# Freewheel/Ratchet Clutch - Combining multiple torque inputs

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1. Apr 6, 2016

### raniero

Hi, I am required to design a tandem bike where multiple torque inputs will be present at different speeds since persons on the bike will not be cycling at the same rotational velocity. From my understanding of freewheel clutches to transmit all the torques produced by the cyclists using freewheel clutches these need to cycle at the same exact velocity, which is impractical. Is this correct ?

If so, what mechanical components should I use to combine the multiple torques produced ?

2. Apr 6, 2016

### Staff: Mentor

To combine power sources that are turning at different rates, you need to use transmissions/gears. If the rotation rates are continuously variable, you will need to use a continuously-variable transmission, which is a bit much for a bicycle. Instead, consider combining them with discrete gear sets. Basically something like a standard bicycle uses, but with to sets of pedals and gears.

3. Apr 6, 2016

### raniero

Combining multiple sources with discrete gear sets won't allow the passengers to stop pedaling while other passengers are still cycling.

4. Apr 6, 2016

5. Apr 6, 2016

### raniero

So, if I use one like the picture below, and say I apply 3 different torques to 3 different gears on this freewheel, the output torque of the shaft should be the addition of these 3 torques right?

I am still unsure how this will allow one of the 'engaged'/used gears to stop rotating while the other are still rotating.

6. Apr 6, 2016

### Staff: Mentor

No, you need one gearset and freewheel per pair of pedals.

7. Apr 6, 2016

### raniero

That's what I was thinking originally, but I was concerned that if the passengers do not cycle at the same rotational velocity then the slower ones would not be doing any work.

8. Apr 6, 2016

### OldYat47

I'm assuming you are using a standard tandem setup for the drive chain. That is, rider #1 has a single chain ring, rider #2 has two chain rings ("input" from rider #1 and "output"). You could observe the rider's pedaling rates and alter the size of the chain rings. For example, if rider #1 pedals 10% faster than rider #2 you could size rider #2's chain rings 10% larger than rider #1's chain ring. Same drive chain passing speed, different rotation rates. That's the simplest solution, but it does mean that all riders have to pedal at the same time (if not the same rate), and changing riders will change the gearing relation. To achieve independence between the riders you will need a freewheel clutch in the chain ring hubs.

9. Apr 6, 2016

### raniero

I was just seeing this animation of a freewheel clutch:

Having the pedals instead of the gear teeth and the rotating shaft as the yellow part, wouldn't it mean that if the shaft is rotating faster than pedals the mentioned pedals wouldn't be doing any work (since the red part would never be able to remain engaged in the inner teeth) ?

On the other hand, if the shaft is already rotating and now the pedals rotate at a higher rotational velocity than the shaft, suddenly they put a torque on the shaft resulting in an acceleration of the shaft. As the shaft accelerates, the other pedal which was originally rotating the shaft first, is now too slow to keep up with the new rotational velocity. From my understanding this means that at any given time only one moment from one pedal is actually transferred to the shaft unless both pedals are rotating with exactly equal rotational velocity (which is highly impractical). Am I missing something ?

10. Apr 6, 2016

### OldYat47

Yes. The axle (or in your description the pedals) have to be rotating as fast as the sprockets to add any torque. Otherwise that rider is just spinning his wheels (sorry, couldn't resist). You may want to search "infinitely variable bottom bracket". There's a bunch of stuff out there that could give you inspiration.

11. Apr 6, 2016

### CWatters

Pity you can't get left and right handed gear sets, put one each side of the bike.

12. Apr 6, 2016

### Tom.G

Are there any small torque converters on the market?

13. Apr 7, 2016

### OldYat47

All torque converters I know of are quite inefficient, except for automotive ones that lock up close to input/output speed matchup.