Stringbike

i appreciate your analysis. ga

I also appreciate your analysis ... another GA

The cam was intriguing; it could help the leg stroke/torque profile of the rider to match the propulsion requirements more smoothly. However, the multiple "strings" made the whole assembly too bulky, and I don't know off hand of a flexible but non-stretchy material better than roller chain. The gear-change scheme was not shown.

(I'll have to remember this the next time somebody asks for a mechanical project.)

I have seen all the videos on youtube. Personally I think too much problems will encounter on such high-tech of a simple paddling system. No chain-cover. Over 80% people in my Country wear shilwar/pajamas and they will tend get entangled.habit of back paddling too particularly, while idle/going down the slope, cords will entangle.I dismiss.

Interesting, but it provides a mass of places for dirt to get trapped, as well as massive cams waiting to get caught in trouser legs (I have never worn special clothing for cycling other than a hi-viz vest). I much prefer internal hub gears, (which this seems to have - to bulky for standard hub) as they have a single shifter and can be set at any speed, even at rest. They use a thicker chain, which stays on the same pair of cogs, so can be covered to reduce lubrication requirements.

google "levocyclette", much older, much simpler, and as (in)efficient

Interesting. Seems it could be biomechanically slightly more efficient, using the leg where it is strongest more of the time.

OP here.

I am surprised by the quick dismissal of the "Stringbike" by CR4 posters. Although posing no immediate threat to chain drive systems, the Stringbike does have certain advantages over prior art:

- Low maintenance: anyone who has ridden a bike knows how a dirty/rusty chain reduces efficiency. No one likes to clean and lube a chain and so, although necessary, it is often neglected. The Stringbike cables require no lubrication and would be a cinch to clean. This is not trivial.

- Inherent efficiency: With the advent of multiple chain rings and larger and larger rear gear clusters, chain skewing and accompanying efficiency losses are a real problem with derailleurs. With the Stringbike, there is never any skewing of the drive cables, they are always in alignment with the front driving members, regardless of the gear ratio.

- No gear ratio overlap: A 27 gear derailleur actually has only about 18 distinct gear ratios due to gear ratio overlap. The Stringbike has no gear ratio overlap.

- Stationary gear changing and shifting gears under load: Neither of these is possible with the derailleur. Both are possible with the Stringbike (At a stop sign in high gear? Just drop it into low and go).

- Simplicity: The entire Stringbike concept (crank/lever/cable/drum), is far simpler and uses fewer components than the derailleur. The derailleur is familiar, but it's not simple. When the drawings for the first derailleur arrived at the patent office, the Examiners probably had a good laugh (He wants to do what? Flip a chain from sprocket to sprocket on the fly?). With the derailleur's conglomeration of sprockets, springs, levers, pins and screws, idler wheels, pull cables, and other bits and pieces, it's a wonder that it works at all. But over the years they have been refined and tweaked until these improbable contraptions are actually quite reliable. They still have problems though, as cited above, and they appear to be near the limit of their practical development (How many more sprockets can be feasibly clustered on the rear axle?).

The "levocyclette" referenced by one poster, was a treadle-action drive, not the rotary crank action of the Stringbike.

Some of the other issues raised about the Stringbike are quibbles. Regarding gear ratio range: there appears to be no reason that the Stringbike design couldn't be modified to increase the range of gearing. Regarding multiple freewheels: Today's freewheels are simple, reliable, and maintenance free. Regarding paired drive cords: With further development, the paired drive cords could probably be reduced to one.

In the current embodiment of the Stringbike, the rear cable-drum freewheels are spring-biased to re-wrap the cables on the drums at the completion of each stroke. However small, this spring biasing would add to pedaling effort. This is a barrier to racing applications. Surely though, the wrapping/unwrapping of the cables could be accomplished without springs. This seems a solvable mechanical problem.

One final comment: Although the gear changing details are unclear in the videos, it's obvious that the various gear ratios are obtained by changing the cable pull point on the lever arm. Again, with further development, if that pull point could be made sliding rather than indexed (but still stable when in position), this would be an infinitely variable transmission - the holy grail of bicycle power transmission.

I think the Stringbike is a very exciting development.

Hi! Bud,

Firstly, I can understand your attitude to the derailleur, but it works so wonderfully well, when in correct adjustment. It's much like the love/hate relationship I have with the reciprocating piston IC engine.

I like the principle of the stringbike. I was lucky enough, some years ago to come across a lever-arm pedalled bike using, basically very simple. No rotary crank, just treadles, much like the modern gym stepper, in application. You all probably know about the thing. I'll try not to bore you too much.

