Fork maintenance failure

[Chuck35]

I recently did seals only on mine. I filled the forks all the way with atf, removed the dust seals and retaining rings, put the caps on and proceeded to compress the fork in a hydraulic press. (as detailed in the manual). This worked like a charm. The lesson learned is if a seal cocks on the way out, push down on the high side while continuing to apply press pressure. I would imagine the bushings would be much easier to do with the seals out.

 

[Northwoods Snowman]

I can't believe I just read this whole old thread, lol Informative, but it's impressive how many times someone will say the exact same thing over and over and over.

So which thing was getting said over and over and over?

Lol, I'm not reading it again to figure it out; I've since forgotten.

On to the other little fact we have not discussed so far.... one installs the lower bushing and assembles it into the lower fork tube. It COMPRESSES as it is installed. How much gap is left in that "piston ring" I do not know, but I do believe there is some as they tend to go in fairly easily. 'Spoze if we had a way to actually measure, we'd be smarter, but the theory is that we have to have enough clearance down there for the bushing to ride on a film of oil. Now on to the middle bushing..... one installs it by sliding it down the inner fork tube, then pushing it down with a special tube tool into it's seating area. Here, the bushing actually EXPANDS while it seats (aha, Grasshopper), so there is actually more clearance on this middle bushing once installed. Are the clearances here similar to the clearances on the lower bushing?..... I suspect similar but precision measurements required unless someone has access to the design drawings.
Back to removal of the middle bushing, all the bushings have nice square edges so when the lower slams into the middle bushing it should not normally go inside but catch enough to unseat the middle bushing. But remember the middle bushing has EXPANDED and has some clearance, so one shouldn't be too rammy on that first attempt to unseat it. Use only as much force as needed, don't be excessive. Once unseated, you're good to go.

Unproven theory, the middle bushing is there to prevent flex of the inner fork tube as forces try to "bend" it (between the lower and upper bushings) when you hit hard bumps or heaven forbid have an encounter with a bambi......... superior Gen II engineering of course..... I'll tell you a Gen I fork story later.......

You're thinking backwards. The lower bushing has to be expanded to fit over the inner fork tube and then the spring force of the bushing is what hold it on the fork tube. The middle and upper bushings are compressed as they are pressed into the outer fork tube. The ID of the outer fork tube is slightly smaller than the OD of the bushing to create the interference fit and hold the bushing in place. The inner fork tube would have to apply force to expand the middle and upper bushings; exactly like what happens when you get the lower bushing jammed into the middle one and it locks the fork tubes together.

I did my forks a couple weeks ago and here are some observations:

• One fork I heated the outer tube to help get the bushings out and that worked well. The other I didn't bother to heat and while it took a couple more blows to get the bushings out, they still came out fine. Key was to not hammer the crap out of it; just repeated soft to medium blows. Bushings were undamaged.
• at 14K miles the bottom bushing was totally worn though the teflon while the middle and upper were perfectly fine.
• I assembled the second fork with only the upper and lower bushings so see what kind of friction there was. I did not put the seal in and there was so much play between the fork tubes with brand new bushings I was quite surprised. I went ahead and put the middle one in before final assembly.

I reassembled with all bushings installed. I figure I'll pull the forks back apart when I change the oil at 30K or so and see if the lower bushings have worn out again. If they have, I will try leaving the middle bushing out at that time and check again at the next oil change.

 

[Fred W]

• I assembled the second fork with only the upper and lower bushings so see what kind of friction there was. I did not put the seal in and there was so much play between the fork tubes with brand new bushings I was quite surprised.

You also did not put any suspension oil in, right? I've also noticed that the fork feels kind of "loose" when the bushings are dry. How much did the "slop" reduce with the third bushing when dry? Completely or just somewhat?

Will be interested in your impressions should you choose to go with two bushings next time.

 

[RaYzerman19]

I think we're all talking the same thing, perhaps I didn't explain it all that well. The lower bushing is expanded to put it on the fork tube, but then it is compressed slightly as it is inserted into the bore, so it can't come off the fork tube. The middle bushing is installed while the fork tube is in, and is compressed initially to fit inside the bore, then expands slightly as it seats in the slightly larger diameter made for it. That's enough to let the lower slip inside the middle occasionally during removal, and jam as you mention. The upper bushing is compressed to fit inside the bore, and if you notice, the ends of the upper bushing have zero gap and takes significant effort to seat it. And remove it, thus causing the lower and middle to potentially jam even more.

The lower has the most slop, the middle a bit of slop and the upper has the least slop. All ultimately ride on a film of oil however the lower has enough movement to cause the most wear as it moves the most (mostly fore/aft), and the teflon is worn off. I agree the majority of bushings removed appear to have little wear on the middle and upper. The lower and middle are the most likely to be damaged by removal, but the upper (if you were really cheap) could be re-used.

