Fork maintenance failure

[Old Guy]

I tend to over-simplify things, but Fred's discussion made me remember my high school geometry -- the part about two points will always fit on a straight line. Add a third point to that line, and it has to be absolutely perfectly in place to fit on that straight line.

With that in mind, I can see that with two bushings, even with normal manufacturing tolerances, will be in a straight line. Add a third, and they won't be.

And those tubes definitely flex. If not, why would anyone even think a fork brace would help?

 

[MCRIDER007]

Interesting analysis Fred but I think you have the wrong parts flexing...but I'm not a engineer so I will leave that to someone who is. The true test of the GEN1 vs GEN2 design is how it works in the real world and to do a good comparison you need to ride a GEN1 and GEN2 back to back and have everything else the same (tires, springs, fork oil,damping). I pretty much did that after I got my 08 because I kept the 05 for 3 years. It wasn't a apples to apples comparison because the 05 had aftermarket suspension on both ends but it quickly became apparent that there wasn't anything superior about the 05's ability to go around a corner and I don't think the 08's slightly longer swing arm was the sole reason it cornered better than the 05. I will add that when I finally transferred the aftermarket suspension to the 08 it worked far better in the 08 than it ever did in the 05.....and the 05 with OEM suspension handled like a slug in comparison. I'm keeping the middle bushing until I ride a GEN1 that handles better than my GEN2.

 

[MCRIDER007]

With that in mind, I can see that with two bushings, even with normal manufacturing tolerances, will be in a straight line. Add a third, and they won't be.

I think this only applies if all 3 bushings were pressed into the lower fork tube, in this case the top 2 bushings are pressed bushings being used to keep the fork tube in a straight line, the 3rd bushing is being as a guide to reduce flex and to reduce stiction between the tube and the lower fork leg.

I wish we could get an explanation from the engineers who designed the fork.

 

[Old Guy]

With that in mind, I can see that with two bushings, even with normal manufacturing tolerances, will be in a straight line. Add a third, and they won't be.

I think this only applies if all 3 bushings were pressed into the lower fork tube, in this case the top 2 bushings are pressed bushings being used to keep the fork tube in a straight line, the 3rd bushing is being as a guide to reduce flex and to reduce stiction between the tube and the lower fork leg.

I wish we could get an explanation from the engineers who designed the fork.

That makes perfect sense.

Thanks.

 

[08FJR4ME]

It looks like they didn't fix the error in their ways on the GenIII's either. They are still using the three bushing design. After assembling all the components on the forks and before putting any oil in the tubes there seemed to be some side to side movement in the inner tube. I was actually quite surprised by how much. But hell, nothing leaked and I just put over 2,600 miles on them and they worked just fine for my EOM trip. I no longer bottom out even when two up. So I think there is more forgiveness in the clearance’s than one may think,

I think relating this to pole vaulting may be a better reasoning than the friction theory. Different pole vaulter’s will use different poles. One pole vaulter that may be taller and heavier will use a different length pole versus someone who is shorter and weighs less. Each pole will be a different length and stiffness custom built for each case. So because of the longer chassis wheelbase and added lbs. that the GenII's three bushing design use could be the reasoning behind the added bushing.

Something to contemplate,

Good night all,

Dave

 

[HotRodZilla]

A couple of select jackasses have actually pole vaulted their FJRs around here...Both GenI and GenII. It didn't turn out good for any of them. Therefore, I recommend another analogy.

 

[900gc]

The FJR fork is stout despite what generation it may be. Peter Kates @ GMD computrack Boston who did my forks over and has rebuilt 1000's of forks over the years, and does a ton of suspension work for Loudon racers, agrees that the middle bushing is basically superfluous in the gen 2/3. His opinion I know, but he does know a thing or two about this stuff.

Now in my fantasy world of wondering, wouldn't it be nice to hook up deflection and stiction sensors to the fork with the middle bushing in and then removed, put it through some kind of repeatable test, and compare the results?

