What you are describing is a high preload spring setup. You are correct that adjustments to the spring characteristics change the optimal rebound damping settings.

Your particular approach may not suit everybody. Personally, I did a 24 hour race with negative set lower than positive and ended up with numb hands for a month because of the harshness. I now tend to favour negative higher than positive setups and don't have such problems.

Commenting on:

THE GOAL
Simply put, rebound damping should smooth out the trail chatter. You should glide over smaller imperfections and feel more connected to the surface. If you feel evey little bump and your fork is pogo'ing, then you don't have enough rebound damping. Once you smooth out the ride, however, you'll be able to crank much more efficiently--all of your energy will be focused on forward motion--so it's a worthwhile goal, IMHO.

I find that the most noticeable characteristic of a well setup rebound setting is the level of grip achieved by the front tyre (i.e. when the chatter disappears, it stays in better contact with the ground). If the rebound is too fast the front tends to wash wide. The subtlety in the setting can be one or two clicks on a typical adjuster.

The signs of packing down I don't find as obvious. Sometimes I only notice when I back off the damping and notice the front end sitting higher and feeling more active. Again, the threshold setting can be determined within one or two clicks.

There is a difference in behaviour depending on whether your fork has a dual-flow (high speed and low speed circuit) rebound. The dual-flow type designs have a wider sweet spot.

The biggest improvement I made to my fork tuning was to mark a baseline setting on the adjusters so that my cumulative twiddling is all relative to a known "in the ballpark" setting. It saves me from gradually drifting away from the known good region on he basis of kiddign myself about subjective riding impressions.

YMMV

 

What you are describing is a high preload spring setup. You are correct that adjustments to the spring characteristics change the optimal rebound damping settings.

Agreed. I'm familiar with your sag/pos/neg observations and found your contributions in this thread especially useful.

Adding rebound by letting out the neg. air is a contradiction to me, since you have the rebound knob which does that job. By letting the neg. air out, you're just losing softness of the fork (=dampening), which contradicts its whole purpose.

I wouldn't call it a contradiction; I'd just say it's another option. If you look at XC racer Sam Schultz's SID settings, for example, he runs 110 pos and 100 to 105 neg (source: MBA Apr2012). Obviously, he rides for a living and what's tolerable for him may not work for recreational riders.

But it sounds like you two agree that higher negative pressure provides relief during longer rides and rocky terrain. While I don't have much experience with either, I'll definitely keep it in mind should I find myself facing such conditions.

And it sounds like, we're in agreement with regards to the desired outcome:

I find that the most noticeable characteristic of a well setup rebound setting is the level of grip achieved by the front tyre (i.e. when the chatter disappears, it stays in better contact with the ground). If the rebound is too fast the front tends to wash wide. The subtlety in the setting can be one or two clicks on a typical adjuster.

The signs of packing down I don't find as obvious.

That's been my experience, as well.

Thanks for sharing! :thumbsup:

 

I think that most important aspect of tuning is the type of trail you'll be riding.
Having no negative air on my local trails would cause your hands to fall off, since they're mostly rocky.
Adding rebound by letting out the neg. air is a contradiction to me, since you have the rebound knob which does that job. By letting the neg. air out, you're just losing softness of the fork (=dampening), which contradicts its whole purpose.
I tried once riding with equal negative and positive air, it wasn't a pleasant ride. Added 15psi over positive and it was a world of change.
This is my setup, if anyone can find it useful:

My weight w/ gear: 85kg/187lbs
Type of fork: 2012 Revelation RL 150mm dual air
Positive/negative air pressure: 90/105psi
Rebound clicks from turtle: 6 (it's about middle way to fastest)
Type of local trails: technical rocky singletracks, some slow, some fast paced with occasional roots and drops.

 

I don't know if I'm the only one, but it weems you have everything wrong.

