document updated 12 years ago, on Jun 24, 2011
This is a new type of bike. I call it a "bipedal-speed". It has two single-speeds, similar in spirit to the flip-flop hub, the dinglespeed, and the retro-direct drive. Like the retro-direct, it can shift in real time. However, it's designed to be sturdier, for BMX applications.
The ultimate goal is to build a hybrid bike that can be used 5% of the time for BMX skateparks, and 95% of the time for urban commuting.
- There are TWO independent drivetrains on this bike, connected simultaneously. :) How? Read on...
- The rear hub will have two freewheels on it, one on the left side, one on the right. Unlike the retro-direct, both freewheels are pointing in the same direction.
- The cranks have a chainring on BOTH sides. There are two chains, one on each side. So both freewheels are constantly engaged.
- The gearing on the left side is for "cruising speed" (henceforth referred to as "cruising-side"). The right side is geared much lower, as with a traditional BMX trick bike (henceforth referred to as "trick-side").
- (sidenote: It's called "bipedal" because its locomotion system is based on two chainlines (analogy: two legs) being active at the same time (analogy: they both extend all the way to the ground). Alternate names: "Direct-direct drive".)
- When both chainlines are engaged, the lower-gear effectively does nothing, because it's spinning much faster. It's as if you were coasting down a long hill at 30mph, and were pedaling normal speed — when the wheel speed is much lower than the driven speed, it just freewheels.
- So, to switch over to the lower gearing, you disengage the cruising-speed chainline. This is done via the coaster-brake modification.
- While the cruising-speed chainline is disengaged, the trick-side chainline is much more secure. It now is exactly the same as any BMX bike.
- Chainline stability — the explicit goal is to, whenever a comprimise is required, to set up the trick-side to be more secure than the cruising-side. If comprimises are required, they will be made to the cruising-side only.
- Coaster brake modification. How do I do this?
- Comparisons to the retro-direct drive:
- It's potentially sturdier.
I don't know for sure, but I've read that some people have had a few problems with the chainline being secure on them. (??TODO: discuss this assumption with others)
- There is no backwards-pedaling required.
Retro-directs are usually configured so that the gear you use more often is the foward-pedaling one, and the less frequent gear is the backward-pedaling one. However, for a BMX / commuter hybrid, I really don't want to pedal backwards on either of them. The commuter-gear is the one I'd use 95% of the time, so I don't want that to be backwards. (??TODO: discuss this assumption with others) And I DEFINTELY don't want the trick-gear to be backwards — that potentially means having to re-learn all the tricks again when you switch to a normal BMX bike. (??TODO: discuss this assumption with others)
- Brainstorming possible caveats:
- Lateral wheel alignment — With normal single-speeds, you move the drive-side of the axle as far back as it can go in the dropout, to tighten the chain as much as is desired. You then adjust the non-drive-side of the axle so that the wheel is true, and doesn't rub against the brakes. However, with this, you want to tighten BOTH chainlines at once. So the chains have to be VERY close to the same length. Still, there will be a comprimise, and it will probably be on the cruising-speed side, so that the trick side is as sturdy as possible.
- Spoke tension — Most rear wheels are built with a clear distinction between drive-side spokes and nondrive-side spokes (with the drive-side spokes getting tighter and tighter as the wheel ages). Will it be a problem if both sides are drive-side spokes?
- This should be the same as flip-flop hubs, correct? Flip-flop hubs have two drive-sides.