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Rear mech hitting spokes
Posted: 11 Sep 2023, 11:13am
by Mr Tom
I've kind of appointed myself official mechanic for my friend at work's bike as I enjoy trying to fix it! She said the other day when she changed gear the rear mech hit the spokes, and I was trying to think how to approach fixing it or even looking into it.
My first thought was that it was the limit screws, but then if they were out of adjustment the chain would get pushed off the biggest cog into the spokes (I found this out the hard way on my bike years back!). Her bike doesn't have the plastic disk (dork disk?) to prevent that. Otherwise maybe it's been bent when it was locked up in public so it just hits the spokes in that position now.
Just wondering if I'm not thinking of something really obvious. I'm guessing that I'd either have to bring my workstand to my work at try it in that, or maybe best is just advise getting it looked at by a bike mechanic in a shop to be safe.
Re: Rear mech hitting spokes
Posted: 11 Sep 2023, 11:25am
by rareposter
Mr Tom wrote: ↑11 Sep 2023, 11:13am
My first thought was that it was the limit screws, but then if they were out of adjustment the chain would get pushed off the biggest cog into the spokes (I found this out the hard way on my bike years back!).
Not really - it can be just far enough out of adjustment to push the mech into just catching the spokes (especially when standing out the saddle and the frame/wheels flexing) without actually sending the mech and chain into them.
Or it could be a bent rear mech hanger, wheel not in the dropouts properly, broken rear axle...
Depends a lot on what kind of bike it is (basic BSO vs decent commuter) and what state it's in - frankly I'd not get involved and would just suggest a bike shop. You run into all sorts of issues around liability etc once you start "fixing" other people's bikes.
Re: Rear mech hitting spokes
Posted: 11 Sep 2023, 11:29am
by rjb
Most likely scenario is a bent derailleur hanger. Caused by a knock or a fall.
Look from behind and see if the mech is hanging straight.
Search on the forum for how to straighten it.
Re: Rear mech hitting spokes
Posted: 11 Sep 2023, 11:49am
by Jupestar
yes bent derailluer hanger or RD would be my first check. but always worth asking a couple of questions.
has teh wheel been out recently? as teh bike been dropped? Have you moved any of the screws.
RD in the spokes is very nasty, for bike and rider you don't want that to happen.
Re: Rear mech hitting spokes
Posted: 11 Sep 2023, 12:00pm
by Norman H
Whatever the cause, it needs urgent attention and she should avoid using the largest sprocket until it's fixed. If the mech catches in the spokes it can do a lot of damage. If its the limit screw the rest of the indexing should be OK but if it's a bent hanger the indexing on all the other gears will be a little bit off.
I would start by adjusting the limit screw so that there is clearance between spokes and dérailleur cage and then see if you can get it to index in all gears. If adjusting the limit screw throws the indexing out its a fair indication that the mech is bent.
The mech hanger thread should be 10mm X 1mm and a suitable rear hub axle makes an improvised alignment gauge. Use a largish adjustable spanner to realign the hanger.
Re: Rear mech hitting spokes
Posted: 11 Sep 2023, 12:11pm
by Mr Tom
rareposter wrote: ↑11 Sep 2023, 11:25am
Mr Tom wrote: ↑11 Sep 2023, 11:13am
My first thought was that it was the limit screws, but then if they were out of adjustment the chain would get pushed off the biggest cog into the spokes (I found this out the hard way on my bike years back!).
Not really - it can be just far enough out of adjustment to push the mech into just catching the spokes (especially when standing out the saddle and the frame/wheels flexing) without actually sending the mech and chain into them.
Or it could be a bent rear mech hanger, wheel not in the dropouts properly, broken rear axle...
Depends a lot on what kind of bike it is (basic BSO vs decent commuter) and what state it's in - frankly I'd not get involved and would just suggest a bike shop. You run into all sorts of issues around liability etc once you start "fixing" other people's bikes.
Hey, yeah I agree about liability. Really I'm exaggerating, my repairs have been giving her a new bell, trimming her brake cables and putting crimps on, checking her back wheel was in correctly (it wasn't), and finding a nut for a rear light bracket. So - nothing very technical! If I went further I'd make it clear that I'm not a pro mechanic.
Re: Rear mech hitting spokes
Posted: 11 Sep 2023, 12:30pm
by alexnharvey
Just ask her out
Re: Rear mech hitting spokes
Posted: 11 Sep 2023, 1:17pm
by thirdcrank
I'd agree with those referring to the rear hanger, not least because it's something I've overlooked with one of my grandchildren's Islabikes.
For starters, you don't want the mech catching in the spokes as that risks possibly serious damage to either or both.
Beyond that, the "sacrificial hanger" was something I'd never heard of. I asked in a proper LBS when they were too busy to sort it immediately but suggested the hanger and Eureka
Put another way, modern hangers are often made to deform a bit to save the mech from more expensive damage. If you know what you are doing, it may be possible to realign the hanger but replacing it may be your best option.
(Sowerby Bros, Mirfield)
Re: Rear mech hitting spokes
Posted: 11 Sep 2023, 3:37pm
by pwa
My wife's bike had the rear mech gently tapping a couple of spokes once, and after deciding the limit screw was correctly adjusted, I trued the offending spokes away from the mech by about 1mm. Then I adjusted other spokes to restore the straight tracking of the rim. It took a few phases of adjustment and ended up with the rim about 1mm further away from the driveside, and possibly therefore a tiny bit off-centre, but it got rid of the tapping, the rim brakes still worked fine and the bike's handling wasn't affected.
Re: Rear mech hitting spokes
Posted: 11 Sep 2023, 4:02pm
by Mr Tom
The thing is, I think she's reluctant to let me do too much in case I'm doing it to get in her good books, which I'm not. The reality is it brings genuine happiness to my day getting to fiddle with a bike instead of whatever I should be getting on with at work. If I ever find anyone who has an interest in bikes at work I always chat to them about it.
Re: Rear mech hitting spokes
Posted: 11 Sep 2023, 4:09pm
by Carlton green
I usually use a hub gear now but before ‘I saw the light’ I used derailleur gears for many years. Before checking the limit screw I’d just check how worn the jockey pulleys are, it maybe that a worn (floppy) top pulley has resulted in in-appropriate adjustment to the limit screw. Giving things a good clean before starting work is always wise, helping other people to do the job themselves is even wiser.
Mr Tom wrote: ↑11 Sep 2023, 4:02pm
The thing is, I think she's reluctant to let me do too much in case I'm doing it to get in her good books, which I'm not. The reality is it brings genuine happiness to my day getting to fiddle with a bike instead of whatever I should be getting on with at work. If I ever find anyone who has an interest in bikes at work I always chat to them about it.
That’s wise of her; whilst you are a genuinely kind hearted and mechanically interested soul there are many unscrupulous people in this world.
Re: Rear mech hitting spokes
Posted: 11 Sep 2023, 4:24pm
by rareposter
Mr Tom wrote: ↑11 Sep 2023, 4:02pm
The thing is, I think she's reluctant to let me do too much in case I'm doing it to get in her good books, which I'm not. The reality is it brings genuine happiness to my day getting to fiddle with a bike instead of whatever I should be getting on with at work. If I ever find anyone who has an interest in bikes at work I always chat to them about it.
I hate to get all kind of "professional" here but you've got a woman who is reluctant to let you do too much to the bike and (I'm assuming) an employer that is paying you to do a job rather than disappearing off to the bike sheds to fettle with a fellow employee's bike; things like that tend not to go down too well with HR.
The fact that you're asking on here for "what could be wrong with this bike?" also suggests that, while it may bring you genuine happiness to be trying to fix it, you might not know exactly what you're doing or what to look for - both she and your employer are unlikely to be thrilled to bits if you "fix" it and she then ends up falling off, whether it was anything to do with you or not - partly because it'll be quite difficult to prove it wasn't your intervention that made things worse.
I've been in similar situations at work - being "the bike guy" who always rides in, you become the one-stop-shop for any colleague to start asking all sorts of crap about bikes from "can you just...?" to "what sort of bike should I get...?" to the more harmless but thoroughly annoying "you mean you ride in every day?!!!" and "ooh you should be in the Tour de France!".
Point her in the direction of the nearest decent bike shop. I get that there's "being kind / being helpful" but there's also a world of complexity if you balls it up or you try to fix it but can't.
Re: Rear mech hitting spokes
Posted: 11 Sep 2023, 4:45pm
by Jupestar
Yep if you can identify the problem and then send her in the direction of the nearest bike shop, you could well save her £100 on a gold service - the outcome of which will be another £25 on a replacement hanger! Of which she will hopefully be grateful.
Re: Rear mech hitting spokes
Posted: 11 Sep 2023, 7:13pm
by Mr Tom
rareposter wrote: ↑11 Sep 2023, 4:24pm
Mr Tom wrote: ↑11 Sep 2023, 4:02pm
The thing is, I think she's reluctant to let me do too much in case I'm doing it to get in her good books, which I'm not. The reality is it brings genuine happiness to my day getting to fiddle with a bike instead of whatever I should be getting on with at work. If I ever find anyone who has an interest in bikes at work I always chat to them about it.
I hate to get all kind of "professional" here but you've got a woman who is reluctant to let you do too much to the bike and (I'm assuming) an employer that is paying you to do a job rather than disappearing off to the bike sheds to fettle with a fellow employee's bike; things like that tend not to go down too well with HR.
The fact that you're asking on here for "what could be wrong with this bike?" also suggests that, while it may bring you genuine happiness to be trying to fix it, you might not know exactly what you're doing or what to look for - both she and your employer are unlikely to be thrilled to bits if you "fix" it and she then ends up falling off, whether it was anything to do with you or not - partly because it'll be quite difficult to prove it wasn't your intervention that made things worse.
I've been in similar situations at work - being "the bike guy" who always rides in, you become the one-stop-shop for any colleague to start asking all sorts of crap about bikes from "can you just...?" to "what sort of bike should I get...?" to the more harmless but thoroughly annoying "you mean you ride in every day?!!!" and "ooh you should be in the Tour de France!".
Point her in the direction of the nearest decent bike shop. I get that there's "being kind / being helpful" but there's also a world of complexity if you balls it up or you try to fix it but can't.
Yeah I think that's all very sound advice. I do appreciate that it's a massive responsibility fixing anyone's bike apart from your own. I mean I kind of know how to troubleshoot things and have repaired most stuff on my bike, fiddled around with hub gears and even done a complete build from the frame, but I think sometimes there are just those issues which aren't obvious unless you're a mechanic. Like as I mentioned before, adjusting the limit screws wrong on my own bike and finding out that it shifted the chain right off the biggest cog and jammed my wheel, or stuff like mudguards getting caught on the tyre and chewed up. So yeah, agree with you!
Re: Rear mech hitting spokes
Posted: 12 Sep 2023, 10:31am
by ANTONISH
You are both work colleagues.
You need to keep things on a professional basis.
She may regard your interest with suspicion - did she approach you to find out what was wrong with her bike?
Possibly she mentioned the problem in passing and you offered to help - of course you have also mentioned her reluctance to allow this so perhaps you are being too persistent ?