Reflections on the potential of human power for transportation

Saturday, September 29, 2012

Transcending the Pedicar: The EcoVia, Part 2

Human Powered Commuter Vehicle Criteria

1.       Weather Protection
2.       Statically Stable
3.       Reasonable Cruise Speed
4.       Cargo Carrying Capacity
5.       No Wider than a Bicycle
6.       Same Height as an Auto
7.       Comfortable posture and ease of entry
8.       Two-wheel drive
9.       Car-type Wheels
10.   Electric Assist for Hills


The EcoVia 2.1 was well on the way to becoming a vehicle that satisfied the HPCV criteria. Unfortunately I did not feel it satisfied #7. I did not consider the riding posture as comfortable as it needed to be. The bottom bracket was higher than the seat and it needed to be below the seat, at least four inches below if possible.
EcoVia 2.2
I had fallen victim to “the recumbent design problem” where the pedals wanted to be where the front wheel was. (Refer to “Recumbents and Convergent Evolution” below.)

http://lefthandedcyclist.blogspot.com/2012/03/recumbents-and-convergent-evolution.html
For reasons of leaning functionality the wheel layout was fixed at one front and two rear wheels, with the front wheel steering. I wanted a compact package, so having the cranks behind the front wheel was rejected. So too was putting the cranks before the front wheel because of the skittish handling that came along with that location.
My first thought was to squash the pedal path, thus lowering the upper position of the pedal without causing interference with the wheel in the lower position. In his article “The Development of Modern Recumbent Bicycles”, Dave Wilson presents three sketches of mechanisms that produce elongated pedal paths.
 The simplest of these is known as a crank-rocker (actually crank, connecting rod and rocker) mechanism in kinematics and historically as a treadle mechanism. It can be made very rigid, since there is only one link between the pedals and the frame. It also has a fixed range of motion with built in decelerations and accelerations at the ends of the stroke. The literature seems to suggest that this type of reciprocating motion produces more power than levers pulling chains over one-way clutches. Recall the latter approach was used in the Pedicar. (Refer to “Pedicar Technology”, below.)

After rotary cranks, the treadle drive may be the most often used method of converting leg motion into rotary motion.  It predates cranks being used to propel velocipedes. It was used in the first attempts to build a safety bicycle and it is used in most children’s pedal cars. My favorite period picture of a treadle driven bicycle is Oscar Egg’s recumbent form the 1930’s. Egg had set the hour record several times on a conventional bicycle and he competed with Francois Faure in recumbent record attempts.























My implantation of the treadle mechanism for the EV2.2 was a bit different from previous embodiments. I fabricated a pedal that would allow connections to both sides of the pedal’s axle. The second connection was formed by brazing a bolt to the pedal’s steel dustcap. The pedal mounting was reversed with the normal thread being mounted outboard of the pedal. The rocker link was connected to this thread. The other end of the rocker was supported by a pair of Igus bushings, which, in turn, were held by a tube that was brazed to a rectangular frame that surrounded the front wheel. This frame did dual duty, since it also contained the pivots that would hold the front tilting faring. As result of the outboard location of the rocker links, they did not interfere with the front-wheel's ability to turn. The other, inboard end of the pedal was attached to a bent connecting rod whose other end was attached to a conventional crankset with the arms shortened. The connecting rod was bent to clear the top of the tire. The entire mechanism was very compact and it positioned the rider’s feet in a low and very comfortable position.
As successful as the packaging turned out to be, the performance was anything but. You see, I had overlooked one critical fact. Almost all the historical uses of treadle drives were on bicycles with fixed gears. The motion of the vehicle carried the pedals over the dead spots at the ends of travel. Problems with the dead spots became painfully obvious when attempting to climb even the most gradual of hills. As a bicycle moves progressively slower when the rider climbs increasing grades, the rider must apply the propulsive torque over a greater portion of the pedal stroke. (An explanation of this will come in a future post on why bicycles climb steep hills so poorly.) With the treadle mechanism, the rider can only apply a torque through the middle of the stroke. Pedal forces required to produce torque near the end of stroke become enormous and forward motion is lost.
I had an old Bullseye elliptical sprocket with a 1.56:1 ratio. I thought I could use it to modify the sinusoidal pedal velocity to improve hill climbing. Mounted in one orientation it would slow the pedals in the middle of the stroke and speed them up near the end.  This is the orientation I expected to improve performance but to no avail. Rotating the elliptical sprocket 90deg also showed no improvement, and I concluded that the round sprocket gave the best performance, which was still woefully inadequate. Oddly enough, however, the cable-clutch system used on the Pedicar, while lacking in power output due to limited pedaling cadence, did work well on steep hills because the output torque was constant except at the extreme ends of pedal travel.  While this approach would have solved the hill climbing problem, its limited power output would have restricted the speed of the vehicle. So much for unconventional drives…

