Tilting, narrow-track, recumbent tricycle with variable stability (patent pending)

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Award Winning Research

The current physical implementation, designed and built entirely by members of the UWM undergraduate student ASME HPVC team uses a moving bottom bracket generously donated by Cruzbike, in which the pedals and crank move as the front wheel steers, but this is not an integral part of the design, and a non-moving bottom bracket would work just as well. The roll cage is required for the competition.

What is it?

A tilting, narrow-track, recumbent tricycle with variable stability:

  • “Tilting” means it leans into turns, just like a bicycle, which makes it stable at high speeds even with a narrow axle track.
  • “Narrow-track” means it doesn’t take up the whole bike path. The rear wheels are only 18 inches apart, and this can reduce air drag.
  • “Recumbent” means you recline in a fully-supportive seat, which makes it comfortable to ride, can reduce air drag, and allows for a full enclosure.
  • “Tricycle” means you can ride easily at low speed and stop without putting your feet down, which is also necessary for a full enclosure.
  • “Variable stability” means you can control just how stable or not it is at any time.

What are the problems it solves?

Upright bikes:

  • Poor aerodynamics. At 20 mph, up to 90% of rider energy goes to overcoming air drag. Cannot be effectively faired because of instability in cross winds.
  • Poor ergonomics: Many riders experience difficulty with their seat, back, neck, and/or wrists.

Recumbent bikes:

  • Can improve upon the main shortcomings of upright bikes, but most people find them difficult or impossible to ride, especially at stops and starts.
  • The low and horizontal seating position increases the lean rate and requires quicker reaction from the rider.
  • The large tiller effect, caused by handlebars that extend far behind the steering axis (head tube) that act like a tiller on a boat, in that one moves the bars to the right in order to turn the front wheel to the left, and vice versa., found on many recumbents requires learning a motion different from that required by upright bikes.
  • Can be faired but not fully, without aid stopping and starting, because of need to put feet down when stopped.

Recumbent trikes:

  • Must be low and wide to avoid rolling over in a turn, which negates any aerodynamic gain, and requires a wide path.
  • The rider, tires, wheels, and frame are all subjected to large side loads in a turn.

Recumbent bikes and trikes:

  • It is harder for a rider to lift themselves out of the seat when riding to avoid the shock of a bump than on an upright bike.
  • The low seat makes it harder for less-agile riders to get into and out of.
  • Both tend to be larger than upright bikes, which makes storage and transportation more difficult.

Existing tilting trikes with parallelogram mechanisms, springs, tilt-locks, or active control:

  • The parallelogram linkage is at least as complex as the swing arm and bell crank linkage but does not provide the inherent suspension feature or folding feature described below.
  • Springs are either too soft to provide static stability or too stiff to allow free tilting necessary for easy cornering. Varying stability implemented with springs would likely require changing the preload, which would require the same magnitude of force that the spring generates.
  • Tilt locks are binary, either off or on, and provide no way to right a lean.
  • Active control requires the added weight, complexity, and expense of a power source, actuators, sensors, and a controller.

Why is this vehicle a solution?

A tilting, narrow-track, recumbent trike with variable stability combines the best of all three vehicles to eliminate the drawbacks of each and avoids the pitfalls of other existing tilting trikes:

  • Ergonomics, aerodynamics, narrow track, and leaning into a turn of a recumbent bicycle.
  • Static stability of a rigid tricycle and higher seat when going slow or stopped.
  • Can be fully faired to provide protection from weather and dramatically reduce power required to overcome air drag.

The swing arms also:

  • provide an inherent suspension, which significantly reduces the impact the rider feels when going over bumps, without the weight, expense, and complexity of explicit suspension components.
  • provide the ability also to vary the seat height with stability so that the seat can be high when stable, to facilitate getting in and out, and low when unstable, to minimize air drag.
  • fold forward to reduce the length of the vehicle for transportation and storage, unlike the alternative parallelogram tilting mechanism.

How does it work?

Unstable mode, where the trike leans freely, exactly like a bicycle

  • The rider creates the lean by countersteering, exactly as on upright and recumbent bicycles.
  • The swing arms are both connected to a bell crank in an arrangement that causes one wheel to rotate upwards exactly as much as the other wheel rotates downwards.
  • The trajectory of the seat is a circular arc as the trike leans, exactly like that of a bicycle. The straight up position is an unstable equilibrium. It is theoretically possible to balance it there, but practically impossible, like balancing a pencil on its tip. It works in MATLAB, but good luck trying it at home.
  • There is some small amount of friction in the swing arm and bell crank bearings, but it is negligible. This can be seen by trying to make the trike stand upright on its own. It is about as difficult as making a bicycle stand up on its own.

Stable mode, where the trike is statically stable like a chair:

  • Simply moving the locations on the bell crank where the tie rods from the swing arms connect changes the trajectory that the seat takes as the trike leans.
  • At the right spot, the seat actually rises as the trike leans.
  • This makes the straight up and down orientation the low spot and so a stable equilibrium.
  • The trike can still lean freely, but now it is more like a pencil hanging from its tip instead of balancing on its tip. If pushed to the side from the straight up and down orientation, gravity pulls it back to the straight up and down orientation.

Semi-stable mode, where the trike behaves with just the amount of stability and instability the rider wants:

  • The rider can position the tie rod connection point anywhere between completely stable and completely unstable to fine-tune the handling for the current riding situation.
  • Completely stable for riding down the hall in EMS at 3 mph.
  • 50/50 for riding at 10 mph and stopping and starting with traffic between campus and the Oak Leaf Trail.
  • Completely unstable for zooming down the Oak Leaf Trail at 20 mph, dodging squirrels, dogs, and pedestrians.

What is the protectable intellectual property?

  • The variable stability and the rider control of it.
  • Tilting trikes have been around for decades.
  • Tilting trikes implemented with swing arms and bell cranks were patented by Curtis L. Prince in 1987.
  • Tilting trikes with swing arms, bell cranks, suspension, and a tilt lock were patented by Carlos Calleja in 1997.
  • Tilting trikes with swing arms and bell cranks have been promoted by Henry Thomas on the Jetrike website since at least 2007. There he publishes a detailed description of his kinematic analysis of the linkage, and his goal of neutral stability, but he appears never to have made the leap to variable stability.
  • What is new is the exploitation of varying the linkage geometry to vary the stability.

What are the applications?

  • Zero emission commuting. Many people do already ride bicycles to work, but this is still a small fraction of the commuting population, and some are inhibited by discomfort or exposure to the elements. A fully enclosed, recumbent vehicle that can take dedicated bike paths can remove barriers that keep people way.
  • Mobility and exercise for people who simply cannot ride traditional bicycles. Current handcycles are predominantly rigid trikes, which means that have to be some combination of wide, low, or slow. A tilting, narrow-track vehicles does not have to have any of those attributes.
  • Motorcycling for riders who have lost confidence in their ability to keep a stationary or slow-moving motorcycle upright. Rigid motorized tricycles that preserve the upright riding position of motorcycles must be wide, 55 inches or more, to prevent rolling over in a turn, and subject the rider to large lateral accelerations that riders on two-wheeled vehicles never experience. Continuously variable stability enables the rider of a tilting motorized tricycle to handle their machine with confidence at all speeds without the need for a wide stance or lateral accelerations.

Finally, thanks to the sponsors that helped make it possible: