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The Tuesday Tune Ep 21 - High and Low Speed Rebound Adjustment

This week on the Tuesday Tune we're looking into how HS and LS rebound adjusters work, overlap, interact and can be tuned.

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As mech. engineer I understand you, but as a biker I would say that you should turn things around when thinking about rebound vs compression.

At comp. we want to know what force we want (or can survive) when hitting certain object at certain speed...

At rebound the force is predetermined by spring, at certain point in the travel spring produces certain force -> at certain point in travel we have certain rebound speed (at given setting). I know in practice it is important how deep in travel rebound is started, but acceleration phase is (I assume because of relatively small unsprung mass) quite short.
I think that question is, what speed of rebound we want at certain part of rebound stroke?

From my "sitting at the table end thinking" point of view, I think that we wish higher rebound velocities deeper than sag to recover from hits as fast as possible (high speed adjustment is important here). And than controlled rebound speed around the sag and shallow in travel to adjust bike behaviour... faster -> more pop, slower -> more control (low speed come in to play here).

I can be totally wrong, but I would like to here your opinion on "speed/sag" point of view.
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I understand where you're coming from in terms of trying to work out where we want the correct speeds for sure - the difficulty is that there is another layer or two of considerations for each type of motion here:
1. Rate of extension will always be faster if the wheel is moving downwards to follow the ground, rather than pushing the rider up
2. Pushing the rider up quickly enough to destabilise the bike requires a lot of force, which can only really occur deeper into the travel. As a result, relatively heavy damping at low force values (and lower velocities) inhibits the wheel's ability to follow the ground when higher in the travel, with little effect on the deeper travel, higher force events.
3. Even deep into the travel, if the wheel is firmly on the ground and we are essentially lifting the rider, the rebound speed that this is occurring at can be occurring slower than motions occurring where the wheel is dropping away to follow the ground earlier in the travel even though the spring force is far higher. Balancing all this out is the key.
4. Statistics matter in terms of dynamic ride height - it is the average ride height, and everything that contributes to raising or lowering it over a short space of time has a considerable effect on the system, and as a result, the suspension's ability to both remain stable and keep the wheel on the ground.
5. No matter what, rebound velocities will always be higher deeper in the stroke. The momentum that they can give the sprung mass by the time you get back near the sag point is a huge factor when considering instability and traction, and preventing that momentum buildup can keep the rider closer to the ground and therefore give the wheel a better ability to follow it.

The truth is that springs and dampers are a completely imperfect system. There's a reason why some manufacturers run progressive rebound and others run digressive rebound - and that reason is as simple as people prioritising different aspects of control.
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@VorsprungSuspension: Thank you for detailed answer, I forgot that usually rider is on the bike too, so forces on damper are not nearly directly related to position...
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So the HSR knob should be turned all the way in and work on the LSR to get the desired rebound speed you want. What if my LSR is 2 clicksin and I would not want it get faster (coz I did a parking lot test and the rebound speed is already spot on), but I notice that the bike's rebound is SLOW in High Speed chatters, what do you recommend? turn the LSR a few more towards fully open or start opening up the HSR knob?

My understanding is to set the LSR to my preferred number of clicks and if the HSR is still slow while the LSR is already in its sweet spot, do not mess with the LSR anymore and just start opening up the HSR
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@VorsprungSuspension
Hi Steve,

your videos along with Race Tech's Motorcycle Suspension Bible are probably the best source of information on suspension.
After watching this video several times along with the one where you compare CCDBA and X2 I have a question:
Does LSC affect LSR (and vice versa)?
I am probably wrong but the low speed needle ports seem open all the time and I didn't see a check valves covering them.

Thanks!
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I have a Vivid R2C. They use the term Beginning and End Stroke rebound. Is this Low Speed and High Speed Rebound or the inverse?
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Beginning stroke = low speed, ending stroke = high speed.
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Nice job