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Going chainless with the Stringbike

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September 27, 2010

When force is applied to the pedal, the stainless steel, triangular-shaped swinging mechan...

When force is applied to the pedal, the stainless steel, triangular-shaped swinging mechanism pulls on the rope and rotates a drum on the rear wheel to give forward momentum

Image Gallery (13 images)

At first glance, a proposal to replace a bicycle's familiar chain and cog drive with one that uses string may sound like lunacy, but that's exactly what's been done to produce the Stringbike. The system features freewheel mechanisms on either side of the rear wheel connected by polyethylene rope to a precisely positioned, symmetrical swinging arm that drives the bicycle forward. When the unit on the right is driving the bike forward, the other is being returned to its starting position and vice-versa which is said to result in greater efficiency and makes for a more comfortable, easier ride.

Stringbike's roots lie in a postgraduate challenge at Budapest Technical University in the early 1990s, to think outside the box and overcome some design issues relating to chain-driven bicycles. One team proposed a chainless system involving freewheels on either side of the rear wheel, which is driven by a sophisticated wired pulley mechanism. The aim was to overcome inefficiencies of a chain and gear system – where the drive mechanism is placed on just one side of a bicycle – that have gone relatively unnoticed due to the ubiquitous nature of this design.

An early kidney-shaped Strinbike prototype

An earlier, now abandoned Stringdrive design

The design has undergone numerous changes since development proper began in 1994 and various problems led to a couple of the designers giving up and moving onto other things. Work started again in 2006 with Michael Lantos and Robert Kohlheb at the helm. Using computer-aided-design software to help iron out some of the issues, they ended up with the Stringbike you see here.

The Stringdrive system consists of a drive unit on either side of the bike frame. Each unit is positioned in mirror symmetry to the other, so when one is moving forward, the other is moving backwards. When force is applied to the pedal, the stainless steel, triangular-shaped swinging mechanism pulls on the rope and rotates a drum on the rear wheel to give forward momentum.

Future incarnations of the system may see different transmission ratios being used on eith...

The system uses the kind of durable, high density polyethylene rope used in sailing. Lantos told Gizmag that the rope does have "a little elasticity, and this has been calculated in the process when the task of driving is shifted from one side of the frame to the other one." The rope has a 3mm diameter and a breaking strength of over 900kg (1,984 lbs). There are stronger ropes available, but the designers reckon that the maximum load (greatest transmission ratio, highest pedal force) on the four cords is only about 450kg (992 lbs).

The system is designed so that when one triangular-shaped eccentric disc is pulling the wheel's drum forward, the other is going backward and returning the rope to the starting position. The symmetrical system "shifts the task of driving unnoticeably from the right to the left sides and vice versa" to offer a familiar, more comfortable, experience for the rider.

The following demonstration shows the system in action:

and this animation offers a clearer picture of exactly what's happening:

The Stringdrive alternating drive system benefits from 19 different transmission ratios – the outer position gives maximum and the inner gives minimum ratios – which are selected by moving the rope wheels up and down the notch positions on the swinging arm. A major achievement with the final design is the ability to freewheel backwards.

Lantos explained: "Normally if you use two differing freewheel clutches in the rear wheel they allow rotation of the wheel in forward direction only. As no bike can be commercialized that cannot be pushed backwards, this simple-looking task had to be solved, and it was technically very difficult."

When one rear wheel drum is pulled forward, the one on the other side moves backwards

The Stringbike has undergone lots of intensive testing over the development period, "including operating in a test machine under load over 10,000 km [6,214 miles], then riding by several bikers under very different conditions." The system is sealed against weather and dirt but the rope, swinging arm and outer casing of the drums will need some attention – soiling of clothing from oil should be avoided though.

Only one manufacturer has so far been licensed to use the system, Schwinn Csepel, but actual commercial manufacture has yet to begin. Future incarnations of the system may see different transmission ratios being used on either side of the bike to cater for those who have a weakness in one leg.

