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The chainline

September 8, 2011

Switching from multispeed to singlespeed not only involves switching the freewheel, you will almost certainly have to alter the position of the hub on the axle. This is in order to set the chainline. On a multispeed bike the chain will go through varying degrees of movement side to side, depending on the gear you select. This is fine to an extent as the chain has the ability to bend slightly side to side, but you usually don’t want to use your largest chainring with the largest sprocket (nearest to the hub) because this will begin wear the sprockets. It’s less efficient and a weaker arrangement.

An example of an extreme chainline. Note the lateral movement the chain goes through.

So, given that for this conversion I have effectively forced my bike to use a gear closest to the hub, I need to adjust the position of the hub to bring the gear into line with the chainring.

How do you do that? well, another illustration from Sheldon Brown will help (see below).

The hub of the wheel sits on the cones nearest the centre of the axle. The locknut to locknut dimension will fit the spacing of the rear of the frame, so the position of the hub is dependent on the amount of space between the locknuts and the cones on each side of the wheel. The hub for this axle will sit slightly right of centre with respect to the frame. This is fine because the dish of the wheel can be set to compensate so that the rim is centred with respect to the frame. I’ll cover this later.

A picture of the rear axle with uneven spacers

The first job therefore is to find a position that the hub can sit on the axle so that the singlespeed freewheel will be in line with the chainrings on the bottom bracket. You can calculate this using the distance from the centre tube and the chainwheel, and matching that to the distance from the centre of the axle (or half the locknut to locknut dimension) to the middle of the sprocket – Sheldon Brown has a good page on this method, but for this first run through I did it by eye. (In the final adjustments, I’ll be more precise). I had two spacers from the original axle spacing – one 20mm wide and one 7mm, which allowed space for the sprockets when it was a multispeed bike. I needed to adjust this, but with only two sized spacers I wouldn’t have much luck. Online these things are ridiculously priced, so I went down to the friendly people at Fitzrovia Bicycles to see if I could get them. They found some brand new 1, 2 and 4mm spacers for me (which relieved me of only £3 compared to £6 or more online for fewer spacers), and on a second visit they gave me a 12mm one for free (that’s what a local bike shop will do for you!). With a range of sizes I could play around with the axle spacing adding different sized spacers to each side until I got the perfect chainline.

In the meantime I needed to adjust the dish of the wheel to bring the rim central. All I did for this was go round the entire wheel using the spoke tool to tighten the spokes on the non-freewheel side and loosen those on the freewheel side by a turn to start with, then half a turn until it was centred. This moved the rim away from the freewheel side, and therefore compensated for the new axle spacing. Since I dont have any of the workshop tools to check centring perfectly, I used my eye and the brake pads to find when the rim came tool close to the pads. This method also allowed me to check for any bending in the rim and correct that too by adjusting the spokes at the point of the bend. I’ve since found out that one mistake I made was not oiling the spoke nipples which meant that the turning of the spokes was sometimes hard, but it worked ok for now.

The chain attachment is next, and then I can take it for a test run!

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