From all that I have replaced, I go straight to the heat for the upper bushing before I start the moderate slide hammer process you correctly recommended. If I get jamming, most times I've been able to unjam. Only had one so far that required my special V-block method, which most likely was the scenario enountered by the originator of this thread.

 

[Fred W]

So we all agree that the Teflon coating of the lower bushings wear out on 2nd Gens (in relatively few miles).

In contrast, Here are the bushings and seals that I removed from my 1st Gen at 82k miles.

Note that the Teflon coating (inside surface of the thinner upper bushings and outside surface of the wider lower bushings) is completely intact and those bushings could have been re-used. In fact I saved these when I installed new parts as I think they could be reused later on.

 

[Brodie]

Since the lower bushings tend have the most clearance, has anyone thought about shimming them to fit closer to the bore they ride in? A stainless steel shim can be cut and placed on the fork pipe recess , then the bushing gets slipped on; it will be captured with no way of sliding out and getting in trouble. Trial fitting would be the best way to figure out how thick of a shim to use, perhaps .001 or .0015, . The idea is that a closer fitting assembly spreads out the contact pattern surrounding the ring, hence less of a wear point on the teflon. I remember doing that on both my Honda Ascot and Yamaha Venture front ends. I got that idea from one of the Rags, Cycle I believe, back in the late eighties.

Brodie

 

[RaYzerman19]

Fred how do you define relatively few miles, Gen 2 vs. Gen 1? I'll add further that I have seen lower bushings like yours that have worn evenly, and some that have worn only front and rear. This makes me think yours were perhaps spinning during use thus spreading the wear more uniformly. I'll qualify that by saying of all the sets I've seen, I did not keep track whether they were Gen 1 or 2, but perhaps should have.

Brodie, interesting..... not sure what to think of that idea or what is ideal. My inclination is to say just because the teflon wears out on the inner part of the lower bushing, it's the outer part that does all the sliding up and down in the lower fork housing and where we'd be more concerned about wear. Your shim would just be the wear part instead of the teflon, but it would indeed reduce clearances, if we believe the clearances to be too sloppy (we don't have the engineering drawings, so all we can do is theorize).

 

[Fred W]

There is no teflon on the non-sliding side of the bushings (inside of the lower or outside of the upper).

 

[Northwoods Snowman]

• I assembled the second fork with only the upper and lower bushings so see what kind of friction there was. I did not put the seal in and there was so much play between the fork tubes with brand new bushings I was quite surprised.

You also did not put any suspension oil in, right? I've also noticed that the fork feels kind of "loose" when the bushings are dry. How much did the "slop" reduce with the third bushing when dry? Completely or just somewhat?

Will be interested in your impressions should you choose to go with two bushings next time.

I actually assembled the first fork with three bushings before I checked play with the second one. I don't remember specifically checking for play with all three bushings so I can't really say how much the difference was. Second fork was dry as I was trying to check clearances and didn't completely assemble. You are correct though, the fluid film does take up some tolerance between the surfaces.

The lower bushing is expanded to put it on the fork tube, but then it is compressed slightly as it is inserted into the bore, so it can't come off the fork tube. It is not actually compressed. If it was it would bind. The ID of the outer fork tube is a hair larger than the bushing. Large enough to allow a slip fit, but NOT large enough to allow the bushing to expand and slip off the inner fork tube. Think of it this way, (I'm just throwing out numbers as I don't know actual dimensions) the bushing fits in a .030" recess in the inner fork tube and has a clearance fit of .005 with the outer fork tube. If the bushing is pressed out against the outer fork tube it still has .025 of the inner fork tube holding it in place; it can't physically go anywhere.

The middle bushing is installed while the fork tube is in, and is compressed initially to fit inside the bore, then expands slightly as it seats in the slightly larger diameter made for it. That's enough to let the lower slip inside the middle occasionally during removal, and jam as you mention.

