Saturday, June 6, 2009

Modern direct-drive recumbent bikes

When a friend of mine told me he was thinking about buying a new recumbent bike, I began surveying the choices available on the market to help him find the right one to meet his needs. Human powered transportation is a subject close to my heart. In the process of reviewing the incredible variation of bikes, I came upon the "direct drive" recumbent bike design that appeals to me (my motivation to help was not purely altruistic). A direct drive bike has the crank axle mounted on the front fork, inside the hub of the driven front wheel; no chain is needed. This front hub also houses a transmission system to allow a wide range of gears for slow or fast speeds.

The direct drive approach, though not commonly seen on bikes today, actually preceded the use of the chain drive, but was overshadowed by it as bicycle designs evolved due to the primitive stage of technical developent of hub gears at the time. The design of the transmission is probably the most complicated part of the whole bike, but today it is no longer a limitation. Planetary gear hub, Schlumpf Speed-Drive, Rohloff 14-speed hub, Sachs "Elan" 12-speed hub, and Biria continuously variable transmission hub: these are some reliable hubs that may be adapted to use in a direct-drive bike, with modification. The overall appearance of a direct drive bike is very clean and simple, and the compact drive system dramatically improves cargo carrying capacity. Though not yet available, were such a bike to enter the market, and if it were designed as carefully as Garnet recommended in his paper, I am sure it would be well received, especially by myself!

One disadvantage of this design is the width of the tread, more commonly known as the q-factor. This is the distance between the pedals that the rider's feet must straddle. Since the geared hub must be within this space, the q-factor cannot practially be reduced beyond the limits imposed by current technological ability to create a small geared hub.

Kretschmer, Thomas (2000). "Direct-drive (chainless) recumbent bicycles.", Human Power, no. 49:11-14.
Garnet, Jeremy (2008). "Ergonomics of direct-drive recumbent bicycles.", Human Power eJournal, article 17, issue 05.
Photo of Kretschmer's bike
Another bike built by Stefan Daniel

No comments: