Tech FAQ: Time for 2×9
I am a mid-pack, expert-level XC racer and I currently ride a 2008 Specialized Stumpjumper Comp 29er. I want to convert my Stumpjumper to a 2×9 over the winter and I would like to know what parts you would suggest I go after to complete my build. In addition, I know they are personal and based on the rider’s abilities, but I also wondered if there are any good 2×9 gearing combos that you can suggest. I ride and race throughout Michigan (think Iceman Cometh Challenge) with a few races in Wisconsin, so terrain is pretty much fast and rolling — big ring stuff all the way.
First, here are my goals for a 2×9 conversion:
• I never use it and I want to trim whatever weight I can, so I want to get rid of my granny ring. I am really looking for optimum performance and reliability in a 2×9 drivetrain.
• My bike is currently outfitted with Sram X9 trigger shifters, Sram X9 mid cage rear derailler, Shimano M581 LX front derailler, and a Shimano M542 two-piece bottom bracket paired to Shimano A/A/S 2 piece cranks. I am on a budget, so if possible, I want to keep this project as simple and buy as few parts as possible.
• I like my X9 triggers and I want to be able to keep using them. I’m hoping that my conversion can be achieved by adjusting the stops to get rid of a shift on the front. I’m also hoping that configuring my 2×9 can be done using my current cranks.
So, is there a popular setup (including front and rear gearing) that you can suggest?
Great question! You should be able to just get new chainrings and use the entire rest of your system as is, with perhaps a change in chain length.
The goals riders like those on the Gary Fisher team were trying to achieve with 2X9 before SRAM XX came out were similar to yours — lower weight with quicker, more reliable front shifting, and closely related to that, bringing the chainrings closer to the frame and hopefully the feet, too (reducing Q-factor). You, of course, cannot reduce your Q-factor without buying a new crank, so we’ll let that one go. And I think you want to reconsider getting rid of your granny ring. Instead, I think you want to keep the granny and the middle chainring and get rid of the big ring, provided you have room for a bigger granny. Having the chainrings closer to the frame will make your shifting quicker by using the portion of the front derailleur’s swing that is moving more across (i.e., “normal” to the plane of the frame) and less up.
As for gearing, you should know that one reason that all of the SRAM XX front chainrings shift so quickly is that the big ring is 1.5 times the size of the inner ring. This means that every pair of teeth on the inner chainring line up with (are centered next to) three teeth on the big ring. So the gear combinations are 26 X 39, 28 X 42, 30 X 45, etc., and the 26 X 39 would have 13 shifting points, the 28 X 42 would have 14, and the 30 X 45 would have 15, thus speeding shifting. But of course, the tooth profiling and the added shifting ramps on XX are designed for these pairs, and you would be finding chainrings that would not be designed for this. But I still believe that you could take advantage of this fundamental geometrical feature to get quicker shifting, were you to pick one of these combinations of chainrings.
I think the Fisher and Trek team XC riders using 2X9 combos a few years ago often did couple Shimano MTB front derailleurs (for weight savings) with SRAM shifters and rear derailleurs, so you’re already there. I think you could do this cheaply and easily and get what you’re seeking.
Technical writer Lennard Zinn is a frame builder (www.zinncycles.com), a former U.S. national team rider and author of numerous books on bikes and bike maintenance including the pair of successful maintenance guides “Zinn and the Art of Mountain Bike Maintenance” – now available also on DVD, and “Zinn and the Art of Road Bike Maintenance,” as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.”
Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and feeding and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn.
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