There are a lot of myths circulating in various forums about the importance of chainline. Chainline refers to how straight the chain runs between front and rear sprockets. In single speed set-ups and where an internal hub gear is used, it’s often possible to get a near perfect chainline. Sheldon Brown explains chainline in three articles, the lead one of which is here. With derailleur gears, the general guidance is to set up a straight chainline based on the position of the middle sprocket on both back and front, where there are 3 on the front, or in the middle on the back and between the 2 front chainrings in the case of a double, or aligned with a single front chainring. The problems with running the chain at an angle include lower efficiency, greater chain wear and noise. But just how big an issue is it?
This paper, written in 1999 by members of the American Society of Mechanical Engineers (ASME) throws up some interesting conclusions. The reason that a ‘poor’ chainline is cited as reducing pedalling efficiency is that there is increased friction causing energy to be wasted as heat. The paper doesn’t argue with this point but the conclusion is that these losses are negligible compared with non-thermal losses. The paper describes how an experiment was set up to measure efficiency relating to 3 characteristics of a chain drive: the ratio of sprockets and their relative alignment (chainline), variations in input power and rotation rate (cadence) and lubrication, or lack of it. The detailed results make very interesting reading. The authors concluded that chainline offset and lubrication have a negligible affect on efficiency, at least under laboratory conditions and no significant efficiency differences could be detected between a lubricated and dry chain. (I wonder if a dry chain might actually last longer because grit and grime would not adhere to it and cause greater wear?) The major factors affecting efficiency are spocket size, larger sprockets providing better power transfer, and chain tension. Most importantly, there is a reciprocal linear relationship between chain tension and efficiency, the higher the tension the higher the efficiency. The latter makes a huge difference. With a chain tension of 305N the experiment revealed a drive efficiency of 98.6%. When the tension was reduced to 76.2N, efficiency fell by nearly 18% to 80.9%. Don’t worry about getting your chainline accurate to within mm, don’t worry about lubrication, but do keep the chain well tensioned. I don’t know how the figures above relate exactly to chain slack but 305N is pretty taught!