Bicycle Mechanics - Adjusting suspension fork

I don't know alot about suspension forks as I am primarily a roadie but here goes:

Recently I was given a Rock Shox Jett XC Long Travel suspension fork which I installed on my MTB bike (previously had a rigid fork). The suspension fork is very new and hardly used, so I am told.

I have downloaded the manual from the Rock Shox website on this fork which describes maintenance and how to adjust.

The manual talks about sag and how to measure and adjust for it. It states that i should place a zip tie on the inner tube of the fork, just above the seal, sit on the bike, get off, then measure the distance from the seal to the zip tie. The result is about 10-12mm on average.

This manual states that the normal amount of sag for this fork is 13-20mm. So I try to adjust the plastic nut on top of the fork and I find it makes no differance. I understand that turning it counterclockwise decreases the spring tension thereby giving the fork more travel. So I turn, remeasure the distance, no difference, I keep trying until it will no longer turn. Still no difference. So I back both off clockwise this time, until they stop, remeasure, no difference. I make a mark on both nuts to indicate my starting point. I turn both nuts evenly on both sides of the fork and measure the distance for each full turn. No difference still 10-12mm.

I try test riding the bike with the adjuster nut turned either all the way clockwise, counter clockwise, OR roughly halfway between both extremes and I still find no difference in the feel of the fork nor the measurement between the seal and the zip tie.

The manual seems vague. If all riders are a differant weight then sag varies from person to person i.e. heavy rider=more sag. But the manual just states that sag should be between 13-20mm, how does the relate to a heavy rider vs. a light rider. Shouldn't there be some sort of graph stating that between such and such a weight the sag should be this much, etc?

Why is there no difference in feel or sag measurement when I adjust these nuts (or whatever ya call 'em)?

Please help!!

Digger.

The Jett uses elastomer springs - basically little sponges that compress and spring back. The first thing is that the elastomers are set for a rider of a certain weight. If you are lighter than the factory "norm" the fork with ride stiff for you. If you are heavy, it will ride soft. You may be able to purchase elastomers of different densities to obtain the desired spring rate. Check your owner's manual.

Elastomers are temperature sensitive - which drove me crazy with my Jett. They're soft when it's warm and hard when cold. This means it's impossible to keep your fork consistent during times of changing temps, like in the fall. If it's really cold, maybe below 50F the fork may have virtually no spring. When it's hot - say over 80F it will have lots of motion.

The Jett doesn't have a lot of adjustment to it, and the preload knobs on mine made very little difference in its performance. It's a low-end fork suitable for bike paths and light trail duty. On rocky, rooty singletrack it's marginally better than a rigid.

I had taken the nut off last night to have a peak inside the fork. It was a spring about 4-5 inches long, not a rubber elastomizer (sp).

But yeah turning that adjuster knob seems to make little difference.

I am a roadie anyway, and do little MTBing, so the fork should do me fine. Just trying to learn the basics of suspenion forks.

One thing I notice is, when I stand to pedal I am bouncing up and down on the fork, drives me nuts, aren't I losing power/forward momentum?

Digger

One thing I notice is, when I stand to pedal I am bouncing up and down on the fork, drives me nuts, aren't I losing power/forward momentum

Yup! Welcome to the real world of forks!:)

My bike has an RS Psylo shock that puzzled me. The manual said diddley-squat as it was a warranty thing in 9000 languages. A plastic cap cunningly concealed the rebound adjuster on the bottom of the right leg (it should have been removed by the lbs) and some protective black labels covered an important diagram on the bottom of the fork leg as well (they too should have been removed by the lbs). Anyway, what I'm trying to say, is that what you describe is normal, but can be minimised. On the Psylo at least, though I don't know the shock you have, I can set the left leg U-Turn adjustment to the minimum. On the bottom right leg there is a rebound adjuster that I set to its lowest (stiffest) setting, and the red cap on the top of that leg gives me a bit of additional control as well. So, on the road, I have a stiffer front setup so I do get more crank effort getting through to where it's needed. There is still some bobbing, but it's manageable.

