Tandem Cranks and Associated Questions

[mikegrundvig]

Ok, I feel like a moron asking this, but I'm just not getting it...

I've read about people using tandem bike cranksets as they get a chainring on each side. The "drive side" is on the right and can still be geared with a motor driving the left side. This makes perfect sense and seems pretty clean. The pedals and motor are both allowed to coast together via the rear freewheel. Then a second freewheel on the motor ensures that pedaling doesn't drive the motor as well. Now for my first dumb question - wouldn't this full configuration force the pedals to spin with the motor as there is no freewheel in there? That seems pretty nasty to me though I know the StokerMonkey setup seems to work that way. Seems like the freewheel at the rear isn't enough - you need one up front.

Now I've seen people talking about freewheel cranksets but all configurations I've seen generally sacrifice one of the chainrings on the front to attach the motor. Isn't there a compromise solution with both? IE: freewheel crankset on one side using a tandem setup to give you a chainring on the other? That seems a really good way to keep your gears on the one side and really keep the chain and motor system out of the way. Has someone done this already? Thanks!

-Mike

 

[vanilla ice]

Yes you're right. The tandem left drive cranks do not have any freewheel integrated in to them so your pedals would spin with the motor power.

 

[amberwolf]

On the first question, yes, like the StokeMonkey system, pedals always spin with the motor. My CrazyBike2's original drivetrain was like this, because I couldn't put the freewheels on my jackshaft at the time.

However: If you are willing to run the motor and pedals separately to a jackshaft, and from there to the regular bike drivetrain, then you can easily give each their own freewheel, so neither drives the other.

If you have a longtail bike there's plenty of room for this, as well as on some crank-forward regular bikes. On a typical upright bike there isn't much room to do this, though, unless you run the jackshaft up under the seat and then back down to the rear wheel, which could cause problems with rear derailer operation vs seatstay/chainstay positions.

On CrazyBike2's intended design, the pedal chain (from the front BB) and motor chain were both on the *left* side, running separately to their own chainrings with freewheels mounted on the left side of the rear BB, which was the input to the regular bike drivetrain (minus the cranks, but with the front triple). So either or both could drive the bike and shift thru *all* the gears, without sacrificing any shiftable gears, and without backdriving either motor or pedals at any time.

It would require modifying an existing bike to do this, possibly significantly, or building one for the purpose as I did CB2.

There are other ways to do this, too, but that's how I planned to do it for CB2.


Now, that said, with the motor you don't actually need all those gears, as it's fairly efficient throughout a wide range of RPM, once you get to a certain point, so you could get by with two or three in a fairly wide ratio.

If you still want all the various gears for pedalling, though, you can then do it a little differnetly.

Use a multispeed internally geared hub (IGH) for most of your gearshifting, in the rear wheel, and use two sprockets on it. The motor sprocket or the pedal sprocket could be setup without a freewheel, as long as the other has one (might have to do this since there may not be enough space on the hub's input to put both freewheels). Then simply add the freewheel for the other one on the actual output shaft of the motor, or on the cranks of the pedals.

You'd still have to have a derailer type of chain tensioner for the pedal chain, if still shifting thru front chainrings since the chain will grow/shrink, but none is needed on the motor.

The catch is for a high-powered bike you may need to put a slipper clutch or similar on the motor part, so it doesn't shock-load the IGH and strip it's planetary gears or sun gear if you apply too much power too quickly.



Another way to do it is the way I am doing it on my new bike build, by putting the iGH into the frame, and running a regular rear wheel with single-speed on it. Pedals will only have a single front ring, and all shifting will go thru the IGH for motor and pedals.

Mine is more complex than it could be because I want no chaingrowth issues while still using a relatively simple swingarm setup for rear suspension, so I have an extra jackshaft thru that pivot, but it's not needed on a hardtail or if you don't care about the chaingrowth issues and use a derailer-type tensioner.