Tilting, narrow-track, recumbent tricycle with rider controllable tilt rate

Photo gallery on Flickr with scenes from the construction process in the CEAS MakerSpace and the 2019 ASME Human Powered Vehicle Challenge.

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, the agressively recumbent seat is merely repurposed from a previous competition, and neither are integral to the tilting tricycle technology.

What is it?

A tilting, narrow-track, recumbent tricycle with rider controllable tilt rate:

  • “Tilting” means it can lean into turns, just like a bicycle, which enables it to go around corners without rolling over to the outside, even with a narrow axle track or a high seat.
  • “Narrow-track” means it doesn’t need much pavement. The rear wheels are only 18 inches apart, tucked behind the rider, and this can also reduce air drag.
  • “Recumbent” means the rider reclines in a fully-supportive seat, which makes it comfortable to ride, can reduce air drag, and even 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.
  • “Rider controllable tilt rate” means the rider can control whether it is free tilting like a bicycle, non-tilting like a tricycle, or anywhere in between. The rider does not control the tilting directly so there is no need to carefully pick just the right angle to balance in a turn. Instead, the rider balances it just as they would a bicycle, but they can select just how docile or agile a bicycle it feels like.

What are the problems it solves?

Upright bikes:

  • Poor ergonomics: Many riders experience difficulty with their seat, back, neck, and/or wrists. A survey of 900 Portland, OR residents found that 13% “were physically unable to ride a bicycle,” and this includes “about 40% of the respondents age 65 or older.”
  • 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.

Recumbent bikes:

  • Can improve upon the main shortcomings of upright bikes, but many people find them too tricky to ride, especially at stops and starts.
  • The low and horizontal seating position increases the lean rate and requires quicker steering reaction from the rider. It also make it harder to push off when getting started.
  • May be faired, to reduce air drag or protect the rider from the elements, but not fully, without aid when stopping and starting, because they cannot balance on their own when stopped.

Recumbent trikes:

  • Must be low and wide to avoid rolling over in turns, which requires a wide path and negates some or all of the aerodynamic gain caused by the low seat. Also, the center of mass should be closer to the pair of wheels than the single wheel.
  • Subject the rider, tires, wheels, and frame to large side loads in a turn.
  • Subject the rider to undesirable lean angles or roll accelerations on tilted or uneven pavement.

Recumbent bikes and trikes:

  • Both can be more difficult for less-agile riders to get into and out of because of the low seat.
  • Both tend to be longer than upright bikes, which can make storage and transportation more difficult.
  • Both make it harder for a rider to lift themselves out of the seat when riding to avoid the shock of a bump, compared to upright bikes.

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 or folding features 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 rider controllable tilt rate combines the best of all three vehicles to minimize 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.
  • No need for center of mass to be closer to the side-by-side pair of wheels, but instead can be located for optimal weight distribution and braking performance.
  • 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 tilt rate so that the seat can be high when not tilting like a tricycle, to facilitate getting in and out, and low when free tilting like a bicycle, to minimize air drag.
  • Can fold forward to reduce the length of the vehicle for transportation and storage, without any additional, explicit folding mechanism, unlike the alternative parallelogram tilting mechanism.

How does it work?

Free tilting mode, where the trike leans freely like a bicycle:

  • The rider causes it to tilt by countersteering, and controls the tilt angle with the handlebars, 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 move upwards exactly as much as the other wheel moves downwards, which enables the vehicle to tilt to the side.
  • The trajectory of the seat is approximately a circular arc as the trike tilts, similar to that of a bicycle. The straight up position is an unstable equilibrium, just as with bicycles, and it is theoretically possible to balance it there when not moving forward, but practically impossible, like balancing a pencil on its tip. It works on paper, but good luck making it work in real life.
  • There is some small amount of unavoidable 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.

Non-tilting mode, where the trike is statically stable like a rigid tricycle:

  • Simply moving the locations on the bell crank where the tie rods connect from the swing arms changes the trajectory that the seat takes as the trike tilts.
  • At the right spot, the seat actually rises as the trike tilts.
  • This makes the straight up and down orientation the low spot and so a stable equilibrium, as with a rigid tricycle.
  • The trike can still tilt 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.

Slow tilting mode, where the trike tilts at a nice and leisurely rate, as a traditional upright bicycle would, or even slower:

  • The rider can position the tie rod connection points anywhere on the bell crank between completely non-tilting and completely free tilting to fine-tune the handling for the current riding situation.
  • Completely non-tilting for riding in a driveway or parking lot.
  • Nice and slow tilting for riding at 5-10 mph and stopping and starting with traffic.
  • Completely free tilting for zooming down the bike path at 15-20 mph.

What is the protectable intellectual property?

  • The variable rate tilting 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 wonderfully-detailed description of his linkage kinematic analysis, and his goal of neutral stability, but he appears never to have made the leap to vary the linkage geometry while riding and the benefits that can offer.
  • What is new is the exploitation of varying the linkage geometry to vary the tilting rate and straight-up seat height.

