tandem 1peice BB cog help..

Joined
Feb 3, 2017
Messages
81
Location
australia
So I made a sort of tandem, it worked for a bit in testing but now the chain comes off whenever I use the front pedals.
I'd love some suggestions.
I think the cog/spindle is warping from the big force applied by the way i have designed it.

The nub on the cog catches on the the pedal arm and forces it to rotate the rear pedals.
Because its a 1peice bottom bracket, i don't want to weld the cog on the left side, because that would make it all unservicable.

  • I don't want to drill a hole into the pedal arm.
  • i could possibly weld on a ton more strengthening to the cog.. i also have solid cog that might be more rigid..
  • i could add a second nub to brace the other side of the pedal arm (would that do anything?)
  • i could look for a 3peice pedal assembly that will fit in this bottom bracket (is this possible? Are they interchangeable?)
 
Joined
Oct 19, 2012
Messages
1,563
Location
Wakefield, UK
Most bottom bracket assemblies will swap just fine. You could swap to a three piece and weld the cog to the square taper on the inside leaving the outside for the arm. Alternatively you could use the inner cog on the other side to go to the other side of the rear leaving you with just a double at the front. That may catch the front derailleur though so you'd then need to route the chain away from the derailleur via a pulley close to the front or you could weld a cog small enough to miss the derailleur on the outside of the three subject to enough clearance with the crank.
 
Joined
Feb 3, 2017
Messages
81
Location
australia
Great thanks. I really hope i dont have to buy another parts bike to throw in the pile, but 3peice swap sounds like a plan. Long story short, the front chain has to be on the Left side.
 
Joined
Feb 3, 2017
Messages
81
Location
australia
Hmm nope. This one has a giant 55mm diameter BB. All my other bikes (including 2x 3peice parts-bikes!) are smaller.
So maybe i can weld a smaller 3peice bb inside the big bb..
Or try to turn a 1 peice into a 3peice..?
 
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