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About two weeks ago I took my bike to the local bike shop to get serviced. It was previously slipping gears under load and at the end I couldn't use the lowest gear going up hill it would just slip constantly.

I mentioned the crunching / slipping to the guy at the counter.

When I went to pick up my bike I was charge $170 bucks for two new set of brakes, replaced one bolt type thing which was apparantly shredded (called a cone or something along those lines) and to have all the gears properly set. I was also told that the rear derailleur arm was bent and this was straightened.

The bike only cost $100 from a reliable friend who only rode it ocassionally and the bike is 3 years old. So the service was comparitively costly compared to the cost of the whole bike.

In the past few days it has started slipping again under load which is what it was doing previously and it won't hold gear at all when in the very lowest 1 1 combination it always slips to 1 2. It is also very noisy when I sit in my average riding gear which is either in 4th or 5th in the rear gears. The crunching is slightly dangerous because it can happen taking off from traffic lights or when I put extra pressure on to leave an intersection / roundabout etc. when cars are waiting behind me.

The other thing is when I picked up my bike I asked about the slipping and the response was kind of like "ah, yeh, ah, that should be OK now" which sounded to me like they didn't take it for a significant ride after repairing.

On the other side, these guys are supposed to be reputable, they have been around for a few years and the bike is not a supermarket bike it was actually purchased in their shop and in their defence the bike worked and sounded like a dream when I took it out of the shop so it is possible they did take it for a test ride even if they didn't sound convincing when they said so.

I'm only an amateur so it is hard for me to describe what the problem is, it's like a split second crunch and it doesn't move down or up gear. Sometimes it feels like it is just slightly out of gear and then after riding for 2 minutes it will crunch for a second and then it will not crunch again unless I change to a different gear. Sometimes I can just sense that it isn't quite right and swith back and forward between gears. Although, when I look down at the rear gears it all looks settled even if it doesn't feel right.

I'm also not sure if it is the rear derailleur cogs where it is slipping or the gears, I have no idea because I can't visually spot it and can't tell exactly where the sensation is.

I would appreciate any advice on what my next steps should be?

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The first thought would be worn chain and cogs, but that's easy for the shop to check (takes all of 30 seconds) and normally would not be a problem for about 2000 miles. Any idea how many miles are on the bike? But the noise you describe sounds like simple misadjustment. Take it back, have them readjust it, and have them check the chain and cogs for wear. –  Daniel R Hicks Jan 12 '12 at 12:18
    
"I was charge $170 bucks for two new set of breaks" Why did they swap your brakes when you clearly had a transmission problem? –  cmannett85 Jan 14 '12 at 10:51
    
@cbamber85 - I also had a break problem. This was valid. I just wanted to give a breakdown of what was in the costs. –  xiaohouzi79 Jan 16 '12 at 0:22
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3 Answers

Can it be just a case of cables stretching a bit after they adjusted them, so the gears are constantly in a half-shifted state? Noisy gears would indicate that I think, especially taking into account that the bike had been ridden only occasionally so the chain/cassette are unlikely to be worn and the gears worked fine after adjustment. The tendency to auto-shift from the largest cog to the second-largest also indicates that the cable is too long.

Slightly tightening the cable using the nut near the shifter may fix the problem I believe.

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There are some pitfalls beyond the correctly pointed worn teeth, mentioned by Colin Newell:

  • Derailer hanger might be misaligned, which causes some gears to not properly engage;
  • The "bent" derailer, which was straightened by the shop, might not be totally straight yet, even if it looks so (there is a parellellogram that needs to be perfectly aligned to work properly);
  • Rear shifter cable might not be running free inside the housings, which might leave the derailer between gears, or might let the gears slowly slip to a higher one as the lightly stuck cable slowly slips back inside the housing;
  • The gears might be mismatched, for example, the shifter might show one gear number and the actual gear is other, because the cable is much shorter or much longer, or the adjusting screw was overturned and "shifted" the whole system one whole gear up or down;
  • Much more odd stuff would be: incompatible chain-and-sprockets, bent rear axle, excentric or misaligned ("wobbling") cassette, chain with "hard links" which slip over sprockets' teeth.

I hope it is not too mystical for you to check, but surely it is worth trying to grasp the essence of bicycle drivetrain mechanics, so you will be a much more powerful and self-reliant cyclist.

If you find something, write a comment or edit your question, and don't forget to accept your preferred answer!

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I've had one from the 'odd stuff' - loose pulley wheel. –  Karl Feb 6 '12 at 6:05
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That sounds like the rear cogs are worn, the likely fix is a new chain and set of cogs for the rear (you have to replace them both at the same time otherwise the problem will persist, perversely even worse).

I suspect the bike shop fixed/replaced the stuff that was obviously visibly knackered and hoped for the best. In truth I suspect they needed to do what they did plus replace the cogs and chain, but they were hoping that wasn't the case because the bill would have been even higher.

In my experience bike shops don't tend to actually ride the bikes so they can miss stuff I wish they wouldn't. That's one of the reasons I learnt to do my own maintenance.

In general the trick with bike shops is to check your bike before you sign off on the work, i.e. I ask to ride my bike around to test it before I leave the bike shop (and pay). If I do find I still have a problem after leaving the bike shop and paying I go straight back and explain the problem and say it's not quite sorted yet. They are normally very reasonable and will sort the problem without additional charge, assuming it doesn't require new parts.

Your best bet is to go back to the bike shop and explain the problem isn't quite sorted and if possible demonstrate it to them. It will cost more money though, it sucks but unfortunately bikes aren't generally that cheap when it comes to maintenance (compared to the initial cost of the bike).

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Surely the best thing to do is to go back to the shop and demand some extra attention. I think they will not oppose to do so, if you don't wait too long to do it. +1 –  heltonbiker Jan 12 '12 at 11:12
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