Reflections on the potential of human power for transportation

Thursday, March 15, 2012

Recumbents and Convergent Evolution

By the late 1870’s bicycle innovators realized that there needed to be a safer alternative to the high-wheel bicycle that had become so ubiquitous. All manner of design approaches to lower the rider and move him backward prevailed, each with the intent of preventing headers, the rider pitching forward over the handlebars. These included gearing mechanisms so the size of the drive wheel could be reduced while keeping the ratio of pedaling speed to vehicle speed the same. Four bar linkages were used that moved the rider rearward as well as levers with overrunning clutches. Separate crank arms being coupled to the wheel by separate chains allowed the pedal location to be lowered, and there was the novel approach to use a single chain to drive the rear wheel. This of course was the safety bicycle and not long after its appearance, the other approaches faded away. A common design was converged upon and human-powered vehicle technology moved forward.
Now if someone were to have asked me, I would have told them that recumbents haven’t converged on a common design that satisfactorily addresses what I will call "the recumbent packaging problem"; that being, that for a proper weight distribution, the pedals want to occupy the same volume as the front wheel. But after a bit more reflection and staring at pictures of the participants in last summer’s “Roll Over America” velomobile tour, I realized that recumbents have converged on a common design, and a pretty good one at that.
I saw a post on about an Italian designer who was developing a rear-steering-recumbent bicycle.!/2012/01/rws-recumbent.html .  He was trying to solve the recumbent packaging problem.

 Now there a number of solutions to this problem. It turns out that rear-steering is the least feasible of those solutions. I should know, I spent 10 years building rear-steerer prototypes.

The Plywood Flyer from 1977

 The VelAero from 1987

In the end, the VelAero, could be balanced at moderate to high speeds, but could not be ridden slowly without weaving drastically. So I wrote up my results, published them in “Human Power” and moved on.

One place to put the cranks is behind the rear wheel, like the Avatar and many other long wheelbase recumbents. The upside is a very comfortable riding posture and ease of balancing. The downside is a very long vehicle that is difficult to transport and a lightly loaded front wheel that can slip laterally on all but the driest of surfaces. Detractors to this layout claim the chain is too long. I disagree. It minimizes the angle the chain makes with the sprockets in the extreme gears and it extends chain life. 22 years of riding an Avatar, breaking the seat twice, sawing through a front derailleur, breaking a crank and breaking the frame but never having to replace a chain.

The other pole is to place the cranks far ahead of the front wheel. The vehicle shown is the aptly named Hypercycle. It is very compact but also suffers from poor weight distribution with a heavily loaded front wheel. The heavily loaded front wheel also resulted in very skittish handling.

The cranks could be located well above the front wheel like the classic racing Velocar of the 1930’s. The package is compact, and the front wheel loading is similar to a long-wheel base design. The downside is that the rider’s feet end up a significant distance above the ground, especially if the front wheel becomes large. This makes it difficult to get started from a stopped condition.

The cranks could overlap the front wheel with the bottom bracket near the steering axis. This is a picture of the final crank location of the EcoVia leaning tricycle. This limits the steering angle, but surprisingly, for medium wheelbase layouts (40-48”), you don’t need much more than +/-20deg. of steering lock for all but the tightest of turns. George Georgiev has made good use of this fact with his Varna speed demons, where the rider’s legs and cranks overlap the front wheel to the extent that only a few degrees of steering lock are available.

Another way of having the cranks overlap the front wheel, without limiting steering lock to the degree above, is to allow the cranks to pivot with the steered wheel. The worked satisfactorily for high-wheel bicycles, where the direction of the pedal force was roughly parallel with the steering axis, but with recumbent posture the pedal-force direction is closer to being perpendicular to the steering axis. Thus pedaling forces will perturb the steering and make the vehicle difficult to ride. (See the post “Arm-Power and the Avatar” below.)

A more complex version of the cranks moving with the steered wheel is having the cranks pass through the front hub. In the Evolution Bike above, the cranks drive an infinitely-variable transmission in the hub. An interesting note, the concept recumbent shown above is nearly identical to a Polish entry in Prof. David Wilson’s “Human –Powered-Land-Vehicle Design Contest held in 1969. That vehicle used a hub transmission as well but most interesting, the laid-back steering axis passing through the contact patch of the front wheel was the same. Convergent evolution? In both designs the pedal force is closer to being parallel to the steering axis than perpendicular, so pedaling perturbations to the steering might be manageable. A most attractive package, but again the driving wheel is seeing significantly less than half of the vehicle’s weight. Another issue with hub-centered pedals is that the bottom bracket height is half the wheel diameter, so the wheel must be large enough to prevent the rider’s heels from hitting the ground when pedaling.
And yet another variation of hub-center pedals it to fix the cranks and pivot the wheel using a hub-center steering approach.  The steering lock is again restricted by the rider’s legs but the posture can be comfortable and steering is not perturbed by pedaling. Of course you must couple the cranks to the rear wheel (or front wheel for that matter). This approach requires a lot of non-standard components and has rarely been used.  