 

[08FJR4ME]

superfluous: When something is so unnecessary that it could easily be done away with.

Had to look that one up.

Better to err on the side of caution than do without. (JMO)

Pole vault analogy: All I could come with at the time Zilla, it was late and the brain cells weren't firing on all cylinders.

Dave,

 

[Fred W]

What other parts would possibly be flexing?

While I can't explain why a second gen's forks would feel so much better to MCRIDER007 (of course the fact that he had just spent $15k on a new bike might make it feel better even if it wasn't. Just kidding!

) there are just far too many other variables in that "head to head" comparison to say that the perceived improvement had anything whatsoever to do with having a third bushing.

Old Guy did a great job of summarizing my theory, and did so with far fewer words and no lousy pencil drawings. Yes, the top two bushings are mounted in the (stiff) lower tube and the third bottom bushing is on the inner tube. So yes there is a small amount of tolerance even with all three installed. Having some of that tolerance is critically important. Some space is required for the oil to maintain a film between the sliding surface of the bushing and the machines surface it is sliding against. When the space is gone you will have friction, and friction is very bad for suspension performance.

The fact that the second gen bushings wear the teflon layer off so quickly is prima facie evidence that this is what is happening during use. If the oil film were being maintained, the teflon would last forever, or at least much longer like it does on the 1st gen 2 bushing design.

The aftermarket suspension tuners all think removing the middle bush is a good idea. I'm betting that they have dome back to back comparisons of the same fork in the same bike with and without the 3rd bushing to come to those conclusions. Or someone with a 2nd gen could do the same if they were predisposed to making the comparison. Fortunately (for me) I am not in that position as I can't install a third bushing on my 1st gen

 

[gixxerjasen]

This may sound stupid but did you remove the inner rods prior to trying to seperate the for legs? Don't ask me how I know this!

Which parts are you talking about? The guts? Yea, there's nothing I can see left inside the fork leg. Used my impact gun to spin off the bottom bolt (made sure the gun was spinning the right way first) and had no problems with that part.

Good luck, I hope this solved your problem. Please don't get out the torch!!!!! Things will ge much worse.

The idea is the opposite of freezing your bearings before pressing them in. A little heat (not like some I've read who heated their forks till they were orange) will help release those inner parts. Didn't work though, but I was chicken and probably didn't use enough heat.

When there is a lateral load on the fork leg the inner fork legs will flex a little, not just in the part that you can see between the lower triple clamp and the top of lower fork leg, but all the way along its length, even in between the two triple clamps and between the two bushings (greatly exaggerated)

Fred, are you sure that isn't a diagram of under exaggerated 1970's fork technology?

I think relating this to pole vaulting may be a better reasoning than the friction theory. Different pole vaulter’s will use different poles. One pole vaulter that may be taller and heavier will use a different length pole versus someone who is shorter and weighs less. Each pole will be a different length and stiffness custom built for each case. So because of the longer chassis wheelbase and added lbs. that the GenII's three bushing design use could be the reasoning behind the added bushing.

As a high school pole vaulter I was excited at a pole vaulting analogy, buuuuuuut, man was that a stretch.

Having some of that tolerance is critically important. Some space is required for the oil to maintain a film between the sliding surface of the bushing and the machines surface it is sliding against. When the space is gone you will have friction, and friction is very bad for suspension performance

Of course, it also seems that having some space might allow one bushing to partially slide into the other bushing.

Headed to Stadium Yamaha here in just a bit. I've already been pricing upper and lowers online in case it looks like the labor to get those apart will be too high. Hopefully it won't come to that but it's good to know where you could have just bought new rather than fixing the old. Sigh...

 

[MCRIDER007]

What other parts would possibly be flexing?
While I can't explain why a second gen's forks would feel so much better to MCRIDER007 (of course the fact that he had just spent $15k on a new bike might make it feel better even if it wasn't. Just kidding!

) there are just far too many other variables in that "head to head" comparison to say that the perceived improvement had anything whatsoever to do with having a third bushing.

