To The Point - Rebound Damping

Jan 15, 2013 at 0:07
Jan 15, 2013
by Mike Levy  
 
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Sante Pelot grew up in Santa Cruz, California, and has been riding mountain bikes since 1986. He attended the University of Oklahoma where he wrote his master’s thesis on a magnetorheological damper design for tractor-trailers. Pelot has worked at FOX since the beginning of 2007, and currently spends most of his time working on FIT dampers (he’s handled every one since 2009, except for the Terralogic and iCD models). He is responsible for design, development, tuning and testing - all the way up to handing off the new damper for production.


What is rebound damping and why is it important?

Let’s start with a bit of background. Damping is the conversion of kinetic energy
(energy associated with a mass in motion) into heat. Typically, that damping
is achieved by forcing oil through tight spaces. This configuration yields a
damper that is velocity sensitive (the force generated is proportional to the axial
velocity of the damper shaft
). A bicycle shock consists of a spring and a
damper. Springs are position sensitive (the force generated is proportional
to the axial position, and is not affected by velocity
). A spring stores energy
and a damper dissipates energy. In some cases, a spring can be velocity sensitive
(an air spring is),) and a damper can be position sensitive (such as our Boost
Valve dampers
), but let’s ignore those complications for this discussion. Let’s also
ignore that when you remove the spring on your downhill shock and depress the
shaft, it returns, just like a spring would, but slower. This is a by-product of the
damper architecture and is not important here.

Rebound damping controls the return of the wheel after a compression. It is a
bit simpler than compression damping, as typically it is only working against the
stored energy of the spring, so the forces acting on it are a bit more predictable.
It is important because without it, your wheel would rebound too quickly. Specifically,
you would get bucked all the time and your wheel would drop into every single hole
it could find, sending you over the bars more than necessary. Faster rebound does
not equate to a faster rider.

A wild ride can be the result of having your rebound speed set too quick.

A wild ride can be the result of having your rebound speed set too quick.



How is rebound damping controlled?

All rebound circuits feature a hole or “orifice,” as we like to say. During the
rebound cycle, the rebound check valve shuts, which blocks flow through the
compression circuit, forcing oil through this orifice. For adjustable rebound
circuits there is a tapered needle that fits through the orifice. We design the
rebound circuit so that (for all practical purposes) all of the rebound damping
occurs at the orifice. Thus, by moving the needle in or out on a threaded rod,
the cross-sectional area that the oil must flow through is changed, altering
the force that the damper produces at a given velocity. All of our FIT dampers
feature a patented, fluted rebound needle that breaks rebound flow up into six
flow paths which helps reduce the noisy “rebound woosh,” which most of you
have probably learned to ignore.

Most of our dampers also feature a rebound shim stack which is a parallel
damping circuit, tuned to open at a certain pressure. Without a shim stack,
the designer must choose between not having enough rebound damping
at slow shaft velocities, or having too much rebound damping at fast shaft
velocities. Sometimes you can get away with no shim stack, but not always.
A poorly tuned shim stack can behave worse than an orifice-only circuit.

Something you probably don’t know is that our rebound circuits offer both low
and high-speed damping adjustments. They are adjusted at the same time to
optimally complement each other. We do this by sizing the needle so that
it affects the entire damping curve, instead of just the low-speed portion of the
curve. I haven’t met many people that would actually rather spend time setting up
their bike than riding it, so we try to make it easy for you!

FOX 2013 suspension CTD Float

As with all FOX suspension, rebound speed is adjusted with the anodized red dial. The useable range for a given person is roughly 3 clicks of adjustment.



How vital is it that rebound be adjustable?

If you have a full-time tuner working for you, then it is not at all important. Since
most of us don’t have that, it is very important. On a bicycle, the rider makes up
two-thirds of the system weight at a minimum. This means that variations in rider weight
have a huge effect on which spring will be used, and thus how much rebound damping
is required. Therefore, it is imperative that rebound damping is easy to adjust and that
it has a wide range. Compression damping is not as sensitive to fluctuations in weight,
because the (adjustable) spring is helping during a compression stroke.

So remember, we design a very wide rebound range to deal with the large weight
differences between our customers. No single person should think that they can
use the entire range. The useful range for a given person is only about 3 clicks.



How does rebound damping relate to the other setup elements such as spring rate and compression damping of a fork or shock?

