Fork Stiction Tuning

[Concourier]

I was lucky enough to participate in a recent Tech Day (I use "participate" loosely-I kinda just watched) when I remembered a long forgotten tip for reducing stiction at the front end as we were working on forks. I was first shown this technique in the pits when a friend of mine was WERA racing in the 1990's.

Now, I did do a search for "fork tuning" and "stiction" across the forum and didn't see this tip specifically, so forgive me if this is redundant, but at least its here if you want to try this the next time you have your forks out. Not trusting my memory (and you shouldn't) I found these specific instructions by a Mike Nixon online and adapted them here to closely match what I've done in the past:

Assemble the fork, leaving out the oil and springs and the wheel and fender.

Very lightly snug the triple clamps.

Bolt in the axle, tightening it in its clamps without the wheel.

Now raise this axle /slider assembly all the way to the top of its travel and let it go. It should drop to the bottom of its travel unaided.

If it doesn't budge, you've got some serious binding somewhere. If it's just a little slow, tap the top of one of the fork tubes with a soft mallet to move the tube downward in its clamps a smidge (0.010"-0.020") and try the drop test again.

If there's no change, tap some more. If still no change, raise the slider/axle assembly all the way to the top of its travel and tap on the bottom of the leg upward, which will move the tube upward in its clamp, and check with the drop test again. Try with the other leg also. You're hunting for the best alignment.

Because of manufacturing tolerances, the best internal alignment may very well occur when one of the fork tubes is incrementally higher than the other. Say 0.050"-0.080".

It's far more important to align the fork's parts internally, than it is to worry about what they look like on the outside.

Just remember to fully tighten the triple clamp bolts when finished.

Install the front fender and try the drop test again.

Don't let the fender influence fork movement. Reshape its inner bracket, shim between the fender and the sliders--whatever it takes to get the same drop speed with and without it.

You could also try sourcing fork seals with fewer sealing lips and sanding the upper wipers, but I never went to these lengths.

 

[Fred W]

I'm not understanding what is being adjusted here. Raising and lowering the fork leg in the triple clamp will not change the forks' alignment (angles) to each other, which is what you are trying to achieve to eliminate fork stiction, right? What am I missing here?

 

[wfooshee]

Yeah, the only thing being adjusted is the alignment at the limit.

 

[Concourier]

I raise and lower, say, the left upper in the clamps while the right remains static until the forks are no longer binding at the axle, if indeed it ever was.

The assumption is that they are true and parallel, otherwise the internals should be addressed. I think this is just ensuring that the actual lengths are the same from clamp to axle so the forks move in tandem with minimal binding.

 

[bluesdog]

I raise and lower, say, the left upper in the clamps while the right remains static until the forks are no longer binding at the axle, if indeed it ever was.

The assumption is that they are true and parallel, otherwise the internals should be addressed. I think this is just ensuring that the actual lengths are the same from clamp to axle so the forks move in tandem with minimal binding.

I think you're saying that the axle should form a perfect 90` with each fork, and one way to check this is to follow the procedure you have outlined, yes? If there is a difference in the axle-to-triple tree measurement, the axle will be slightly skewed, which could lead to all manner of suspension and handling problems, and possibly uneven front tire wear. Ideally, each upper fork tube should form a perfect 90` with the triple tree; however it's more important at the axle.

 

[Concourier]

I think you're saying that the axle should form a perfect 90` with each fork, and one way to check this is to follow the procedure you have outlined, yes? If there is a difference in the axle-to-triple tree measurement, the axle will be slightly skewed, which could lead to all manner of suspension and handling problems, and possibly uneven front tire wear. Ideally, each upper fork tube should form a perfect 90` with the triple tree; however it's more important at the axle.

I'm no suspension warlock, but yes I think that's part of it. Also, the tolerances of the internals (damper rod etc) will be different between the left and front and you're compensating for that and any wear as well. I've performed this without thinking too deeply about it - just knowing that the forks slid up/down much easier after I did this than before (haven't done this on the FJR yet).

Here is the link to Mike Nixon's site - I suspect he explains it better than I. He's being Vmax specific, but like I said, I was shown this by WERA folks down in the pits also:

https://www.vmaxoutlaw.com/tech/fork_stiction_tuning.htm

Scroll down to step three

And yes, this would be in addition to assuring parallel alignment top to bottom.

I edited this to include: You made me think of something else - this would also compensate for the triple clamp not being absolutely square, either due to tolerances or age or getting tweaked somehow.

