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Mountain Gear

Technical FAQ: The science, art and magic of cassettes

Lennard Zinn discusses mixing up gearing, power meters and chainsets

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Editor’s Note: Lennard Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn.

Dear Lennard,
I’m planning to move from a 26-inch Trek Top Fuel (3×10) to a Trek 29 inch Super Fly (2×10) this fall.  I would like to move the FSA SRM crank-based power meter from my Top Fuel to the Super Fly.

The Trek Super Fly that has the following 2×10 crank specification:
·         Crank: Shimano Deore XT M785
·         Bolt Circle Diameters: 104/64 mm
·         Standard: 38/26
·         Chainline:  48mm

My FSA SRM crank has the following specifications:
·         Crank: FSA K-Force Lite (SRM) 3×10
·         Bolt Circle Diameters: 104/64 mm
·         Standard: 22/32/44
·         Chainline:  48mm

I found these chainrings at Blackspire:
·         Outer Chainring:  104/38t
·         Inner Chainring:  64/26t

Can I put these chainrings on my FSA crank to use them on the Trek Super Fly?  I’m thinking it should work since the Bolt Circle Diameters and chainlines match.
— Allen

Dear Allen,
Yes, you can, although it is possible that the 26-tooth chainring will rub your chainstay. If it happens, it would be because the chainline of a triple crank is measured (from the central plane of the frame, of course) to the center of the middle chainring, while the chainline of a double crank is measured to an imaginary plane centered between the two chainrings. So the 4-arm 64mm BCD mounting face on the FSA crank will be a bit closer to your frame than the one on your XT crank is.

As you can see in the photo, I’m doing the same thing on my own bike; those are 26-38T 64/104BCD Black Spire chainrings on my bike with a plastic Gamut bash guard in the outer-chainring position. Works great; no clearance issues.

Look at how much space you currently have between the teeth of your 26T; if you don’t have at least 3mm, I’d guess that you’re going to have rub with the FSA SRM crank. Of course, you can always run a smaller inner ring on the FSA if that happens. Or you can use the outer two 104mm BCD mounting platforms for your chainrings, but the smallest inner ring you can get for that size is probably a 32T.
― Lennard

Dear Lennard,
I have a Trek Fuel Ex 9.9 2011 with a BB95 bottom bracket.  If I wanted to change the chainset to say a SRAM X0, which one would I buy?  There seem to be several options including GXP and BB30.  I find the whole bottom bracket thing confusing.
— Cath

Dear Cath,
Buy the GXP. BB30 won’t work on that bike. You’ll need Trek’s GXP bearing set and adaptors.
― Lennard

Dear Lennard,
I’ve been contemplating something you discussed recently — marrying a 50/34 compact crankset with an 11/36 cassette.  It’s a double-ring setup that gets me very close to my current 30/32 low, and it would allow me to use the SRAM double-tap levers, which intrigue me.

But I’m a bit confused by the cassette offerings.  The hideous cost of the cassette aside, SRAM chose to start off their XX 11-36 cassette with 11/12 cogs, which means they have to make it up by putting a nasty jump in the middle.  Shimano was a little smarter with their new XT 11-36 cassette.  They did an 11/13 jump in the beginning – smart in my book.  But even so, they finish off the cassette with 24/28/32/36 cogs, which means a 17% jump from the 24 to the 28.  If they had done 24/27/31/36 cogs, they would have evened things out and pushed the one larger jump to the bottom end, which in my experience is OK.  Any idea how these companies make their cog selections?
— Steve

Dear Steve,
I asked both SRAM and Shimano about this, and I got an answer from Shimano’s R&D director Wayne Stetina and SRAM’s mountain bike product manager Chris Hilton.

Shimano’s Stetina says, “I’ve been told by our engineers that Shifting ramp technology to get clean shifts is far more difficult in odd => odd tooth combinations other than 2-tooth difference, compared to even => even numbered teeth for larger steps. You can also put in as many gates as the tooth difference between cogs. Also, Steve is looking for a Road system.  Since the 10-speed Dynasis system is not designed or contemplated for road use, we also wanted to keep all the steps as even as possible.  For MTB you don’t want a bigger jump at the very bottom end.”

SRAM’s Hilton says, “Cassettes are a tricky product. They seem pretty straight up, but they are full of science, art and magic. You have to balance trying to get the right steps between gears, while also making sure you have good shift synchro for those gears. There is also a development timeline and significant tooling. Plus you have to factor in the user and the chainring gearing being used. It’s amazing how few riders know anything significant about their gearing. Inevitably, whatever you make will lead to people asking for something different.”
― Lennard

Read also:

Tech FAQ: What’s the big deal with 650b?

Tech FAQ: Fine-tuning entry and release with Crank Brothers pedals

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