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The Fixed Innovations Axle Explained

This page explains the "theory" behind the Fixed Innovations axle. After reading it you will understand better how the axle increases the number of single-speed gears that will work on your vertical dropout frame.

(To order a Fixed Innovations axle, click here.)


With a single speed, and particularly with a fixed gear, the chain has to be tight. That's because you don't have a derailleur to take up slack in the chain. Slack makes the chain slap around if you're lucky; if you're unlucky it can allow the chain to come off and lock the wheel causing a crash.

Chainpath with derailleur Chainpath without derailleur

Now we come to dropouts. Vertical dropouts have become very popular in recent years for some good reasons: they tolerate lower-strength (a.k.a. "trick" or "lightweight") quick releases, and they hold the axle firm against the huge forces generated by super-low mountain bike gears. And they make changing wheels easier.

vertical dropout horizontal dropout

Unfortunately, they also prevent you from adjusting the distance between your axle and your bottom bracket. That is, they make it impossible to adjust your effective chainstay length and so to take up any slack in the chain -- once you've removed your derailleur.

Whether a bike has vertical or horizontal dropouts, the slot for the axle will be 10mm wide. This is because rear axles have traditionally been 10mm in diameter. But in recent years many hubs have been built around cartridge bearings, the most popular of which is the 6001 which has an internal diameter of 12mm. So many cartridge bearing hubs have axles that are 12mm in diameter along most of their length but narrowed to 10mm at the ends in order to fit the dropouts.

The Fixed Innovations axle is just like "traditional" axles for 6001-cartridge-based hubs but for one change. As illustrated in the pictures below, the 10mm end sections are located touching one edge of the 12mm main axle body instead of in the middle.

FI axle view 1 FI axle view 2

This means that as you rotate the axle within the dropout -- as you can do when the quick release is open -- the 10mm section stays in place but the rest of the axle -- and the hub and cogs -- move relative to the bottom bracket. The difference between the longest and shortest effective chainstay lengths on any frame using this axle is 2mm.

Here are a couple of small screen shots from the Java version of FixMeUp!. The light blue regions in the center show the gears that are available on a frame with 16.5" chainstays and a standard axle (left) and with a Fixed Innovations axle (right). Note that the difference between the effective chainstay length at the two ends of the blue region is only 2mm.

Chart with vert. drop Chart with FI axle

The region you get without the axle is only about 1/4 the size. Check out one of the versions of FixMeUp! to see how this works for your bike.

Copyright 1996-2007 by Eric House & Fixed Innovations
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