"small bump sensitivity", I think I get it now (maybe)

[RebornRider]

Despite being nearly as old as dirt, I started serious mtb riding only about 3 years ago. I'm trying to learn the language and to develop some tech skill. Small bump sensitivity shows up in (probably) every suspension review, video, and thread I've seen, but I had no feel for what that term meant.

About a year ago I replaced the original air shock on my Turbo Levo with an EXT coil. I did this because my regular trail has long stretches of *very* rough hardpack and washboard. These are not just braking bumps, rather they are the result of California weather and clay (a few months of rain and rapid runoff followed by baking in the sun for the rest of the year). I did a lot of shock adjustments, but I couldn't smooth out the washboard and avoid bottoming out in the bigger drops (with the same settings). I'm happy to say that the coil made a significant improvement in the choppy bits.

Now to the point: Last week I put the air shock back on so I could send the coil back to EXT for routine service. On a stretch of slightly bumpy and pebble-y fast downhill fire road (at about 25 mph), I finally felt it! The loss of small bump sensitivity in the air shock meant that I felt every little bump and pebble through the bars and the seat. It seemed very subtle, and not something that would motivate the change to a coil if it was the only factor, but at least I understand the term. Maybe.

 

[apac]

Getting both initial stroke sensitivity and mid stroke support seems to be the hard thing to get with an air shock. I have a fox X2 and its a very very fine balancing act being able to get an amount of both. To get a good amount of one you lose the other.

 

[Mikerb]

Getting both initial stroke sensitivity and mid stroke support seems to be the hard thing to get with an air shock. I have a fox X2 and its a very very fine balancing act being able to get an amount of both. To get a good amount of one you lose the other.

because it is not just controlled by the shock........the kinematics of the bike also play a role.

 

[apac]

because it is not just controlled by the shock........the kinematics of the bike also play a role.

How do you know from your bikes kinematic points what is a suitable shock to buy to get the best out of it?

 

[Mteam]

How do you know from your bikes kinematic points what is a suitable shock to buy to get the best out of it?

You can look at the leverage ratio and how it changes throughout the rear suspension travel to get an understanding of whether a coil might suit better than an air shock, and whether an air shock with a smaller or larger air chamber might suit.

or just ride it with what the manufacturer supplied - which is probably going to be good enough for most people (inc me)

 

[Alexbn921]

Small bump compliance is all about friction. A coil has a less break away friction and better small bump compliance.

Servicing your fork regularly will reduce a significant amount of friction too.

 

[Mikerb]

You can look at the leverage ratio and how it changes throughout the rear suspension travel to get an understanding of whether a coil might suit better than an air shock, and whether an air shock with a smaller or larger air chamber might suit.

or just ride it with what the manufacturer supplied - which is probably going to be good enough for most people (inc me)

.............or the shock used on any model further up the range of the bike you bought.
I think it is the case that some brands select a tune that has too much compression, especially on lower cost shocks that have limited user adjustability. The majority of shocks bought aftermarket have medium tune throughout so if you go down that route and get a shock with LSC and HSC you can get it dialled regardless.

 

[R3Z3N]

The wording for "small bump sensitivity" is very vague.

If you have a sensitive touch, you feel the smallest of changes.

So is "small bump sensitivity" referencing rider, frame or shock?

To clarify, what people mean is the fork/shock is compliant at low speeds/low oscillations. It is ACTUALLY less sensitive for the rider.....you are muting small chatter from your riding experience.

We need to get back into facts vs feelings and get these frames on dynos.

 

[RebornRider]

Small bump compliance is all about friction. A coil has a less break away friction and better small bump compliance.

Agree that coils have less breakaway friction, but I've also seen that the effective spring rate for air springs is higher than the "equivalent" coil in the first part of the travel. I'm pretty sure I saw this in a Vorsprung Suspension Tuesday Tune video. I'm looking for it.

 

[RebornRider]

So is "small bump sensitivity" referencing rider, frame or shock?

I've always *assumed* we were referring to the spring/damper assembly. IOW, I think small bump sensitivity means that the spring/damper can absorb the impact of small bumps without passing them through to the frame (and then the rider).

I've been a Systems Engineer in product design for about 30 years, and one of my tasks is to write product requirements. Ambiguity is not our friend! Ambiguity is why it's hard for newbies like me to learn a new language such as MTB Speak.

 

[Mikerb]

Agree that coils have less breakaway friction, but I've also seen that the effective spring rate for air springs is higher than the "equivalent" coil in the first part of the travel. I'm pretty sure I saw this in a Vorsprung Suspension Tuesday Tune video. I'm looking for it.

initial breakaway for both air and coil is already achieved when riding because each sits c 25% into travel just by SAG.The primary difference between air and coil is that air is progressive and coil mostly linear. All suspension settings are a compromise but it is possible to make that compromise work for any given rider weight and one reasonably similar set of riding terrains. The compromise will show up when you ride a completely different type of terrain..........unless you reset the suspension for that ride!

 

[RebornRider]

initial breakaway for both air and coil is already achieved when riding because each sits c 25% into travel just by SAG.

