exception) the behaviour of the compression and rebound circuits is not the same in that all the oil is forced through the rebound adjusters, but not all the oil is forced through the compression adjusters. Why this is relevant - the range of adjustment of both LSR and HSR is huge and overlapping substantially. You can achieve damping curves that are very nearly identical with different combinations of LSR and HSR - what we are trying to do is cut out the redundancy of having two different setups that you could stumble upon that behave pretty well identically on the trail, by providing a specific method of tuning each of the HS and LS circuits. It might be that you're quite picky about pedal bob for example, in which case you might find that fully open LSR, even with max HSR, feels more "bouncy" than you like at low speeds - at this point you're getting into the "more advanced tuning" mentioned in the blurb of the original post

@thuren for sure there needs to be a good range of rebound adjustment available, whether it is internally adjusted with the valving or externally with a dial. It does strike me as a bit odd that you're 200lbs and running a Lyrik wide open in rebound though... if the valving is stock (and knowing what you do, it probably isn't!) that is a very particular setup.

 @VorsprungSuspension: how does the CCDB differ in tuning practice?

 @VorsprungSuspension: My Lyrik IS bone stock. Never felt the need to mess with it! That said, might just be one of the odd ball of the batch with less range. It IS fast by normal standards for sure though. Just how I like it.

 @Sponger: it doesn't massively, just the CCDB coil has a rebound shim stack on the main piston that the other DB series and X2 dampers don't have. That shim stack does assist with linearising the rebound damping to a certain degree, but the result there is that the effects generated by each of the HS and LS adjusters become even more similar to the point where there is considerable redundancy due to the overlap of the two.

 @VorsprungSuspension: thanks you.

 @thuren: wow, 115 psi in a Lyrik with no rebound damping? Not saying it is wrong, just different and interesting. I have a 2017 Pike, also stock, 100 psi, 2 volume spacers and I run rebound 6 out from closed, 210lbs body weight. This is for trail riding, not overly aggressive and I like a composed comfortable fork. My fork feels like a pogo stick if I open the rebound all the way.

 Lighter weight oil has a greater effect on low speed than high speed. Oil flowing through a port is much more viscosity sensitive than shim stacks.

 @carym is on the money. Lighter oil primarily affects the low speed damping.

 @VorsprungSuspension: @carym:
Thanks for the info - that's how the change feels on the bike. Good to know theory and practice are lining up

 If that's the case I can only assume you're on a Vivid, in which case yes you'll only notice the difference in particular situations on the trail. The difference between HSR fully open and fully closed is hugely noticeable on an X2 or CCDB damper when bouncing on it, unless your LSR is completely open in which case it may be a little more difficult.

 @VorsprungSuspension:
If so, if I can adjust rebound properly with hsr closed, then it's enough and hsr doesn't affect jumps drops etc?
Thank you for video and reply.

 @jungjoonc: I think you have the right idea there.

 Not sure exactly what you mean there - lag in what specifically?

 @VorsprungSuspension: in other words, force rise time. Delayed time to reach peak force.

 @cjdean1: that kind of lag is typically referred to as hysteresis, which is predominantly to do with the elasticity of the shock including valve displacement etc. The CCDB and X2 do display a certain amount of that due to the relatively large displacement of the high speed poppets/rod valves compared to shimmed valves. It isn't specifically related to the adjustment range.

 Possibly, what do you want to know?

 Rapid Recovery, depending on how it is configured, is intended to create lower HSR by preloading the shim stack, which means you can get more LSR with less HSR. Not every example of it actually does this though.

 Generally speaking yes.

 What about Y- fronts?

 @poah: definitely don't want much dampening in those regardless of speed

 They only have one rebound adjuster so it's not specifically relevant there. Check out our past videos on HS and LS damping and you'll see the similarities and the differences.

 Similar line of thinking but not quite the same process - check out our previous episodes on HS and LS damping, they cover compression setup in a lot more detail.

 I saw those just wanted to see the dyno again lol. @VorsprungSuspension: