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Osiris
Starting Member

USA
27 Posts

Posted - 07/29/2017 :  05:53:31  Show Profile  Reply with Quote
Some time ago I got a close look at a WAW owned by someone in my area. I was surprised at the sheer volume required just to create clearance for the rider's feet and knees while pedaling, and that got me thinking about ways to minimize the bulbous front end. I started by creating some 3-D models in SolidWorks which could be animated. Using a human model based on my own dimensions, including a size 11 cycling shoe and 165mm cranks, I constructed a model of a conventional pedal arrangement:



By animating the assembly, I managed to create a volumetric model showing the amount of space required for pedaling inside the shell of the vehicle:



I then came up with an alternative pedal arrangement which considerably reduced the vertical distance that the rider's knees and feet would need to travel throughout each pedal stroke:



Here's an animation I created: https://www.youtube.com/watch?v=KZfWaRvcaI4

The result was an outer shell that was more aerodynamic, but without much of an increase in the length of the nose:



This was just a fun project for me, so I make no claims about how well it would work in reality.




Grant-53
recumbent guru

USA
544 Posts

Posted - 07/29/2017 :  08:47:23  Show Profile  Reply with Quote
There would be two more bearings in each stroke. The gain would be in the reduction of height more than a pointed nose. Thanks
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purplepeopledesign
recumbent guru

Canada
685 Posts

Posted - 07/29/2017 :  20:46:00  Show Profile  Reply with Quote
AFAIK, just about every builder/racer/experimenter that has tried a linear drive has gone back to standard cranks. Even if one assumes that those prototypes had better aerodynamics, it follows that the performance of linear is so much less that the combination is still less efficient that circular in a bigger package.

:)ensen.

Those who claim to be making history are often the same ones repeating it.

Video of my trike
http://www.youtube.com/watch?v=MdSLRD_2vzc
Photos of my trike
http://www.flickr.com/photos/purplepeople/
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Osiris
Starting Member

USA
27 Posts

Posted - 07/30/2017 :  06:23:58  Show Profile  Reply with Quote
"AFAIK, just about every builder/racer/experimenter that has tried a linear drive has gone back to standard cranks. Even if one assumes that those prototypes had better aerodynamics, it follows that the performance of linear is so much less that the combination is still less efficient that circular in a bigger package."

I haven't read anything showing that linear drives generally perform poorly compared to circular drives. Do you have a source for this?

The term "linear drive" has been used to describe just about anything that departs from traditional circular pedaling, ranging from various types of oval to straight line travel paths. There have also been a variety of different approaches used to achieve non-circular pedaling. It would be a mistake to lump all of these together when making a general statement about their relative efficiency.
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carolina
recumbent guru

USA
601 Posts

Posted - 07/30/2017 :  10:15:32  Show Profile  Visit carolina's Homepage  Reply with Quote
Some years ago in Europe

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velosRus.com

Edited by - carolina on 07/30/2017 10:18:32
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warren
human power expert

USA
6119 Posts

Posted - 07/30/2017 :  20:19:02  Show Profile  Visit warren's Homepage  Reply with Quote
If you just think about it from a momentum point of view you can picture why the linear drives perform worse than conventional. Conventional drives preserve the momentum of your legs going around and around, but with the linear drives that momentum is disturbed if you are pedaling in a triangular path, or even lost completely if you are pedaling in a true linear path. If you want a smaller pedal box, just use shorter cranks. You can use down to about 130mm without much power loss. You do have to pedal at higher RPM.
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alevand
human power expert

USA
2940 Posts

Posted - 07/31/2017 :  04:21:25  Show Profile  Visit alevand's Homepage  Reply with Quote
Linear drives have been around since the beginning and actually predate chain drives. What Andrew has drawn is not a linear drive, but more elliptical with a/b about 2/3. The start/stop only occurs at the slider end of the mechanism. To get the same height the short crank arms would be 110 mm from 175 mm. There have been many attempts at using linear drives in faired bikes, but none performed as well as the round crank.
https://www.youtube.com/watch?v=P32YBt1FWLE
http://www.recumbents.com/wisil/gNickGreen/nick_green.htm

https://www.youtube.com/watch?v=P32YBt1FWLE



C:
Tony Levand

Edited by - alevand on 07/31/2017 04:30:37
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Osiris
Starting Member

USA
27 Posts

Posted - 07/31/2017 :  05:09:08  Show Profile  Reply with Quote
quote:
What Andrew has drawn is not a linear drive, but more elliptical with a/b about 2/3. The start/stop only occurs at the slider end of the mechanism.


