Warning, Technical Text follows.
Bob Durst’s Differential Drive Steering System.
It is possible, of course, that others have invented this system at various times in history, and I would not be surprised if it has been used in, say, real battle tanks. But Bob came up with it in his garage in Corvallis, so let’s hoist an organic, non-alcoholic beverage in his honor. Here goes:
The DDSS uses no conventional steering parts or methods. Nothing gets turned sideways, and there is no braking.
Instead, there is a second drive system that propels the two sides of the vehicle in opposite directions.
First, let’s consider the main propulsion system. From the pedals (or collector shaft, if multiple riders), the power goes thru a chain to a differential of the go-kart / old lawn tractor type. From there, the power goes out the two exposed axles to the wheels. Entirely straight-forward as we covered in an above post.
The second system begins with a hand crank -- one hand is enough -- made from a bicycle crank set. It is important to grasp that this hand crank will not serve as a steering wheel, nor as a tiller, but will be rotated continuously during all turning motions of the vehicle. From the hand crank and common bicycle chain ring, this chain goes to its own differential, which is mounted parallel to the main drive differential. From both output shafts of this Steering Differential the motion is then transferred to the Main Drive Shafts thru two chains. In other words, the steering system is a parallel, or redundant, low power drive system, terminating at the wheels just like the main drive system.
Here comes the magic: It’s in those two chains that connect the Steering Differential Shafts to the Main Drive Shafts. One of them runs backwards. (Tah-dah!) I call these two chains the Magic Chains.
Reversing the motion of a chain can be done two ways.
Bob runs his left Magic Chain in a figure-8. He has guide plates on the sides of the sprockets and he simply replaces the chain when it becomes worn from rubbing against itself.
The other way is to run the chain over the “back side” of the sprocket. This is easily done by having two idlers at points beyond the sprocket. The new KSR Contraption built at the Oregon Institute of Technology in Klamath Falls, OR, uses this arrangement.
Now we need to comprehend what actually happens.
Imagine you are pedaling straight forward while holding the Steering Crank still. The Main Drive Shafts turn The Magic Chains and the shafts of the Steering Differential turn in opposite directions. That’s the key. I repeat: Thanks to one “Magic Chain” being reversed, the shafts of the Steering Differential turn in opposite directions. Thus, the innards of the Steering Differential go on Mr. Toad’s Wild Ride, while the housing of that differential stays still.
Stop the vehicle, and just sit there for a moment. Now start turning the Hand Crank. The Hand Crank only, to keep it simple. This motion goes to the Steering Differential. The Steering Differential Housing turns. The two Steering Differential Shafts turn -- in the same direction. But when the motion arrives at the Main Shafts, the Magic Chains have reversed the rotation on one side and the Main Shafts turn in opposite directions. The vehicle turns in place around its own center. Children cheer with delight and adults clap their hands in appreciation of True Glorious Folly.
Apply yourself to both systems and you get any combination of forward motion and turning motion you desire. The only thing you have to learn is which way to spin the Hand Crank to go left, and which way to go right. If you must have logical steering input, you could put a bevel drive there.
I could polish this text a bit, but I have to get ready for a trip.
Anyone who is serious about building a DiffDriSS really ought to take the trouble to attend a Kinetic Sculpture Race and meet Bob and see his Contraption in action. Your safest bet will be at the (Leonardo) daVinci Days Festival of Art and Science in Corvallis, Oregon 14-15-16 July 2006.
If you don’t want to wait that long, the Klamath Falls machine will probably race in the Grand Championship Kinetic Sculpture Race in Humboldt County, Northern California, 27-28-29 May 2006 (Memorial Weekend).