Yesterday, I took delivery of a Rotor 2InPower MTB crank, and I looked at the installation instructions. They call for 35–40 Nm to fix the chainring to the crankset in their instructions. But I don’t think there is a way to use a torque wrench in this instance.
Rotor, in their own installation video, does not use a torque wrench and I wouldn’t even know to attach one in this particular instance. I cannot use a ratchet attachment like this one and the video instead uses a bb wrench.
Am I missing something or is this just something I need to “eyeball”? If so, what does 35–40 Nm feel like?
On a standard 10mm hex key such as Wiha, 40 Nm is enough force at the handle to lift a 20 litre bucket of water off the ground. With an extra long tool like the HT-10from Park, it’s enough force to lift a 12 litre bucket off the ground.
I use a standard 3/8 torque wrench with a 3/8 square x 10mm hex impact driver: both commonly available from non-cycling tool suppliers for reasonable money: around $AUD100 for the torque wrench and under $AUD20 for the driver.
Alternatively, 40 Nm is somewhere between good’n’tight and effin’tight.
I bought a Rotor 2inPower SL crankset a couple years ago and after figuring I wanted to be nice to this crank bought a Tekton 3/8” drive click type torque wrench off the amazon. Found out I had been over tightening my other cranks so while I don’t use it often I feel it’s money well spent.
FWIW eccentric British motorcycle genius Allen Millyard has a brilliantly simple method of ensuring that your torque wrench is in spec. It’s at about 2:00 in this video Allen Millyard
I am sorry I misread your original question. I have never had to deal with that Rotor power meter crank, I thought you were referring to a central crank bolt. My apologies.
FWIW the general principal remains valid: you can gauge the torque by multiplying the length of the spanner used to turn the adapter ring (torque arm) by the force applied.
So there are fasteners where torque actually matters (cylinder head bolts, main bearing and rod bearing bolts) and those where they’re simply trying to stop people from stripping the bolt but have the thing tight enough that it doesn’t come undone. Chainring bolts are the latter. Crank arm to spindle bolts are the former.
With a normal short bb tool say 125mm in length, 35Nm is about as tight as a normal person will be able to pull without risking slipping.
I also don’t think most things on a bike that call for 35 are really that critical to get to 35, including crank bolts, but I’ve never used rotor. If I couldn’t get a torque wrench on it I’d just retighten after the first ride
Torque is really just an approximation of the end result, which is a set preload or bolt stretch for a fastener. Another method is to use an angle measure. So you tighten to snug, 10-20 ft/lbs and then turn the fastener an additional amount to ensure you get the desired bolt stretch based on mechanical properties. It eliminates the vagaries of lubricated torque readings. Most things on a bicycle don’t really require this level of detail. Engine builds do.
SRAM do a bottom bracket tool that includes a cutout for a 3/8” driver to allow use of torque wrenches, and is of a design that the torque wrench wouldn’t interfere with the spindle. Unfortunately it’s for SRAM 12 notch BBs so wouldn’t actually work for you, and I’m not aware of anyone making a 16 notch version. But in principle the same solution would work. SRAM DUB BSA Bottom Bracket Wrench | Tredz Bikes