Right, with this particular geometry there would be no stopping of the pedals at the top and bottom of the pedal stroke. The eliptical path in which the pedals rotate can be made flatter or taller by simply varying the length of the rocker arms.

quote:
To get the same height the short crank arms would be 110 mm from 175 mm. There have been many attempts at using linear drives in faired bikes, but none performed as well as the round crank.


Do you mean that the faired bikes using these systems didn't perform as well, or that pedaling in perfect circles has been proven to be more efficient for some reason?

Edited by - Osiris on 07/31/2017 05:11:07
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alevand
human power expert

USA
2940 Posts

Posted - 07/31/2017 :  13:33:57  Show Profile  Visit alevand's Homepage  Reply with Quote
I don't know, but your knees and thighs certainly are not going in a circle on a round chain drive, only your toes. There are elliptical chain rings that claim improvement over round.


quote:
Originally posted by Osiris

quote:
What Andrew has drawn is not a linear drive, but more elliptical with a/b about 2/3. The start/stop only occurs at the slider end of the mechanism.


Right, with this particular geometry there would be no stopping of the pedals at the top and bottom of the pedal stroke. The eliptical path in which the pedals rotate can be made flatter or taller by simply varying the length of the rocker arms.

quote:
To get the same height the short crank arms would be 110 mm from 175 mm. There have been many attempts at using linear drives in faired bikes, but none performed as well as the round crank.


Do you mean that the faired bikes using these systems didn't perform as well, or that pedaling in perfect circles has been proven to be more efficient for some reason?



C:
Tony Levand
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Ivo.M
Starting Member

Germany
28 Posts

Posted - 07/31/2017 :  19:29:12  Show Profile  Reply with Quote
How about this?

http://moeve-bikes.de/en/cyfly
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purplepeopledesign
recumbent guru

Canada
685 Posts

Posted - 07/31/2017 :  22:17:50  Show Profile  Reply with Quote
That crank system is not European, but a Vacuum Velocipede by Dave Berkstresser, an American.

AFAIK = I am my own source.

Every single one of the multiple linkage drives I've ever seen at various recumbent events has been a one off. The inventor never bothers with more. I've seen a number of speed bikes with linkage drives and cannot recall one that ever impressed with a reasonably fast 200M sprint run. Even the ones that I've seen only in pictures have never gotten past the novelty phase.

My own take comes from a simplified analysis of a mentor. Every pivot point is a source of friction. If a regular crank loses X watts at each bearing, that is 3X watts lost for the crank and pedals. Add a bearing, add X watts. So.. for example that Vacuum has 3 extra bearings per side for a total of 9 bearings and triple the friction of a regular crank with pedals. That's a very big penalty for improved torque over specific sectors.

:)ensen.

Those who claim to be making history are often the same ones repeating it.

Video of my trike
http://www.youtube.com/watch?v=MdSLRD_2vzc
Photos of my trike
http://www.flickr.com/photos/purplepeople/
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Osiris
Starting Member

USA
27 Posts

Posted - 08/01/2017 :  05:54:20  Show Profile  Reply with Quote
"Every single one of the multiple linkage drives I've ever seen at various recumbent events has been a one off. The inventor never bothers with more. I've seen a number of speed bikes with linkage drives and cannot recall one that ever impressed with a reasonably fast 200M sprint run. Even the ones that I've seen only in pictures have never gotten past the novelty phase.

My own take comes from a simplified analysis of a mentor. Every pivot point is a source of friction. If a regular crank loses X watts at each bearing, that is 3X watts lost for the crank and pedals. Add a bearing, add X watts. [/quote]

The rarity of such designs showing up at races could suggest that they don't work well, or it could be for entirely different reasons. It could be that the designer just never built another racer. Or it could be that he settled on a different hull shape that doesn't benefit from making the pedal stroke longer and flatter.

In this particular case, I wanted to stick with a conventional 3-wheel velomobile, but one that does away with the traditional bulbous nose section. The benefits are a reduction in overall weight, a lower center of gravity, less susceptibility to crosswinds, and better aerodynamics. The cost of such a system is some increase in friction owing to the two additional joints.

The question then is, would the added friction cancel out all the other advantages? I seriously doubt it. Of the three forces the rider has to overcome (rolling resistance, mechanical friction, air resistance), drag forces resulting from air resistance are by far the greatest. The power loss caused by two additional bearings is microscopic by comparison. For example, my M5 CHR has three idler wheels that none of my diamond frame bikes have. Those extra idlers obviously add friction as the chain moves over them. I have tried various ways to measure how much friction, but it's too small an amount. It doesn't show up at all on my power meter, which suggests that it's less than 4 watts (the smallest amount my power meter seems capable of registering).
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purplepeopledesign
recumbent guru

Canada
685 Posts

Posted - 08/01/2017 :  07:05:14  Show Profile  Reply with Quote
quote:
Originally posted by OsirisThe question then is, would the added friction cancel out all the other advantages? I seriously doubt it.


Let's agree to disagree then. Good luck with the build.

:)ensen.

Those who claim to be making history are often the same ones repeating it.

Video of my trike
http://www.youtube.com/watch?v=MdSLRD_2vzc
Photos of my trike
http://www.flickr.com/photos/purplepeople/
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Speedbiker
human power expert

USA
3761 Posts

Posted - 08/01/2017 :  09:09:29  Show Profile  Reply with Quote
Every attempt at linear drive (or eliptical) destined for competition was done for the reasons you list. The best engineers in our sport tried. All results were disappointing. But don't let the poor results of people smarter and more experienced than you stop you! Also, don't let the stellar performance of conventional pedaled machines change your mind, either.
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Osiris
Starting Member

USA
27 Posts

Posted - 08/01/2017 :  10:30:56  Show Profile  Reply with Quote
quote:
Originally posted by Speedbiker

Every attempt at linear drive (or eliptical) destined for competition was done for the reasons you list. The best engineers in our sport tried. All results were disappointing. But don't let the poor results of people smarter and more experienced than you stop you! Also, don't let the stellar performance of conventional pedaled machines change your mind, either.



Where to start. First, the type of vehicle I had in mind isn't a single purpose machine designed to set speed records, as I've already explained. If that had been my goal, the result wouldn't have been anything like a 3-wheeled velomobile. It would likely have been a fully enclose two wheeled vehicle in which the rider lies on his back, and has a computer screen in place of a canopy. A design like that doesn't require anything other than a circular pedal arrangement. It should be no surprise then, that this is precisely what we see in these sorts of craft. But that says absolutely nothing relevant about the efficacy of eliptical drives in other types of vehicles that were NOT designed simply to set speed records.

Edited by - Osiris on 08/01/2017 12:33:54
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alevand
human power expert

USA
2940 Posts

Posted - 08/01/2017 :  12:59:51  Show Profile  Visit alevand's Homepage  Reply with Quote
You can jump up and down saying round drives are best, but what about proof and analysis.

The roundy round drive covers pi times the diameter circumference every pedal stroke, where the linear drive covers only the diameter in stroke, therefore the round drive has 3 times the distance where other leg muscle groups, not active in linear, such as calf, motion can provide tangential forces. That's my get anyway.

