Originally Posted by FarHorizon
Out of curiosity, has anyone made a front-wheel-drive recumbent with rear-wheel-steering?
Yea, a bunch have probably been built over the last ~100+ years or so.... by home-builders.
It would seem that having the drive train concentrated in the front of the bike (where the power is developed) would be more efficient than having to route a mile of chain to the rear wheel.
The Cruzbike design evolved out of this concept, but it still uses conventional front-wheel-steering geometry.
What are the physics of rear wheel steering? Would such a bike be rideable? What type of geometry would be needed to provide adequate rear-wheel-trail for stability? Does anyone make such a contraption?
The steering would probably need to be under-seat, with an offset tiller bar used to move the rear "fork." The idea is intriguing...
The desirable state of steering geometry is that a vehicle exhibits "positive stability", which means that the steering wheels will tend to return to a straight line as well as hold a straight line on their own, at any speed--even though some minimum speed may be required for this stability to function.
The mathematics for analyzing steady states of mechanical systems is Eigenvalue charts. There is one present in the article below, about halfway down the page:
The Eigenvalue charts for mechanical-only
rear-wheel steering shows that none devised so far can maintain positive stability
. Furthermore, it appears as though such a design is not possible.
The best that can be done is to locate the majority of the vehicles mass (the rider's center of gravity) very close
to the steering axis, and then the steering becomes fairly neutral--it does not tend to deviate from a straight line on its own, but then, it does not return to a straight line on its own either. These bicycles may be ride-able at typical speeds but companies generally refuse to manufacture and sell them as performance or general-use bicycles. Some are sold as cargo or commercial vehicles with cart-axle-type steering (such as tadpole vending trikes) but these are only intended to be used at VERY low speeds, and on level ground.
Note the distinction here:
1) it is not
impossible to build a rear-wheel-steer bicycle that is ride-able; there's several designs that work.
2) it is probably
impossible to build a rear-wheel-steer bicycle that exhibits positive stability
A number of people have done #1 and think they've solved some great cosmic mystery--when they simply didn't have a full understanding of the problem.
What technically defines "what kind of steering" a vehicle has is its Eigenvalue chart; the common variations of front, mid and rear-wheel steering show typical kinds of charts. A casual way (that usually
is correct) is to observe what wheels pivot relative to where the vehicle's mass is located. For the Kalle bike--the frame is two sections, and the rider's mass is fixed to the front section--so the rear section + wheel pivots to steer, and it is a RWS bicycle. (with the rider's mass located almost centered on the steering axis)
The Eric Wannee RWS bicycle pages show some bicycles and trikes that tried to use steering systems other-than-front-wheel-steer. Most of these toys were taken off the market and recalled due to reports of sudden crashes and injuries.
A lot of home-builders seem to think that if they're building a trike, it doesn't need to have positive stability since "it's got three wheels"--and they are likely to learn the truth the hard way. Wear your helmet!
The Wannee pages have a bunch of RWS trikes that were recalled because they tended to jack-knife and roll over at high speeds and/or rough ground. And note that many of these were children's riding toys, so "high speed" may have not been higher than 5 or 10 MPH....
You can build a negatively-stable bicycle or trike if you want; it may be fun and useful at low to moderate speeds. But it's never going to be safe at high speeds and/or rough terrain, and you should think really really
hard before you sell it to anyone else.
The Python is not called a "low-racer" because even tho it should have very low aero drag, it's design is negatively stable: it is unsafe at high speeds. So it is called a "touring" recumbent, not a racing recumbent. It tends to jack-knife at high speeds. And even tho the frame design is rather simple, no company has ever built and sold this design.