Some shocks have lock out. My rear shock does, so I can get a 95% hardtail effect, 90% rigid front configuration which makes road use a little easier. A pain to adjust, but hey-ho, there's no such thing as a free lunch, eh?:)

Originally posted by digger
I had taken the nut off last night to have a peak inside the fork. It was a spring about 4-5 inches long, not a rubber elastomizer (sp).

But yeah turning that adjuster knob seems to make little difference.

One thing I notice is, when I stand to pedal I am bouncing up and down on the fork, drives me nuts, aren't I losing power/forward momentum?

My Jett, as does my son's, has elastomers, so your's is different, and better for it. You may still be able to buy different springs than the factory provided one to give you a better ride.

Suspension bob is part of the mountain biking world. High-end forks have a lock out, which turns the fork rigid when activated, and damping controls, which controls fork movement. If you consider your bob excessive, then you may need a stiffer spring.

Personally I think the whole front suspension bob/efficiency thing is a bunch of bs. I fail to see how the fork compressing can rob you of forward momentum. I agree it can be an unpleasant feeling for those not used to it. I suggest this experiment, get rolling down a flat road at a fast speed, then stand up and bob the fork up and down, does the bike slow down a lot when you do this? mine doesnt, in fact I cant detect any noticable effects. The suspension fork gives you better control in bumpy situations and fatigues you less, thereby effectivly making you more efficient. Rear suspension bob is another story, but that is for another thread. :beer:

I'm not sure if the suspension...bob, for lack of a better word, does rob me of forward momentum, it seems as if it does. But it doest matter anyway, I use it for offroad.

Anyway, back to my original question/problem:
Adjusting the fork seems to make little difference in the tension on the spring, and I was wondering if I was doing something wrong? Is something busted?

I guess I could look into new springs, but how do I know which spring is right for my weight (180 pounds)? Perhaps the tension is ok now, and I just don't realize it?

Like I said, the manual I downloaded from Rock Shox website discusses sag. But turning the adjuster nut makes no difference in sag, it still hovers around 10-12mm and the manual says 13-20mm - which seems an awfully large range to me. I thought one had to adjust the fork depending on one's weight?

Digger

Personally I think the whole front suspension bob/efficiency thing is a bunch of bs. I fail to see how the fork compressing can rob you of forward momentum.

Maybe that's partially true. Let's say I want to smack down part of a wall. I'd use a 5lb hammer (more proof that a 5-pounder will solve every problem a hacksaw cannot:)) and a chisel. I place the chisel on the mortar line. Hammer smacks chisel. Most of my whack gets to the mortar. Let me use a chisel with a spring coil halfway along it's length. I smack chisel. Coil partly compresses. Less whack gets to mortar line. Result. Harder work. I know I've taken a crude example, but I think in this case it probably is true?

On the bike, I honestly don't know for sure anymore. By the time the front wheel has started to turn, then it's in reponse to the rear wheel turning. As the front wheel is independent and can only move in response to what forward momentum the rear wheel delivers, then I'm confused about this. :confused:

All I know, is that to me, it does make a difference. I can't explain, it just seems to, that's all. If I stiffen the front setup to minimise the bobbing, I do seem to ride faster, but it does not make sense.

All I know is this. I don't tell Einstein about relativity, he don't tell me how to ride a bike.:D

OK, I'm a roadie and don't have an extensive knowledge of suspended bikes, but I know a bit of physics :)

I think the inefficiency due to bobbing is more to do with loss of energy from the pedaling effort. The earlier post which mentioned bouncing the fork whilst running downhill seems right - there is no way this could steal a significant amount of energy.
The energy that is lost from suspension is due to the pulsed nature of pedaling, when you push at the start of each leg's pedal circuit, the front of the bike rises due to the slight acceleration - energy is used raising the front of the bike against gravity, and compressing the rear spring if the bike has dual suspension. When the end of the push is reached, the bike returns to its "normal" position on the springs. Here, the damper absorbs this energy (without dampers the bike would be too bouncy) and it is turned to heat. So there is a portion of your pedaling effort that never reaches the road, but is absorbed by the damper. Of course if the rider could have a perfectly smooth pedaling action, the bobbing motions would not occur, and the effort would not be lost this way.

