front fork sag

[lurch12_2000]

I've been searching previous threads on this but can't quite find the specific answer when measuring sag when I sit on the bike vs off.
With the bike OFF the centerstand when I sit on it with all my weight it will drop 1" or 26mm vs no weight and OFF the centerstand.
Am I supposed to compare it this way or my weight OFF the centerstand vs the bike ON the centerstand with no rider and tilted back with weight off the front end??
I'm just seeing if I need to do anything with spacers and fork springs before I take it apart. I already drained and replaced the fork oil (with 20w) last season but didn't notice any real difference.

 

[Steve]

Replacing the fork oil will not change the ride height unless you fill the entire fork and it can't compress at all. :shock:

Hopefully you are a LARGE person, as 20w oil will make the forks rather stiff in response to small bumps like manhole covers and expansion joints. I even found 15w to be too stiff for me in my Wing, and for my wife in her 850L, so both of them are using 10w now, with much smoother ride. I suppose that if you have a habit of scraping footpegs in the corners, it might be better with heavier oil, but we spend more of our time rather vertical.

To measure the fork sag, you want to compare the fully-extended length to what it is when you are going down the road. You can measure the fully-extended length either on or off the centerstand, but if you measure it off the centerstand, be sure to pull up on the handlebars to make sure the forks are fully-extended. To measure the compressed length, sit on the bike in your normal posture, with as little weight on your feet as possible. I will hold the front brake and bounce the forks a bit to get them to settle a bit past the 'stiction' point. Have a friend measure the forks in that condition. You are shooting for no more than 25% of fork travel being used. Since most forks have about 6" of travel, you are looking for 1 to 1 1/2" of sag.

When you get around to checking the rear shocks for sag, remember that travel is only about 4", so you are looking for 3/4 to 1" of sag.

.

 

[PAULYBOY]

One Man Suggestion

One Man Suggestion

How bout this suggestion for doing the measuring when you can't find a helper. Take a wide tie strap, zip strip, tie wrap, whatever you call the things that bundle electric cords together, and wrap it around one fork loose enough to slide up, but tight enough not to slide back down too easily. Adjust it so it is as close to the fork lower as possible, then when you sit on the bike and do the shimmy brake thing, it will stop at its farthest compressed point. Then you can measure the distance, and get the difference between the 2 numbers.

 

[Steve]

That would work if you wanted to find the maximum compression, but for setting the 'sag', you only want it at normal ride height. Because the fork seals tend to stick just a bit, you want to bounce the forks just a bit to overcome that and let them come to their happy resting point. The wire tie around the fork leg will record the maximum, not the medium.

NOW, if you bounce the forks into a normal ride height, then have your friend slide the wire tie down to the top of the lower fork tube and get off the bike carefully without disturbing it, you will be able to get it back up on the centerstand and measure the difference.

.

 

[lurch12_2000]

Replacing the fork oil will not change the ride height unless you fill the entire fork and it can't compress at all. :shock:

Hopefully you are a LARGE person, as 20w oil will make the forks rather stiff in response to small bumps like manhole covers and expansion joints. I even found 15w to be too stiff for me in my Wing, and for my wife in her 850L, so both of them are using 10w now, with much smoother ride. I suppose that if you have a habit of scraping footpegs in the corners, it might be better with heavier oil, but we spend more of our time rather vertical.

To measure the fork sag, you want to compare the fully-extended length to what it is when you are going down the road. You can measure the fully-extended length either on or off the centerstand, but if you measure it off the centerstand, be sure to pull up on the handlebars to make sure the forks are fully-extended. To measure the compressed length, sit on the bike in your normal posture, with as little weight on your feet as possible. I will hold the front brake and bounce the forks a bit to get them to settle a bit past the 'stiction' point. Have a friend measure the forks in that condition. You are shooting for no more than 25% of fork travel being used. Since most forks have about 6" of travel, you are looking for 1 to 1 1/2" of sag.

When you get around to checking the rear shocks for sag, remember that travel is only about 4", so you are looking for 3/4 to 1" of sag.

