I have a very basic Deluxe RL on my Levo, especially compared to the DBair on my old bike. Surprisingly it feels rather good! I gather it’s a custom tune by Specialized for the specific bike and size.
The thing that’s now puzzling me, is that my LBS pointed out that I’m running the rebound very very slow. I’ve just tweaked it whilst riding to get the feel
I want, but I noticed when off the bike as you push it down to sag and release it comes back up super slow.
When I’m standing on the bike and bounce it’s on the slower side for sure but in a controlled way, not totally lifeless. And when I push it hard to hop or jump it springs back nicely (to the point I hit a gap jump last week for the first time in ages - I have a really big mental block with them).
I’m guessing that the rebound knob only affects the low speed rebound? But what’s going on in there? I’ve found info on the Super Deluxe but does the piggyback change everything?
I’m guessing that the rebound knob only affects the low speed rebound?
Correct. Once shaft speed reaches a certain point, the majority of oil flow will be though a shim stack which the knob doesn't change (it alters the size of a bypass past the shim stack).
Running the slow speed rebound at a very slow setting won't affect return from larger hits much, but it will allow the shock to pack down over repeated smaller stuff and thus negatively affect traction.
Correct.
Incorrect. The low speed governs high speed as well - as oil will still flow through the low speed circuit during the high speed stage. A lot of shock designs have such high flow flow speed rebound ports that Vorsprung recommends simply closing off the high speed circuit as much as possible and only using the low speed port to tune the rebound. This will work for certain designs that use preloaded sprung valves for controlling the high speed circuit - eg the DHX2 - I don't think it would with a Grip 2 damper though as the external high speed adjustment act much more like changing the actual shim stack.
https://www.pinkbike.com/news/vorsprung-tuesday-tune-ep-21-low-high-speed-rebound-video.html
If they work like that then they are badly designed. Although there's much less difference between low and high speed for rebound than compression, so you can get away with just using a simple orifice valve, it's still better to have properly separated circuits.
Not really. The low speed circuit always flows oil no matter what - it simply has an upper limit to the amount that it can flow. If you reduce the amount that the low speed can flow then you reduce the total amount of oil that can flow at high speed as well. On top of that, preloading a shimstack using a high speed adjuster only changes the point at which the high speed rebound circuit opens up - it doesn't stiffen the damping rate.
Watch the video.
If you cut start reducing the amount that the low speed can flow then you reduce the amount of oil that can flow at high speed as well.
Technically true, but if it's a significant factor, then in my mind the high speed circuit is poorly designed and should allow higher flow rates.
"Running the slow speed rebound at a very slow setting won’t affect return from larger hits much, but it will allow the shock to pack down over repeated smaller stuff and thus negatively affect traction."
I was aware of this but hadn't put two and two together regarding a recent observation. My rear tyre is a 29 x 2.6 Butcher - it's a lot of rear tyre! But I'd noticed that it doesn't have as much grip in the turns as I'd expect - not a problem because it helps me get this big bike around tight corners and I'm used to riding a hardtail. But I suspect that it's the slow rebound causing the reduced traction, not the tyre!
If I only rode my local trails I'd leave this be because it does work very well. But I'm concerned at what might happen through the rockier stuff at uplift centres etc.
"Not really. The low speed circuit always flows oil no matter what – it simply has an upper limit to the amount that it can flow. If you reduce the amount that the low speed can flow then you reduce the total amount of oil that can flow at high speed as well. On top of that, preloading a shimstack using a high speed adjuster only changes the point at which the high speed rebound circuit opens up – it doesn’t stiffen the damping rate.
Watch the video."
I've watched these videos before. And I'm pretty certain that Steve is talking about shocks with external control of LSR and HSR, not shocks where there is a knob for LSR but the HSR can only be changed by messing with the shim stack, it isn't preloaded by an adjuster.
Technically true, but if it’s a significant factor, then in my mind the high speed circuit is poorly designed and should allow higher flow rates.
Why would you want the high speed rebound offering higher flow rates and consequently less damping force than the low speed? If you want to reduce crossover, reduce the size of the low speed ports.
LSR and HSR, not shocks where there is a knob for LSR but the HSR can only be changed by messing with the shim stack, it isn’t preloaded by an adjuster
True. But most off the shelf shocks are designed with low speed rebound ports that suit a wide range of wider weights, hence they do effect high speed rebound quite significantly.
"Why would you want the high speed rebound offering higher flow rates and consequently less damping force than the low speed? If you want to reduce crossover, reduce the size of the low speed ports."
Doesn't high speed damping always allow higher flow rates than low speed? Bear in mind that fluid resistance in laminar flow increases roughly with the square of fluid velocity, so high flow rate doesn't mean lower damping force if the velocity increases enough. NB: My Mech Eng degree was 20 years ago!
Doesn’t high speed damping always allow higher flow rates than low speed? Bear in mind that fluid resistance in laminar flow increases roughly with the square of fluid velocity, so high flow rate doesn’t mean lower damping force if the velocity increases enough. NB: My Mech Eng degree was 20 years ago
Well yeah, but that might make a few people's heads explode. Adds complication for people including me hahah, you of course need higher flow to deal with increased speeds. I felt what the poster was getting at was damping force.
You need to slow down the shock more at the end of the stroke more than at the top, which from what I gather is one of the difficulties of speed sensitive rebound? There's no real way to separate the rebound damping positionally with a speed sensitive damper. Hence you can't have a damping circuit where by you can massively slow down the end stroke and keep the beginning stroke nice and fast. You're always mucking about with the crossover to find a compromise.
