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# Differential input Tandem Bicycle

Allow individual pedaling cadence while eliminating distrust.
 (+4) [vote for, against]

The tandem bicycle is a relatively simple machine. Usually the front person pedals and the power is transmitted by chain to the rear crank set. Here the second person pedals and the sum of both inputs is delivered by a second chain to the rear wheel. Both crank sets are locked together by the chain so that they both turn at exactly the same rate. This is potentially problematic since some cyclists spin quickly at 90rpm or so while others prefer a slower torquey-er cadance of 60-70 rpm. An additional problem is that the front cyclist may never know if the rear cyclist has decided to lift their legs up and have a bit of a break.

So, instead of locking both crank sets together, each pedal input drives the input of a differential. These inputs are summed and fed to the rear wheel. Now, this would allow two different pedaling cadences to be accommodated, and the cyclists could even have independent gearing. What the system does require is torque input from both cyclists, if one cyclist removes their feet from the pedals, then the tandem input from the second cyclist will simply spin their pedals backward. Anything else is fine though, so one can even stop pedaling and as long as they hold the pedals the second can still drive the rear wheel.

 — bs0u0155, May 27 2019

Prior Art Complicated_20De-synchronised_20Tandem

In case anybody thought I was joking in my anno [notexactly, May 28 2019]

I'm sure the back-driving of pedals could be prevented with some sort of non-reversible clutch mechanism.
 — MaxwellBuchanan, May 27 2019

 This is a complex and overengineered but incomplete solution to a simple problem, so [+].

 The optimum solution is to have the front rider's pedals drve one side of the rear hub through a standard righthanded deraileur gear cassette.

 The other side of the hub also has a deraileur, this time lefthanded. The rear rider's sprocket drives this gear. Both sides have standard freewheels.

 The rear forks will need to be wider, but not very much.

 This allows the two riders to pedal at different speeds using different final drive gear ratios, under individual control.

Most of this can be done simply by installing standard components the wrong way round in a modified frame; the chain from the front sprocket might need a intermediate idler.
 — 8th of 7, May 27 2019

 //sort of non-reversible clutch mechanism//

pffft. And miss the opportunity to install a Torsen limited slip diff? Or a ball diff? or a standard one with custard filling?
 — bs0u0155, May 27 2019

 Viscous couplings aren't wonderfully efficient ...

 Each rider's pedals should drive a generator. Depending on terrain, energy is either supplied to the final drive, or a portion can be diverted to a battery to help on hills.

 Going downhill, the final drive can recover energy by regenerative braking.

The fact that it uses electrickery meand that each rider's energy input can be monitored and displayed to any number of decimal places, affording endless opportunities for discord and criticism.
 — 8th of 7, May 27 2019

 //affording endless opportunities for discord and criticism//

 A commendable feature.

You could create an interesting long distance race with a tandem electric hybrid. Presumeably the speeds could be quite high, electric regen braking will harvest much of the lost energy on descents and can even be added to. It would be fun to watch the rear cyclist pedalling furiously to store extra energy while the front guy concentrates on not crashing. When to deploy the energy can get super tactical and complex. You'd probably want to ascend quickly and descend super slow to reduce non-linear aero losses. It might be a good idea to have one cyclist and one physicist to model the energy budget.
 — bs0u0155, May 27 2019

You could also achieve the desired result by adding two extra wheels (one behind the other) in the middle of the tandem, and then using an angle grinder to split the frame inbetween them. This has the advantage that, not only can the two cyclists each choose their own cadence and effort, but one of them can cycle to the pub while the other slogs up and down a hill.
 — MaxwellBuchanan, May 27 2019

Been there, done that - see link.
 — xenzag, May 27 2019

 // rear cyclist pedalling furiously ... while the front guy concentrates on not crashing. //

 There is an opportunity to add a further layer of unnecessary complexity.

 It is not unreasonable to assume that each rider will have some sort of display presenting them with energy and performance data.

 If this display is a generic video device such as a tablet computer, then imaging captured from a camera at the front of the tandem can be fed to it.

 With the replacement of the rear fixed handlebars with a pivoting set interlinked to the front set, both riders can steer and the rear rider can see the road ahead. In fact, this system makes "head down" riding a great deal easier.

 Unfortunately such video technology also allows a rear view canera to be incorporated, disclosing the stealthy approach of a large mechanically-propelled vehicle driven with murderous intent and calculated malice.

 An intercom system would be a wise addition to the installation, and would add comparatively little to the load on the primary DC power bus.

Since the video displays may be vulnerable to rain, insect debris, avian excrement, or being smashed with a wheelwrench by road users who hate and detest all vermin on bloody push bikes, cluttering up the roads, bastards all of them in their stupud lycra shorts and silly hats, acting like they own the place, arrogant bastards, kill them, kill them all, yea, slay even the children lest the evil persist, fire, blood and slaughtet, red hail upon thr plains of Meggido .... <coughing>, er, augmented reality goggles may be a viable alternative.
 — 8th of 7, May 27 2019

*makes mental note to not go near the Borg Cube after having developed a spacefaring bicycle*
 — notexactly, May 28 2019

 xenzag,

Clear prior art. Will delete in a day or so.
 — bs0u0155, May 28 2019

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