Six Speed MTB

This season I’m running six gears on the freehub of my Gary Fisher Superfly. I decided to use six gears instead of the normal nine for a few reasons. First, six is the max # of gear cogs that can be run on most single speed rear hubs. And why would I want to run a single speed hub? Because the wheel can be built without dish (laterally stiffer/stronger) and a better chainline (no cross chaining). The laterally stronger wheel build also means that I can ride a lightweight rim with less risk of damage. Another very small benefit is reduced weight from eliminating a few gears. I figured these benefits combined would outweigh the downsides of fewer high gears and more shifting to the big front chainring.

Now for the how… For starters, I’m still using the stock unmodified SRAM XO shifters, front derailleur, crank, chainrings, and chain. For the hub, I’m using the lightweight and strong DT-240 singlespeed version. For the cassette, I could have choosen loose gear cogs of my choice and put them together with standard 9 speed spacers. However, this would have limited the number of gears to five. Also, the hub has a lightweight aluminum freehub body, and if individual cogs aren’t pinned together, the pedaling torque will cause them to eat into the freehub body over time. To cut down on potentially damaging the freehub body, I choose to use the pinned together 18-32 gear cogs from an 11-32 cassette and only one small loose gear cog. This gave me the following 6 sizes: 15, 18, 21,24,28, 32. In order to fit all of these gears on the shortened singlespeed freehub body, the back of the cassette spider was milled down by clamping the cassette in a bench vice and using a facing tool. I’m not sure how much metal was taken off of the cassette spider, but if too much is taken off and the cassette ends up too close too the spokes, spacers can always be used on the freehub to push it back out.

That was the easy part. The tricky part was modifying the SRAM XO rear derailleur to stop before pushing the chain into the spokes. The modifications consisted of longer b-limit & lower limit screws, and a stop fabricated for the derailleur. A small hole was drilled into the derailleur parallelogram and it was tapped. The picture below shows the new screw holding the new metal stop. This was required because the longer lower limit screw by itself ended up interfering with the high travel when adjusted to properly limit the lower travel.

So how does it work on the trail? The initial adjustment was kind of tricky, but once the limit screws were dialed in, basic shifting was unchanged from a normal SRAM XO setup. The most noticeable initial difference is the ability to shift down all six gears with one thumb flick. Also, with large 29 inch wheels, I have yet to find a trail situation where I needed the missing larger gear ratios. I ran out of gears on a few faster pavement downhills, but XC races are won climbing, and that’s where this setup shines. Since all six gears are available with any front chain-ring combo, I can stay in the middle ring without shifting to the small ring on climbs longer than with a standard setup (I never used the 32/34 combo in the standard 9 speed setup due to chain rub). The clunky shift down to the small chainring ring is probably one of the biggest equipment related time zappers in a race, so this is a very good thing. Finally, if I do end up on a course requiring all nine gears, a standard nine speed wheel/cassette can easily slip back into the dropouts and shift flawlessly after a few minuntes of shifter/derailleur adjustments.

Much Thanks to Ace mechanic Tyree at the Homewood store for getting this to work!

Omar
www.ozonexterra.com