Practical Advice for Transportation Cycling

Sometimes, all that matters is getting from Point A to Point B as cheaply, safely and efficiently as possible. You don't need a fast bike, you don't need a pretty bike, and most of all you don't need an expensive bike, you just need one that works.

Monday, February 11, 2013

Roadside Repairs: Suddenly Singlespeed

Sometimes, especially on an old clunker, you might find yourself with a serious shifting problem. This can be because of damage from a crash, something that happened while your bike was parked or just failure of an old component. But for whatever reason your ability to change gears is compromised.

In these cases, the repair may be beyond what you can accomplish at the side of the road, so your best "get home" solution is to lock your bike into a usable gear, usually one in the middle of it's shifting range.

The key to this is knowing how limit screws work.
Rear derailleur with clearly labeled high and low limit screws
The limit screws control how far a derailleur can move at either end of its swing. If a limit screw is too far out, the derailleur can overshift and drop the chain off the cogs, if the screw is too tight, the chain will not be able to move from one cog to the next.

Normally, this is a bad thing, but if you're looking for a quick fix to get you home, tightening the limit screws so the chain is stuck on a particular cog is just the thing you need to do.

First, you have to make sure the shifter cable is not an issue. If the problem is a damaged or broken cable to begin with, you probably don't have to worry much, but if the problem stems from the shifter, you'll probably have to cut or disconnect the cable using your multitool.

Once you've done that, you can simply tighten down the limit screws, while turning the cranks so the chain moves, until the chain moves onto a cog near the middle of the gear range
This may be easiest to accomplish if you turn the bike upside down and balance it on the seat and handlebars.
If you're doing this with your rear derailleur, you probably only have to turn the High limit screw, as the spring tension of the derailleur will automatically force it towards the high (outer end, smaller cogs) end of the gear cluster. A few turns of the screw will bring the chain towards the middle of the gear range, rather than forcing you to ride in your high gear the whole way home.

If you're working on the front derailleur, and you have a triple crankset, the derailleur spring tension is set to move the chain towards the lowest cog, so you'd simply tighten the Low limit screw until you got the chain to sit in the middle chainring (if it's a double crankset, the lower gear is probably adequate to ride home anyway, but triple cranksets have a super-low "granny gear" range which can make moving at any speed difficult).

Of course, you'll have to re-set the limit screws when you're fixing the actual problem, but being able to perform this simple hack can save you a long walk home!

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