Old Guy did a great job of summarizing my theory, and did so with far fewer words and no lousy pencil drawings. Yes, the top two bushings are mounted in the (stiff) lower tube and the third bottom bushing is on the inner tube. So yes there is a small amount of tolerance even with all three installed. Having some of that tolerance is critically important. Some space is required for the oil to maintain a film between the sliding surface of the bushing and the machines surface it is sliding against. When the space is gone you will have friction, and friction is very bad for suspension performance.

The fact that the second gen bushings wear the teflon layer off so quickly is prima facie evidence that this is what is happening during use. If the oil film were being maintained, the teflon would last forever, or at least much longer like it does on the 1st gen 2 bushing design.

The aftermarket suspension tuners all think removing the middle bush is a good idea. I'm betting that they have dome back to back comparisons of the same fork in the same bike with and without the 3rd bushing to come to those conclusions. Or someone with a 2nd gen could do the same if they were predisposed to making the comparison. Fortunately (for me) I am not in that position as I can't install a third bushing on my 1st gen

The other part that could be flexing is the lower fork leg....and the point of flex would be at the upper bushing(s).....but I think you would need a computer simulation to verify. I didn't spend 15K on my GEN2, it was quite a bit less, but I never expected there to be such an overall improvement, that was a very pleasant surprise, and I was not the only one who rode my bikes back to back and noticed it.

I would expect that the teflon layer on the lower bushing would last actually last longer....which it clearly does not, at least not on the original lower bushing, but that could also be the result of the fork re-design if Yamaha decided to cut costs by not polishing the inside of the lower fork tube as much on the GEN2 or decided to go to a tighter tolerance (and went too far). It may be possible that the second lower bushing will last much longer than the first but I haven't heard of anyone's real life experience.

I don't put too much value on the aftermarket suspension tuners opinion(s) because they are not engineers, most interested in cutting costs, and don't have any way to accurately test something like this. They are experts in springs and damping curves as long as they have a rather expensive computerized shock dyno at their shop but that only measures vertical forces and prints damping curves.

If you are interested in making a real world comparison, buy a plane ticket to Pasco next spring/summer and you can ride my much improved GEN2 with all 3 bushings in place over some of the best motorcycle roads in the US. Just be prepared to spend the big money after you get back cuz that old 05 of yours just will not feel the same.

 

[08FJR4ME]

I just recently serviced my forks for the first time. 59,000 miles! So as far as the 10K milestone I think that may be subjective. I will post pic's. of the teflon bushings later on.

Dave

 

[yamafitter]

Fred,

You stated that GP Suspension does not install the middle bushing. I found that interesting since when Dave at GP Suspensions did my forks in 2010 the middle bushing was installed. I mentioned that Traxxion were not installing the middle bushing and Dave was NOT keen on that idea since he felt the forks of the Gen II benefitted from the extra stiffness the middle bushing provided but there was a trade off with some extra stiction. The benefit of the extra bushing is that the distance between the lower triple clamp and the next lower support point is reduced. This means that greater forces are required to cause the forks to defect the same amount in a lateral direction which improves handling. The fact that the middle bushing wears as much as it does proves that the bushing is transferring these lateral forces from the fork tube into the outer leg increasing the effective stiffness of the forks to lateral forces.

 

[Fred W]

Fred,
You stated that GP Suspension does not install the middle bushing. I found that interesting since when Dave at GP Suspensions did my forks in 2010 the middle bushing was installed. I mentioned that Traxxion were not installing the middle bushing and Dave was NOT keen on that idea since he felt the forks of the Gen II benefitted from the extra stiffness the middle bushing provided but there was a trade off with some extra stiction. The benefit of the extra bushing is that the distance between the lower triple clamp and the next lower support point is reduced. This means that greater forces are required to cause the forks to defect the same amount in a lateral direction which improves handling. The fact that the middle bushing wears as much as it does proves that the bushing is transferring these lateral forces from the fork tube into the outer leg increasing the effective stiffness of the forks to lateral forces.

