Chain wear.

14 12 2014

It could be argued that the chain is the single most important component of a bicycle. In some ways, other than the presence of two wheels, it is what makes a bicycle a bicycle. The chain transmits drive to the wheel from the pedals.

Abused and poorly understood, it bears some examination.

On an off-road bicycle, the chain is subject to both mechanical wear from use and the abrasion caused by dirt and water.

Chains on modern bicycles are marvellous. They measure only a few millimetres wide (Shimano 10 speed chain 6.2mm), yet they work exceptionally hard and (at least in the case of Shimano) are highly durable.

The chain and sprockets are designed so that the rollers match the shape of the teeth and the valley between them – when meshed the contact surface area between the two are maximised. This is ideal for the transmission of drive force and minimising wear. They have a pitch of 1/2″ or 12.7mm – this is the distance from one roller to the next.

Chains do not stretch. But with use, the rollers, pins and bushings wear. This allows elongation beyond the 12.7mm pitch and the wear of the chain, rings and sprockets increases exponentially. It is the pin wear that has the most effect. The teeth on the rings and sprockets are slightly triangularly shaped, which means the distance between drive side surface areas of the teeth is slightly greater at the top of the tooth compared to the bottom. When all is ideal, the roller sits at the bottom of the valley between the teeth, but with wear, the rollers ride higher on the teeth, there is less surface area in contact and thus the increased rate of wear and friction.

The solution is to replace your chain often. The magic number is elongation of 0.5% requires replacement. Many years ago, one of my favourite cycling gurus, Charlie Cunningham, wrote a piece on drivechain wear that stuck in my head. In essence, with frequent chain replacement and scrupulous lubrication, the rest of the drivechain will last a long time, even in muddy conditions.

I have owned and used numerous ‘chain checkers’. These tools make the process of checking chain wear quick and easy. To be fair, even using a ruler is quick, but it is also open to user error and I’ll admit I like the replace: yes/no binary answer a good checker gives. Most are placed on the chain in such a way that they push two sets of pins/bushings/rollers apart in order to measure the % elongation of the chain over a certain number of links. We are most interested in pin wear and tools that work in this way include roller and bushing wear in the result. As such, the checkers are designed to give a fairly conservative assessment so that you do not end up using a chain for too long and causing tooth wear. But it is inaccurate.

Shimano make two checkers. The TL-CN41 and the TL-CN42. I have the latter and it works to push the two sets of pins/bushings in the same direction and thus gives an accurate pin wear measurement. It is easy to use and although it is relatively expensive, it will save you money in the long term by prolonging the drive chain life.

The following resources were invaluable in learning about chain wear:

Chain wear measuring tools.

Sheldon brown: chains

63xc: fixed gear chains 101.

Peter Verdone: when to replace chains.

Charlie Cunningham’s (new) site can be found here. It has details of some of his builds – fascinating!

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