I couldn't believe the efficiency of the machine. Only when one is able to apply continuous effective effort to the back wheel, and not have the nearly 40% losses incurred in a rotating crank set-up ('stringy' included), is it possible to appreciate the inefficiencies of all the machinery which relies on reciprocation for propulsion.

Of course I bought the bike.

Loved it.

The major trouble was that it was made in China and as I gradually removed the various components which were not up to scratch and failing, I eventually had a wondrous machine. My son used to delight in racing others and mostly winning.

The great trick was that it could be "short stroked", thus allowing increased frequency

and so allowing greater top speed with a fairly high ratio (numerically).

Ratio change was very similar to that of the stringy, in our case it didn't need a movement of the back wheel, or 'pedals', as both the stringy and the derailleur do.

"A 27 gear derailleur actually has only about 18 distinct gear ratios".

BS.

It has 27.

I guess your view of the USPO is a little askew.

I like the basic principle of the stringy, but I do have reservations on the "strings".

Unless they're made of expensive Kevlar composite they have a very definite flexure lifespan( I've spent a very long time with cables running over sheaves). Look at the dynamics. Personally, I can produce about 230lbs steady force on the 'lever' at it's maximum effort position (90deg to the axis of effort) and then calculate the tension stress on the strings. That's why they're using twinned strings.

And that's why, universally, a roller chain is preferentially used to transfer power to the drive on pushbikes, with all it's inefficiencies.

My old "treadly" used dual chains, one each side of the rear wheel, operating on sprocketed sprag clutches ( freewheels) both of which I had to replace with quality units. I looked for roller bearing one-way clutches, but couldn't find any that I considered appropriate, without extensive re-machining of the hub. Once I got quality components in the machine it was 'bullet proof'. And a joy to ride.

I do prefer the action of the lever-arm propulsion to the crank of the 'stringy'. It's way more efficient, and only takes minutes to get familiar with.

Lack of public acceptance killed it's availability.

And failure of various inferior made bits and pieces. ( won't mention that country name again).

Oh! And I've never had a "dirty/rusty chain" on any of the bikes I've owned, pedal or power. Only a fool would.

Cheers,

Stu

- Possibly, for a recreational rider, a treadle action has some appeal, but it will never be accepted by serious cyclists because clip-on pedals enable application of force through 360 degrees. For this reason Stringbike has retained the standard rotary pedal/crank arrangement.

- According to Stringbike, their cable drive has been subjected to the equivalent of 10,000 kms of riding. The cables showed no signs of stiffening or other deterioration. A typical bike chain would be worn out long before 10,000 kms.

- An earlier Stringbike prototype used a single cable on each side. Did they change to paired cables for strength? I don't know, but an outcome of this strategy is the maintenance of a constant in-line force between the rear drum and the front driving pulley. This eliminates the chain skewing problem that plagues derailleurs.

- "I've never had a dirty/rusty chain on any of the bikes I've owned..." Nor have I, but that doesn't mean I enjoy the chore of cleaning and lubing the chain. I would happily give it up if it wasn't necessary - and on the Stringbike, it isn't.

- I own a 27 speed bicycle. I had heard cyclists referring to ratio overlap, and was unsure if this was true. So one day I calculated all of the 27 gear ratios. You are correct, there are 27 ratios, but about a third them are so close to others that to call them separate gears is meaningless. Ratio overlap is a real and recognized problem with multiple front chain ring derailleurs. The Stringbike has eliminated this problem.

Ratio overlap is a real and recognized problem with multiple front chain ring derailleurs.

Oh come now. Yes, some ratios overlap, but this is not a "real and recognized problem"; it is simply a recognized condition. There are also ratios that are unusable because of too-great chain crossing angles. This too is recognized, but not a problem. The rider quickly learns to use the best gear for the conditions. Not once in my riding have I thought: "Gosh, ratio overlap is a problem."

On the other hand, there have been times when I wanted both higher and lower ratios (especially in the days of ten-speed bikes) -- and this is a real problem with the current Stringbike. Apparently, there are serious issues to overcome to get the Stringbike to parity with current tech. If these issues were trivial, then they would have been overcome prior to promotion of the prototype, if one assumes that the promoters are of average intelligence. (Why would a savvy marketer bring out a complicated and expensive product that cannot improve upon the status quo?) The trend over the years has been to greater and greater number of ratios and increasing range from low to high. Few people want to buy five speed bikes today. Many people have wanted both a higher high and a lower low, and the market has responded.