EcoVia 2.3
I realized that if the bottom bracket was almost touching the tire, I could drop the bottom bracket height from 26” to 20”. If I kept the bottom bracket close to the steering axis and spaced the crank arm as far apart as I could, I might have enough front wheel lock to have a viable solution. I bought the widest triple-crank bottom-bracket axle I could find. I coupled this crank to an intermediate crank (left over from the treadle drive) on the left side and tensioned that chain with a spring-loaded chain tensioner. The right side of the intermediate-crank carried triple chainrings.
The acid test was riding the two miles to the library. The overlapped-crank drive passed with flying colors, the only crank/tire interference occurring during very tight, slow turns.
 In addition to lowering the bottom bracket, I changed the lower mounting position of the shock to raise the seat and I moved the seat back forward by adding an extra bend near the top. The result, while not being as ideal as the Avatar 2000, was a great improvement, enough to make the EV2.3 comfortable to ride.
All of the EcoVias used caliper brakes on all three wheels. Per tradition, the left lever actuates the front brake. The right lever actuates both rear brakes through a linkage that balances the forces applied to each rear brake. The travel of the right lever was increased to produce the greater cable motion required to actuate two brakes.
The middle cable from the brake lever is attached to a toggle linkage. The ends of the toggle linkage are attached to brake links that pull cables attached to the individual brakes. As the toggle flattens out, the forces on the brake links increase. If one caliper clamps the rim first, its brake link stops moving and the toggle motion continues to move the other brake link until its caliper clamps the rim. This balancing mechanism would be unnecessary if hydraulic disk brakes were used. Unfortunately there is no room for the disk rotor on the cantilevered hubs.
Now, actuation of the rear brakes had an unexpected consequence.  During leaning, each wheel rotates slightly with respect to its wheel beam. Since braking prevents this rotation, the act of braking the rear wheels inhibits leaning.  So during an emergency braking maneuver, when leaning is undesirable, it is automatically prevented. 


Below is a picture of the suspension-shock attachment locations, the wheel-beam-reverser linkage and the disk brake sector used as the lean-lock mechanism. The vehicle is in its upright position.
















Below are two pictures of the vehicle leaned over.



















Below is a detail of the seat. The lever on the rider’s left engages the motor. The lever on the rider’s right engages the lean lock.
Electric Hill Assist
The electric hill assist uses a geared Astroflight cobalt motor that drives one of the rear tires through a 2”dia. aluminum puck. The puck is brought into contact with the tire by pulling upon the motor engagement lever, above. The gear ratio of the motor is 2.7:1. At 24v, the motor can put out 675W at 3430rpm at the motor rotor. The results in a vehicle speed of 7.6mph for a vehicle payload of 300# climbing a 15% grade. The speed should top out about 15mph on flat terrain. The batteries and motor controller are not incorporated at this point.





















Faring
The faring frame is made up of a ½”dia. tubular steel loop and ½ x1/8”aluminum strips bolted to the loop and pop riveted to each other. ¼ x 1/8 ” aluminum strips are pop riveted to the wider strips. A polycarb windscreen is pop riveted to the ½” strips and a foam nosecone made up of laminated insulating foam is bolted to the front of the loop. At this point the faring frame remains uncovered. I am soliciting suggestions for a heat-shrinkable material to cover the frame. 


 The faring tips forward to allow for the rider to get into the vehicle.

The rider sits at about the same height as the driver of a typical minivan.