In regards to spring rate, the stiffer the spring, the more rebound damping will be
required. For compression damping, depending on the damper architecture, there can
be a noticeable effect on compression damping from the rebound setting. This typically
isn’t important because there is a very narrow range of rebound settings that a given
rider will use, and most people will set-and-forget (which is completely valid once you
find a good setting
). Therefore, a rider will be able to find a compression setting they are
happy with after setting rebound. Also, conveniently, heavier riders will require more
rebound damping, which also results in more compression damping. So it is self-adjusting,
in a sense.

The order of setup should always be: measure sag, adjust spring until proper sag is attained,
adjust rebound, adjust compression damping. This follows the order of importance of each
element: the correct spring rate is the most important, then the correct rebound setting, then
the compression setting. Other interesting tidbits are that dampers are tuned so that, in general,
rebound damping is much higher than compression damping and rebound shaft velocities are
about one-third of compression velocities.

Rebound damping can be controlled via an orifice, a shim stack, or a combination of both.

Rebound damping can be controlled via an orifice, a shim stack, or a combination of both.



Does one type of rebound setting suit everyone?

There is a very limited range of rebound velocities that result in a good setup. Some
people may prefer rebound on the slower side of this spectrum, and some on the faster side.
This is worth repeating: the useable spectrum for a given rider is only about 3 clicks wide. But
it is easy to have a bad rebound setup that will result in making you a slower rider. We are talking
about damping of frequencies generated between 5 and 35 mph at displacements of 1 - 8 inches
here (most of the time; special circumstances may require deviations from this rule), so either your
rebound setting does a good job at these frequencies, or it doesn’t. When it comes to rebound
damping, there are right and wrong setups. Riding style will only dictate which end of the useable
spectrum you should use.


What are the telltale signs of an incorrect rebound setting? What about a proper rebound setup?

If rebound is set too fast, it will result in your wheel falling into more holes, instead
of skipping over the top of them, as it should. It will result in a bike that feels skittish and
uncontrolled, and will be scary off of jumps. You will get bucked, you will bounce upon
landing (even if you kept it rubber-side-down), and it will be a wild ride. At high lean angles
it can result in loss of traction.

If rebound is set too slow it can “pack down,” where successive hits result in the wheel
getting deeper and deeper into the travel. This will result in not having enough travel to
deal with subsequent hits. Most people don’t realize that a symptom of too much rebound
damping is harshness on compression. This is due to the bike riding deeper in the travel,
and your hands and feet working against higher forces in the springs. A common fix for
harshness is to reduce compression damping, but if you are packing down, it will exacerbate
the problem. Another symptom of slow rebound is your bike sitting too deep in turns
(especially fast berms). If your bike has the rebound set properly, you won’t be thinking about
rebound while you are riding.

Remember, take the time to set up a bike properly. Don’t trust that the person at the demo
booth or shop did it correctly. If they ask you for your weight, and don’t measure sag, they
are doing it wrong. If they change the spring pressure and don’t check rebound, they are doing
it wrong. And don’t forget to check the tire pressure! Tires can have a huge impact on how
suspension feels. The best bike in the world can ride like a department store bike with bad setup.


www.ridefox.com

Follow Mike Levy @MikeLevyPB
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91 Comments

