My Rotor MTB crank arrived last week and I finally had time to install them across an evening and a morning (we have three kids). As usual, Rotor’s stuff is top quality.
Rotor officially lists the crank to be compatible with 11-speed Shimano drivetrains and with BSA68 BBs. Hence, I thought I was in the clear. But to me it seems that the chain line sits much more outboard than with the native XTR cranks:
It gets very noisy and it doesn’t seem, hmmm, whatever the polar opposite of ideal is. Is my chain line too extreme? I cannot remove any spacers on the drive side, I have only included a 0.5 mm plastic washer to protect the BB from rubbing against the crank.
The other factors that contribute are the oval 38-tooth chain ring and it seems to me that the BB is several mm wider than the XTR one it replaced. On the plus side, I have a 11–40 cassette in the rear.
Is that chain line rideable? Should I get a smaller chain ring, perhaps a round one?
PS Turns out, I needed a new crank anyway, I hit stairs with my crankarm last Thursday or Friday and this happened:
Hence, my XTR M9000 crank is toast. It was good, because this way I could tell my wife I replaced the crank because the old one was broken — and not because I wanted a power meter on my hardtail and I didn’t like 175 mm cranks and wanted to switch to 165 mm like on my road bike.
Have you measured the chainline? You can measure the center of the chainring to the centerline of the down tube or seat tube. If you would like to take the time, you can reinstall the old crank as well, just to verify its chainline measurement to compare.
Are you concerned mainly because of the drivetrain noise? I ask because Rotor chainrings can be loud at steeper chain angles. It’s almost like the inside faces of the chain plates are squeaking across the tooth faces.
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Unfortunately, measuring the “before” would mean I’d have to reinstall my old cranks, etc. The chain angle looks more extreme, but I haven’t measured anything.
Yes, they are much louder. I had to extend the chain, too, because the old chain length was woefully insufficient.
The (round 38-tooth) Garbaruk XTR chain ring I had before had an additional offset (with size) to make sure the chain line is still acceptable.
Some chainrings come in 0, -3mm, and -6mm offsets, with the -6 being closest to the bicycle’s centerline.
Example: Direct Mount Chainrings for SRAM 3-bolt Cranks – Wolf Tooth Components
From that page:
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There are 3 offsets:
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Boost: 0mm offset for 55mm chainline required for SRAM Transmission
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Boost: 3mm offset for 52mm chainline or Transmission compatibility with Super Boost bikes
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BB30: 0mm offset for BB30 short spindle cranks for 49mm chainline
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Standard MTB: 6mm offset for non-Boost mountain bikes, GXP and long spindle BB30 cranks for 49mm chainline
Trailcraft says:
We like our standard chainring at 6mm offset for ALL of our bikes (boost and non-boost) but a 3mm offset chainring might work better for you based on where you typically ride in the cassette. The 6mm is going to offer better chainline in the bigger (easier) gears, while a 3mm offset might be better in the lower (harder) gears like downhill/enduro type of riding. The 3mm (boost) vs. 6mm (standard) is heavily debated on the internet forums, but for us a standard 6mm offset seems to offer a great chainline for those who spend most of their time in the easier (bigger) gears. You can see such discussions here.
Trailcraft Direct Mount Cranks | Lightweight Bike Cranks
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Count me amongst those that prefer the 6mm offset. My typical riding terrain means majority of time spend in lower half of gear range, with a decent chunk of that in largest three sprockets, so optising for that makes most sense, particularly given that’s also when the chain has to deal with the most acute angles (shortest run from chainring is when mech cage is angled forwards the most)
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To my knowledge only Rotor itself makes chainrings for their OCP mount. And they do not make chainrings with different offsets.
They do make chain rings for various BCDs. But you are right in that they do not specify or mention the offset (like e. g. Garbaruk does with some of their chain rings), I don’t think I have a choice.