The purpose of rebound damping is NOT to smooth trail chatter. It's purpose is controlling the speed at which your fork gets back to its resting position. Depending on terrain type, you'll want a slower or faster rebound. The general rule is that the more successive bumps you encounter, the faster the rebound you'll need to prevent packing up an help keeping your tire on the ground.

The purpose of a negative spring on forks is NOT to control rebound speed, it's purpose is to set the small bump compliance of your fork, or to say it otherwise, absorb trail chatter. Sure it affects rebound speed so it has to be used in conjunction with rebound damping depending on the frequency on the bumps and desired feel.

Maybe I'm not reading you right but I think you got things reversed... You are right on the outcome however your understanding is off and your way of setting up your fork is very weird...

 

I don't know if I'm the only one, but it weems you have everything wrong ...

... Maybe I'm not reading you right but I think you got things reversed... You are right on the outcome however your understanding is off and your way of setting up your fork is very weird...

LOL No problem.

I agree that my approach is different (to some degree) than the prevailing wisdom or setup advice. That was the point of this thread--to challenge the manufacturers recommendations because they fall well short, IMHO, of explaining what it is we should be aiming for with regards to effective rebound damping.

For example, how do you know if you have better grip? What signs on the trail would you look for? And why do XC racers use less negative pressure?

Just to clarify, I agree that rebound damping is used to control the speed of the rebound. But if you want to alter its force, then adjusting negative pressure is an option.

 

I would love to see a diagram that describes how the negative/positive spring pressure works.

I'd also like to see a spring rate/travel plot for a series of different air spring forks and pressures.

DG

 

There is such a graph around MTBR forums that shown exactly that. I'll try to find it.

 

I am interested in this as well. I have a Dual Air RS Reba and have just kinda randomly picked a rebound setting. My last fork was a spring so not many adjustments to make. The thing that really is confusing me is the pos/neg air. I adjust the pos for sag, then I add in neg air and it affects the sag. If I add in neg air it affects the rebound. If I add in pos air it affects the compression. If I add in too much neg air I loose travel. Every time I play around with one air setting it affects so many other ones its hard for me to get a baseline down. Then I setup rebound for one air pressure, and I change something and my rebound feels different.

I have played around with alot of different pressures (70-150 psi) and everytime my fork feels different. And since its mountain bike riding I do a fork setting do a ride with a couple of good downhills. Ride uphill and I am tired. Go home change some fork settings come back next week. Ride the trails but can't really compare because I don't remember exactly how it felt the week before. Without a ski lift/shuttle I find it very hard to compare suspension settings since so much time inbetween rides has occured.

How do you guys test your suspension, or maybe you are doing longer rides than I am. Do you stop in the middle of a downhill, tweak your settings ride for alittle then stop and do it again? Alot of my downhills are steep technical that are over in under 2 min, then a 30 min climb. So its hard for me to get enough runs in to test.

 

See this thread for some good info on setup: Help dialing Reba

Here's an excerpt:

Just to give you an idea of what is possible, in my model (based on a 150mm fork) the following combinations give the same sag:

80+ 46-
90+ 67-
100+ 89-
110+ 110-
120+ 131-

Out of all of those combinations, 80+ 46- will have the highest preload and will blow through its travel the easiest. It has the lowest spring rate.

120+ 130- will have the lightest preload (possibly a bit sucked down). It will have the firmest mid stroke feel and the highest spring rate.

As for setting rebound (how and when), I'll let others chime in since I don't do a whole lot of DH.

 

See this thread for some good info on setup: Help dialing Reba

Here's an excerpt:

As for setting rebound (how and when), I'll let others chime in since I don't do a whole lot of DH.

hmm link isn't working for me. When I first got it I read a thread on rebas and it there were very conflicting methods (kinda like this one) and I tried them all. That's why my range was from 70-150 psi but it was still hard to compare since so much time between rides. Does your link give a good guide on setting up air pressure? I am willing to try but I need a good method to narrow it down. Other wise it feels like I am shooting in the dark.

 

Here you go: Help dialing Reba

 

Interesting.

I'm still not wrapping my head around how the positive and negative air chambers work, conceptually/mechanically. Is it basically a piston in a pressurized tube, with different pressures above and below the piston?

I'm not following how this is better than having the top of the piston pressurized and the bottom vented to atmosphere (like an air bag suspension on a big truck). Can someone explain it like I'm 5?

Looking at the spring rate graphs, and assuming that they are rate graphs (rate vs displacement) as opposed to spring check graphs (force vs displacement) the fork starts out with a high rate at full extension, then the rate falls as the shock settles in to the sag point, stays kinda-sorta linear for 40mm or so, then starts ramping up.

OK, so the differential sets where in the fork travel the quasi-linear bit is, and the absolute sets the rate in the area between full extension and the ramp-up.... maybe that's what the negative chamber does, it lets you move the linear portion around...

DG

 

Is it basically a piston in a pressurized tube, with different pressures above and below the piston?

Yep

I'm not following how this is better than having the top of the piston pressurized and the bottom vented to atmosphere (like an air bag suspension on a big truck). Can someone explain it like I'm 5? ...

Not trying to be dismissive, DG, but I was really seeking input from others on how they approach rebound adjustments, what issues they're trying to mitigate and their ultimate handling objectives. Guidelines, observations, anecdotal information.

As you can surmise, the change in volume results in pressure changes. Since the positive/upper chamber is larger (as in the second example), the changes are not as dramatic as those realized in the negative spring. How this affects linearity, opposing forces, etc. would probably be a good subject to search or discuss in a separate thread.

Any thoughts or experiences you might want to share regarding rebound adjustments, though, would be greatly appreciated.

 

Any thoughts or experiences you might want to share regarding rebound adjustments, though, would be greatly appreciated.

My thoughts are that idea damping grip has been shown to occur at a damping ratio of 0.65 to 0.75 or so.

But in order to calculate damping ratio, you need to know the natural frequency of the system.

And to do that, you need to understand how the spring works. A linear spring, as used in most automotive applications, is easy to work out. Air springs, being purely progressive, are a trickier beast. And air springs in the MTB application, which have more complicated mechanisms, will be even more interesting.

I'm already starting to understand why MTB applications have position-based damping circuits; it's a requirement of the changing spring rate. Flat damping with a velocity-regulated digressive knee probably won't cut it.

In other words, help me wrap my head around how the spring functions, and I'll calculate you a damping curve from first principles - which is what you want, right?

DG

 

In other words, help me wrap my head around how the spring functions, and I'll calculate you a damping curve from first principles - which is what you want, right?

DG

Not exactly but I definitely appreciate your approach. I'm looking for more of the test & tune strategies to determine effective damping. If I were at the track, I'd look at air pressures (cold and hot), tire temps, exit speeds, lateral g's and, of course, lap times. With that data, you can make informed decisions with regards to setup,, rgiht? But on an MTB, it's been a bit of a challege (for me, anyway) to determine because there really is no data to work with.

Which takes us back to the goal. If we're not chasing better lap times, then what exactly are we after? If we can nail that down, then I think we have better chance of securing a desirable outcome.

 

Unfortunately... the RS rebound damper isnt very good. You'll never get it perfect, because its not very tunable and works in a narrow range.

Rebound is all about control. If you've ever lost rebound on a longer travel fork, its just plain scary. On a RS damper, it becomes pretty subjective and rider based. Some people like a slower fork (its more poppy), some people want it faster to absorb chatter without packing up. Settling somewhere in the middle is a good compromise, but it'll probably pack up in the roughest chatter.

The DA spring really should be left completely out of the rebound damping equation. You really want to set your spring so it works well as a spring, and let the damper do its thing separately. Messing up the spring curve to try to get back damping will give you a worse ride.

If you upgrade to the blackbox shimmed damper, everything changes and you really open up options.

 

I'm looking for more of the test & tune strategies to determine effective damping.

Those don't work.

Time and time again, at least in the auto racing context, empirical strategies for setting damping have been proven to arrive at the wrong solution. Give the driver access to the knob, and they invariably crank the rebound way, way too high. I had one Mustang guy with a set of Konis who not only broke my "highest ever rebound force" record, but produced a very strange damper plot - until I figured out that the rebound forces were deflecting the shock dyno fixture (I doubled the cross section on the fixture and the plot returned to normal)

Similarly, I had a customer complain that his C5 was handling "a little weird" and that he thought that it might be different left to right (but wasn't sure) Dynoing the shocks showed the usual super high rebound, but on top of that, it showed his uber-guru shock t00n3r had installed the left side shimstack upside down, so the shocks were mirror images... and it was just "a little weird".

Drivers are just not good at sensing damping details - and that's on a billiard table smooth race track, never mind pounding along over roots and rocks and whatnot.

No race team does the old Koni "add compression until it chatters, back it down a little, then add rebound until it feels right" process that was de rigur for setting shocks for decades. Everything is natural frequencies, damping ratios, and suspension speed histograms these days. The driver isn't allowed to opine on settings; he reports what he feels, that is cross-referenced against the data, and the engineer makes a setup change.

Now the MTB case is both simpler and more complex. Simpler, in that there is no lateral weight transfer to deal with. More complex, in that the number and size of bump impulses are WAY larger, travel is several times larger (the race car saw maybe 40mm bump travel) and the location of the CG can move 6" higher and 5" forward by the rider standing up.... there's a couple of PhDs waiting to happen in MTB suspension analysis (good Lord, what about the tire...) and if I ever go back to school that's what I'd like to do. But in the meantime, much can be learned by looking at the suspension from first principles, and THE most important first principle is the spring.

The spring reacts to bumps and the dynamic situation, the damper controls the spring. If you don't know what the spring is doing, there is no point in talking about the damper - except in odd cases where you aren't allowed to touch the spring so need to fake spring dynamics with damping (SCCA Stock classes) but which do not apply to mountain bikes.

You'll never get it perfect, because its not very tunable and works in a narrow range.

I'm inclined to agree. Assuming that the rebound adjustment is a low speed bleed, and given that you've got an air spring with some sort of nonlinear rate, rebound damping is only going to be optimized over a narrow range of spring rate - which on an air spring, also corresponds to a position. I imagine that - although I might have it wrong - that the best case is getting the rebound at or near 0.7 at the travel amount when the bike is at max cornering G.

MTB suspension is really fascinating, and it isn't easy or obvious.

DG

 

MTB suspension is really fascinating, and it isn't easy or obvious.

DG

That's what I'm sayin'!

Given the variety of surface conditons, equipment and riding technique, I'm inclined to believe that optimal suspension setting represents a compromise: What works best for certain conditions will produce less desirable results in others.

For example, compression damping is always compromised. While one setting might speed up your descent over bumpy sections, it may compromise power transfer while pedaling on flat sections or climbing. So you need to choose where speed is most desirable--descents, flats, corners or climbs--and adjust accordingly.

Similarly, finding the best compromise for rebound damping is a matter of isolating when optimization is important and the improvement you're looking for, e.g. speed, grip, comfort, etc. Getting it right for high-speed descents may be your primary goal, for example, but it's likely you'll give up speed in other sections.

 

I'm inclined to believe that optimal suspension setting represents a compromise: What works best for certain conditions will produce less desirable results in others.

Based on my race car suspension work, that isn't necessarily true - or perhaps it is better stated as "the degree of compromise required is of a much smaller order than is commonly expected".

One of the ways you could tell if the damping was right was that the car got comfortable. Sure, stiff springs and stiffer tire sidewalls made for a pretty firm ride, and no race car will ever be considered plush - but it wasn't jarring, or crashy, or teeth-rattling. The car would soak up a hit and carry on, unpreturbed.

