Originally Posted by

**LWaB**
Are you sure rmfnla?

39t x 25.4/2 = 495.3mm circumference/ PI = 157.7mm diameter

With a 144mm BCD, this gives 13.7mm total difference in diameter between the centre of the chain and the centre of the chainring bolts. Slightly less than 7mm difference in radius, I don't think this can work.

With 135mm BCD, the total difference is 22.7mm, giving just over 11mm radius difference, still quite tight.

Repeating the calcs with 41t and 144mm BCD gives 21.7mm total difference, which almost matches the difference of a 39t 135mm BCD chainring.