Wednesday, May 02, 2007
Continuous Torque Drive
A few weeks ago I wrote about Earle Jones and his continuous torque crank set. That original post is here:
Earle loaned me one of his bikes and after swapping the bars, stem and saddle to make the bike layout similar to what I'm used to, I rode the bike for a total of 150 kilometers over the course of several days.
I can't claim that this was a particularly scientific test. Ideally, to compare the difference the crank makes, you would take two identical bikes that differ only in the crank mechanism and ride those bikes over the same course, under similar conditions. My commutes are always somewhat different in terms of traffic and what load I'm carrying, weather and other factors. These factors, combined with the fact that I was riding bikes that differed in far more ways than just the cranks, probably account for enough "noise" to make any particular numbers meaningless.
Nonetheless, everybody wants to know, do the cranks make a difference? Did they make me faster or did they make the riding easier? Were my commute times dramatically faster or slower?
The answer is, for me, the numbers came out about the same. On one commute I was a little faster on Earle's bike but the bike had faster coasting tires than my regular commuter. On the other hand, maybe I was better positioned on my regular bike. I really can't say with certainty. Another day I was faster on my "regular" bike.
My impression is this: the cranks make a difference, but I don't think that difference makes much of a difference.
Let me explain.
If you look at the pictures at the top of this post, I think you can get an idea of how the cranks work. In the first photo, the cranks are horizontal. If you think in terms of a clock face, one crank is at 3:00 and the other is at 9:00.
Looking at the second photo, you see that when one crank is at 6:00, the other crank is no longer 180 degrees off from the other one, it is actually 22.5 degrees off from top-dead-center. Instead of being at the 12:00 position, it's more like at 1:00. The cams in the crank force this motion.
In the next two photos, you see the cycle repeated. When the cranks are horizontal, they are 180 degrees offset, but when one crank is vertical, the other is 11 degrees ahead. The net result is that the cranks eliminate the dead spot in the pedal stroke.
When I first rode the cranks, they felt odd. Since the cranks vary their relationship to each other, one foot is effectively pushing the opposite foot around. Initially this felt like a pulsing in the drive train, but I soon got used to this and it felt "normal." One thing I did notice was that I tended to climb in a higher gear. My theory is that difficulty in getting a pedal past the top dead center spot in a normal crank is what cues my brain to down shift. With Earle's cranks, that cue isn't there. But the cranks do smooth out the effort, so I didn't feel I was straining my knees in any way. What feels like surging is actually, the instinctive surge that I'd put into a "normal" crank set. I realized this after I'd gotten used to Earle's cranks, I actually thought at one point that I'd broken the cranks because I wasn't noticing the surge any more. However when I spun the cranks I saw they were still working and when I switched back to riding my bike with the "normal" crank, it felt odd. I felt the dead spots in the stroke and now it felt like my effort was surging.
And I think that revelation is the bottom line about these cranks: they do work, they let me put out continuous torque.
And I don't think that matters.
Physiologically, I'm not torque constrained. If I gear higher, I turn the cranks a little slower. If I gear lower, I spin a little faster. The net result, for me, is that I climb at similar speeds. If I'm not applying constant power throughout the stroke, is that a bad thing? Maybe I'm getting a micro rest every revolution? The human heart beats in pulses. Is it bad to pedal in pulses? I don't know. My lungs breathe in and out in a distinct rhythm. I'm legs and lungs and heart and I eat food and drink water and I ride a machine that lets me go further and faster than I can go by myself. But does this extra clever crank let me go extra fast or extra far? I kind of wish it did. Like Fox Mulder, I want to believe. But I don't think it does. not for me. I think that I can adapt to continuous torque cranks and conventional cranks and at the end of the day the difference is not much.
Earle's cranks are similar to the Rotor Cranks which are reviewed here:
Throughout history, similar ideas keep coming up. Here is Tom Traylor's story of similar cranks:
And here's what Bryan Ball of Bent Rider Online had to say about Rotor Cranks:
All these cranks are clever solutions to the same problem. Perhaps for those racers trying to gain a few seconds in a time trial, a drive with cams is the answer they are looking for. But for me, riding to work or riding around, I just turn the cranks which turn the chain which turns the wheel. I don't think about my pedal stroke, I don't think about the dead spot, I just ride my bicycle. And that, perhaps, is why these clever cranks have not taken the world by storm. If you solve a problem that people don't have, the world won't beat a path to your door.