Not trying to be a brat (an ill-mannered immature person, not a sausage) , but this is not correct. Initial breakaway refers to the transition from no relative motion to achieving relative motion. It's a function of the higher seal friction in an air spring damper versus a coil damper. The air spring damper requires a bit more force to get the piston moving in the cylinder because of the higher seal friction. Unless there are worn spots in the damper, this "sticktion" will be the same everywhere in the travel. AFAIK.

 

[Mikerb]

Not trying to be a brat (an ill-mannered immature person, not a sausage) , but this is not correct. Initial breakaway refers to the transition from no relative motion to achieving relative motion. It's a function of the higher seal friction in an air spring damper versus a coil damper. The air spring damper requires a bit more force to get the piston moving in the cylinder because of the higher seal friction. Unless there are worn spots in the damper, this "sticktion" will be the same everywhere in the travel. AFAIK.

fair enough but when you are riding the shock is in constant motion. Yes of course a shaft runing in seals will result in some resistance but hardly significant when you consider the force being applied to it. A coil shock still has a shaft running in seals ( the damper)!

 

[RebornRider]

Regarding the variation of the spring rate over its travel of an air spring (not including leverage ratio changes in the rear suspension geometry), this is not the clip I'm looking for, but he makes the same point. I'm still looking for the clip that includes spring rate graphs for an air spring and a coil spring. My recollection is that the typical air spring is stiffer than an "equivalent" coil in the first part of its travel, softer in the middle, and stiffer again at the end of its travel.

Vorsprung Suspension Tuesday Tune video clip

 

[RebornRider]

fair enough but when you are riding the shock is in constant motion.

I think part of small bump sensitivity comes into play when the shock is *not* in constant motion. I'm imagining a fire road that is smooth enough that there is not enough force coming through the suspension to overcome air shock sticktion. The small ups and downs of the wheels following the ground are passed through the frame to the rider. This is what I felt on the fire road I mentioned in the first post. It was a small magnitude "jitter" that I didn't feel in my year on the coil. Hitting a pebble resulted in a small sharp jolt passing through the frame to the rider that was very noticable because it wasn't there with the coil. (I'm talking about my regular trail that I ride every week. I know it well)

The coil was less harsh, no doubt about it. I'm just trying to learn how to express that difference in MTB Speak.

 

[RebornRider]

I never know if I should edit an existing post or add a new one. Here's a new one.

I found the Vorsprung Suspension video I was looking for. I recommend watching all of his Tuesday Tune videos (I have, FWIW), but this is the segment I had in mind for this thread:

Vorsprung - air vs coil spring rates

Edit for those who don't want to follow the link. The straight line in this graph is represents the force needed to compress a linear coil spring, while the curved line represents an air spring. One of his points is that the air spring will be stiffer than an "equivalent" coil in the sag neighborhood.

 

[Mikerb]

I think part of small bump sensitivity comes into play when the shock is *not* in constant motion. I'm imagining a fire road that is smooth enough that there is not enough force coming through the suspension to overcome air shock sticktion. The small ups and downs of the wheels following the ground are passed through the frame to the rider. This is what I felt on the fire road I mentioned in the first post. It was a small magnitude "jitter" that I didn't feel in my year on the coil. Hitting a pebble resulted in a small sharp jolt passing through the frame to the rider that was very noticable because it wasn't there with the coil. (I'm talking about my regular trail that I ride every week. I know it well)

The coil was less harsh, no doubt about it. I'm just trying to learn how to express that difference in MTB Speak.

3 psi difference in tyre pressure would make the difference between feeling that and not. Its likely small stones on a fireroad would not produce enough force to lift the weight of the chainstays/seat stays let alone move the shock. ps a lot of people underestimate the influence of rebound/damper especially when it comes to small bump sensitivity....ie have too much rebound set.

 

[RebornRider]

3 psi difference in tyre pressure would make the difference between feeling that and not

Agree that the air spring feels like I'm running 35 psi in the tires rather than my customary 25 psi. Lots of small magnitude but sharp jitter coming through the seat and bars that I didn't feel with the coil. Sounds like a difference in small bump sensitivity to me.

Regarding rebound, I always start at the midpoint of the adjustment range, then tune based on oscillation after riding off a curb. Then I might further adjust based on how the bike feels on jumps. I can't speak for lots of people. Just me.

 

[Planemo]

fair enough but when you are riding the shock is in constant motion. Yes of course a shaft runing in seals will result in some resistance but hardly significant when you consider the force being applied to it. A coil shock still has a shaft running in seals ( the damper)!

The damper does indeed have seals and yes they generate stiction but they are also much, much smaller than airspring seals and therefore have much less surface area to overcome. Ergo I found that there was virtually no perceptible stiction in my damper shaft when I had my lowers off.

Stiction in airspring seals is a very real thing. It is after all the single reason why negative air chambers were designed - to help overcome stiction in the first 35% or so of travel (so includes small bump travel). Its not limited to just the sag portion of travel.

Yes in an ideal world the shock should be in constant motion but its not. Every little bump requires a direction change therefore a transition from static to moving. Its all those little transitions (and stiction points) that we are trying to overcome.

