Design Issues

Direct drive bike 4

The recumbent design challenge

The position of the front wheel of a recumbent bike is a real challenge for the designer.  In the recumbent riding posture, the rider’s feet are in front, positioned where the front wheel would be best located.  From a design perspective, this causes a conflict between the ideal front wheel position and the location of the pedals on the bicycle’s frame.  This conflict has dictated recumbent bike design, and led to the categorization of the type of recumbent bike currently on the market (Stegmann (2002)). The long-wheelbase (LWB) recumbent avoids interference between the front wheel and the pedals by placing the front wheel ahead of the pedals; the short-wheelbase (SWB) by positioning the front wheel behind the pedals; and the compact long-wheelbase (CLWB) by positioning the pedals above the front wheel. A tadpole recumbent tricycle addresses the conflict by having two front wheels and placing one on each side of the pedals.

The direct-drive solution

The direct-drive recumbent design transforms this wheel/pedal interference problem into a solution. Since the pedals are best positioned at the ideal front wheel location, why not simply drive the front wheel with the pedals directly? By attaching cranks, pedals, and a hub transmission to the front axle, the design problem is solved. In addition, the long cumbersome chain is eliminated.

Two issues need to be addressed for an effective direct-drive recumbent design. First, an effective direct-drive hub must be developed and commercialized; and second, there needs to be a clear understanding of the pedal force effects on the steering and the best design practices to address these effects.

The need for a hub

Regarding the first hurdle, currently there is no direct-drive multi-speed hub on the market, though many designs have been proposed. Given the increased interest in direct-drive, the need for such a hub is increasing. Velotegra addresses this need with a unique hub design that is also usable on a regular bike, and that seeks to reduce wear and friction within the hub.

Pedal force misconceptions

Turning to the second design hurdle, the pedal force affects the steering because the pedals are mounted on the front wheel crank axle which turns with the steering. This is called a moving bottom bracket (MBB) bike. These pedal forces must be resisted by the rider through the handlebars. A rigid frame between the seat and the pedals is seen by bicycle designers as vital for efficient pedaling, so having a steering pivot between the rider’s seat and the pedals is dismissed by most bicycle designers as inefficient. But a rider frequently applies the greatest pedaling force when standing off the seat with the pedal force resisted by the hands through the handlebars. So a rigid frame between the seat and the pedals may not be so vital. Furthermore, perhaps the actually pedal force transmitted back through the handlebars is not a large amount.

Best design practices

To address this second hurdle, Velotegra’s proprietor did a study of the pedal force feedback and handling of direct-drive recumbent bicycles (Garnet (2008)). The study recommends an optimal frame and fork design to reduce pedal force feedback, improve handling, and augment the overall user-friendliness of the bike. With this recommended geometry, the pedal force that is fed back to each of the rider’s hands can be as low as 12 percent of the total applied pedal load. The study highlights the following design aspects:

Handlebars: The handlebars are best arranged in the above-seat steering position with the rider’s arms outstretched ahead, allowing the arms the greatest mechanical advantage. An inverted V or W shape to the bars gives the greatest leverage to the rider’s arms for a given overall width, while providing the necessary knee clearance.

Optimum head angle: Inclining the head angle reduces the pedal force feedback. However, this is not due to a closer alignment of the steering axis with the applied pedal forces, because there is a corresponding decrease in the mechanical advantage of the rider’s arms in resisting these pedal forces. Rather it is due to an increase in the fork offset which gives the outward lateral pedal forces greater leverage in cancelling the main pedal forces. However there are handling difficulties with shallow head angles, so the head angle should not be too low.  The study indicates that although there is an advantage to reducing the head angle, there is no benefit in reducing it to less than 56 degrees to the horizontal.

