alleviating play in shock bushings with aluminium foil (on a bmc superstroke)....

what i used is thicker than conventional foil, it's actually the back of a tube patch, but from what you're saying the difference that makes will most likely be a matter of minutes.

anyway, it is a thru-axle.

actually the bushing is visibly tarnished, kind of surprising, given the amount of time i've had the bike, and the dry conditions it's been ridden in. disappointing, but given that it was one of the 600us frames on jenson, i'm not feeling too hard done by. i'll post a picture of it next time i take it apart, which will probably be tomorrow or the next day when the foil has disintegrated after i've ridden it around to the local shops to discuss the problem.


unfortunately it's a hard tail world over here, so i doubt i'll get much help. also, the level of knowledge is pretty low in the bike shops....

thanks for the feedback

aha.


it seems i put things incorrectly. sometimes a little knowledge is worse than none.


anyway after an hour or two lying in bed pondering the whole thing i realised that what i thought was the bushing, was actually the shock mount. the reason for this mistake was that it was the space between the reducers and the bolt that were moving and had play, rather than the bushings.


one of the reducers was pushed so tight into the bushing that it took me twenty minutes and a hammer ot get it out. so it seems that due to the tightness of the bushings the shock was pivoting on the bolt, leading to the reducers and the bolt wearing down, as well as the point where the side of the reducers touch the frame. not ideal


i guess this could also be explained by the shock bolts not being torqued sufficiently. they were tight, and i haven't adjusted them since i got the frame except to check that they were tight, but i didn't use a torque wrench, so i can't be sure it was torqued high enough, i guess. so perhaps this whole issue stems from that

also it looks like there might be some misalignment on the rocker mount... will look at that again later


anyway, there's some wear on the upper shock mount, but practically none on the bottom mount, so i'm thinking maybe if i gently sanded (with 1000+) either the bushing or the part of the reducers that sits in the bushing, that that would allow the shock to move more easily where it's meant to move.

i could also try getting new reducers, which is perhaps the proper solution, seeing as i still need to lose the play caused by the wearing of the reducers in the wrong places.

here are some pictures. the worn reducers and bolt are from the top. i've shown the most worn sides on the frame, other sides, seem fine. strangely the almost impossible to extricate reducer was on the bottom, and it doesn't show wear in the wrong places. this doesn't bode well for my theory and maybe suggests misalignment?so what do you think, does this seem a likely situation, or have i gone astray?


anyway, thanks for the help so far.

ps. it's clear now that my bushings are on two piece spacers, not a thru-axle.

seadna said:

aha.

it seems i put things incorrectly. sometimes a little knowledge is worse than none...

ps. it's clear now that my bushings are on two piece spacers, not a thru-axle.

Click to expand...

Just to be clear, the pictures you've posted are of the reducers and shock-mount bolts not the DU sleeve type bushing in the eye of each end of the shock. This is what's typically referred to as "mounting hardware". It appears you've damaged the reducers as well with your "hammer" removal process :nono: At the very least you're going to need new reducers. Your bolts appear to be shouldered (not threaded on the contact area) so that is a plus. I'm not sure if you will be able to get "off the shelf" thru-axle style reducers to fit your frame and I'm not sure where to send you to look either. It's possible that a competent machinist could turn something for you on a lathe. Probably would be cheaper then trying to import something. See if you can find a machine shop and take in your linkage and old reducers and see what can be done :thumbsup:

thanks

yeah i know that they are the reducers, and that the du bushings are still in the shock eyes.

i admit that the hammer removal was not my finest moment. in retrospect two or three alternative ways of getting that off have come to mind... i have an offhand tinker and bash approach to matters mechanical, which doesn't really cut it with more expensive parts.

anyway, i'll try to pick up new reducers, which should be possible if they're a standard size, as there is a fox dealer in town. let me guess though; they aren't a standard size!

otherwise i'll track down a machinist, but bikers, of all people, should be aware of how inconsistent mainland chinese machinists can be. especially here in the west... still, worth a shot, and could even be interesting to look into.

thanks for your help, it's pretty cool, and without it i would be quite stranded, as local shop owners seem, on average, to know even less than i do... i guess we're all learning

Looks like you have an alignment issue.
You should go through all your suspension bearings and make sure they are installed properly, and in good shape. I've seen 2 different frames with poor alignment. 1 had a bearing that wasn't seated properly from the factory, and caused wear similar to yours. The other had a bearing that was destroyed and threw the alignment off.


