tips for setting fork preload sag

[kelham]

can you guys throw me some tips for setting my preload sag on my marzocchi 55 rs fork? a few questions come to mind, however any other advice is appreciated.

1. once I've got my zap strap on, do i want to be seated or standing on the pedals? should i be stationary or should i roll around outside?

2. if i'm shooting for ___% sag, should I measure it on the fork, or just eyeball it?

3. my preload is adjusted with a 3 or 5mm allen bolt on the top right of the fork. how sensitive is the adjustment (i.e. quarter turns or muliple full turns)?

thanks!

[CraigH]

http://www.performancebike.com/bikes...ForkAdjustment

[kelham]

Originally Posted by CraigH

http://www.performancebike.com/bikes...ForkAdjustment

To set the pre-load on a fork:

1. Turn the compression and rebound damping as low as they will go
2. Loosely attach a zip tie to one of the stanchions so it will barely stay in place
on the stanchion
3. Move the zip tie all the way down so it is against the wiper seal
4. If your fork manual does not specify the range of motion for your fork follow
these steps:
a. fully compress the fork by applying body weight, you should feel the fork
bottom out
b. if the fork does not bottom out reduce the pre-load and try again
c. measure the distance from the top of the wiper seal to the bottom of the zip
tie
d. write this distance down so you do not forget it.
5. Find a wall to lean against or a strong friend to help support you and your bike,
gently get on the bicycle and sit on the saddle with your hands on the handlebars
just like when you are riding. Try not to compress the fork while getting on, just
once you are seated.
6. Dismount the bike and measure the distance from the top of the wiper seal to the
bottom of the zip tie. This is the amount of sag
7. Multiply the range of motion measurement from your manual or from step 4 by .25.
This is the suggested sag
8. If the suggested sag is greater than the measured sag, decrease the spring pre-
load. If the suggested sag is less than measured sag, increase the spring pre-
load.
__________________________________________________ _

Won't the zip tie prevent the fork from completely bottoming out and cause the measurement to be off by the width of the zip tie? (Assuming the wiper/seal runs all the way to the crown...or does it not?)

[CraigH]

BTW, the directions to measure fork travel are assuming it is an air fork and that you have drained the fork of air.

To do this with a coil fork you have to remove the spring.

You only need to do that if you don't know the travel of your fork.

Most forks have a few mm from top of dust seal to bottom crown at full compression.

[kelham]

Originally Posted by CraigH

BTW, the directions to measure fork travel are assuming it is an air fork and that you have drained the fork of air.

To do this with a coil fork you have to remove the spring.

You only need to do that if you don't know the travel of your fork.

Most forks have a few mm from top of dust seal to bottom crown at full compression.

OH. I have a coil fork. I have 160mm of travel. So i should set the sag to 40mm?

Update: OK, even with the preload bolt backed all the way off, I only get about 20mm of sag from a seated position in my living room. That's 12.5%. It doesn't make sense to me, because a couple of weeks ago had to dial the preload down a lot to keep the fork from feeling too soft on the trail.

[DirtySanchez]

There are a lot of instructions and recommendations as to setting sag. I found that most of these recommendations are terrific if yer screwing around buck naked in yer garage fawning over your new steed; you know, air humping it and the like. But the true test is when you ride and what you ride. Go by feel. Sometimes a stiffer less compliant fork is needed for the terrain and style of riding; sometimes more plush. Sometimes it's counter-intuitive where the more plush the fork, it actually performs faster and better. It's a hands-on situation. Typically, 25% sag F/R for a standard AM style ride; up to 40% for DH. I found with the latter - DH - I stiffened the front end up with compression, and lessened the preload. Reasons for this are the bike is doing what it's doing best when it's aiming downhill which means - typically - more pressure on the front.

Long paragraph. I know this.

[kelham]

It's just strange, becasue if I dial the preload all the way off, the fork is so soft on the trail that it bottoms out and is unrideable. Yet, that only gets me 12.5% sag (which would imply too stiff). Does this make sense? Morgan told me that maybe the fork is sticky and therefore giving a false sag reading (but it feels pretty smooth to me).

[Kanza]

Are you just sitting on the seat? you need to be in the attack position. Bounce the fork by weighting the front a few times, then in the attack position set the sag. You might want to do this leaning against something.

[Bryce]

^^ this. Attack position or at least the standing position that you'd normally ride downhill in. Its easier if you can get a friend to help.

[biggles604]

Originally Posted by kelham

It's just strange, becasue if I dial the preload all the way off, the fork is so soft on the trail that it bottoms out and is unrideable. Yet, that only gets me 12.5% sag (which would imply too stiff). Does this make sense? Morgan told me that maybe the fork is sticky and therefore giving a false sag reading (but it feels pretty smooth to me).

When you go downhill, your weight will be much more forward, and when you brake, most of the inertia will be absorbed by the front wheel. If you are riding flat trails, then you need less preload than riding downhill trails. I see so many people who run their fork so that it feels good pedalling around on the driveway, but it's then too soft for trails.

[kelham]

Originally Posted by biggles604

When you go downhill, your weight will be much more forward, and when you brake, most of the inertia will be absorbed by the front wheel. If you are riding flat trails, then you need less preload than riding downhill trails. I see so many people who run their fork so that it feels good pedalling around on the driveway, but it's then too soft for trails.

I hear ya, but then why do the manufacturers tell you to set your sag this way?

[Bryan]

Don't forget you're gonna control the bottoming via your damper not your preload.

Also when setting sag, your gonna wanna bounce up and down and get the bike to settle, which should overcome the intial stiction the fork may have.

[kelham]

Originally Posted by Bryan

Don't forget you're gonna control the bottoming via your damper not your preload.

Also when setting sag, your gonna wanna bounce up and down and get the bike to settle, which should overcome the intial stiction the fork may have.

Can you elaborate? My fork only has adjustment for preload and rebound. Damper isn't in my vocabulary yet.

[scotter]

the following applies to coil sprung suspension. air sprung suspension operates under the same principles, but with some caveats.

the purpose of preload is to set sag, which has no effect on the suspensions tendency to bottom out. preload is a static load placed on the fork or shock, which "reserves" some travel for filling in dips in the trail, as opposed to bumps, which the rest of the travel would be used for. the force required to completely compress your fork is not altered by preload, and so setting it too high or too low should have no effect on the suspensions tendency to bottom out. as springs compression tends to progress (somebody please correct me on this point if i'm wrong), you may notice that the suspension is slightly firmer in the beginning of the stroke with more preload, but this shouldn't be significant.

as for damping, this is the suspension's hydraulic mechanism by which it slows or seemingly resists compressing or in the case of rebound damping, decompressing. we use compression to alter how firm the suspension feels. even with lots of compression damping, the weight required to completely compress the suspension will be the same, but because of the slowing of the speed to achieve this, the ramping up of the compression is gentler and the rider tends to absorb impact with arms and legs more. the suspension fluid isn't able to move through the suspension as quickly and so this keeps the fork or shock from bottoming. rebound damping is just the opposite, slowing the suspensions return to a non-compressed state.

your fork probably has an internal adjustment for compression damping, either by allen key or by shim/washer stack. higher end suspension usually has an external knob to adjust compression in both high speed and low speed hits. the adjustment for compression usually opens or closes a valve allowing suspension fluid to flow more or less quickly, thus providing slowing to either the compression or rebound of the suspension.

i think i've gotten everything right, but anybody please feel free to correct me on any of these points.

[Bryan]

I'm gonna guess that the forks compression is controlled via the oils viscosity. A thicker oil will slow things down a bit an then you adjust your rebound to suit. If you like the way the fork runs in the first part of the stroke with proper sag, you might want to raise the oil level. This will keep the compression damping the same while greatly increasing te bottom out resistance. Raising the oil level decreases the size of the air chamber. If your not comfortable opening your fork up, you might want to take it to a shop. If you're in the LM, just take it to naz at marzocchi in north Vancouver and he can set you up