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 tilt rate 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.

When will they be available?

We are making progress towards commercialization, but it will likely take a while.

  • We did present our results so far at BMD 2019, in September, where the concept was well received.
  • We will be showing our one working prototype at Recumbent Cycle-Con, in October, where we hope to find interest in licensing the technology from existing tricycle and recumbent bicycle manufacturers.
  • We are enrolled in Milwaukee’s I-Corps program, in November, which “links business mentors with academic participants to investigate the prospect of commercializing research ideas.”
  • We are actively developing a second vehicle to enter in the ASME HPVC at E-Fest North next April.
  • Finally, we have reserved the url PantherTrike.com, under which we hope to market our own versions.

If you want one, please don’t hesitate to let us know and be sure to indicate your color preference.

What it is not:

There is no “active control” of the tilt angle. There are no sensors, actuators, or electronics of any kind, nor is there a way for the ride to control the tilt angle directly short of putting their feet on the ground. When underway, the rider controls the tilt angle exactly as they would on a bicycle, indirectly by countersteering.

There is no explicit suspension. There are no springs, dampers, or components specifically designed to flex, although the paracord tie rods to provide a little bit of compliance.

The tilting mechanism is merely a mechanical linkage and the rider varies the geometry by moving where the tie rods connect to the bell crank, which changes the behavior of the vehicle as described above.


A Note on Terminology:

Stable and stability:

I try to avoid using “stable”, “unstable”, and “stability” because their meanings are fuzzy in this context. In bike dynamics, “stable” may refer to the self-stability of the uncontrolled vehicle or directional stability or roll stability of the vehicle without or without a rider. Plus, it is not yet clear exactly how the mechanical properties of a bike influence how “stable” a bike feels to a rider. Instead, what we are varying with this tilting tricycle is the rate at which it tilts simply by changing the trajectory the seat follows as it tilts.

I still do use “static stability” to refer to a rigid tricycle and our tilting tricycle when it is in non-tilting mode. Here it means it will return to its static equilibrium, straight up, after being displaced.

Tilt, lean, roll, etc.:

There are many terms used to describe the rotation of a vehicle about its longitudinal axis:

  • Cars roll (hence the development of anti-roll or anti-sway bars)
  • Ships list
  • Bikes lean
  • Planes bank
  • Mechanisms tilt (“About 9,750,000 results” for “tilting mechanism” on google vs “about 7,940,000 results” for “leaning mechanism”, and the Wikipedia articles are titled “Tilting three-wheeler” and “Tilting train, not “Leaning three-wheeler” or “Leaning train”.)

Thus, I try to use “tilt” and “tilting” consistently to refer to how this tricycle mechanism operates. The one exception is using “rollover” to refer to the type of action, usually undesirable, that rigid vehicles, such as cars and tricycles, may experience.

What we mean by “rider controllable tilt rate”:

First, the tilting (leaning or rolling) behavior of a bicycle can be crudely modeled as an inverted pendulum, and the roll acceleration of an inverted pendulum is approximately inversely proportional to its length.

Thus the taller a bicycle is, the slower its roll acceleration tends to be, and the easier it is for the rider to control its roll rate, which is just a fancy way to say “balance it”. Conversely, the shorter a bicycle is, the quicker its roll acceleration, and the quicker the rider must respond to any tilting (leaning or rolling) behavior.

If the seat of our tilting trike follows a circular arc from the straight-up position, just as the equivalent bicycle would, the tilt rate of the trike will be the same as that of the equivalent bicycle. Since the tilting tricycle has recumbent seating, the seat is low to the ground, like that of a recumbent bicycle and much lower to the ground than that of most upright bicycles, it is like a short inverted pendulum and has much quicker roll acceleration than most upright bicycles.

We can vary the effective bell crank dimensions on our tilting tricycle, by moving where the tie rods connect from the swing arms, which changes the trajectory that the seat takes as the trike tilts. If the seat trajectory changes to follow an arc with a larger radius, it acts like a taller bicycle or a longer inverted pendulum, and this slows its roll acceleration.

At one extreme, the tilting tricycle acts just like the equivalent recumbent bicycle, and the rider cannot even detect that there are two wheels behind the seat. At the opposite extreme, the seat actually rises as the tricycle tilts and so is statically stable, just like a rigid tricycle.

One interesting detail is that between these two extremes is a particular bell crank geometry which causes the seat to neither rise or fall as the trike tilts and so is neutrally stable: if tilted to the side, it simply stays there, neither tilting further nor returning to straight up. Researchers at Cornell University specifically built such a vehicle and explain that it can be “balanced or steered but not both.” We tend to avoid riding in this configuration, and luckily, it is easy for us to skip right over it.

Finally, thanks to the sponsors who helped make the first prototype possible: