Afternoon with a Rotorized Brompton: a cautionary tale (long)
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?
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The Rotor crankset sounds very interesting, but there aren't enough pictures on the net for me to get a clear idea of what the problem is. If you took a hammer to your frame and pounded the offending bit of steel out of the way, would it still be possible to ride the bike? That is, can you do this without ruining the alignment of the frame? If so, would it be safe? If it's worth a try, what's the risk of failure (how much does a new Brompton rear triangle cost)? I suspect you would be better off to experiment with this crank on a different bike first, and only adapt it to the Brompton later, if you find it indispensable.
What kind of pedals are you using? Brompton stock crankset is lighter than expected.
In general, I use a Dimension cage on the right (with Power Grip) and the stock Brompton folding one on the right. For the moment, I've got the Dimension cages left and right on the Rotor; I don't have a socket set, and don't want to take chances with the folding pedal bolt.
And you're definitely right about the Brompton's crankset. That swaged ring, in addition to dispensing with a spider and bolts, provides a flat face with lots of clearance in that chainstay area. The B's L pedal and BB do cancel those weight savings out a bit, though.
The Rotor crankset sounds very interesting, but there aren't enough pictures on the net for me to get a clear idea of what the problem is. If you took a hammer to your frame and pounded the offending bit of steel out of the way, would it still be possible to ride the bike? That is, can you do this without ruining the alignment of the frame? If so, would it be safe? If it's worth a try, what's the risk of failure (how much does a new Brompton rear triangle cost)?
Believe me, I wouldn't dare try such major surgery on the triangle myself. While that area is actually fairly beefy-looking (given the huge-diameter chainstays, and equal-sized chainstay brace), the amount of metal that would need to "move" is pretty substantial (~38.0% of the stay diameter).
I think one would need to cut that area down, then weld a plate back over the relieved area for reinforcement. Way beyond my abilities, to be sure.
And, if all else failed, a new B triangle could be procured...albeit to the tune of $142.
Originally Posted by rhm
I suspect you would be better off to experiment with this crank on a different bike first, and only adapt it to the Brompton later, if you find it indispensable.
I've been knocking that idea around myself. I've been knocking around the idea of "fixing" an old Elance frame I've got in the back room, and the Rotor might be fun to try there. If the RS4x can make a freewheeling bike feel like a fixed gear, I'd imagine it would make a fixed gear feel like relentlessness squared.
And I do know for a fact that people have singlespeeded the Rotor, so I imagine it would work fixed. And if not, at least I've got grist for another thread.
i know this was ages ago but i've now acquired some rotor cranks but reckon with the derailleur system on the S2l i can probably get away with a wider chainline - or am i being stupid and it won't make any difference? - you got any more pics of your project with the cranks fitted?
Wouldn't an Ovalized chainring get a lot of what you seek, done more simply?
Rotor makes those too, and the Welsh Machine Shop, Highpath Engineering
can make any configuration of chain ring that you wish. ovals and whatnot..
The Q-Rings appear to be a severely watered down version (of the original concept) for the purpose of successful commercialisation. Not to start a flame war, but from what I've read they seem to be the least effective of all non-circular chain rings on the market today.
Here is a good independent study of the Rotor crank system. The power gain is much more modest in comparison to Rotor's claims. The interesting thing, however, is that the cranks load the lower limbs' musculature and joints differently, a fact which is consistent with the claims for better efficiency.