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Sep 122009
 

Joe Pusl did an interesting modification to his Santa Cruz Blur. Check it out.

I recently changed my Santa Cruz Blur over to a 2×9 setup (2 front rings, 9 speed in rear). I have attached some pics of this project. For 2010, SRAM has done this (2×10 in their case) for their XC racing group called “XX”.
See http://www.sram.com/en/XX/products

This is done for 2 main reasons: Narrower Q factor (distance between pedals / stance width) and better shifting (less cross chaining since only 2 rings). My Dura Ace double road crankset has a Q factor of 145mm. My M760 XT crankset has a Q factor of 175mm. That’s a huge 3cm difference that my knees didn’t like.
See http://www.julmtb.com/index.php?menu=qfacteur for more Q factors.

SRAM actually offers 2 different Q factors (156 or 166mm) depending on what the width of the chainstays on your bike are. The 2010 XX crankset is not available yet (as far as I can tell), but when it is, it will be big $.

So I set out to make a road crankset work. A road triple (Ultegra) has a Q factor of 155mm. Notice that this dimension is about the same as the narrower of the 2 SRAM options, so it won’t work on all MTB frames. The total BB width of a road double setup is 90mm (68mm BB + cups) The total for a road triple setup is 100mm (90mm + 10mm of spacers). I am using a 6mm spacer on the spindle drive side (comes w/ triple crank), + 4x1mm spacers (from a cogset), 2 on each side of the BB (pics 1 & 2).

The key issues are crankarm clearance and chainline.
The 2x1mm spacers on the non-drive side are used to keep the non-drive arm from hitting the frame (pic #3). The other 2x1mm spacers on the drive side are used to help the chainline and get the total spacing up to 100mm. Road bike front derailuers come closer to the seat tube and hence the rings are placed closer. The chainline is defined as the distance from the center of the seat tube to the center of all the rings. This is 45mm for a road triple and 47.5 to 50mm for a mtn triple.
See http://www.sheldonbrown.com/cribsheet-bottombrackets.html#chainline for chainline info.

Using the Ultegra crank w/ the 6mm spacer (w/o other spacers) w/ a 68mm BB shell (like I have; some mtb’s have 73mm) would give a 45mm chainline, just like on a road bike. Adding the 2x1mm spacers brings it up to 47mm, which is pretty close (pic #4).

I removed the big ring and swapped out the chainring bolts w/ ones that were shorter (made for a single ring). I kept the middle ring (39t), and changed out the little ring to a 26t. These are the same size rings as SRAM’s 1st of 3 options for their double. See pics 5-6.

They claim that 1.5:1 for the ring sizes is optimum for the most usable gear ratios.
The reason Dura Ace will not work is that the minimum little ring is 30t due to a bigger BCD.
See http://www.sheldonbrown.com/cribsheet-bcd.html for BCDs. Ultegra is 74mm inner, 130mm middle/outer (standard 5 bolt rings).

I’ve only ridden it a few times so far, but it really feels a lot better to me.
If you have any questions, send me email.
-JP

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  One Response to “Build your own 2X9 Mtn Bike Crank System – Reduce your Q factor”

  1. Cool article. Which model ultegra did you use?
    Do you know the weight of this setup?

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