I don't know, but an outcome of this strategy is the maintenance of a constant in-line force between the rear drum and the front driving pulley. This eliminates the chain skewing problem that plagues derailleurs.

This reads like fiction. There is no constant in-line force. The Stringbike applies (roughly sinusoidal) force variation to one side then the other. The forces range from essentially zero to quite high. The frame would tend to wag from side to side, if insufficiently stiff (just as a standard bike frame does). With the Stringbike, however, both the left and right sides of the frame must be engineered to avoid flex under drive pull. The derailleur system, on the other hand, has the potential for being optimized with a lighter left side. (The trend in high performance motorcycles has been toward eliminating one side of the structure entirely -- thus monoshocks and single-sided swingarms.)

Some questions that appear to be unanswered in the promo materials:

• What is the actual, measured, drive efficiency? How does this change after the cams and rollers have worn?
• Have there been time trials performed with standard bike vs Stringbike, using well-matched professional riders? Particularly convincing would be a track comparison, using a single transmission speed.
• What is the actual weight of the entire assembly -- in other words, precisely what are the weights of identical bikes equipped with the two alternatives?
• Has the bike been tested by any of the bike magazines?

This is the sort of info that needs to be presented, if the Stringbike concept is to be taken seriously. It the Stringbike implementation is lighter and more efficient, then this could be a winner. Similarly, if it is the same in these respects to current tech, but less costly to implement, then it could be a winner. However, the promotional materials don't make either case.

I am surprised by the quick dismissal of the "Stringbike" by CR4 posters.

I could have made my dismissal longer, I suppose.

The spring return issue on the ropes is one I did not mention, to avoid coming across as too negative. A system to do this without springs (but instead by rider energy) could be engineered, no doubt, but would be unlikely to be simple. A positive action return would be helpful, so that if a freewheel becomes sticky (as one certainly will) the ropes would not wrap around the wheel hub, with spring tension being too low to counter the friction.

I also did not mention that the freewheel devices add drag that would otherwise not be present. In a standard bike, the freewheels only come into play when coasting. In the Stringbike they operate all the time, adding a small amount of drag.

I also did not mention the issues with internal friction in ropes. As you probably know, Kevlar and Spectra ropes are comparatively stiff in bending, because the load bearing core has a lot of sliding friction. A single rope per side would be simpler, but would incur even higher friction.

The promo materials do not make strong cases re the issues you bring up. For example:

- Low maintenance: The system appears to be high maintenance, putting a large number of components under high load. In most engineered products, the cams and rollers of this unit would be in an oil bath, and made of heavy, hard and high strength metals, rather than the aluminum typically used for a bicycle front sprocket.

- Inherent efficiency: They don't make this case. Without direct numeric comparisons, one would assume that the bending friction of a rope is higher than that of a chain. This is the main reason that belt drives are less efficient than chain drives.

- No gear overlap: This is not a meaningful issue. The wide range 15 to 18 usable speeds of standard bike are better than the narrow range 10 of the Stringbike.

- Stationary gear changing: To quote the promo material: The change of gears will slightly drive the bike forward, therefore shifting when the bike is moving or rolling in a non-driven (idle) way is easy. If the bike is stationary, the shift can be made either by slowly rolling the bike forward, or by slightly turning the pedals backward, or by raising temporarily the rear wheel from the ground. This seems more complicated than you have described -- and seems a lot like a standard bike.

- Simplicity: This is a really tough sell. The only heavily parts of the standard bike are the chain and the sprockets, and the freewheel. The freewheel is lubed and well protected. The Stringbike has multiple heavily loaded exposed parts.

Regarding gear ratio range: there appears to be no reason that the Stringbike design couldn't be modified to increase the range of gearing. Why then, promote a substandard version? Perhaps getting a more useful range is not easy. The rear drum is already quite small, meaning that the front cam would need to be larger and heavier.

Regarding paired drive cords: With further development, the paired drive cords could probably be reduced to one. But the prototype went from one to two, and you seem to be extolling the virtues of two, not one. The current ropes are already at the leading edge of high tech ropes, so it is not as if a simple change from crummy rope to good rope can be made.

Regarding multiple freewheels: Today's freewheels are simple, reliable, and maintenance free. And two are twice as heavy and twice as costly as one.

Again, with further development, if that pull point could be made sliding rather than indexed (but still stable when in position), this would be an infinitely variable transmission - the holy grail of bicycle power transmission. I agree that this could be desirable. "Holy grail" might not be the right term, because there are existing CVTs for bikes.

