But that doesn't stop me so I decided I would construct my own, so after seeing some more videos I figured out how they work, but before I could design and manufacture an equivalent product I need to make a fast prototype for testing. This instructable is the way i made that prototype which really exceeded my expectations and you could use it to make your own and save a lot of cash.
But what exactly are this things, well to my understanding they have a freewheel mechanism between the axle of the crankset and the cranks which make each crank independent from each other so you need to actively maintain the 180 degree separation of the cranks.
This supposedly makes you go faster (hence the price tag), but I really don't care about that, they just seem fun to me.
Step 1: Materials
1.- Cheap chinese steel crankset. I used the one that came with my bike but a new one cost about $11.00. You could buy one new or salvage one from another bike if you don't want to destroy your actual crankset.
2.- 2 cheap chinese frewheels. One speed freewheel BMX type, I got these for $0.90 each at the chinese store
3.- Mighty Putty (HI BILLY MAYS HERE!!!)
4.- A handsaw, a file, a grinding stone and welding equipment.
5.- Patience. (Yoda: "The boy has no patience." Obi-Wan: "He will learn patience.")
Step 2: Cut the Cranks
Just cut the crank arm and file the part that attach into the bottom bracket so it can hold the freewheel. I was very lucky that the inside diameter of the freewheel was almost the same diameter of the round part of the crank arm.
Cut enough material to make sure that the axis of rotation of the two pieces is concentric and parallel mine is misaligned by 0.4 mm, which I considered it was good enough. The front part of the freewheel (the one with the brand on it) need to point to the right side of the bicycle in the two arms.
Cut the crank arms to the correct length and file some of the teeth of the freewheel so you can connect the crank arm to the exterior race of the freewheel.
This is a good opportunity If you want to change the lengh of your cranks if you are too tall or too short. I suggest you read this http://www.nettally.com/palmk/crankset.html
Step 3: Assembly
I used the epoxy because that way when I go to the welder he will not misalign my cranks, What he did was to put a piece of metal between the crank arm and the exterior of the freewheel an weld the center of the crank arm to the interior of the freewheel. He charged me $4.00 for that.
Just be careful not to weld the freewheel mechanism together.
Step 4: Enjoy
From the little testing I have done so far I draw the following conclusions
1.- You feel which one of your legs is the weakest because you lose your coordination. In my case the right leg is the weak one
2.- You feel muscle pain from other muscles that probably you never exercised before. In short you feel like you when stop riding for the winter and start again.
3.- Is a little bit difficult to maintain coordination but I think I'm getting better at it.
4.- You could drive both of your legs in parallel like a kangaroo, is funny as hell and very hard.
5.- I haven't got outside to test this in traffic. I will get out as soon as I feel more confident.
Now I would expose the disadvantages of this design:
1.- Is ugly as a neimodian. Well yes, but if you are or have access to a good welder you could skip the epoxy and just weld the components and it will look better, also you could file all the teeth of the freewheel, but as I said this was the first prototype maybe I made another one.
2.- I don't know how much radial force and torque the freewheels are designed to withstand. but seem they are holding up well.
3.- The freewheels have a backlash of 16 degrees. I suppose that is desirable to have this backlash as minimum as possible but I don't know if this is small enough.
I will test this design for at least 2000 km or until I get bored to see what else comes up and probably will make another design a little bit more refined, but all things considered I think the results are good for just an afternoon of work and less than six dollars of total expenditure.
Here is a sample of how they work.