Ed

Good reply, Ed. I can see what you are getting at. It's much clearer now. I remember now that if I pedal steadily there is a point where the ride is smooth. But once I go into a sprint type of mode, the jerkier action does cause more bounce, even though I'm riding on the same type of surface, like a sidewalk, for example.

Sorry Digger, but this thread turned into something else.

I don't think anyone doubts that rear sus bob is energy robbing, however I understand the question about the front.

One of the MTB magazine editors in this month's issue said he thought front lockout was unnecessary - even for the pros. They think they need it, so they have it, but it's likely that they would go just as fast without front lockout. I thought it was a good point.

My fork - Manitou Mars Elite - doesn't have lockout, and I don't feel I need it. I just don't experience much bob, or I don't notice it.

I can't help thinking, though, that anytime a fork compresses without need (you didn't hit a bump) energy is wasted. Maybe it didn't hinder forward motion, but it may have unnecessarily sapped - wasted - some of the rider's energy. What caused the fork to compress on a paved road? The energy came from somewhere and was wasted compressing the fork without need. I don't know the physics and math, but intuitively it seems that energy was wasted, and for a racer that's always bad.

Still, even though I race XC, I don't want to bother with the lockout.

I'm not sure if the suspension...bob, for lack of a better word, does rob me of forward momentum, it seems as if it does. But it doest matter anyway, I use it for offroad. digger

as Ed commented, it doesn't rob you of momemtum, but pedalling efficiency as your leg power is used to compress and lenghthen the fork instead of pushing the bike forward.

it depends a lot on what kind of riding you do whehter this makes a big (or tolerable difference) or not.

For instance, IF you do long steep paved climbs (say >10% and 500m or more), ESPECIALLY when standing, you will notice a major loss in power and thus, a considerably slower time than a bike w/o a suspension fork.

as someone else said, the negative "bob" effect from rear suspepnsion is generally worse than the front. but from my experience WHEN STANDING the front is worse than the back.

Since March i have a FS '02 Specialized FSR XC with both front and rear lockouts. I personally rarely use the rear lockout (only for long road sections) but use the front quite often for road sections or climbs that don't have many obstacles.

my front fork (Fox Float RL) locksout with the twist of the top knob which i can do with one hand while riding (although i have to reduce pedal-power a little). as for needing it in XC racing:: i think it depends on the course... plus, i believe they have some systems where both front and rear lockout happen with the touch of a button on the bar (expensive so i haven't checked it out)

as to the original question of adjusting a fork: there are tons of different forks that use springs, elastemers or air plus all kinds of combinations and oil weights...

in general there are 3 adjustments that a fork MAY have:
1) basic travel/stiffness adjustment by increasing the elastimer stiffness, increasing the air pressure, using heavier/lighter oil weight, or replacing springs with stiffer/spongie-er version
2) adjusting preload which determines how much tension the system is preloaded with --- making it "tighter" means the fork won't react until a bigger force is applied
3) adjusting rebound which has to do with how quickly the shock springs back --- too fast and it throws the bike around funny and can even make you crash from the after-effect and too slow and it feels sluggish

anyway, each fork differs tremendously in how much each of these things can be adjusted and what side-affects doing so has...

for instance, on my Fox Float fork, i can precisely adjust the sag by changing the air pressure, but for most spring/elastimer forks, especially the basic and less-expensive ones like the Jett, the adjustments are pretty minimal (other than buying new elastimers or springs and changing them out which is not an adjustment but a change in my eyes)

Sorry I got this thread off on a tangent, but I think I can help with the original question. I used to have a Jett fork on my bike. The preload adjustment knobs didnt seem to affect the sag, but they affected how plush the fork felt. I think this might have something to do with the stiction this lower end fork has. There is some initial resistance to movement in the first portion of the travel because of some binding/friction in the bushings. Anyway, I wouldnt worry about it. The amount of sag they recommend is so little, it will have little or no effect on the handling, I would just go out and enjoy it.