And if you think about it, that makes sense. If you are trying to optimise grip, then you want to minimize the effects of bumps on the chassis. You want to car to "stay in shape" over the bumps rather than bounding around or getting upset and snapping loose or whatever. Grip and comfort are, within reason, two parts of the same elephant.

(without getting into the dicussions of ride frequencies and human comfort which is a different and unrelated kettle of fish)

In a mountain bike context, we all know that there are some obstacles you can ride fully seated, and others that you have to stand for, using arm and leg articulation to absorb the hit. A better tuned suspension would allow you to stay seated for some of those hits, sucking up the bump and preveting the transmission of the shock to the rider (and throwing him off the seat)

The spring just sees displacement and produces a force in some way related to the amount of displacement. The shock just sees piston velocity and produces a force in some way related to the speed of the piston. The suspension cannot and does not differentiate between a root on singletrack or a pothole on a fire road - and that's good, because (assuming that amplitude and frequency are the same for both) it means getting the damping right for one means the damping is right for the other.

If you have the right damping for the set of all possible frequencies and amplitudes, then the suspension will quite literally handle anything.

The hard part isn't the damping; the hard part is determining the operating envelope (that set of frequencies and amplitudes) I had that on the race car. I don't have it for the MTB.

DG

 

If the fork rebound damper has a simple adjustable bleed valve without a shimstack, there is huge compromise, you have to pick faster or slower rebound, you can't tune it for "most conditions".

If it has a shimstack, then you have more tuning range.

 

If the fork rebound damper has a simple adjustable bleed valve without a shimstack, there is huge compromise, you have to pick faster or slower rebound, you can't tune it for "most conditions".

If it has a shimstack, then you have more tuning range.

Oh, agreed - most of the tuning is in the shimstack. The bleed is for making little nudges.

Which is a good point for the OP - if the base curve is all wrong, no amount of twiddling the rebound bleed will fix the problem.

DG

 

So how would one determine if the base curve is all wrong? Or the relevant operating envelope?

 

You need suspension position data from the bike on a typical trail (which can produce suspension velocity data) to get the operating range of speeds. This can also be used to produce suspension speed histograms which can fine-tune valving.

You also need to know the natural frequency of the suspension, which you get from spring rate, motion ratios, and weight distribution.

Complicating things are the nonlinear air springs (assuming constant weight distribution, natural frequency will change according to where you are in the travel) and the fact that weight distribution can change by the rider moving around. Simplifying things is the fact that most of the operating range will be close to static ride height, and large travel events will tend to be transients.

I wish I had 6 months of free time, a battery operated data logger, access to a shock dyno (again), and a very high speed camera. The stuff that could be learned....

DG

 

Oh, well. Guess I'll have to go back to chalk on the sidewalls.

Thanks, anyway.

 

I've got a RockShox Domain RC Fork and a Fox DHX RC4 rear w/ 600 lb spring (I'm about 180lbs). I'm pretty new at this stuff... I see references to pos/neg pressure and I'm not sure whether this applies to my equipment because all I can find is one schrader type valve on my RC4 and no valve on my forks... Could someone fill me in, maybe I need to take something apart to find the other valves?

 

I believe the Domain RC is a coil sprung fork. Links to the related manuals are listed at the bottom of the page:

Domain RC | SRAM

I don't know much about Fox products. Perhaps someone else will chime in or you'll be able to locate similar support documentation online.

 

Negative springs don't apply to coil sprung forks or shocks. They are only on air springs as coils don't have as much inital stiction and harsh top-outs versus a air fork (with positive spring only) given the linear nature of a coil spring.

With that said: 600lbs spring for a 180lbs rider seems like a lot. How long is your shock's stroke and how much travel?

 

Thanks joeinchi! I found a bunch of reference material...

PissedOffCil: I'll have to measure the travel, on the RC4... From what I'm reading, it seems like air springs are considerably more versatile and able to tune much more precisely for a rider's preference... looks like I'm gonna have to revise my letter to santa! ;]