 

[Alexbn921]

I think part of small bump sensitivity comes into play when the shock is *not* in constant motion. I'm imagining a fire road that is smooth enough that there is not enough force coming through the suspension to overcome air shock sticktion. The small ups and downs of the wheels following the ground are passed through the frame to the rider. This is what I felt on the fire road I mentioned in the first post. It was a small magnitude "jitter" that I didn't feel in my year on the coil. Hitting a pebble resulted in a small sharp jolt passing through the frame to the rider that was very noticable because it wasn't there with the coil. (I'm talking about my regular trail that I ride every week. I know it well)

The coil was less harsh, no doubt about it. I'm just trying to learn how to express that difference in MTB Speak.

What you are feeling is the reduction of friction. Every time the suspension starts to move it has to overcome a shelf of friction. AKA Breakaway force. Air has 2-6x times as much friction depending on long it's been since service.

Fork test. Put weight on your bars slowly increasing until your fork moves. A fresh fork will slowly since into it's travel with no jutter. A fork that needs service will hold your weight and then jump to a stop.

Even the bushing used in a rear shock can effect feel. I swapped out to needle bearings on my high rotation link and had to add 20psi to the shock. The difference was night and day.

 

[Mike D.]

Not trying to be a brat (an ill-mannered immature person, not a sausage) , but this is not correct. Initial breakaway refers to the transition from no relative motion to achieving relative motion. It's a function of the higher seal friction in an air spring damper versus a coil damper. The air spring damper requires a bit more force to get the piston moving in the cylinder because of the higher seal friction. Unless there are worn spots in the damper, this "sticktion" will be the same everywhere in the travel. AFAIK.

"Not trying to be a brat (an ill-mannered immature person, not a sausage)"

Wurst Metaphor In Thread Winner.

 

[KnollyBro]

Agree that the air spring feels like I'm running 35 psi in the tires rather than my customary 25 psi. Lots of small magnitude but sharp jitter coming through the seat and bars that I didn't feel with the coil. Sounds like a difference in small bump sensitivity to me.

Regarding rebound, I always start at the midpoint of the adjustment range, then tune based on oscillation after riding off a curb. Then I might further adjust based on how the bike feels on jumps. I can't speak for lots of people. Just me.

I am trying to understand why you care about how your bike feels when riding down a fire road that has small pebbles on it or is it just the point about recognizing small bump sensitivity? Also, if you swapped out the coil shock for an air shock (for service) and noticed all this, are you running an air or coil fork? The part of the trail where you feel the sharp jitter coming thru the seat could be solved by standing up

 

[RebornRider]

I am trying to understand why you care about how your bike feels when riding down a fire road that has small pebbles on it or is it just the point about recognizing small bump sensitivity? Also, if you swapped out the coil shock for an air shock (for service) and noticed all this, are you running an air or coil fork? The part of the trail where you feel the sharp jitter coming thru the seat could be solved by standing up

It was just an observation. Not a complaint. I'm trying to learn the language. Please reread the first paragraph of the first post.

FWIW, I run a Smashpot coil in my Rockshox fork for exactly the same reason I run a coil in the rear suspension.

 

[KnollyBro]

It was just an observation. Not a complaint. I'm trying to learn the language. Please reread the first paragraph of the first post.

FWIW, I run a Smashpot coil in my Rockshox fork for exactly the same reason I run a coil in the rear suspension.

Congratulations on your epiphany. It was a long thread to read. If you reread my first sentence, it asked if you were still on the same topic or just happy you figured it out. Happy trails!

 

[1oldfart]

This in my opinion is a nice rainy day question.
My usual answer is ride more.
Just take 30 minutes on a bikes forum.
Most bikes are begging for an upgrade from their average rider.
You might get what you look for with a different PSI
You might get what you are looking for from different tires.
Or just improving with hours on the saddle.
This is just a suggestion but a looser set of hands will avoid transmitting all vibes.
Use your saddle to steer your bike.
I like 2.6 to 3.0 i do not care what the trend is.
I like a 600 mm bar, yes you guessed it, i do not care what the trend is.
Try stuff, find what you like in real life, no need for charts if you ask me.

 

[Mikerb]

If indeed you are mostly trying to learn the terms used in mtb, small bump compliance primarilly means suspension that is plush enough at the beginning of its stroke ( after SAG) to respond to small undulations whilst maintaining tyre traction. The suspension, coil or air, does nbot of course know one bump from another! What we are really talking a bout is trail conditions that cause a relatively slow compression of the shock and mostly at the initial 3rd of the total stroke. Typical trail conditions are therefore small undulations and shock/fork compression pushing into a berm and that is adjusted using the low speed compression controls.
It really does not cater for running over a gravelly surface since your rebound setting would have to be zero. There would be no time for the suspension to compress and rebound unless the rebound was set really fast..probably wide open.......and that would be like a pogo stick on regular small bumps!! As I said a bove, the main suspension for continuous gravel stone hits is the tyre and a suitably low pressure is all that is required to absorb it.........although maybe not on a DD carcass.......but then if you are running that heavy a tyre you really do not care what a fireroad feels like!