  • + 88
 That was an AWESOME writeup. Great work! I love reading about the more techy side of mountain biking systems.
  • + 14
 smae, im a bit of a nerd and I find all of this tech wizardry very interesting, I would love to see an indepth look at hoe BrakeForceOne brakes work, them things are complicated!
  • - 18
 did you forget a H in there somewhere?
  • + 6
 I'd say this is as good as tech tuesday Big Grin
  • + 2
 im completely the opposite i normally think sod it, get on and ride, but i found that an interesting read. ill be making some checks on my bike.
  • + 1
 gazsutton: truly true truth Wink , but on the other side big truth is that good working suspension can make you go faster, with more certainity and with less energy drain from your body, so you can eventually go longer (sounds like advertisement on viagra or so, but isn't Smile ).
  • + 1
 if i got my 09 boxxer re-built and i still have a top out? what does this mean? is my fork Forked?
  • + 1
 with top out (after fork is compressed the fork goes back to full length too quickly) ...rebound is too fast. slow it down. Sometimes it just needs one or two clicks. But without seeing the fork in hand it could be other problems
  • - 1
 i run my suspension at the fastest rebound cuz im hardcore. if i wanna go faster, i pump faster. thas wassup. ha great article, stuff like this really helps when you dont have someone with a stop watch telling you definitive differences in your speed and times
  • + 2
 You mean the suspension on your old ass big hit?
  • + 2
 that's where it all began! money don't buy you skills homeboy
  • + 3
 Yea, but the reason you need your rebound so fast is prolly cuz the oil in you dropoff hasnt been changed in 5-6 years.
  • + 1
 thanks folks kinda helped, rebound it slow. After fork is just sittin with no weight on the bike i can lift the bike and the fork will lower.
  • + 1
 i dont own that bike anymore, my other pics show a more recent bike, although old broken down oil might make it rebound faster
  • + 1
 When was the last time they had a tech tuesday?
  • + 1
 @imdabomb - TT has been retired for 2013, although it may return down the road.
  • + 2
 Noo!! It was like my favourite! It will be missed
[Reply]
  • + 13
 I'm glad to see this its amazing how many people spend a fortune on forks and have no idea what they are for, and further still couldn't even imagine what a rebound damper looks like.
  • + 5
 For me, that was a Captain Obvious's article. But it's always good to repeat the basics for beginners and others.
  • + 3
 ya pwcutajar that would be me I have no idea what forks that cost a fortune are good for honestly I am probably just worthy of a cheap domain 180 instead of my friends high end fox 180 that has been pushed (cost small fortune). I try messing with the fork but by mid ride I just say F it and ride have fun and not care how it feels lol
[Reply]
  • + 13
 Speak for yourself Sante, but I cant get enough of that "rebound whoosh" that you speak of. It's up there with the awesome sound of a quality hub freewheeling.
  • + 4
 Kind of like a blow-off valve on a turbo. sounds kind of cool. I have to say though, now that I have a rear mech with one way clutch, it's nice to hear the tires on the trail and surroundings more than the bike.. The chain slap and whooshing are a little distracting. Makes me think of the bike under me which distracts from total focus on the trail ahead.. That's just me though.
  • + 1
 Is that what the guys in the vids have (clutch rear mech)? Cause even with a tyre wrapped around my chain stay all I hear is clangabangaclang of my rear derailleuer no matter how fast or slow my rebound is. Suspension is trial and error. Don't set your rear rebound super fast and compresss the bike as much as possible into the transition before a dirt jump or you will do half a front flip and land on your face like I did. Now I slow that shit down. Thanks 661 for me still having a face.
[Reply]
  • + 9
 really like these. I love learning more about mountain bikes! More of these please!!!
[Reply]
  • + 4
 good to see this write up, on the other hand it comes a day after i got off the phone with the guys at fox due to rebound problems with my Fox Float 34 29er, it doesnt have the FIT cartridge its more of an open bath system that does not work at all, they told me "we really don't have a lot of experience with these set up....????what??? you make them yet dont have experience???!?! what kind of "george bush" bullshit answer is that!
I called the guys at Push and they were way more helpful yet also did not work with the "non"FIT dampner system, now i am aimlessly trying to fix my own fork
thanks FOX you make me want to put ROCKSHOX on everything i own
[Reply]
  • + 6
 Thanks very much. I don't have a mechanic that I can pass my bike to and say "It's just doesn't feel dialled". That's where articles like this come into their own.
[Reply]
  • + 6
 So now we get to read about what components do, but don't get to learn to work on them? I want tech Tuesday back!
  • + 3
 more tec-Tuesdays!
  • + 1
 Yes. More tech tuesdays please. This article is very well-written and simplifies/clarifies adjustments to the most important components on our bikes.
[Reply]
  • + 4
 I hope Fox hired this guy to eventually bring smart magnetorheological fluid filled shocks to the market. Say good-bye to suspension tuning.
[Reply]
  • + 1
 I love to read articles about damping, but didn't anybody else notice the plot holes in this one? For instance...
"At high lean angles it can result in loss of traction."
How does fast rebound result in loss of traction at high lean angles??? This would have the tire on the ground more, resulting in higher traction. More logic please.
Also, not everybody wants their bike to skip over the top of everything world cup DH style(though I happen to like it like that), lots of riders just don't have the nerve to go that fast over the rough stuff.
  • + 4
 This is pretty standard suspension workings. At high lean angle if your rebound is too fast it is going to push your wheel away from your bike at the slightest loss of traction. The faster the rebound the further away your wheel will travel over the same bump changing your line. Slow the whole process down and your rear wheel will stay on your chosen line and still suck up the bump.
  • + 0
 Thanks, Fatboyslim, I could see how that would help hold your line at medium lean angles, where maximum traction is not an issue. Having a hard time understanding how extra air time for the tire helps at high lean angles which are at the edge of available traction. If you set up your race car so that the front had slow rebound, the front would just lose some traction over bumps and push/understeer. Why would a mtb be different?
  • + 2
 Hey Foghorn. Who you calling fat? Its FASTboyslim! Hahahaha. The above will have less effect on medium lean angles. Your suspension will be operating in a vertical axis. High lean and your suspension will now be working closer to a horizontal axis. High lean angle and your slip angle can be changed loads but medium lean not so much. A car set up is a lot different so a bad comparison. A motorbike would be a bit better to compare but still not good as the motor has a huge influence over your suspension setting as to corner exit and entry.

