Front or Rear Wheel Steer?

As a first time builder, you will no doubt be considering all possible permutations for your trikes design. This will inevitably lead to questions about rear wheel steer (RWS) -- Does it work?

The Culty FWD RWS delta trike

The attraction most certainly is the simple, fixed drivetrain and direct chainline that a RWS delta trike affords.

RWS exhibits classic oversteer behavior

However a quick Google search will probably put the idea to rest, most authorities on the subject have concluded long ago that RWS creates oversteer, and oversteer is unstable and dangerous at speeds above about 10kph. As the illustration above clearly shows, the path along which a RWS trike travels has all the hallmarks of oversteer. Another problem illuminated by the diagram above, is that the rear of the trike moves towards any obstacle you may be trying to avoid -- not so good if that is another vehicle or pedestrian.

The Sidewinder FWD RWS tadpole trike

Nevertheless, many designers have attempted RWS with varying degrees of success. Take for example the Sidewinder shown above, which is a commercial RWS, FWD tadpole trike. The obvious question -- Given what I have just been told, how can this be safe?

RWS FWD simulation

The image above shows the simple simulation I created to verify the Sidewinder design (download the Juice file here). For a RWS trike, it has surprisingly good handling. Find out about running Juice simulations here.

Countering the oversteer

This has been accomplished by moving the rider CoG forward, which counters the RWS oversteer with weight distribution that favors understeer, making the trike fairy evenly balanced. However this also moves the rider CoG in front of the forward tipping axis, which compromises deceleration performance under braking forces. Despite this limitation, it is a cleverly resolved RWS design with stable cornering characteristics.

The SoonW T3 RWS FWD delta trike

As the above photo illustrates, the biggest drawback of a RWS delta is that the rear of the trike moves toward the thing you are trying to steer away from. This is not so great if the thing happens to be a parked car or a pedestrian. Nevertheless, one day I would really love to have a crack at RWS. Owners describe the shear pleasure of riding a trike that turns like a speed boat, your body sinking into the seat as you turn.