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Reverse K-drive

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    Posted: 17 January 2019 at 5:48pm

I've been reading on K-drive and something simply made no sense to me.

Why the secondary cranks are co-rotating?

That effectively preserves leverage though the ellipse. If you are willing to go to ridiculous lengths with your drivetrain, why not drive them with a pair of sprockets, not a chain, and make them counter-rotating instead?

Of course, with crank dimensions like on K-drive that would result in an outrageous leverage curve - like an oval chainring that is likely about 4 times as long as it is wide, but making secondary cranks shorter and main cranks longer should fix it.
It will also make the pedal path more oval, less like a linear drive that comes with a set of caveats...

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Post Options Post Options   Thanks (0) Thanks(0)   Quote AlanGoodman Quote  Post ReplyReply Direct Link To This Post Posted: 17 January 2019 at 10:32pm

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Balor Quote  Post ReplyReply Direct Link To This Post Posted: 18 January 2019 at 10:16am
Bah. I think I've not stated my case clearly, I am not native English speaker.

As it is indeed clear from this video, when in power phase, the 'secondary cranks' rotate WITH the crankset, as in - 'speeding your pedals along'.
That compensates for drop in leverage when cranks 'collapse into themselves', hence leverage curve stays relatively flat (or maybe even dips in the middle - admittedly, I cannot build a mathematical model and go by intuitive understanding of leverage).

When your cranks are maximally extended (dead spots), second cranks rotate against the direction of rotation, hence decreasing leverage - again, reverse of we want, if oval chainrings are of any indication.

Of course, you might be an oval rings sceptic, in this case disregard (but there is a a LOT of data on those available, including very plausible scientific models - but few of them are 'oval' enough or perfectly rotated - Q-rings, for instance, simply do not work).

With a counter-rotating K-drive, you are not limited in "ovality" (as in - variation of leverage though the stroke) unlike when it comes to oval rings where a chainring 10 cm wide and 1 meter long is simply unviable :).
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Balor Quote  Post ReplyReply Direct Link To This Post Posted: 18 January 2019 at 10:21am
Let's elaborate:

Instead of 'speeding your legs along' at power stroke  counter-rotating secondary cranks will instead slow them down, further decreasing your leverage (in addition of subtracting it's length from main cranks) just like an oval ring, and exactly where you can take it - the center of power stroke.

At dead spots your combined crank length is greatest AND rotation of the cranks will speed them along as well, basically dragging your legs though the spots.

Of course, with crank dimensions like on K-drive that would result in an outrageous leverage curve - like an oval chainring that is likely about 4 times as long as it is wide, but making secondary cranks shorter and main cranks longer should fix it.
It will also make the pedal path more oval, less like a linear drive that comes with a set of caveats... something like 14 cm main crank, 3 cm secondary crank (will also make the system much stiffer).

Would make a great system for small wheels or streamliners btw - I presume it would make above mentioned system 'feel', when used with 32/24 MTB crankset on 20" wheels, like 'kind of' a road compact double at height of power stroke, and you will likely be able to get away with a simple road cranks on a streamliner.

In effect you'll be spending much more time at power stroke phase.
Given that original idea is about 100 years old, I wonder if anyone already tried that... and what was the result.   
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AlanGoodman Quote  Post ReplyReply Direct Link To This Post Posted: 18 January 2019 at 10:48am
No idea to be honest - The K-Drive works but is heavy, relatively complex and a little prone to breakdowns.
For most people standard short cranks are probably a better bet.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Balor Quote  Post ReplyReply Direct Link To This Post Posted: 18 January 2019 at 10:51am
When it comes to recumbents, our most-perfused muscle group is glutes - provided you don't put too much weight on them, hence seat with lots of recline/area might be a great *performance enhancing* device (see Railgun seat).

Of course, not everyone is equally affect by recumbent-specific blood drop in elevated legs (our circulatory system, IN THEORY, has ways of compensating - but it seems my organism is particularly bad at it, for instance), but given that number of bent/DF riding athletes that have absolutely the same power numbers on both platforms is very small (most show a varying drop in power, especially when high BBs are concerned) - it makes sense to maximise the work phase that our most perfused groups are best at doing - pushing.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Balor Quote  Post ReplyReply Direct Link To This Post Posted: 18 January 2019 at 10:59am
Originally posted by AlanGoodman AlanGoodman wrote:

No idea to be honest - The K-Drive works but is heavy, relatively complex and a little prone to breakdowns.
For most people standard short cranks are probably a better bet.


Yea, I love short cranks. Solve ALL kinds of problems. What they do NOT do is emphasize different muscle groups.

Of course, same can be achieved (and likely we already do it in a way, subconsciously) by varying muscle contraction speed though the pedalling circle, speeding up at some regions (power stroke) slowing down in dead spots - hence you don't have to work as hard, and this is why conventional cranksets are actually quite good, if not strictly 'optimal'.

It makes sense that adding lots of linkages/pivots to the system may optimise biomechanics, but generate more friction than watts you gain with the former... 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AlanGoodman Quote  Post ReplyReply Direct Link To This Post Posted: 18 January 2019 at 11:20am
You will be welcome at any of our events if you are ever over this way...

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Balor Quote  Post ReplyReply Direct Link To This Post Posted: 18 January 2019 at 12:45pm
Oh, I'd love to, I really admire Burrows for instance (and take his '90 deg angle is best' ideas to heart) - if only just to meet people :).
None of my bents is 'racing grade'... neither I am, truth be told. I'm more of an obese ultracyclist and a tinkerer that outsources most of his tinkering - a living oxymoron, ehehehe.
 
In my case - above mentioned power drop with high BB - I would be unable to compete with someone who is more gifted (or less cripped, truth be told). Low BB kills aerodynamics, though K-drive is good in this respect - you can have BB reasonably level with the seat and maintain adequate aerodynamics even without fairing. That is the reason why I began researching 'alternative drivetrains', btw - but it seems to me that a relatively small modification to design should make not just better aerodynamically, but biomechanically as well, provided that resulting extremely steep levelerage curve will not turn out to be 'too much of a good thing', which is entirely plausible... not to mention this system is way too complex for me to make or order done (last I heard commercial implementations are south of 1000$).
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RoyMacdonald Quote  Post ReplyReply Direct Link To This Post Posted: 18 January 2019 at 5:37pm
Miles says the loss over the traditional rotary drive due to the K-drive is 10%. But the aerodynamic gain being able to keep your feet within the Kingcycle fairing is considerabley more. 

I found the K-drive is hard on your legs though and harder to spin at a higher cadences. The Beano just uses short cranks. I do have short legs though. Short cranks do feel nice and comfortable to me.

Roy 
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