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April 10, 2010 4:16 PM   Subscribe

Second Hand Kid's bike issue: Can you adjust a pre-set handle-grip gear selector, or is it always a sign of another issue?

My step son bought himself a second hand bike the other day - a fairly cheap LA Bicycles mountain bike thingy with 6 gears. He is delighted with it, but had a small crash the other day (yes, we bought a helmet) and has issues with the gears not selecting cleanly. Unfortunately, because this is his first bike with gears he wasn't used to them at all so I can't be sure if the problem started when he fell off, or just became apparent when I put the chain back on and checked it afterwards.

So. I tried to adjust it today, and while I have decent experience with derailleurs from my childhood, it was a long time ago. Longer than I'd like to admit. I struggled with it for a while, but have come to the conclusion that either:

a: The cable is stretched or sticking and preventing proper adjustment (so replace or lubricate the cable)

b: The derailleur is bent/damaged/pooped

c: The handle bar grip adjuster (twist grip style) is out of adjustment (Or broken - it's very stiff, but only really when the derailleur is connected)

So. What I have done so far:

I can adjust the system so it selects both extremes of the gear pack fine. However, I can only get it to accurately select one half of the gear set accurately at a time. it seems that no matter what I do, the cable length either at the 1-3 gear range is wrong, or that part works and the lower 3 is wrong in the other direction. If I can select 6 - 5 - 4 fine, then it will either jump or not select one of the next three. If I fix 1-2-3, then I can't shift from 6th to 5th - it won't react and then will jump to 4th. The only way to get it to work is to adjust the length of the cable halfway the travel, which is obviously not acceptable.

I am confident it's not an issue with the limit stops, there is no spring tension adjuster on the mechanism (presumably a cheap one) and the cable seems to travel easily through the outer when not connected to the derailleur (it doesn't seem to be jammed up to me).

So going on basic logic and elimination, it appears that the spacing of the 'clicks' on the twist grip are at a different spacing to the cable lengths required to accurately select the gears. However, this doesn't seem very likely to me as a failure mode.

Now, I imagine that a cable tension issue would also cause that (different loading/spring tension on the cable affecting this through the travel?) but don't know if this is definite, or sign of some other possible cause. I've been through it repeatedly and checked as best I can for bent things, but everything I have read online is suggesting that if things don't make sense once you adjust them, then something is bent.

So I can't imagine that the detentes on the selector need adjusting (or even that it is possible), but does anyone have any evidence to point toward one solution or another? I am toying with just fitting a non-detente lever to make the gears usable cheaply, but if I can get it to work as designed easily, then I'd rather do that.

Anyone got any better ideas?
posted by Brockles to Sports, Hobbies, & Recreation (2 answers total)
IAAFBM (former bike mechanic)

Brockles, there's no way a crash would change the spacing on the shifters, that much is certain. So either somebody put a mismatched shifter on the bike before you bought it, or something else is up. Much more likely the latter.

To me this sounds like either a sticky cable or a bent derailleur (hangar). Both of these problems can be caused by a crash. I know you say that you can't see anything bent, and that the cable housing seems to slide over the cable smoothly, but bear with me.

If you view the bike from low and behind the rear wheel, does the rear derailleur seem to be bent inward a little? This is one of the most common ways a crash can affect shifting. It can be a little hard to see - it's most obvious when the derailleur pulleys are close to vertically below the freewheel. If you do see a bend there, try to bend it back out gently and see if that improves things. (I'm assuming you have an inexpensive bike and that the derailleur uses a built-in mild steel bracket to attach to the axle - if that's not the case, it would be better to have a shop do the bending.)

The other possibility is a cable kink. This can be hard to see - it might be inside the shifter in some gears, but out in the housing in other gears. Some parts of the housing would move smoothly over the cable, but the one spot where the kink is would be causing enough friction to interfere with the smooth operation of the indexing. The fix for a kinked cable is a new cable (and maybe housing too).

If you can't find either of these things, then yes, a non-indexed shifter would likely fix the problem but kids need a little instruction in how to use them to avoid skipping exactly like what's happening when the shifter's not adjusted.
posted by richyoung at 4:45 PM on April 10, 2010

Since we don't have an "I agree" button on AskMe I just wanted to say that I agree with richyoung - these are the classic symptoms of a bent dérailleur hanger. If it is a cheap bike you can probably bend the rear drop out or the dérailleur hanger pretty easily using an adjustable wrench. You can always bend it back.
posted by ChrisHartley at 8:45 PM on April 10, 2010

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