When the faring frame is covered, the total weight of the EcoVia will be about 100#. 60# is for the tricycle proper, 20# is for the motor and batteries and 20# is for the faring and mounting framework.
I consider the EcoVia 2.3 a proof-of-concept vehicle. All the features that comprise the design work, but features were added piecewise instead of being integrated as in a production vehicle, or even a prototype. So, before discussing how the EcoVia transcends the Pedicar, I would like to paint a word picture of what a productized version of the EcoVia would be like.
·         11-36 tooth, 10-speed cassettes are readily available. (The EV2.3 uses a 12-32 tooth, eight speed cassette.) Attaching one of these to the central driveshaft and using a single chainring would make an intermediate bottom bracket and secondary crankset unnecessary for flat terrain and would greatly simplify the drivetrain. For hills, the motor drive would provide the extra climbing ability. (Sram now makes a 10-42t, 11 cog cassette, but at a cost of almost eight times the 11-36 model, it is not a cost-effective alternative.)  Instead of using bar-end-shift levers at the ends of the handlebars, a twist grip or a trigger shifter could be used. Elimination of the bar ends would allow the width of the faring to be decreased at least by three inches.
·         Recall that the frame is made up of two pieces. The upper frame is a seat tube which is supported by a pivot at the fork end and a shock absorber at the rear. The lower frame is a 2”dia. tube that supports the fork, pedals, driveshaft, wheel beams and banking linkage. Much of the lower frame was carried over from the rear-steering EV1. If the lower frame is redesigned, it can be made significantly lighter. The weight of the rider is supported by the seat-tube pivot in front and the shock in the rear. The seat pivot loads go almost directly into the front tire through the fork. The shock loads go almost directly to the rear wheels through the wheel-beam-reverser linkage. So the lower frame does not carry these loads. The lower frame holds the head tube stationary, reacts the chain forces between the cranks and the driveshaft and fixes the lower shock mount and the reverser –pivot with respect to the drive-shaft tube. These loads are small enough that a space frame made up of thin-wall ½”dia. tubing can be substituted for the 2”dia. tube in the current version. This should significantly reduce the weight, since three, ½”dia-.028wall steel tubes weigh .45#/ft, while one, 2”dia-.065 wall tube weighs 1.4#/ft. The seat  tube could be dipped in the middle to aid in step over when getting onto vehicle.
Refer to the sketch below.




The second major change to the lower frame is to support the wheel beams directly on the drive shaft using ball bearings instead of using an intermediate tube and bushings. The over all goal is to try and reduce the weight of the tricycle proper to about 50#.

·         Currently the motor drives only one rear wheel through a puck. To take advantage of the dual-wheel drive, the motor must be coupled directly to the driveshaft. This can be accomplished by mounting a large diameter gear on the driveshaft and driving it with a small gear on the motor. The motor can be pivoted out of the way to disengage the gears. A ratchet could be used on the motor gear so the motor could stay in continuous engagement, but then the ability to drive the vehicle in reverse using the motor would be lost.
·         Currently, the faring pivots on a frame mounted ahead of the front wheel. If the faring was to pivot at the back of the vehicle, above the storage rack, the front frame could be eliminated and the weight of the faring reduced. The goal is to reduce the faring weight to 15#. This approach also opens up the area around the front wheel to make swapping out the wheel in the event of a flat tire easy.
·         The overall weight goal for the tricycle, the faring and the motor drive is #85. Since much of the weight for the motor drive is in the batteries, a weight reduction in this feature is unlikely
·         The EcoVia would be sold as a base tricycle, with the motor drive and the faring as individual options. This would allow the cost of the base tricycle to be minimized.
So how would a production EcoVia compare with the Pedicar?
1.       The pedal drive would be more efficient.
2.       Because of the compact and streamlined faring , the EV could attain higher speeds.
3.       Because of the ability to lean into turns, the EV could corner at these higher speeds without the danger of tipping over.
4.       The EV would weigh about 2/3 of the Pedicar.
5.       The rider posture of the EV would not be as comfortable as the Pedicar, nor would the entry and exit be as easy.
Improvements in four out of five categories makes the EcoVia a clear improvement over the Pedicar.
So the concept is proven. What needs to happen now is to find an agency with the resources and interest to take the EcoVia into production. Possibly a reader of this blog?

Hephaestus