Never a week goes by without at least a half dozen bikes with shifting issues. Most of the time it is an easy fix, a slack cable or a tweak of the limit screws, (see a previous post). Sometimes a bent derailleur hanger is the culprit and, occasionally, it is a simple case of 'it is worn out’!
If we have checked all the above and it still has not fixed the problem, then chain, cassette and chain-ring wear is likely' the problem. Checking chain wear is a relatively simple procedure, there are quite a few chain checkers on the market, none of which are necessary or even reliable, the best way to check a chain is to lay it lengthwise on a bench and measure it over 3ft using an ordinary yard stick that measures 3 feet in inches. Line the center of a chain pin on the 1 inch mark and pull it taught; now look at the 36 inch mark, a chain pin should be centered on it. A new chain has its pins exactly a half inch on center, if it is not, then, how far off is it. Here is my rule of thumb on chain wear.
⅟₁₆ inch past = Fine, absolute minimal wear. If you are one of those people who like to slap on new chains every five minutes, then now would be a good time. I, personally, am not a follower of that cult.
⅟₈ inch past = Still fine, lots of wear left, however you are probably past the point of just renewing a chain.
⅟₄ inch past = Time to renew the chain, cassette and chain rings.
As the chain wears and the rollers start to migrate away from each other they wear the teeth of your cassette cogs and rings to suit their new dimension. Once the chain has reached that ⅟₄ inch mark it has done a lot of reshaping and a new chain will have no chance of adapting to the new tooth profile and the old chain will be having a hard time hanging on to the teeth in your favorite gear combinations, the ones that are worn the most, under heavy pressure. So, end result, suck it up and open up the check book…
Just recently we had a Time Trial bike in for the very problem of slipping under pressure. This particular bike had lots of underlying issues as well and ended up taking a good deal of my Sunday afternoon, however, I will not bore you with them now. The bike did bring up an issue which surfaces from time to time though, especially among club riders and weekend racers who are prone to swapping wheels and cassettes from bike to bike. After I had addressed each of the underlying problems and adjusted the derailleurs the thing still gave me a fit shifting in certain gear combinations. The chain was showing minimal wear so I persevered with trying to fine tune it, to no avail. On closer inspection I found a Connex chain, which suggested a renewal at some point, a Sram cassette of a different vintage and chain rings of a Shimano system, which was probably original equipment After talking to the owner of the bike I learned that the cassette was recently borrowed from a friend, the chain was possibly renewed by the previous owner of the bike before it was sold and the crank and rings were, indeed, original.
The moral of this story is; keep track of your drive-train components. Swapping things around on bikes that wear at different rates causes mismatched parts. Modern, high end, gear systems are finicky, hell they barely get along with each other at the best of times and, they definitely prefer to stick to the components they know. Grab the chain off one bike and the cassette from another and there’s gonna be trouble, with a capital T…