Aside from the complex appearance, (which people will either like or not) the essential advantage seems to be limited to not having to lube a chain. The market will determine if that advantage outweighs the disadvantages.

I think the Stringbike is a very exciting development. I'm happy for you. For me it falls into the "kinda neat" class. My bike performs essentially flawlessly, already. If an invention could solve the sore butt, the neck aches from the awkward riding position, the leg aches, low endurance, low speed, etc. then I could get almost excited -- because those are, for me, real problems.

GA from me.

Essentially my thoughts too.

The Stringbike is 'neat' but will, sadly, suffer the same fate as my old 'treadly'.

I only run to 21 speed derailleur nowadays, but I'm about to re-engineer that to make it easier to ride. Add an electric motor.

One thing about the spring return of the strings- the treadly incorporated a 'string' yoke linking the 'pedals' together over a small sheave. One down pulls the other up. The drive chain lower ends were/are anchored to the lower end of the 'lever', with small extension springs to compensate for the change in geometry in gear selection.

Same could be done with the Stringbike, with a little engineering.

The Stringbike is going to get really 'clunky' when the cams and followers get worn.

And, it unfortunately does suffer the same inefficiencies as every rotating crank, reciprocator driven machine.

I found the torque change info expressed very eloquently, diagrammatically, at: SDV Drive with Oval Pedal Motion by Tetsu Iwatsuki. (Google)

The upstroke of a clipped shoe adds very little to the applied torque on the crank, quite contrary to popular belief here.

Thanks for listening,

Cheers,

Stu.

- Ratio overlap and chain skewing are not "problems" for recreational riders, but they most certainly are for serious cyclists. Racers must always try to stay in gears with the least chain skew, and they must know where ratio overlap occurs and plan their shifts accordingly. These are not issues with the Stringbike. Twist the grip shifter one way or the other - that is all the rider needs to know. What could be simpler?

- "there are existing CVTs for bikes" True, but none as simple as an infinitely variable transmission would be if based on the Stringbike concept.

- Durability: tested 10,000 km under load and still going strong...

- Stringbike mechanism exposed to elements: So are all the components of a derailleur system. Dirt build-up will affect the operation and efficiency of both. But restoring cleanliness and efficiency to the Stringbike is much easier and simpler than cleaning the multitude of chain links, sprockets, levers, and other bits and pieces that comprise a derailleur mechanism.

- Range of gearing: Even if, with further development, the Stringbike range of gearing does not equal that of a derailleur, this is not grounds to dismiss the entire concept. Not every bike and riding style requires a wide gearing range. Many bike couriers ride machines with just one gear. A road bike does not need the deep lows of a trail bike. A trail bike does not need the steep highs of a road bike. Stringbikes and derailleurs can co-exist, just as internal hub gear bikes and derailleur bikes now co-exist.

- My quote: "...a constant in-line force between...etc" I agree that I didn't express this well. From Stringbike's website: "By this arrangement (the paired cords), the resulting forces of the ropes always fall in the central plane of the swinging member." Compare this with a derailleur, where, as a result of chain skewing, the resulting forces of the chain certainly do not always fall in the central plane of the front chain ring. Translation: higher efficiency for the Stringbike.

- My quote: "With further development, the paired cords could probably be reduced to one." For all you chain lovers, the paired cords could be replaced with a right and left lengths of chain as in the "Levocyclette" referenced in an earlier post. The chain return strategy of the Levocyclette could also be employed, eliminating the spring return of the Stringbike. Thus, the rear mechanism of the Levocyclette could be paired with the front mechanism of the Stringbike, combining the advantages of both (no chain skewing, no gear ratio overlap, simple shift mechanism, few components). You read it here first. When I take possession of my new Stringbike I will investigate this possibility. Best Regards.

When I take possession of my new Stringbike I will investigate this possibility.

Let us know how your experience with the bike goes. Of particular interest would be race results -- if the bike is faster and more efficient, the market will respond.

Have fun, and ride carefully.

Me too.

Keenly interested in how the machine performs in the hands of an experienced rider.

Rather hear the story from a user than the Ad. dept of the maker.

Mostly two very different visions of the 'truth'.

Always looking to see better ways of doing stuff.

Cheers,

Stu.

Also,

so what does your Latin signature mean?

I only speak 4 languages. Latin is not one of 'em.

Stu.

It means:

If you can read this sign, you can get a good job in the fast-paced high-paying world of Latin!