Rotor cranks

Recumbents and all feet forward machines

Postby Kalgrm » Thu Feb 14, 2008 4:05 pm

It's not "a lot" faster, but you do get the sensation that something feels different.

The lever arm is shorter, and in my drawings, that is X1 and X2. It's easiest to see it on the recumbent drawing: Imagine the extreme case where the chain ring is the same radius as the crank arm's length. To drag the chain along (say) 30 links, you'd have to do nearly half a rotation of the crank.

Now if the chain ring were only 1/5 the radius, you would need to do three rotations to get those same 30 links dragged through.

If the chain speed was constant, it would mean you'd need to be spinning much faster to keep the same chain speed. (This is exactly the case when you change gears wit hthe front derailleur.)

The Q-rings act like you've got a small chain ring in place at the dead spot and a big chain ring in place at the "power position". However, there is only about 15% difference between the chain rings they are simulating, so you don't get all jerked about in your stroke.

Cheers,
Graeme

(PS - Thanks for that offer to make them for me Harry. I'll keep it in mind if I can't get what I'm after myself.)
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by BNA » Thu Feb 14, 2008 7:54 pm

BNA
 

Postby Uba Tracker » Thu Feb 14, 2008 7:54 pm

No worries at all Graeme, I'm happy to lend a hand (assuming I can). It's been quite a learning 'oval' reading all the contributions to this thread and the one undeniable fact that in a standard crank configuration there is a 'dead' spot, and by dead we're all agreed that this is the lull between opposing power strokes. So.......the mental hurdle that I just can't get my head around is that we can somehow convert this void of mechanical advantage into mechanical advantage. I mean if there is no or diminished input then there is nothing to draw advantage from. Am I making sense? I understand the Rotor Crank principal because of the variable crank timing but the 'Q' rings are posing a challenge.
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Postby Kalgrm » Fri Feb 15, 2008 4:10 pm

G'day Harry,

I'll give it one last go at explaining how I see an advantage in oval rings, since, up until now, I haven't explained it too well (based on the fact that there is still confusion out there ....)

An elliptical ring has the effect of having two chain rings on at the same time (see here for details of the various effective gearing ratios). For example, my 44t q-ring has the equivalent of a 42t ring at the dead spot and a 46.5t ring at the power position. Riding it has the effect of using both chain rings at once, so my feet scoot through the dead spot more quickly and stay in the power position for longer.

In effect, it's like having a graduated way of swapping between chain rings four times per crank revolution, going from 42t->46t->42t->46t, with the 42t taking effect at the dead spot.

The idea is that you spend more time in the power positions and less time in the dead spots per crank revolution.

Okay, that was my last attempt to explain how they work and why I have one (and getting more) on my bike. If that hasn't helped, it's beyond my meagre language skills.

Cheers,
Graeme
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Postby Uba Tracker » Fri Feb 15, 2008 8:06 pm

Oh..................I see, well if you grow tired of extraordinary photography then I am sure a vacancy within education is available. I now consider myself informed, thanks Graeme your patience is appreciated.
best regards
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Postby Birdman » Mon Feb 18, 2008 11:23 am

That picture in the link you provided really helped.

Thanks again.

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Re:

Postby johntolhurst » Wed Apr 21, 2010 12:05 pm

Uba Tracker wrote:If you need help with a drilling template Graeme, let me know the number of teeth you want and the mean major and minor elliptical axis dimensions and I'll see what I can whip up for you.
cheers
Harry

As I understand it, this would require calculating the circumference of an ellipse, and so choosing from a number of approximations because there is no exact formula?
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Re: Rotor cranks

Postby John Lewis » Wed Apr 21, 2010 10:11 pm

Here is Giles pucket's nifty program to do all the work for you.

Well not quite. You have to drill and file. :D

http://www.fleettrikes.com/giles%20puck ... ulator.htm

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Re: Rotor cranks

Postby toolonglegs » Wed Apr 21, 2010 10:26 pm

Meh...my rotor cranks have long since been removed...too heavy...way too much upkeep...and too hard do go from rotors to normal cranks...illegal to race and well...they are broken :roll: ...anyone want some titanium equipped 180mm rotor cranks that need a service :roll: .
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Re: Rotor cranks

Postby johntolhurst » Fri Apr 23, 2010 12:04 pm

John Lewis wrote:Here is Giles pucket's nifty program to do all the work for you.

Well not quite. You have to drill and file. :D

http://www.fleettrikes.com/giles%20puck ... ulator.htm

John Lewis

http://www.ozhpv.org.au/shed/sprocket.htm
This is the original posting.

http://www.cruzbike.com/Fitting_Ellipti ... heels.html
This is my original article on fitting elliptical chainrings.
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Re: Rotor cranks

Postby Joeblake » Sun Apr 25, 2010 11:47 pm

toolonglegs wrote:Meh...my rotor cranks have long since been removed...too heavy...way too much upkeep...and too hard do go from rotors to normal cranks...illegal to race and well...they are broken :roll: ...anyone want some titanium equipped 180mm rotor cranks that need a service :roll: .


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Re: Rotor cranks

Postby toolonglegs » Mon Apr 26, 2010 3:16 am

Joeblake wrote:
toolonglegs wrote:Meh...my rotor cranks have long since been removed...too heavy...way too much upkeep...and too hard do go from rotors to normal cranks...illegal to race and well...they are broken :roll: ...anyone want some titanium equipped 180mm rotor cranks that need a service :roll: .


What's the freight to Perth?

Joe


I will find out.
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Re: Rotor cranks

Postby Becker_11 » Wed Jun 23, 2010 4:41 pm

I had my doubts regarding the seriously short crank arms on my Greenspeed X5 it comes standard with 165mm cranks. Well the guy in the shop explained all about the riding position and spinning lower gears etc and after doing some reading online I understand what he was talking about. Another point and this doesn't relate to performance but if I'm being lazy when riding the X5 my heel can scrape the ground at the bottom of the pedal stroke. I'm a size 10 shoe and using the 165mm arms. Obviously shorter cranks and or smaller feet and this wouldn't be a problem.

I should add it might happen once now and then it's not an every revolution thing, if it was that would get very annoying
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Re: Rotor cranks

Postby bradwoodbr » Wed Jun 23, 2010 6:34 pm

Becker_11 wrote:I had my doubts regarding the seriously short crank arms on my Greenspeed X5 it comes standard with 165mm cranks. Well the guy in the shop explained all about the riding position and spinning lower gears etc and after doing some reading online I understand what he was talking about. Another point and this doesn't relate to performance but if I'm being lazy when riding the X5 my heel can scrape the ground at the bottom of the pedal stroke. I'm a size 10 shoe and using the 165mm arms. Obviously shorter cranks and or smaller feet and this wouldn't be a problem.

I should add it might happen once now and then it's not an every revolution thing, if it was that would get very annoying


Some ideas:
:idea: 1. You may be inadvertantly ankling. That means your heel is leading the pedal on the part of the stroke between 12 and 6 o'clock. 12 being the top position of the crank causing an occasional heel strike.
:idea: 2. In the past I have used 145 and 150mm cranks and there was no down time for me to get used to them, even with my size 10 feet. Speed and power were the same as for 170mm cranks. The advantages, however, are to be believed.
:idea: 3. You can get shorter cranks from Trisled and Greenspeed will make them from existing cranks for you.
:idea: 4. Another trick is to position your shoe cleat as close to the heel of the shoe as possible. This may well stop the occasional heel strike.
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