Rear Chainline

[scituatejohn]

I spent some time searching the Internet for information about chainline. I found some good information about the front chainline, but I did not find any information about the rear chainline. What is the rear chainline when using Shimano 130mm and 135mm 9-speed freehubs? And, Is it desirable to match the front and rear chainline? Are there instances when it is not desirable to match the front and rear chainline? Is the chainline of mountain bike cranks (47.5-50.0 according to Shimano) larger than the chainline for Shimano rear hubs? If so, then why not use a wider (tandem) hub?

 

[boudreaux]

Originally posted by scituatejohn
I spent some time searching the Internet for information about chainline. I found some good information about the front chainline, but I did not find any information about the rear chainline. What is the rear chainline when using Shimano 130mm and 135mm 9-speed freehubs? And, Is it desirable to match the front and rear chainline? Are there instances when it is not desirable to match the front and rear chainline? Is the chainline of mountain bike cranks (47.5-50.0 according to Shimano) larger than the chainline for Shimano rear hubs? If so, then why not use a wider (tandem) hub?

You cannot just stick a wider hub in a frame not designed for it. What you have in the rear is fixed by the hub/cassette being used. Given the curent situation with octalink/isis BB and cranks, there is not much chance or even latitude for mucking with front chainline. If using tapred spindles, there often was alot of latitude for mismatch, and often the chainline was biased towerd the small or big cogs for specific reasons.

 

[Crosser]

The chainline is really important for fixed-gear or single-speed bikes, less so for those with derailleuers. Chainline is usually a center line between the two front chainrings and the center of the rear cassette. This is usually set by the type of equipment on your bike..the crankset and the rear cassette. Check Sheldon Brown's website for a definitive answer to where a MTB and road bike differ.

Tandem hubs are wider at the axle spacing, 140mm is a minimum, 145mm and 160mm are the norm. These won't fit in your frame, mountain or road.

 

[pudster]

Front chainline is measured by taking your seat tube diameter and dividing it by 2 then measuring from the drive side of the seat tube to the center of your chainrings. Next you add the 2 measurements together and this is your front chainline. The rear chainline is measured by taking 1/2 of your hub spacing (should be 130mm for road and 135 for mtn. half would be 65mm or 67.5mm). Next you measure from the inside of the drive side drop out to the center of the cassette. For a 8spd cassette it is between the 4th and 5th cog, for a 9spd it is to the 5th cog and for 10spd it is between the 5th and 6th cog. Subtract the 2 numbers and this is your chainline for the rear. The 2 numbers should measure within 2 mm to be correct. Another way is to put a straight edge on the center of the chainrings and let the straight edge drop to the rear hub and it should land in the center of the cassette. If you measure the chainline and it is correct if your frame is out of alignment bad in the rear of the frame the visual method may show the chainline is incorrect. Hope this helps you out and makes sense.