2x9 ala PVD
The concept for this project was driven by my upcoming races at the 2007 Sea Otter Classic. I plan on competeing in Super D, Hardtail Downhill, and Cross Country on the same bike.
I have been riding my Six-Speed on trail with great results, but I will need more range for the Sea Otter races.
Narrow Range Front Gearing
Finding a crank that will run adequate gearing for a 2x trail bike is a bit difficult. One crank that is almost perfect from the box in terms of gearing is the Shimano FC-6603 Ultegra 10 Speed road triple crank. The smaller gears of the crank are 39 and 30. A very good spread.
This crank has a PVD Custom Bash Ring installed. I usually do not use bash rings, but a section of trail we have been riding lately requires draging ring over a few rocks to clean.
The 5 bolt chainrings are 130/74mm patterned. The 30 tooth is probably a little high. I'm going to change that out to 28t with a 11-32 cassette. Salsa makes some very nice rings that are perfect for this. It would be nice to place a 40 tooth ring in place of the 39, but it is basically impossible to find one with as nice shift ramps as the Shimano 39.
A standard 11-32 cassette would be nice for this bike. Better would be to tighten it up and move the first cog to 13 or 14 for improved performance.
I ended up with a 13-30 cassette at first, but just ordered the 11-32. Needed the lower and higher gears. The cassette was just to close.
The Front Derailure
A standard MTB front derailure will not work with the large road rings unless the derailure is raised fairly high on the seat tube. The cage will otherwise get caught up on the 30t gear.
I elected to try a Shimano FD-R770 front derailure for double. It allows the derailure to be placed low and close to the rings. Both styles of deraulure come very close to the rear tire.
The front derailure needed down tube routing to work. A drill and some tortches solved this issue.
The Ultegra 10 speed Triple cranks configured as a twin gear as I have done produces a chainline of 43.5mm. This is pretty bad. It brings the front derailure very close to the rear tire. Adding an additional 2.5mm spacer brings the line out to 46mm. Any more than this is very difficult to acheive.
The Final Product
Other Crank Options
Stronglight seems to be trying to get back into 2X9 gearing for MTB.
The Oxale Two Crank w/ 108/113mm ISIS bb has a 48/50mm chainline. This is nice, but the 29-44 gearing is a little rough for all but the stronger racers. The bolt circle is 94mm. To bad about the ISIS.