Monday, July 14, 2008

Trike: one in front or one in back?

I knew I wanted a trike so I felt safe on slippery roads, so it felt more car-ish. I bike is simpler but I wanted a trike. However, trikes do have a stability problem. The traditional trike design, called a "delta," with one wheel in front, can easily flip at high speeds. This illustration shows why:



A. Trike moving forward. Blue arrow indicates interia keeping it moving forward.

B. Front wheel is turned, creating a deflection force (pale blue) for the front of the trike.

C. The trike starts to turn. Inertia still wants the trike to move forward, the wheel is still deflecting the trike front to the right. Notice that inertia is now moving across the line between the front wheel and the rear left wheel. This line become a fulcrum for the bike to tip over. In a car, the weight of the car is shift to the front left wheel. In a delta trike, this wheel doesn't exist.

D. Inertia keeps the bike moving forward, by adding a simple torsion to the bike mass, known in highways safety lingo as a roll-over, or just a crash.

In a trike with one rear tire and two front tires (called a "tadpole"), the left front tire does exist, and the rollover is prevented.

Other factors, pro and con:

  • A delta trike requires a more complex power train, some sort of differential to allow the rear tires to spin at different rates in a turn, and or a single axle to transmit power to both tires.
  • A tadpole trike requires a more complex steering mechanism, so the front wheels turn in unison.
  • One poster has said that tadpoles are more inclined to fishtail.
  • The majority of trike designs available from more technically informed people are tadpoles.
  • Tadpoles look cool.

I'm gonna make a tadpole.


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