.

I understand that fork weight of the oil won't affect the compression measurement. I was just saying that the new 20w oil vs the old dirty ?w oil didn't seem to change the feel of the ride.
As far as the sag, when I measure with my weight (170 lbs) OFF the centerstand vs no weight ON the centerstand it is at 85mm (3.35") difference, which according to what you're saying is too much.
Full travel appears to be 7 " (180mm) on my GS1100G, that is when the bike is ON the centerstand and measured from the top of bottom tube to the point at the bottom of the brace. Now I don't know if the full travel is really all of that.

 

[Steve]

There may be 7" of room, but you don't want to dent your fender, so the actual travel is probably closer to 6". Check a service manual or a bike test in a magazine for specs.

You say you have 3.35" of sag. Assuming 6" of available travel, you have already used up over half of that. :shock:
Imagine meeting a 2 by 4 laying in the road. You will have to absorb that 2" bump with only 2.65" of travel. Yeah, it's possible, but not comfortable.
Now imagine that your sag is set properly, with 4.5". Soaking up that 2" bump will be NO problem at all. 8-[

Then you get to the fine-tuning with oil weight. Getting 20w fork oil to move through all the damping orifices is like trying to move your hand through a bucket of honey. Switching to 15w is like the difference of using water instead of honey. Still resists your hand motion, but moves easier. 10w would be even easier. Just as important (maybe more so), is the quantity of oil. If there is not enough oil, the dampers may be operating in air, providing no resistance to motion until the forks are compressed a bit, when they will slam down into the oil, suddenly changing the damping characteristics rather dramatically. If there is too much oil, the air that is trapped above the oil will compress too quickly, which changes the spring rate sooner than designed, also affecting performance. If there is really too much oil, you can approach hydraulic lock-up and the forks will lock up, usually blowing the seals right out of the forks.

.

 

[lurch12_2000]

Went back and checked the front fork air pressure and there was very little. I brought it to 15lbs air and it raised the forks over an inch and then I sat on it. Sag was still 1" from my weight but now there is 5 1/2" of travel total with no weight and off the centerstand. This should put me closer to the 25-30% number for my weight in relation to the full travel.
Not sure what the max air pressure is on these GS1100G forks but it takes at least 10lbs to get it to rise any. Single air valve with a crossover hose.
Next semi-warm day I'll give a it test ride unless you have any other suggestions.
thanks

 

[Dogma]

That would work if you wanted to find the maximum compression, but for setting the 'sag', you only want it at normal ride height. Because the fork seals tend to stick just a bit, you want to bounce the forks just a bit to overcome that and let them come to their happy resting point. The wire tie around the fork leg will record the maximum, not the medium.

NOW, if you bounce the forks into a normal ride height, then have your friend slide the wire tie down to the top of the lower fork tube and get off the bike carefully without disturbing it, you will be able to get it back up on the centerstand and measure the difference.

.

I think this could still be accomplished single-handedly (OK, one person and two hands). I'm thinking you should be able to push the wire tie (or rubber band, whatever) back down with a stir stick or dowel rod once the fork has settled.

I haven't gone through this exercise yet myself, so I'm not sure how much difference to expect between the initial squat and the settled ride height. My ideas for setting the rear end are far less convincing. I won't mention them.

 

[robertob]

I think this could still be accomplished single-handedly (OK, one person and two hands). I'm thinking you should be able to push the wire tie (or rubber band, whatever) back down with a stir stick or dowel rod once the fork has settled.

I haven't gone through this exercise yet myself, so I'm not sure how much difference to expect between the initial squat and the settled ride height. My ideas for setting the rear end are far less convincing. I won't mention them.

This is the way I usually do it. It's not perfectly accurate but close enough for me. I sit on the bike off the centerstand and bounce it up and down, then sit on it as per normal, holding myself against a wall to keep from falling over. Then I push down on the cable tie with a stick & get off the bike carefully to prevent it from moving. Up on the centerstand, then pull up on the bars to get the full distance from cable tie to dust seal.