Bicycle Mechanics - Rear derailleur - broken rubber ring
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11-08-10, 09:57 PM
Hi all, I noticed that this rubber/plastic ring on my rear derailleur is broken and is just hanging in there with the ends fraying out loose as shown in the attached pics. My question is what purpose does that ring serve? Would it affect the security of the RD mount and or cause shifting/other issues if it breaks off completely? Thanks in advance for your advice!
I'm not 100% familiar with that particular derailleur, but it looks like it's a combination spacer and seal. The seal function is probably no loss, but if it's also acting as a spacer, it's loss will allow a bit of play of the upper body on the mounting bolt and some loss of shifter accuracy.
I wouldn't sweat it for the moment, since there's not much you can do about it anyway. If it disintegrates totally ant the mount gets sloppy, you can look for a replacement, either the original, or maybe a washer of the correct thickness. If you can't find an appropriate part, you might be able to make do by winding some dental floss into the gap as a filler.
11-08-10, 11:13 PM
Based on my experience taking apart many derailleurs, I'd have to say that the function of that part is to be both a seal, and a low friction "bushing" of sorts between the cage that houses the pulleys and the rest of the derailleur. Its loss as a seal is not that big of a deal really, but more important is the space it takes up. I have to wonder if dental floss would really do the trick (I don't really see how it would) but worst case scenario, total disintegration might be remedied by just cutting a similar washer out of any plastic sheet you have lying around.
More than likely, the plastic bushing is also used to locate the spring inside. I would say though, act like you never saw that, and move on with your life. You'd probably be better off ignoring it, unless it has a lot of play, or binds.
I have seen people with derailleurs in totall $(*&@% conditions that seem to work just fine. If it ain't broke don't fix it... unless you have lots of money. Also, the number of speeds that bike has will definitely affect whether a tiny bit of slop/misalignment is a big deal or not.
Oh, and clean your bike......
11-08-10, 11:48 PM
Thanks Mike and FB. It's actually a Sora shifter on a 9 year old bike. I'm in fact doing a complete tear down and cleaning of the bike so I just need to know whether I should be extra careful with that piece before I mess with the RD. I guess I'll just keep an eye on it and maybe clip off the frayed end so it won't catch and tear out more of itself later on.
11-09-10, 01:47 AM
It appears to be a bearing for the B-Axle. That part isnt available by itself but you can get a complete B-Axle Assembaly that includes the part. http://techdocs.shimano.com/media/techdocs/content/cycle/EV/bikecomponents/RD/EV-RD-3300-1882_v1_m56577569830609114.pdf
11-09-10, 02:38 AM
Based on what I see, I find it difficult to imagine that getting caught on anything. There really isn't anything that goes around back there, but if you feel like putting the effort knock yourself out. I used to completely overhaul every bike I bought, and now after I've started working as a mechanic and I see people come in with bikes that shouldn't be rideable, and somehow they shift just fine... it has definitely lowered my standards. As far as a learning experience though, it is definitely worth it, just make sure to have a nice set of pliers around (needle nose my favorite) and be ready to get pissed at it. That specific part (the B-tension spring assy.) is the most annoying part to have to disassemble on a derailleur. It is interesting to see that I took apart a 30 year old shimano derailleur, a 10 year old one, and a brand new one, and they all have the exact same pieces and are put together the same way. This is true across brands (for the most part).
Given the prices of parts I have ordered from Shimano before, it might not make any sense to order just that part, being that a 9year old sora derailleur probably wouldn't cost that much on fleabay anyways. I have completely overhauled a ton of derailleurs before (completely disassemble, re-grease etc) and honestly.... it has never been worth the effort. If it works fine.... I don't get the point. If it feels bad though, knock yourself out. I do it sometimes just because I find it therapeutic. It is worth the time to clean the outside though... for pure bling factor.
A sora RD on ebay runs new for $30, so keep that in mind. (including shipping)
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