OK, maybe GP is using them, but many of the other suspension shops are not. And even though I gave my money to GP for suspension components, and they do seem to work pretty well, I'm not sure if they are nay better, worse, or different from any other shop. I think they are just guys that have a clue about what's going on and how to make (more than) a few bucks selling suspension parts. IOW, they ain't nukeular scientists!

And I know you are a fart smeller, a real nukeular power plant guy, Billy, but you must have failed your basic physics classes in High School!

Having the support/contact points close together would not be an advantage in this case. It would be a disadvantage in leverage. The effort is being applied at the far ends of the levers (either at the wheel on the bottom, or the weight of the bike applied at the triple clamps at the top). The ideal situation would be to place the sliding support surfaces (aka the bushings) as far apart as possible to reduce the leverage and minimize the pressure on those surfaces. Kind'a like where the top and bottom bushings are located already.

That middle bushing is a duplication of the upper bushing, and I don't see where it does much of anything... UNLESS you buy into what MCRIDER007 suggested, that the lower fork tube is what is flexing and is what they are trying to restrain.

Just looking at the structure of the lower fork tube (especially as compared to the upper one) I completely reject that theory. But other opinions may smell somewhat differently.

 

[gixxerjasen]

By the way, I wanted to say thanks for the clarification on upper, middle and lower. I suppose my brain sometimes thinks of things in odd ways. I was thinking that forks like on the gen I have an outer and an inner (by which tube it was attached to) and the Gen II would have an inner and an outer upper and an outer lower. Hence when GeorgiaRoller was talking about a lower bushing I was thinking Gen II and the outer lower bushing, which is in fact the middle bushing.

Inside my head...it's like spaghetti and quite scary.

 

[ionbeam]

Yup...I'm under the impression that middle bushing is the reason the GenIIs don't need (benefit from) a fork brace...

First gens don't need a fork brace either. People that make (and sell you) fork braces think that every bike needs a fork brace. For what exactly? What are these magic dog bone brace supposed to be bracing against? The forks actually work considerably better without one in my experience

This is of some interest. This morning I bombed out to work on my trusty FJR and quickly noticed something was 'different' in the way it handled. I deviated from the normal set of ruts and took a little ride down Hawkwood Ln's fine twisty bits and it was obvious something was different. <*****> I forgot that I took off my fork brace on Monday so I could fix my leaking fork seal and didn't take the time to put it back on. </*****> I can definitely say that I noticed a difference between having the brace on and not on. Color me surprised. Now to figure out if that difference is better or worse.

And now, back to the botched fork thread.

 

[yamafitter]

Fred completely missed the point and rather than get into a bunch of childish name calling I'm just going to let him believe whatever. By the way I was a Physics major at University and have a passing knowledge of Newtonian Mechanics.

And with that I'm outa here

Now where is that ignore button?

 

[ionbeam]

...I was a Physics major at University and have a passing knowledge of Newtonian Mechanics...

It's amazing the way some people can hide their education

I notice that the word SUCCESSFUL wasn't used.

 

[MCRIDER007]

Deleted.....screwed by the software again.

 

[08FJR4ME]

Fred,
You stated that GP Suspension does not install the middle bushing. I found that interesting since when Dave at GP Suspensions did my forks in 2010 the middle bushing was installed. I mentioned that Traxxion were not installing the middle bushing and Dave was NOT keen on that idea since he felt the forks of the Gen II benefitted from the extra stiffness the middle bushing provided but there was a trade off with some extra stiction. The benefit of the extra bushing is that the distance between the lower triple clamp and the next lower support point is reduced. This means that greater forces are required to cause the forks to defect the same amount in a lateral direction which improves handling. The fact that the middle bushing wears as much as it does proves that the bushing is transferring these lateral forces from the fork tube into the outer leg increasing the effective stiffness of the forks to lateral forces.

Well Bill it is a bit better than my pole vault theory.

Dave