Seat height: Increasing the seat height decreases the pedal force feedback by aligning the pedal forces more closely with the steering axis.  Providing the rider’s arms remain at the same angle (i.e. providing the handlebars are raised the same amount as the seat) there will be no corresponding loss of arm leverage. However, the seat should not be so high as to lose the recumbent comfort. The feet should still be able to plant firmly on the ground without too much seat cushion depression, and the centre of gravity should remain low enough to prevent over-the-handlebars mishaps under full braking. All this means that for a rider of about 1.8 m (6 ft) height, the seat height should not be more than 560 mm (22 in).

Trail: The trail is the horizontal distance from the steering axis to front wheel ground contact point. Trail causes the front wheel to steer in the direction of the tilt of the frame, restoring balance automatically – even if the bike is ridden hands-free. The amount of trail determines how quickly the steering reacts to a lean of the bicycle frame. In a regular bike, the trail is about 50 mm (2 in). A direct-drive recumbent should be designed with less trail due to a heavier weighted front wheel and a more inclined head angle. However, negative trail should not be used due to instability from ridges and other road surface imperfections. A formula for the trail is presented in the study. The formula indicates a trail of 25 mm (1 in) for a head angle of 56 degrees to the horizontal.

Centering spring: A centering spring is a spring that returns the steering to the center, i.e. to the straight ahead position. A centering spring is required for a direct-drive recumbent bike because the mass of the front assembly (everything that turns with the steering) is much larger than in a regular bike, due to the weight of the rider’s feet on the pedals. Without a centering spring, the steering would respond too strongly to a tilt of the frame or a slight turn of the steering, resulting in an over-correction of the imbalance, and ensuing instability. A centering spring controls these effects, restoring stability and user-friendly handling. The study presents an equation for determining the required spring constant for the centering spring. This spring can be a simple torsion bar placed in the head tube.

Wheelbase: For compactness, it is tempting to place the rear wheel immediately behind the seat back. However this will result in a front wheel which is too lightly loaded, leading to traction problems particularly when going uphill. It is best to aim for a 50/50 weight distribution between the front and rear wheels with the rider on board. This result in a wheelbase ranging from 1300 to 1550 mm (51 to 61 in), depending on the size of rider.

Rigidity: In a regular bicycle, the torsional rigidity of the frame is important to ensure that a portion of the rider’s energy is not lost in frame flexure. For a direct-drive recumbent bike, it is rather the torsional rigidity of the front assembly that counts, particularly from the handlebars to the front wheel axle. This ensures that energy is not lost in structural flexure as the rider’s hands resist the pedal forces. The torsional rigidity can be improved by a fork with a direct connection from the front wheel to the handlebars, with the headset offset behind. This avoids the weakening that results from a relatively narrow steerer tube portion passing through the head tube.

There are other design advantages not included in the study. For example, the centering spring not only improves handling but also makes the bike much easier to walk. One can simply hold the top of the seat back and walk the bike with one hand, leaning the frame to turn with no risk of fork flop. If a tighter turn is required, one simply holds the handlebars with the other hand and forces a tighter turn against the force of the centering spring. In more confined spaces, one can easily place the bike vertically of the front wheel due to the light rear assembly. Simply hold the seat base with the right hand, while holding the handlebars with the left hand with the front brake handle depressed to prevent the front wheel from turning, and lift the bike vertically on the front wheel. Once upright, release the front wheel brake and the bike can be maneuvered with ease. The centering spring keeps the rear assembly upright with no difficulty.

Because front assembly torsional rigidity is required rather than frame rigidity, there are more design options for allowing some frame flexibility to improve the ride, without needing suspension. The 50/50 weight distribution already improves the ride by effectively halving the height of each bump, and careful attention to the frame’s vertical flexibility can improve the ride still further.

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Stegmann, John. (2002). “Chain of Thought”. Velo Vision, no. 8, p. 23.

Garnet, Jeremy M. (2008). “Ergonomics of Direct-Drive Recumbent Bicycles”, Human Power eJournal, Article 17, Issue 5.