About the Author
Paul Ridden While Paul is loath to reveal his age, he will admit to cutting his IT teeth on a TRS-80 (although he won't say which version). An obsessive fascination with computer technology blossomed from hobby into career before the desire for sunnier climes saw him wave a fond farewell to his native Blighty in favor of Bordeaux, France. He's now a dedicated newshound pursuing the latest bleeding edge tech for Gizmag.   All articles by Paul Ridden
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20 Comments

Let's see some efficiency numbers. Chain drives are already extremely efficient with the bonus of being simple and plentiful.

This looks like a solution in search of a problem.

colinshark
28th September, 2010 @ 02:38 am PDT

Considering it can be as reliable, durable and cheap as the traditional chain drive system, then it is welcome.

One thing I'm sure about; It's gonna be a pain in the a$$ fixing it on the go.

Nitrozzy7
28th September, 2010 @ 05:49 am PDT

My thoughts exactly colinshark. I'd love to see some transmission efficiency numbers. While the cam design is quite intriguing, it looks as though there will be some obstruction problems with the strings to overcome. Also, can you shift under power?

The design is quite beautiful but as you said, solution searching for a problem.

Blixdevil
28th September, 2010 @ 06:01 am PDT

I have a concern about the roller. I would hate to see a kid get their toe or finger or just clothing caught in there. The principal is good and clever - but is it safe? That area should have a shroud to cover it.

Don

donwine
28th September, 2010 @ 08:00 am PDT

Too bad they couldn't work out a system that accepted return pedal power such as when a cyclist is clipped into the pedals and can exert force on the upward stroke.

Also, they will have to move the cams in buy 5mm in order to avoid contact with lower leg and shoes.

I would have preferred a shaft drive with all moving parts enclosed within a high-strength single-sided swing-arm. Logically, all gear selections should be at the crank, possibly in the form of a selectable CVT or simple transmission. Also, try dynamic crank-length adjustment rather than cam-oriented efficiency boosters. This will give the riders' muscles a brief rest as other fibers are called into play and pedal power can be maintained, for example in a downhill situation, from the calf muscles (and ankle) rather than just the quads and/or hamstrings.

BMW and Cannondale, among others, have proven that single-sided forks and swing-arms are very reliable. Do all your math at the crank and let the transmission of power and rear hub be simple and require zero maintenance.

Muraculous
28th September, 2010 @ 09:51 am PDT

I see some issues here : What happens when a string breaks? Will it be as easy to replace as a chain with a quick-disconnect link? How do you maintain a constant string tension?

What is the range of transmission? I don't see this providing a wider range of gearing than a derailleur chain drive or hub drive, even if the cam angle can be varied.

Lastly, the efficiency statement is false - serious cyclists wanting to get more out of their bikes use 'clipless' pedals, SPDs or 'spuds', of which there are myriad variations in clever designs like look, time, crank brothers, frog beyond the common shimano type where a special shoe fitted with a cleat engages with the pedal.

The principle of the spud pedal is that you can exert additional torque by pulling the pedal up with one foot while pushing down with the other, this can give you up to a third more torque when sprinting out of the saddle than just pushing down one foot at a time. This string system only seems to allow for torque to be produced by the downgoing crank, which is less efficient.

Otherwise looks very clever, in a heath robinson/rube goldberg sort of way but I don't see the point. This is an effort at reinventing the wheel...

PeetEngineer
28th September, 2010 @ 11:06 am PDT

How about using two independent strings so you can push both feet at the same time for double the power, this would probably work better on a recumbent bike with a back rest to push against.

Anthony Parkerwood
28th September, 2010 @ 11:26 am PDT

It's a variation of the "treadle" drivetrain, probably dates back to the late 1830s. Examples with cloth or leather belts, instead of string, were manufactured in the US as recently as the 1920s, maybe later.

Well, it works. I have no idea if it has any inherent advantages over a chain drive, and changing drive ratios might be challenging (I can think of a couple strategies off the top of my head, but it would make the system unnecessarily complicated).

William H Lanteigne
28th September, 2010 @ 12:06 pm PDT

This works exactly like the worlds first gasoline powered motorcycle. Used two rods and a ratcheting system to pull the cogs of the rear wheel.

This is also reminiscent of the bike the Giant Bikes made back in the 80s.

Ed
28th September, 2010 @ 02:13 pm PDT

It's like a conventional bicycle mated with a Harris vertical bicycle and this is their offspring. There have been some other similar designs that used two levers pumped up and down. In other words there's a ton of prior art they had better include in any patent applications.