 

[Fred W]

I'm still not getting it.

When there is no spring in the fork leg, raising and lowering one of the inner fork tubes should have no effect on the lower fork tubes at all, except at their travel limits. Having a height difference in these parts will have an effect once the springs are installed as one spring will have greater dynamic load and will try to twist the axle at the bottom with that amount of force.

But if it works for you...

Wait!! I just followed your link. You left out the most important part:

Does it drop to the bottom of its travel unaided? If it doesn't budge, you've got some serious binding somewhere. If it's just a little slow, remove the axle, loosen one of the damper rod's allen bolts (either one), turn the slider 90 degrees, retighten the bolt, replace the axle and try the drop test again.

By turning the fork leg (it would be the inner leg, not the outer slider) you will be varying the direction any small bend in the leg, which is exactly what you want to do. I still don't understand the raising and lowering of the inner fork leg though. That makes no sense.

 

[Concourier]

FredW - thanks for adding that bit I missed. Next time you have the forks off, try this. You may find that like me, you'll find this spot where the lowers will slide/drop easier than before. Think of it more like a more precise "axle test".

I went hunting and came across this video. Its long, and BMW specific (I know, I know) but he mentions the up/down alignment at about 1:58:

 

[Fred W]

Yeah, I understand what he's talking about in that.

But I still don't see how you can demonstrate that doing the "drop" test (with the springs and oil out of the forks) like in your first post or the linked procedure.

Think of it this way: With no springs in, the sliders are free to slide up and down the length of the inner tubes until they hit a mechanical limit . When you install the axle you are fixing the distance between the bottom of the two sliders, ostensibly at the bottom of the stroke. raising or lowering one of the two inner tubes will not influence the relationship of the sliders to each other as they will still slide up or down freely. The thing that will cause the forks to stick now is if the inner legs are not parallel to each other

You do want the two forks tubes installed at the same height because otherwise the springs will be compressed unevenly inside during articulation. With modern (not BMW K75) triple clamps it is pretty likely that the top surface will be pretty square and true. If you wanted to check this is the case you could just install your axle (check that it's straight) and be sure that it goes in perfectly aligned. If one of the forks is too high or low the axle will not line up perfectly.

Also, FWIW, the guy in the video is all wet. Front end oscillation is not caused by fork stiction. I have no idea where he got that idea from.

 

[ahchiu]

I'm rebuilding my forks this weekend and will give this a try to see how they slide.

 

[Concourier]

Hey Marcus - let us know what happens. Just because some racers showed me this back in the day and I have been doing this all along doesn't mean its not all wet. I'll be interested to know what you find on the FJR.

I've done this all along on my other bikes, and there's always that sweet spot where the lowers just "drop" on their own with little resistance.

I admit I don't know why this happens, but I've always thought making sure the lowers moved the most freely was a good thing.

I never did do the "spin the stanchion" step that FredW discovered I left out, which is probably why I left it out. It seems an important step.

Maybe on old bikes it doesn't matter?

I do agree with FredW that the more I think about this, the more I can't say why it would work.

also, I added the link to the vid because it was done by "Forks by Frank", a longtime builder and tuner of forks, so I figured he knew what he was doing. That don't mean he's 100% right all the time tho.

 

[ahchiu]

Hey Marcus - let us know what happens. Just because some racers showed me this back in the day and I have been doing this all along doesn't mean its not all wet. I'll be interested to know what you find on the FJR.

I've done this all along on my other bikes, and there's always that sweet spot where the lowers just "drop" on their own with little resistance.

I admit I don't know why this happens, but I've always thought making sure the lowers moved the most freely was a good thing.

I never did do the "spin the stanchion" step that FredW discovered I left out, which is probably why I left it out. It seems an important step.

Maybe on old bikes it doesn't matter?

I do agree with FredW that the more I think about this, the more I can't say why it would work.

also, I added the link to the vid because it was done by "Forks by Frank", a longtime builder and tuner of forks, so I figured he knew what he was doing. That don't mean he's 100% right all the time tho.

I will let you know, I also just ordered the Motion Pro ForkTru Fork Alignment Tool from e-bay but it won't be here in time for the rebuild this weekend.

 

[Concourier]

I will let you know, I also just ordered the Motion Pro ForkTru Fork Alignment Tool from e-bay but it won't be here in time for the rebuild this weekend.

Oh, too bad. I've never had the luxury of using one, I'd have loved to have seen how it worked for you.

Good luck on the rebuild