Once you get the alignment dialed, put in the new reducers with a little bit of Slick Honey grease on them. The reducers are suppose to fit tight. I can't get the upper ones out of any previous shock I've had unless they were worn. If they come out easily, then the bushing needs to be replaced.

The mounting hardware has one purpose, pinch the mounting area tight enough around the reducers so they don't move. Make sure you get that stuff tight enough. If not, you'll start ovalizing the mounting holes like others have mentioned.

well, i'm kind of tired now, so just a quick update.


spent a good while fiddling around with things, noted the following:


the top mount mounts to the shock perfectly, only the bushing moves, no play, feels good.


the bottom mount doesn't look good. no matter how hard i tighten it (within reason) there's slop and the shock isn't turning on the bushing, rather the reducers are turning on the mounting bolt and against the side of the mount on the frame. i tried the bottom bushing and reducers on the top mount and that worked fine, so the bushing is good, and the reducers can't be in terrible shape. (i suspect the top mount is less stiff than the bottom mount, and also has a lot more more grip on the surface, so maybe the reducers still need replacing)

not so good.

so basically i need to somehow tighten the reducers against the frame so that they stop moving.

the sides of the reducers may have been ground down by this turning, as may the frame at the mounts, (that's before i even think of the mount holes having ovalised), so i'll try to get new reducers for the bottom, and see if that enables me to tighten it up so that the bushing becomes the path of least resistance

if that doesn't work i'll get onto bmc, probably (not sure what their customer service is like, but i'll know soon).



but is there any way to lock those reducers in? loctite? spacers between the reducers and frame? if i say that it's the frame mount that's damaged and try to do something to prolong it's life might one of those work. i mean, it seems that if i got the mounting hardware locked down (no matter how i do it) so the bushing worked again it wouldn't do any more damage to the frame.


anyway more investigations on this interesting (to me at least) topic after some sleep. thanks again, nice to be able to throw ideas out there and get critical responses. i'm learning, and even having fun, despite the concern that i'm giving my frame an autopsy!

mtnbiker4life said:

If you're going to make those Spacers the Diameter that inserts into the DU Bushing should be:

1/2" DU Bushing - 0.498" - 0.502"

12mm DU Bushing - 11.966mm - 11.984mm

Click to expand...

Why is the 1/2" style reducer allowed to exceed one half inch, and the 12mm style not allowed to exceed 12mm? That seems odd.

If you are a through-and-through DIY'er, like me, it makes perfect sense to simply add some clamping power with improved mounting bolts and spacers, and reduce the bushing/reducer interface friction by gently sanding either. However, I imagine it's often better to just explain your situation to the manufactuer(s) and get the right equipment, not some makeshift stuff.
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That said, I would source a new mounting bolt out of a stainless steel dowel pin [ see part # 91585A204 at http://www.mcmaster.com/ ] and thread each end just long enough to get a nut and washer on one side and a nut, washer, and lock-washer on the other. An appropriate dowel pin will be slightly oversized, but still hand-fit much more tightly in your frame than a standard metric bolt will, as bolts are generally slightly undersized. IMO, the extra snug fit is worth the process of threading the dowel, because it will work to minimize ovalizing in the frame, should the reducers start slipping as the shock is constantly changing the force on the dowel in opposite directions. The external threads on the pin will allow you to apply proper force on the reducer without fear of stripping any tiny hex sockets. [For reference my frame manufactuer sugguests torquing the M8 socket cap screws and bolts on their mounting hardware to 20N*m http://knollybikes.com/support/torqueDelirium ].
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I would see if you can get slightly wider reducers, as I believe they are commonly made in widths such as 22mm and 22.2mm, for example. It is my speculation that the 22.2mm is made for worn, or looser frames. If that route doesn't work, I would sand down a stainless steel washer to the right thickness.
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If the reducer/bushing interface really is amazingly tight, and you can't get ahold of any different ones, I don't see any harm coming from lubing them, or as a last resort minutely sanding the reducers down. These are the parts that are going to need the most precise fit, and if you were to remove too much material you'd be screwed.
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I hope by posting this I won't lead anyone to ruin... but it made sense to me~