Since this design has no way to provide power by the feet pulling up on the pedals, there's no advantage over the Harris or other lever designs.

Instead of reinventing the wheel, they've reinvented the double freewheel pull action drivetrain. ;)

Facebook User
28th September, 2010 @ 03:05 pm PDT

I would think that since there is still a crankshaft that connects the two pedals, one could still pedal on the 'upstroke', the power would still be transmitted to the cam on the other side of the bike.

I would be interested to know the weight savings (if any) vs. traditional chain and sprocket designs as well as what the maintenance is. I would think that all but the rope could be contained in a case, keeping it safe from debris and wayward toes and fingers.

One thing that the article does point out is that bikes have basically been engineered to death, but the vast majority still use the tried and true chains. I suppose that it remains to be seen if a better way is out there...

aspork42
28th September, 2010 @ 04:44 pm PDT

Sorry but this system looks like a complete waste of time and materials as the chain and sprocket is far more simple and easier to maintain, and a gates belt is even easier. for something really interesting have a look at the pinion bicycle gear box, now there is something inovative.

flibb
29th September, 2010 @ 02:04 am PDT

Hey this article is damn incomplete. I really don't understand 2 basic things in the design:

1) How to "moving the rope wheels up and down the notch positions on the swinging arm" in order to change the ratios? Hoooowww?

2) "A major achievement with the final design is the ability to freewheel backwards.

Lantos explained: "Normally if you use two differing freewheel clutches in the rear wheel they allow rotation of the wheel in forward direction only. As no bike can be commercialized that cannot be pushed backwards, this simple-looking task had to be solved, and it was technically very difficult." What is this? I really cannot imagine the mechanism from his words.

Akemai Olivia
29th September, 2010 @ 02:03 pm PDT

This is almost identical to the failed Facet Biocam design from the late 1970s. Nothing new here except maybe the polyethylene string.

Gadgeteer
29th September, 2010 @ 03:23 pm PDT

The Original Stringbike:

http://zajlada.blogspot.com/2010/09/bicycle-without-chain.html

Lukács József
29th September, 2010 @ 04:43 pm PDT

Akemai,

As noted, this is pretty much an updated version of the Biocam from 30 years ago, which I'm guessing they didn't know about, otherwise they wouldn't have spent their time "reinventing the wheel," so to speak. Changing drive ratios works the same way. The "swinging arms" they refer to are the triangles you see around the crank on each side. They swing forward and back, not round and round. The polished pulleys pull on the strings leading to the rear axle. When you move the pulleys up and therefore away from the bottom pivot, you get less leverage on the strings but more travel with each pedal stroke. Move the pulleys down and there's less travel but greater leverage. Hence ratio change.

Gadgeteer
29th September, 2010 @ 08:15 pm PDT

is this mechanism will work for elderly people riding the bicycle

Facebook User
5th October, 2010 @ 05:31 am PDT

At least it's not one of those dopey Ducati motorcycles...

Mr Stiffy
19th October, 2010 @ 06:23 pm PDT

Some answers (e.g. shifting under power) can be found here:

http://www.freeweb.hu/vonhauser/bike_eng.html

Interpreting "a solution looking for problems" as pointing out inconveniences of a widespread working mechanism that everybody has adapted to without more fundamental questioning, this might be one indeed.

Are we witnessing in these posts the same type of "social resistance" to (or fear of) new designs that made us ride upright bicycles instead of recumbent ones?

Unfortunately, market forces will shape the fate of this design, where it should really be its technical performances.

armando sano
26th October, 2010 @ 07:31 pm PDT

There is, however, at least one obvious reason why it might not be as efficient as a chain transmission: the cables need to be rewinded before they can be pulled again. This implies working against some sort of spring mechanism when pulling, _theoretically_ reducing power.

On the other hand, in a chain transmission, there is friction between the chain links and between chain/sprockets (esp. in the derailleur), which I can imagine could be less with the string design (bending of the cables still adds 'friction' in the transmission).

To know which is more efficient in practice needs some quantitative figures. Or a race...

armando sano
26th October, 2010 @ 07:46 pm PDT
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