Magic Gear


I had a client who wants to convert his Bianchi Premio, with vertical drops, into a fixed gear. For the uninitiated, fixed gear bikes are usually created with bikes that have horizontal drops, thus allowing the builder to adjust the rear wheel forward/backward to get the chain tension perfect.

Horizontal Drops

Many bikes have vertical drops, as the derailler is what creates the chain tension, and adjusting the wheel is unnecessary.

Vertical Drops

It is the issue of building a fixed gear bikes with vertical drops that we are covering here..


There are expensive options to achieve this, including eccentric hubs that allow for horizontal movement to achieve proper chain tension.

Another way to deal with this is by calculating the Magic Gear, which is the calculated combination of chainring and cog that will give correct chain tension at a given chainstay length.  I am using the calculator found at Fix Me Up Online.

(Team Lope attempt at the Magic Gear)

The form asks for an range of chain-ring and cog teeth you are willing to use, as also asks for tire size and chain-stay length.  Chain-stay length is measured from center of bottom bracket to center of rear axle location.  Its admittedly hard to get an exact measurement, but I got within 1/2mm I believe.



The calculator returned the following gear ratios as being acceptable:

48:14
50:16
52:18

Since my client has a 50cm Shimano chainring, we are going that route.


The chain-ring came into the shop as a double chain-ring.  Since only 2 bolts were left on, it was easy to disassemble....however since there were only 2 chain-ring bolts, I ordered a new set.

We are building a wheelset for this project, and once that happens we'll be able to test the validity of this process.

More to come...

Ok, the parts came in today and I just had to test it out.  Before building up the wheels, I decided to mock up the axle in place.



It resulted in a pretty straight chain-line.

Initially, this crank had 2 chain-rings, 50 on the inside and 42 on the outside.  I flipped the 50 to the outside position to avoid clearance issues with the chainstay.


Next step: build the wheels and test ride...

Sunday 3/28/10
Well, I am actually very happy how this turned out.  I got the wheels all built up and trued, and the mocked the bike up for a test ride.  There was a surprising amount of play in the vertical drops.







Thats one tall glass of water.




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