A pedal path that is elongated horizontally to minimize pedal/wheel overlap can be used. The pseudo-linear linkage above was tried on the EcoVia. Notice the two-sided pedal where the rocker link was outboard of the pedal and the coupler link was inboard of the pedal. It packaged very compactly. Unfortunately, the harmonic-velocity linear motion had pedals that came to a stop at the ends of the stroke. This significantly reduced the maximum pedaling cadence and made climbing of any type of hill prohibitive.

To eliminate the extreme velocity fluctuations associated with linear-type drives, but to preserve the squashed pedal path, I played with an egg-drive concept. A triangular link holds the pedal. This link is supported by a rocker link at the rear and a rotary crank at the front. The big circle below the egg is a 20”dia. wheel. The smaller circle behind the egg is a conventional crank for comparison. This approach is obviously a bit over the top in complexity.

And then there is the most unconventional means of addressing the packaging problem, ride the recumbent backwards. The steered wheel is in front and so is the riders head. The drive wheel is in back straddled by the rider’s legs and the vehicle conforms nicely to a teardrop cross-section with rider’s shoulders in front tapering to the rider’s feet at the rear. The idea has been used at least three times. By the inventor Milt Raymond in the mid 1980’s, by Ohio State with their “Buckeye Bullet” at one of the human-powered speed championships and most recently and most successfully with the Eiviestetto, which just wrested the hour record from the Varna Tempest (see the post “Back to the Future” below) covering almost 57miles in an hour.
Of course, there is an “out of the box” solution to the pedal/steered wheel interference problem, but purist may feel the original design intent is violated. Use two front steering wheels, place the pedals between them and drive the single back wheel.

This layout dates back before WW2 with the Fantom from Sweden. Supposedly 100,000 copies of the plans for these were sold. The layout was resurrected by Mike Burrows with his Windchetah in the late 1970’s, again used by the Vector team to break the break HPV speed records in 1980, by Carl-Georg Rasmussen with the Leitra and it has become the default layout for most velomobiles. It was while staring at a picture of a line of recumbents winding through a Portland OR. park at the beginning of the “Roll Over America” velomobile tour that I realized that seven of the nine visible vehicles looked essentially the same. They were based on the Fantom layout. The two outliers were a long-wheel base recumbent bike with a partial-fabric faring and Miles Kingsbury’s Quattro.

Sure, you could argue that a velomobile, by default, is an enclosed trike of the Fantom layout, but since velomobile is a generic term for an enclosed recumbent bicycle or tricycle, this only enforces my contention. The recumbent HPV has converged on this design approach, which I will refer to as nufantom.  
The move toward commercial recumbent trikes appears to have been going on for some time, and now that I think about it, I have seen more Nufantom trikes than recumbent bikes on the local bike trail. It is not surprising. A new rider can adapt to riding a tricycle a lot faster than becoming confident on a recumbent bike. I have only seen a few enclosed trikes, but was recently very impressed with a Quest from Blue Velo.
Converging on a common solution allows designers to concentrate on refinements, since they don’t have to worry about the overall vehicle layout being feasible. The Quest exhibited a lot of refinements to the nufantom approach. These included enclosed wheels for improved aerodynamics, cantilevered wheels all around for ease of flat repair, without removing the wheels, a monocoque chassis, a well thought-out suspension, Burrows-style u-joint steering controls and a very trick cover for the rear derailleur.
 As impressed as I am with the Quest, it doesn’t meet two of my criteria for a Human Powered Commuter Vehicle.  Most significantly it is too low to provide adequate visibility in traffic, and, case in point, I have only seen these vehicles on the bike trails. The other is ease-of-entry and riding posture. I suppose I could get used to the laid back seating, but my old back wouldn’t be too happy without a lumbar support. Entry and exit, however, appear to be extremely difficult, very similar to getting into a formula one car. The upside is the vehicle is statically stable for this process.
I have long felt that the recumbent bicycle is a transient form, evolving toward another vehicle. Streamlined recumbent bicycles will probably always hold the speed records, but for a utilitarian vehicle the Nufantom-style trike seems to be the most popular option.

1 comment:

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