Design principles
Driving the front wheel directly with the pedals causes the pedaling forces to be transmitted to the front fork. Since the front fork does the steering, there is pedal force feedback to the steering that the rider must resist through the handlebars. This force is not excessive when the bicycle’s frame and fork geometry is correctly designed. It is worth noting that pedal force feedback is common in every bike when the rider stands off the seat for extra force and resists the side-to-side frame motion with the hands through the handlebars. Garnet (2008) did a study of pedal force feedback in direct-drive recumbent bicycles. The study highlights the following best design practices:
Handlebars: The handlebars are best arranged in the above-seat steering position with the rider’s arms outstretched ahead. An inverted V or W shape gives the handlebars the greatest leverage for a given overall width, while providing the necessary knee clearance.
Optimum head angle: Inclining the head angle (angle of the steering axis) further back reduces the pedal force feedback, but there is no benefit in reducing it to less than 56 degrees to the horizontal. The higher the seating position, the higher the head angle can be. Generally the head angle should be in the range of 56 to 62 degrees to the horizontal.
Trail: The trail is the horizontal distance from the steering axis to front wheel ground contact point. The 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. It is essentially the caster effect. 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. 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 for a head angle of 56 degrees and about 30 mm for a head angle of 62 degrees.
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 to the steering. A centering spring controls these effects, restoring user-friendly handling and improving hands-free stability. 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: Positioning the rear wheel immediately behind the seat back 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, 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 fork should be designed for good torsional rigidity.