HPV Front Wheel Drive Suspension
Suspension Fork Designs
For Front Wheel Drive Human Powered Vehicles
By Warren Beauchamp
Front wheel drive (FWD) recumbent bikes and HPVs offer many benefits over their rear wheel driven (RWD) cousins. The greatest of these are the compact nature of the system, and the fact that the drivetrain does not need to pass below or the seat, or over the shoulder. This means that the rest of the bike's egonomics can be designed without worrying about leaving space for the drive train. This is very important in a streamlined HPV, as to make it as small as possible vertically you want to be sitting in the bottom of the streamliner body, which doesn't leave a lot of room for chains and idlers.

The major problems with a FWD system are torque steer, wheel slip, and the lack of a good solution for a front suspension. Torque steer and wheel slip are low speed issues, which can be overcome by learning how the system behaves, but a lack of suspension is a real problem when you are cornering at high speeds, or moving at high speeds over a rough surface. Some unsuspended streamliners are actually launched into the air for several feet while cruising over rough pavement or expansion strips. This is not a good thing. The page explores methods for creating a suspension for a FWD vehicle.

Shown is the picture to the right, is a good example of a FWD drivetrain, on the Varna Diablo. As you can see, when the pedals are cranked on, force is applied to the fork in the "up" direction. The major problem with creating a suspended FWD design, is that the drive chain tension would cause the suspension to compress when the rider applies force to the pedals. This results in a loss of power during the power stroke, and when the suspension rebounds during the flat spot in the stroke of the human engine, the power of the rebounding suspension is lost as well.
To solve this problem, the suspension fork must be designed in such a way that the power stroke does not cause the fork to compress. Fortunately, the use of a leading link fork design will do just that. The leading link FWD suspension fork design specifies that the power side of the wheel's drive cog be lined up with the pivot point of the leading link fork.

Rick Wianecke has built this leading link fork as a prototype for his new streamliner. In the picture to the left, forward is to the right. The wheel pivots around the bolt on the left side. The left hand drive cog, visible on the far side of the hub lines the drive force up with the pivot point. 

This picture shows the crown area of the fork. A die spring was used to provide the boing, and the guide rod in the center of the spring slides in a rod end bearing to allow for a slight forward and backward offset as the bike bounces.
This picture shows the fork in the alignment jig. Delrin spacers were used on the pivot points to insure smooth operation.
This picture shows the whole fork. 

As bike suspensions designed for the road need less than an inch of actual travel, the small amount of travel provided by this design is more than adequate.

Alternatively, the springs can be mounted below the suspension pivot point. I was planning on using this design on a new streamliner, but after fabricating some of the brackets I decided it would be too complex. 
Another option is to use a more conventional spring between the fork crown and the head set, and have the steerer tube extend all the way through the head tube on bushings to allow up an down movement. The intermediary gear and the remote steering are mounted to the portion of the steerer tube that protrudes above the headset. The intermediary gear is mounted on a bearing which allows it to stay straight while the steerer tube turns. This allows the secondary chain, intermediary gear, and remote steering mount to all bounce up and down as a unit, while the chassis to remains still. A pivoting arm connects to the intermediary gear to the frame to keep it straight and to prevent torque steer. More info about this headshock system

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