This is sort of right. The same thing happens to the middle bushing that happens to the top (as correctly stated below). If you measure the upper and middle bushing ODs you will find that the middle has a slightly smaller OD than the upper. That's because the bore of the outer fork tube is like an upside down Christmas tree. The very top where the upper bushing seats is the largest diameter. The intermediate section between the two bushings is slightly smaller. The diameter is smaller than the seating area of the upper bushing but larger than the seating area of the middle bushing. This allows a small lip for the top bushing to seat against but will allow the middle bushing to freely slide down to where it seats. The area where the middle bushing seats is a slightly smaller diameter yet. This diameter is slightly smaller than the bushing OD so that when the bushing is driven/seated into place it compresses the bushing. As the bushing compresses that gap it it goes away. This makes the bushing behave as though it was a solid piece of metal that does not want to compress any more. This incompressability is what makes the friction fit between the bushing and the outer fork tube to hold it in place. One thing to note is that the ID of the fork tube is actually a couple thousandths smaller than the OD of the bushing in its "compressed" state, not just the relaxed state. This is where the press fit comes from. There is no "expanding" of the middle bushing until you hammer it out of the seating zone and it springs back to it's relaxed state. There is no lip retaining the middle bushing like the lower bushing on the inner fork tube; if there was you would NOT get the fork back apart without destroying the bushings at a minumum.

The upper bushing is compressed to fit inside the bore, and if you notice, the ends of the upper bushing have zero gap and takes significant effort to seat it. And remove it, thus causing the lower and middle to potentially jam even more.

Absolutely correct. The jamming in caused by the lower fork bushing wedging up under middle. This is where the clearance between the middle bushing and the diameter of the "intermediate" zone of the outer fork tube come into play. If the middle bushing is allowed to expand to much, and/or the lower edge was damaged during the slide hammering, the lower bushing can wedge underneath the middle bushing. How bad the fork tubes seize together is directly proportional to how much force is used to wedge the bushings together. This is why it's imperative to not beat the snot out of the fork when pulling it apart. The harder the impact, the more likely you are to wedge the bushings together.

The lower has the most slop, the middle a bit of slop and the upper has the least slop.

Not sure what this info is based on. There shouldn't be any more slop than what is required for variances in thermal expansion, flex and such. I can't say this is not the case, just wouldn't expect it for various reasons.

All ultimately ride on a film of oil however the lower has enough movement to cause the most wear as it moves the most (mostly fore/aft), and the teflon is worn off.

Linearly, all bushings see the exact same amount of movement. Lateral movement is a result of clearance between the fork tubes and as the clearance is increased the load bearing area of the bushing decreases. A smaller bearing area means higher pressure. More pressure means more friction and accelerated wear. For a better (exaggerated) picture think of a soup can put into a coffee can with both laid on their side. The only contact between the two is a line along the edge. In the forks most of the wear is a function of the lateral force the bushing is subjected to, the area that force is applied to, and the surface finish of the surface the bushing slides against. I don't know how Yamaha finishes the inside of the outer fork tube, but it would have to be hard plated (chrome/nikasil) as the aluminum is too soft and rough to handle the sliding friction. If the sliding surface of the inner fork in GEN II bikes is any rougher (microscopically) than the GEN I bikes that would cause the accelerated wear of the bushing. Perhaps the inner surface was not as finely honed because engineers figured the loading of the lower bushing would be less with a three bushing design and spec'ed the surface finish to be less to reduce cost. Or perhaps the ID is not plated at all. This of course is speculation but I could see it happening. I could not tell when I had my forks apart. In a ideal environment the surface finish of the inner fork tube SHOULD be equal to that of the inner fork tube. However, the bottom of the fork is where all the debris collects in the oil and this debris accelerates the wear of the lower bushing that is bathing in it in a similar way that a rougher surface would.. That's why it's important to perform frequent oil changes to keep the oil clean.

I agree the majority of bushings removed appear to have little wear on the middle and upper. The lower and middle are the most likely to be damaged by removal, but the upper (if you were really cheap) could be re-used.

From all that I have replaced, I go straight to the heat for the upper bushing before I start the moderate slide hammer process you correctly recommended. If I get jamming, most times I've been able to unjam. Only had one so far that required my special V-block method, which most likely was the scenario enountered by the originator of this thread.

Heat is a good idea. Expanding the outer tube reduces the press fit to allow the bushing to be removed easier.

Not trying to debate with you, rayzerman, just want to use my experience as an engineer to help explain to people what's really going on.

Hopefully what I wrote is understandable.

Another thought I'll throw out there. I know there was the idea thrown out there that the third bushing in the fork increases the forces subjected to the lower bushing as the middle bushing counteracts the flex of the inner fork tube (and accelerates the wear of the lower bushing). I don't believe this to be the case. Setting up a crude force vector diagram shows that what ever load that middle bushing is subjected to as a result of the fork tube flexing is actually removed from the lower bushing proportionally to the distances between the bushings. That's not to say the overall friction is reduced, just the lateral loading of the lower bushing.

