The fish sculpture, when it eventually gets converted to being mobile and bike towed requires the crank drive to be perpendicular to the direction of travel. This creates the need for converting the output movement to 90 degrees from the pedaling motion…

Not an easy task using old bike chains and parts and as there seems to be little relevant info available on the internet I’ve included notes on how I achieved it here.

Gears & Support

The input drive is from pedal to five speed freewheel via the original chain, the derailleur is left intact to keep everything aligned and to give a bit of choice for drive ratio.

The output is from a bottom bracket mounted on a piece of scrap steel tube. The other end of the tube is bolted to a seat post which is mounted into a piece of a seat tube. The seat tube is supported on a formed steel ‘strap’ that locks to the bike frame between the dropouts and the rear wheel spindle. The strap has two uprights welded to it for additional support and vertical adjustment.

This support arrangement allows adjustment in both height and orientation to align the two sets of gears. It also cants the bottom bracket sideways so it is directly above the freewheel whilst allowing the bottom fitting to stay as close to freewheel as possible to reduce the space required.

The Chain

This is why it works, the vertical chain is opened and twisted 180degrees then rejoined to form a mobius strip. Result being that the chain ‘wants’ to twist through 90degrees from one end to the other.

There will be a minimum length of chain that will allow this twist without jamming but I just keep the chain to its original length and luckily it worked!

When the system runs the twist tends to make the chain run against the next set of gear teeth on the upper gear train – I toyed with grinding the gears away to prevent this as it was noisy and added a lot of friction but in the end just added a rotating gear guide onto the freewheel end of the system to help with alignment and it works nicely.

The order of which chain is on which side of the freewheel makes a big difference and I’ve found that the setup shown works bet for the gear sizes I used and gives a reduction ratio of 2:1.


90 degree Chain Drive a video by arbarus on Flickr.

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