C:
Tony Levand
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Speedbiker
human power expert

USA
3761 Posts

Posted - 08/01/2017 :  13:04:52  Show Profile  Reply with Quote
Wooo, you sure got me there! Obviously you have built many more human powered vehicles than me. You are clearly light-years ahead of us. Please show us pictures of all the other vehicles you built while developing your brilliance. While you're at it show us the shop full of equipment where you'll build this wonderous machine. Will it be built, soon?
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alevand
human power expert

USA
2940 Posts

Posted - 08/01/2017 :  13:49:09  Show Profile  Visit alevand's Homepage  Reply with Quote
Pipe down Thom, we don't want this thread deleted.

C:
Tony Levand
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Speedbiker
human power expert

USA
3761 Posts

Posted - 08/01/2017 :  19:22:24  Show Profile  Reply with Quote
Sure Tony. He's clearly another guy who wants to be the smartest guy on the forum, but hasn't ever built anything. Let's wait and see if/what anything gets built.
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carolina
recumbent guru

USA
601 Posts

Posted - 08/01/2017 :  19:51:00  Show Profile  Visit carolina's Homepage  Reply with Quote
Let's figure out this huge rubber band recumbent!!

[/URL]

velosRus.com

Edited by - carolina on 08/01/2017 19:51:32
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Speedbiker
human power expert

USA
3761 Posts

Posted - 08/02/2017 :  03:15:18  Show Profile  Reply with Quote
Wow! How do you wind it up? Many questions!
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alevand
human power expert

USA
2940 Posts

Posted - 08/02/2017 :  04:01:39  Show Profile  Visit alevand's Homepage  Reply with Quote
Looks like by backing up with the Flintstone brakes.

C:
Tony Levand
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Osiris
Starting Member

USA
27 Posts

Posted - 08/02/2017 :  05:27:47  Show Profile  Reply with Quote
quote:
Originally posted by alevand

You can jump up and down saying round drives are best, but what about proof and analysis.


Hell, who needs proof and analysis when you've got bucket fulls of emotion and shoddy logic on your side?

quote:
The roundy round drive covers pi times the diameter circumference every pedal stroke, where the linear drive covers only the diameter in stroke, therefore the round drive has 3 times the distance where other leg muscle groups, not active in linear, such as calf, motion can provide tangential forces. That's my get anyway.


I didn't consider using a true linear drive because it wasn't optimal for my application, but I do remember one practical problem linear drive have, and that's the dead spot at the top and bottom of the pedal stroke. If you happen to stop pedaling at just that point, it would be extremely difficult if not impossible to get the pedals moving again. It wouldn't be difficult to solve that problem, but it would come at the cost of adding mechanical complexity.

At any rate, the problem of efficiently converting circular motion to linear motion, or vice versa, is ancient, and many creative solutions have been tried. Below is a working computer model I completed years ago of the 1829 Stourbridge Lion. The sheer complexity of its drive system is mind boggling. If the geometry isn't exactly right, the system simply freezes at a certain point, and you'll spend hours trying to figure out why.



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alevand
human power expert

USA
2940 Posts

Posted - 08/02/2017 :  07:16:10  Show Profile  Visit alevand's Homepage  Reply with Quote
The discussion on linear drive has been beaten to death for many years. I doubt that with your limited experience you will tell us any thing new. Steam engines - Ha. The solution has be found in antiquity, a flywheel.

C:
Tony Levand

Edited by - alevand on 08/02/2017 07:18:22
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Osiris
Starting Member

USA
27 Posts

Posted - 08/02/2017 :  08:16:41  Show Profile  Reply with Quote
quote:
Originally posted by alevand

The discussion on linear drive has been beaten to death for many years. I doubt that with your limited experience you will tell us any thing new.



I'm not proposing to tell you anything new with respect to linear drives. What I'm interested in is learning why an elliptical drive in this particular application wouldn't be a better solution than a traditional circular drive.
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warren
human power expert

USA
6119 Posts

Posted - 08/02/2017 :  09:30:18  Show Profile  Visit warren's Homepage  Reply with Quote
Linear drive is fine if you just want something for tooling around or want to have an engineering project. By all means, go ahead and build and test it and we will be interested in hearing the results.

Unfortunaely real world testing has shown that the aerodynamics gain does not make up for the efficiency loss in the drivetrain.
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