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I have only recently started riding to work and I've been given a bike by a friend which is in reasonably good shape.

For the way things are working out I keep the front gear in the same position and change the back one between 4, 5 and 6.

The trip is very hilly so I like to switch over to 6 as I near the top of a hill so I can continue pedaling as soon as possible at the base of the next hill.

However, the bike changes quite readily between all other gears 1 - 5, except going up from 5 to 6. Going down 6 - 5 works fine. This means more effort when trying to go up the next hill. Sometimes the gear kicks in just before I want to switch back down to 5 again.

There is also a 7th gear on the back that I don't use, so the 6th is not the highest choice.

This problem occurs regardless of if I'm going up, down or on a flat grade. It also doesn't help if I try switching back down to 5 and trying again, it just seems to kick in randomly.

Any suggestions on what I can see could be causing this.

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Please tell us whether this is a derailer-type bike (a chain that moves between different sprockets) or an internally geared hub (only a single rear sprocket with gears inside the rear hub). –  Daniel R Hicks Sep 23 '11 at 4:22
    
@DanielRHicks - It's a derailer-type. –  xiaohouzi79 Sep 23 '11 at 4:27
    
How is it going from 6th to 7th? –  cmannett85 Sep 23 '11 at 6:29
    
@cbamber85 - No problems –  xiaohouzi79 Sep 23 '11 at 6:32
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3 Answers

up vote 6 down vote accepted

How many miles on this bike? If over 5000 miles (8000km) then your rear gear cluster may be worn out. Your symptoms are most strongly suggestive of that.

Otherwise, the first thing to do is to clean the derailer real well. Lean the bike against a wall (derailer facing you) outside somewhere where a few oil stains won't be a problem. Shove some newspaper or cardboard up behind the derailer and drape it over the tire, to keep stuff from dripping on the tire. Then spray the derailer well with a spray solvent (WD40 in the US) and scrub it with an old toothbrush. Dry the derailer well with an old rag, and lubricate with chain oil.

With the derailer clean, swivel it back and forth by hand and notice if it seems to be twisting in parallel with the chain and sprockets or is perhaps bent so that it goes at an angle. If it seems bent (which happens from dropping the bike on its side) then the "derailer hanger" probably needs to be straightened.

Also, pay attention to the shifters and cables. A little spray lube in the shifters and down the shifter cable housings is generally all that's needed here, unless the cables are badly corroded or beginning to fray, in which case they need to be replaced. Be careful, though, about what sort of lube you put on the shifters as most now have plastic components that can be damaged by strong solvents.

And, of course, it's probably time to adjust the derailer, but that a whole different post.

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It's always difficult to tell without seeing it, but since it's shifting well in all gears except one (and in one direction on that one) - check for any bent teeth on the cassette.

It's a good idea to check the state of the derailleur and make sure your end-stops are set correctly as well.

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This is a fairly common problem that happens when the derailleur cable gets past its sell-by-date. Only a small amount of deterioration in the condition of the cable is needed to make shifting less than perfect. If the bike has been in storage for a while then a localised patch of oxidation can set in to affect one bit of the shifting range, where this is depends on what gear the bike was last used in. You can also get a kink in the cable that might have been made when the bike was initially setup. Such a kink will also give unpredictable results.

You can pop the cable out and give it a lube to see if that fixes the problem. Put the chain on the big sprocket and then shift the lever to the position for the little sprocket but without pedalling. This will give maximum slack. The cable guides on the frame should have little slots in them, you can now pull the outers out and be able to move the outers up and down on the inners. Be careful to not bend the inner if doing this. Give the inners a good clean with 'GT85' or other water-displacing light lube, make sure everything is clean and put it all back together again. You now need to make sure that the outers are all of the way in on the cable stops, stress the inner a bit to do this by pulling the inner.

Give the bike another test but expect problems to persist or be even worse. If this has not fixed the problem go straight for a new cable inner and outer. A new cable will be silky smooth, however, this will require tools to cut the cable outers to length and shape the ends nicely. Plus you don't want the inner to have a frayed end. Your regular pliers just won't cut it. If you cannot borrow a nice pair of Shimano cable cutters, take it into a bike shop and get them to do it for you.

With a new kink-free and corrosion free cable your gear changes will be a lot smoother and index properly.

It is unlikely that grime on the derailleur or sprockets is the cause of the problem. Even if the derailleur wheels and sprockets are solidly built up with gunk this should not affect shifting. This is more likely to result in the gears slipping (but staying in gear rather than not indexing across the range).

It is also highly unlikely that you have bent teeth on the sprockets, if you did then you would not be able to ride '6' at all, which you can if the indexing lets you do it.

To summarise, the magic bullet here is a new cable. You can try to clean up the existing cable and that might work, however, putting in a new cable will save you a lot of time and give you gears that are a joy to use.

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