Independent pedaling/coasting system

[WADMW]

Can someone help me with some links to learn the drive mechanism of the 'Da vinci's independent coasting system' and/ or any other available independent pedaling or coasting system.

 

[btrueblood]

The "daVinci tandems" website has some pretty good info on it. An intermediate shaft with two freewheel clutches, seems pretty straightforward. The more interesting idea is the intermediate gear ratio combinations available.

 

[SwinnyGG]

I am pretty familiar with this system- it just uses a freewheel for each rider as the input into the shaft driving the front chainrings. It is relatively simple. If you want more details, google is your friend.

 

[JStephen]

A friend of mine rides one, and I never have seen that it was that big of a deal; it seems to solve a non-existent problem. It does give him essentially a quadruple chain ring instead of a triple, but I don't know but what there's some minor mechanical losses incurred in doing so. Da Vinci will license their system to other bike manufacturers, but I'm not aware of anybody else offering a similar setup. Tandems are a pretty limited market in the first place, so there's not as much motivation for innovation as with single bikes.

A lot of people see them on that da Vinci and assume that they somehow have independent gearing as well, and no, they don't- they're either pedaling at the same speed or one of them's not contributing. It just looks weird because they're not in phase with each other.

 

[SwinnyGG]

"it seems to solve a non-existent problem"

I wouldn't say that. There are significant gearing advantages- bikes with this system wind up having more distinct gears, a wider range, and smaller chainrings, all of which are tangible benefits.

For people who ride tandems on mountain bike trails, which a lot of people do, believe it or not. The stoker can adjust their cranks as necessary to clear obstacles, without the captain needing to stop adding power (or vice versa).

Lastly, for a rider combination with different levels of skill/experience, it means that the captain does not have to use his own power to drive the stoker's legs if the stoker wants to rest. This is significant.

 

[EdStainless]

Or is he refereeing to the continuously variable ratio part of the drive.

Setting up a tandem with two freewheels is no big deal. The question becomes whether you want to give each rider their own gear ratios. I did one years ago where the rear rider could coast while the captain peddled, but the final drive ratio was the same for both.

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P.E. Metallurgy, Plymouth Tube

 

[racookpe1978]

How did you do that?

I had thought of, but not prototype - a tandem where the front drove a left-hand derailleur and left-hand chain and gear set, and the read drove a right-hand derailleur and gear set. On the back axle, both - or one or the other - would have the usual "coast" feature. Either then can pedal at different speeds and with different loadings.

 

[SwinnyGG]

You are describing a system that would be functionally the same as what Ed is describing.

The gearing would never be truly infinitely variable, but the stoker would have a few cadence choices to choose from in order to match output speed with the captain.

 

[3DDave]

Eventually the gearing overlaps, so the claim of 32 speeds is probably down to 18 usable ratios. Since the adaptability of the power plant is pretty good and the power requirements can vary considerably for small changes in slope and wind, making very fine changes to the ratio isn't that valuable.

Sprag or over-running clutches can be used, as can conventional clutches.

Sprag -

https://www.youtube.com/watch?v=Fsp3fm4KHs0

 

[EdStainless]

Either way it takes a third crank spindle.
Here is the best execution of independent ratios that I have seen.

http://www.bilenky.com/#!viewpoint/cb9e

http://www.tandemseast.com/frames/bilenky.html

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P.E. Metallurgy, Plymouth Tube

 

[JStephen]

With a conventional tandem with triple, I'm good from about 5 mph to 30 mph, and there's not much around here that falls outside of that range. (But, if it did, there still bigger rear sprockets, available, too, so you don't have to a da Vinci system to get lower gearing.)

I mentioned a friend had one. They love it, primarily for the independent coasting. And no, they don't go offroad. But that just doesn't strike me as a need. Yeah, if you had a bicycle where your right leg and left leg pedaled independently, somebody would like that, too, though.

On the "left derailleur"- very simple in concept. But tandems mainly use single-bike parts, they're already a small market, so there's a limit as to what you want to produce in the way of specialized parts for them.

 

[WADMW]

Referring to what JStephen Said; should it necessarily be the same speed while pedaling?

 

[JStephen]

I understand on some of the older tandems, the rear cockpit was shorter and you had to have the pedals in sync or the riders could hit each other's feet. I've not checked my tandem to see if that's an issue or not.
Theoretically, if that's not an issue, you should be able to put different size timing sprockets so that the stoker always pedaled faster (or slower) than the captain. But I've never seen anyone do that, either. If there's too much of a cadence mis-match, it's better to just ride your own bike.
It would be an intriguing experiment to build a conventional bicycle where the left and right pedals went at different speeds. Presumably, you'd need clipless pedals or a fixed gear bike for those awkward both-pedals-down moments. It just might be exceedingly irritating to ride, too.

 

[SwinnyGG]

I would also be highly inefficient.. your body is its most efficient when the power output is as close to constant as possible, which is what you get with a normal bike.

 

[JStephen]

We did a 600k brevet over the weekend; the photo is from Meridian, Texas. The bike on the left is a coupled titanium da Vinci that I mentioned above. The bike on the right is a conventional steel-framed CoMotion Primera.

 

[WADMW]

Cool.. thanks!