Since the lower bushings tend have the most clearance, has anyone thought about shimming them to fit closer to the bore they ride in? A stainless steel shim can be cut and placed on the fork pipe recess , then the bushing gets slipped on; it will be captured with no way of sliding out and getting in trouble. Trial fitting would be the best way to figure out how thick of a shim to use, perhaps .001 or .0015, . The idea is that a closer fitting assembly spreads out the contact pattern surrounding the ring, hence less of a wear point on the teflon. I remember doing that on both my Honda Ascot and Yamaha Venture front ends. I got that idea from one of the Rags, Cycle I believe, back in the late eighties.
Brodie

Not to say you couldn't but I wouldn't do it personally/. I would be curious to see some testing with this though.

 

[RaYzerman19]

It's all good and has me wishing I had the collection bushings from the various bikes I worked on. What we need is a good set of drawings or some scrap forks to make some cutaways.... and I'll attempt to verify some things on the next set(s) I work on.

 

[Fred W]

Hmmm... trying to think this thing through.

So, in my mind's eye I see that when there are only two bushings we have a variable length lever made up of the two parts of the fork leg with lateral force being applied at the axle end of the inflexible lower fork leg, being transferred to the more flexible inner fork leg by the two bushings, and then terminating at the triple clamps. With a given side force applied, all of the force will be applied to force side of the lower bushing and to the opposite side of the upper bushing.

I guess that adding the middle bushing would split the opposite side load of the upper bushing in half, but the lower bushing would still have the same force applied, right?. And then if there is flexure of the inner fork, since the upper bushings are located in the inflexible lower fork leg, together they would exert the added force of attempting to restrain the flexure on the lower bushing?

I'm sure I am missing something important here...

 

[gixxerjasen]

What we need is a good set of drawings or some scrap forks to make some cutaways....

You talking about like slicing a fork down the middle?

 

[Northwoods Snowman]

I guess that adding the middle bushing would split the opposite side load of the upper bushing in half, but the lower bushing would still have the same force applied, right?. And then if there is flexure of the inner fork, since the upper bushings are located in the inflexible lower fork leg, together they would exert the added force of attempting to restrain the flexure on the lower bushing?
I'm sure I am missing something important here...

I think you're on the right track. If the inner fork tube stays perfectly rigid then the middle bushing doesn't do anything. Because there is a gap between the bushings and the mating sliding surface the inner fork tube would tilt (ever so slightly) in the outer and would only contact the upper and lower bushings; taking the middle bushing out of play. Since the inner tube flexes in the real world the middle bushing will attempt to constrain the flex. With only the top and bottom bushing the lateral loads would be equal and opposite. Once the tube begins to flex the middle bushing begins to take load. I'm trying to think of a good way to explain this... the axle tries to push the fork forward. The outer tube pushes forward on the lower bushings and backwards on the top bushing due to the moment generated by the axle in relation to the triple clamps. The fork tube flexes in a concave shape facing forward; top and bottom forward, middle toward the rear. With the middle bushing in place and the fork tube flexed, the middle bushing pushes forward on the inner fork tube trying to counteract the flex and straighten the tube. This means that the force exerted by the outer fork tube trying to push the inner fork tube forward is distributed between the lower and middle bushings. The result is less force on the lower bushing since the middle one is taking some of it. The top bushing is still the pivot point and is pushing backwards on the inner tube.

I would like to see a cross section of one of our forks. That would be cool.

 

[Fred W]

Thanks. That explanation helped form a visual for me.

Let's take the example of just supporting the stationary weight of the bike. Due to the rake of the forks, the axle force is at an angle (up and) in the forward direction on the fork leg, so this is pushing the lower leg forward and contact would be at the back side of the lower bushing and front side of the upper bushing. If there is a middle bushing it wouldn't contact anything until the inner leg flexes. When that flexure happens the contact of the middle bushing would also be on the back side, so it must take some of the load off of the lower bushing.

This theory is contrary to the observations that the (much wider) lower bushing seems to be the one that always wears out first. Maybe that is because the lower bushing is sliding on the more porous surface of the inside bore of the alloy, lower fork leg and the upper bushings are sliding on the highly polished surfaces of the hardened steel inner leg. But you would still expect that the lower bushing should last longer on a bike with an added third bushing, not wear out faster. And I'm still having difficulty envisioning how the 3rd bushing improves anything significantly

 

[Northwoods Snowman]

Yes, you would think that the bushings would last longer, so something else is different. I guess would be the surface finish. Somehow the lower bushing is subjected to more abrasion in the Gen II design.

As far as the effectiveness of the third bushing goes think about bending a piece of 1" x 1/4" steel. A piece three inches long is nearly impossible to bend by hand. A piece three feet long can be easily folded nearly in half by hand. Same principle with the forks. The closer the bushings are together the harder it is to get the fork tube to deflect under load which should make the front end more "rigid" and handle better.