The extra air time he speaks of is pointed more to straight line than turning.

This stuff is harder to explain when you cant use your hands to explain the motions.
  • + 3
 WHoops! Sorry, hope your self esteem didn't take a hit! I guess I need to go to Shaolin and meditate on this for a few decades.
  • + 3
 Hahaha, you will get it. Search for some mx tuning sites and have a read. I am going to search for a wonder diet since you called me fat! That was low man.
  • + 3
 Take a free shot then, I deserve it! Lol
[Reply]
  • + 2
 Great info !! What about the rebound adjuster knob on the bottom of the fork leg , my got bent and I had to remove it to get adjustment again !! It's to exposed down there . Ps it's a 2013 ctd fit talas so it's NEW STUFF
[Reply]
  • + 1
 So - they make one damper unit fits all weights. Since its is not linear in action you are stuck with 3 clicks for adjusting out of 18 clicks possible. Why not 3 different damper needle valve units for 3 weight groups and thus increasing the linear characteristics to 9 out of 18 - or better? Still like my DHR 3.0 with spring a lot - very easy tuning and good to ride.
  • + 1
 That would mean stocking 2 more part codes for inventory and that costs a business $$$.
  • + 1
 Thats a bad idea wakaba. Those 3 clicks are are for dry set up. With the same shock and spring set up in wet weather you would use the rest of the 18 clicks adjusment. Not all but you would be looking at the rest of your tuning range.
  • + 1
 Rebound circuits are designed to accomodate a range of riders, not provide a single rider with a load of adjustment. One shouldn't need more than the 3 settings. Once my spring rate and sag are set, I find my rebound setting and pretty much leave it. Conditions shouldn't have an effect- I have never felt the need to adjust for wet/dry trail conditions.
  • + 1
 I have had an Ohlins technician all through my GP career. 2 degree in track temp is enough to warrent resetting. Settings between dry and wet are day and night difference. If YOU are not changing your settings from condition to condition then i suppose you dont need more than 3 clicks.
  • + 2
 Corrected, and you're right. Here in Anch, we get an average of 6.8" of rain between may and aug. I shouldn't and don't know anything about proper wet conditions. I wiki'd london and there's no comparison. You guys get that within a week! I stand way corrected.
[Reply]
  • + 1
 It is good to know that Fox dampers being quieter than the competition is no accident. I dug up a a href=http://service.foxracingshox.com/consumers/Content/Resources/Images/36FITRLC/removeRbndNeedleAssy02.gif>picture of the fluted rebound needle/a> if anyone is interested in seeing it.
[Reply]
  • + 1
 Great article. One thing I've noticed that wasn't mentioned was the huge effect that unsprung weight has on required dampening level. I find that if my wheel or tire combo weight changes by more than 100gms that I have to alter the rebound setting by at least a click.
[Reply]
  • + 1
 Good read. I never knew about too fast rebound damping resulted in pothole bucking.

I always figured that was supposed to happen: the more time your tires are on the ground , the more traction you have?
Need to readjust.

Also I would be curious to know what those three settings of rebound are that Fox thinks are good. They always say adjust to your preferences in their manuals. It would be very helpful to know these.
  • + 1
 those 3 settings are:

