For some time, I've been fixated on the idea of installing a Rotor crankset on my M3L. For those who aren't familiar with them, Rotor's RS4x cranksets have their right crankarms mounted on a 4-bar linkage. This allows each crankarm to move ahead of top-dead-center as the other one reaches bottom-dead-center, thus removing the "dead spot" in the stroke.
My aims in doing this are partly to address some lingering knee issues I have, and partly to satisfy my cat-killing curiosity.
After fiddling around with the numbers, I figured that a RS4x double would be the one that would be most compatible with the Brompton's chainline. After some Googling, I located a used double with 170 mm arms and a 50T big ring--in other words, a match for the stock Brompton setup. (Big up to Momentum Multisport in Honolulu for their help.
I was initially concerned about the weight penalty of the Rotor crankset. Once I took out the B's old crankset and put it on the scale, the numbers shake out as follows:
Stock (Stronglight) crankset with BB, chainring and pedals: 1244 g
Rotor crankset with chainrings and integrated BB, but without pedals: 1264 g
So, once you throw in the pedals, you're up by ~ 370 g or so. Not great, but not nearly as bad as I had feared.
Installation went reasonably well, with one B-related hiccup. After I installed the left arm, I noticed a lot of drag on the crankset as it turned. Closer inspection revealed there was zero clearance between the left shoulder of the BB shell and the pinch-bolt closure of the left crankarm; it was shaving off a layer of powdercoat as it went around.
Steps to solve this problem:
1) Carve the remaining powdercoat off with an X-Acto knife.
2) File the shoulder to brightness with a flat file.
3) Cover the metal with Phil grease for protection.
4) Bevel the square edges of the pinch bolt interface with 10 strokes from a triangular file.
The maiden ride went remarkably well. As I coasted along and slowly turned the pedals, I could feel--and see--a definite push every time my feet went through the dead center points. Under way, the Rotors felt a little strange for the first few seconds, but were fine after that. I was a little concerned that the drive-side arm (which is perfectly straight) would clash with my right foot, since I have a duck-footed stance. This, however, was a non-issue.
Others have said that riding Rotors makes you feel like you're constantly riding downhill; this is definitely true. It makes the gears easier to push, and gives the B that same "relentless" feel that I get riding my fixed gear. Plus, coasting on a Rotorized Brompton is not nearly the character-building experience that "coasting" on a fixed is.
Now for the bad news. When I got back and made the M3L "sit," the rear triangle jammed a couple inches shy of where it would usually stop. Flipping the ring around and recessing the bolt heads didn't resolve the problem.
Closer inspection revealed the Rotor's inner housing was catching on the chainstay. Resolving this problem would require an extra ~7.16 mm of clearance where this occurs. Since I obviously can't move the right crankarm out that much, this means modification of the rear triangle itself would be neccessary to make the RS4x work completely with the Brompton.
Since I feel the Rotor cranks have real potential, I plan to start looking around for individuals and shops that would be willing to alter the right chainstay. Steve Parry and Wildframe are the first two that come to mind. Any other suggestions?
Thanks for reading,