Bichain Fixed/Free Bicycle Drive Train

by Bruce Ingle

Drivetrain details:

• The hub is a steel shell, splined left ball cup Sturmey-Archer AW modified to 2-speed fixed by brazing the planet cage to the left ball cup with a MAPP torch and removing the low-gear pawls. The ring gear is modified to give a neutral position at the "normal" position by grinding away the dogs which engage the clutch at this position. So normally, this would give a 3/4 reduction fixed gear, neutral (freewheeling in both directions) position, and a direct fixed gear. A 15 tooth sprocket is used with the hub.
• The outside of the left hand ball cup is the same diameter as the interior of a freewheel thread, and after brazing, is soft enough that a freewheel can cut threads into it. So, a 16 tooth BMX freewheel is threaded onto the left side. This results in a hub that can be driven via a freewheel on the left and via fixed gear on the right, but the freewheel is reversed, so the hub is used with the freewheel is on the right and the shift chain is on the left. Despite the suspect nature of the mechanical connection, I've cranked the crap out of it and it appears to be holding quite well.

The bearing race inside the ball cup also will have been softened. Tack welding the planet cage to the ball cup, or making a connection with a steel bar, as described in another article, would avoid this problem, but cutting threads would be more difficult. -- John Allen

• At the crank end, two right-hand cranks are used to accommodate chainrings on both sides and an identical pair of pedals (some well-worn Look PP76's, in this case) have the axles swapped to make them compatible with the cranks. Thread locking compound is used to secure the left pedal. Both cranks are arms swaged to 52 tooth rings; the right ring is cut from the arms and replaced with a 22 tooth cog, which fits the lightening holes in the remains of the spider.

  A better, but more costly solution would be to use a tandem synch crank on the left side, as I did on my Gunnar 3-speed fixed gear. -SCB

  A right crank which is integral with the spider would still need a modified pedal, but still would be better,. A swaged crank which drives in both directions is likely to loosen from the spider. I've had swaged cranks welded to the spider to get past this problem. -- John Allen

• A 3/32" old (with high side plates) derailleur chain is used with the 52/15 normal gear on the left and the freewheeling 22/16 gear on the right. The frame has horizontal dropouts and the chain lengths are pretty good match at this gearing, so neither side is very slack.

Ah -- Different chain slack is an issue with a bichain system I hadn't thought of. -- John Allen

• A left Shimano bar-end shifter set up for a center index (insert turned 90°) on the right end of the drop bar is used for selecting gears. It doesn't provide protection for the axle key from overtensioning of the shift cable, but it's preferable to the sudden unintended shift from a bump which can result from using a Sturmey trigger. An unintended shift while coasting on a fast downhill could very easily cause sudden engagement of a fixed gear and a subsequent loss of control.

Interesting results:

• Whenever a fixed gear is engaged, the drivetrain will bind in reverse. This can make trackstanding somewhat tricky, but it works as along as you understand that you have only a little bit of lash (the pawls in the freewheel and the clutch in the fixed gear) to work with.
• When trackstanding uphill in a fixed gear, the freewheel gear is engaged until the lash is taken up between the clutch and dogs in the fixed gear hub, at which point the fixed gear is engaged -- the mechanical advantage changes at this point, and on the setup I've created it's fairly obvious when it happens. When trackstanding downhill, only the fixed gear can be engaged.
• On this particular setup, first gear can be preselected by shifting the lever down from the lowest fixed gear, then engaged by relieving pedal pressure (a little jump will do.)
• I really like this setup. It's a fixed gear that fears nothing...I'll generally leave it in high gear if I'm noodling along but downshift to the lower fixed gear for intersections (to slow down and accelerate quickly) or for power work. The low freewheel gear is great for diving off the road into the woods or loose gravel for a nature break. I'm seriously considering using it for the Deerfield Dirt Road Randonnee, which features some very steep climbs (~20%) on dirt roads.

  It is very mesmerizing to watch while spinning on a workstand.

Possible improvements:

• One possibility is to add a derailleur drivetrain to the right side, which would add more freewheel gears. This only works as long as the maximum freewheel gear does not exceed the minimum fixed gear. I personally don't favor adding the extra maintenance of jockey pulleys and I prefer a narrow tread, which favors no front derailleur, but it might suit somebody else. One advantage would be increased backlash (from chain slap) in the derailleur drivetrain, which would allow the user to operate further in reverse during a trackstand. If a 6-speed freewheel isn't practical for space constraints, a 1- or 2-speed BMX freewheel could be used with multiple chainrings.
• Another possibility would be to install a different ring gear with the low gear dogs ground off instead of the normal gear dogs, which would yield a sequential shifting pattern (1-2-3 instead of 2-1-3)...although I like being able to preselect the lowest gear, and the current setup also allows me to shift to coasting from the highest fixed gear instead of the lowest.

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Last Updated: by John Allen