1- the "Ideal" rebound speed for a given rider after setting sag. Works well in most scenarios FOR THAT RIDER.
2- above setting +1
3 above setting -1
[Reply]
  • + 3
 It's the first article i've red that says its OK to have a faster rebound.....most people say fast rebound is bad...i beg to differ. Really good write up!!
  • + 0
 I've always heard/read to keep rebound as fast as is comfortable without getting bucked or getting that wild ride feeling. I move mine to fastest setting, click it back approx 40%, then ride and adjust as needed.
  • + 0
 I allways used to run my rebound pretty damned slow , sent my shocks for a service and ' tune ' and they come back way faster than i'm used to running , thought it was gonna be really bucky ride but the difference it made in traction is amazing !
[Reply]
  • + 0
 So what the hell is up with the newer CTD forks then? this article is great but the newer ctd forks seem to be very limited to internal adjustments for the shims stack, re compression and setting travel with spacers provided by fox, ex, rubber spacers to push the air cartridge up the fork eg reducing the travel if you go over 80psi the negative coil-spring extends the air cartridge, lengthening the axle to crown and travel, we are looking at adding a second spacer and I want to increase the oil levels over Fox rec levels to make the fork more progressive, quite frankly Im hating CTD ad for such an expensive fork the lack of tuning capability is a slap in the face. If what Im told by suspension tuners is correct still have my doubts but seems the negative spring is not available in other spring rates wth Fox. Fox ctd forks seem to be to be over damped and under specced for compression, ctd is for for budget level riders imo but for the high price forks like Ive aid for my float 34 CTD you should be able to get the compression rebound levels for faster harder level riders, BOS, DVO, XFusion Im seriously looking into if this cant be sorted, I love the 34 chassis, looks, Kashima but the internals seem pretty average.
[Reply]
  • + 0
 good intent! but I would have expecting to hear more on the compression part of things. eg. relation between HSC and BottomOut pressure and progression. Race setup on the WC circuit in terms of rider weight / sag / HSC / LSC. We need a FOX technician to set up the sag? Like I need a SDG technician to tell me how to sit on my *** on their overpriced pieces of plastic.
[Reply]
  • + 1
 Awesome, love the descriptions. However, I still struggle really understanding the nuances (my bad). I think I need to pair this with hands on coaching. I wish I could attend a clinic.
[Reply]
  • + 0
 Not a bad article, but the next time you can add a few diagrams of how a needle and a Shim stack works, a few Dyno plots, etc....... all these picture of Fox are really not needed at all.
[Reply]
  • + 3
 Nice! Like the adjustment checklist.
[Reply]
  • + 0
 Fantastic article! I've always found myself drawn back to the same small section of the 18 click range in rebound, and at least now i know why! More tech articles like this please!
[Reply]
  • + 1
 Very informative article, makes for interesting reading and gives a great understanding of the importance of correct setup. Can we see more of these articles !!!! S
[Reply]
  • + 1
 Is there a guide for the air-spring pressure and rebound velocity? If there is it would be helpful. If anyone knows... it's for a float 32 rl 120mm
[Reply]
  • + 3
 More of this please!
[Reply]
  • - 2
 I smell a f*ckton of elitism in these comments, do you really have to know the advanced details of hydraulic damping to deserve to ride high-end suspension? For most of us 'how squishy are they?' suffices to describe forks, not 'how progressive is the spring curve?' or 'how many oil flow paths are there in the rebound damper?'.

I'm not saying that it's a bad thing to be nerdy about suspension, if anything it'll result in a more dialled setup and therefore a faster bike and by extension, rider. What I am saying is that you can still own a £1000 set of forks without knowing the details of how the damping works, that doesn't make you an idiot.
[Reply]
  • + 1
 Can we get a guide to tell us what weight to rebound set up or something PB?
[Reply]
  • + 2
 Very good article, I think some people will learn from that.
[Reply]
  • + 1
 This is a great idea for a weekly series and rebound dampening is just the thing to get started with. Well done!
  • + 1
 Bugger me, the article is called 'Rebound Damping' and people still insist on calling it dampening. Dampening is what you do to a cloth before you wipe down the table!
[Reply]
  • + 1
 interesting, definitely gonna keep this one around to reference for tuning once mother nature quits pissin on the trails!
[Reply]
  • + 1
 Really really Helpfull thanks.
[Reply]
  • + 1
 Thanks Mike! Cant wait to play around more with my set up this weekend!
[Reply]
  • + 1
 I can see why this guy got the job!
[Reply]
  • + 1
 This is a great idea. Educate me while entertaining me. More please.
[Reply]
  • + 1
 is this sort of like tech tuesday?
[Reply]
  • + 1
 awesome !!! i`d like to read more of that
[Reply]
  • + 1
 awesome article. Keep it up!
[Reply]
  • + 2
 vague and almost useless
[Reply]
  • + 0
 I really like this article. Fox is ahead in the world of shocks! We salute you! Salute
[Reply]
  • + 1
 The more you know
[Reply]
  • + 1
 More...more..more info..
[Reply]
  • + 1
 great writeup!
[Reply]
  • + 0
 Hmm, nothing out of the fairly obvious there really :/
[Reply]
  • - 1
 Holy black penis that's sick
[Reply]
  • - 2
 Wait what? Ziiiiiiiiiip!
[Reply]
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