Recent progress in blending HPV efficiency with practicality

Gerald E. Pease
Human Power
Spring 1989, p. 16-18

It is now common knowledge that IHPV A members have achieved remarkable success in the area of land speed records for human-powered vehicles. It is no secret that these accomplishments, in the tradition of UCI records, have been attained using vehicles totally unsuited for any other purpose. Indeed, a UCI sprint bike would be considered a model of practicality compared with the typical streamliner, which usually requires a pit crew to assist the rider in entering, starting, stopping, and exiting. The fastest vehicles have a reputation for being easily blown over by light cross winds. The lack of adequate ventilation means they are unfit to ride even moderate distances. Thus has efficiency come to be equated with uselessness in the real world, where cost effectiveness and convenience rule above all else.

The most popular bicycle combining practicality with some measure of efficiency is still the lightweight multi-gear diamond-frame Safety concept, available with a wide choice of tires, handlebars, and saddle designs. A current trend appears to be away from efficiency in order to achieve modest improvements in comfort, safety, and durability. This tradeoff is exemplified by the ubiquitous Mountain Bike and its City Bike cousin. The popularity of these machines seems to hinge on their jack-of-all-trades nature, particularly the ability to perform competently on rarely encountered bad road or even off-road conditions. This is somewhat analogous to the current popularity in metropolitan areas of four-wheel-drive trucks, which also are significantly compromised in street efficiency by their rarely used off-road capabilities. But in the case of motorized vehicles, high performance and efficiency are also popular. Where are the Porsches and Ferrari F-40s of the bicycle world?

Enter the darling of the HPV enthusiast, the recumbent bicycle. ln unfaired and partially faired forms, recumbents offer a big improvement in comfort and a worthwhile improvement in efficiency. They are not popular. Not a single recumbent design has ever been mass produced. We know that people say they don’t buy them because they are confused by the lack of standardization and because the racers claim they are no good on hills. They are also usually more difficult to transport, and the ratio of price to perceived quality is not favorable. As marketed, there is no competition class for them (they are not competitive with fully faired racing recumbents), so the flat-road performance edge doesn’t count for much. None of them is a match for the UCI road racer in an out-and-out hill-climbing contest which is, naturally enough, considered by traditionalists to be one of the most important tests of real-world performance.

With the sudden appearance of the Lightning F-40 on the scene and its startling victory in the 1988 Argus Tour, a new standard of efficiency for practical vehicles now exists. This commercially available 15-kilogram streamlined recumbent bicycle is easy to enter, start, stop, and exit without assistance. Ventilation is outstanding for a streamlined vehicle and is adjustable. Best of all, the bike is not blown around by normal crosswinds. In extreme conditions of temperature or wind (over 32 degrees Celsius or 9 meters/sec windspeed) the major part of the fairing, made of nylon Spandex, can be removed and stowed in less than a minute. A worthwhile bonus for touring in cold or wet weather is the protection offered by the fairing, which can be ordered in waterproof stretch Cortex. Other touring options that are available include extra-wide-range gearing, a front drum brake, mudguards, and aerodynamic pannier carriers integrated with the fairing.

The efficiency of the Lightning splits the huge gulf between the partially faired practical recumbent and the impractical full streamliner required to be competitive in short-distance HPV races. The F-40 requires only half the power at the pedals needed by a UCI racer on a flat surface at 18 meters/sec. In other words, we are looking at a new generation of bicycle for touring and long-distance road-racing. These tasks are performed inefficiently by standard bikes and not at all by most fully streamlined recumbents. A legitimate question is whether or not the improvement in efficiency justifies the cost (about double that of a good partially faired recumbent) and the additional inconvenience in transporting by automobile. It was affordable enough for my budget but I’m still working on the transportation problem. A good roof rack should do the job if the Spandex part of the fairing is removed from the bike prior to transporting. This is a relatively minor inconvenience.

The accompanying figure illustrates the efficiency spectrum of existing types of bicycles for which speed is an important design consideration. The Lightning F-40 curve more or less defines the present limit of efficiency for a practical vehicle. There may be some practical tricycle designs with comparable level-road power requirements, but I feel their additional width and lower profile causes them to be too dangerous in traffic, while the extra weight, complexity, and cost may not be justified by the stability advantage. At this point I also think it makes more sense to attempt incremental improvements to the workable streamlined recumbent bicycle design rather than to try to make the fully streamlined racer either more practical or faster. 1 would like to see a shock-absorbing front suspension added to decrease rolling resistance and to improve the ride quality and handling on rough surfaces. The ride quality is presently good, provided that the tires are inflated to touring pressure rather than racing pressures.

The well-known equation for power requirement, P, as a function of level-road speed, v, in windless conditions was used to generate the curves, expressed as

P = av³ + Bv

where

A = (Cd × Af x Da)/(2 × Em)

and

B = (Cr × Wt)/Em

Cd and Cr are the respective aerodynamic drag and rolling coefficients. Af is the frontal area. In each case 1.226 kilogram per cubic meter was assumed for air density, Da, at sea level. Total weight, Wt, was obtained by multiplying the total mass of bike and clothed rider in kilograms by the acceleration of gravity, 9.806 meters/sec² at sea level. Mechanical efficiency, Em, was assumed to be 0.95 except for the Lightning, which has a drive-side idler with precision bearings. For the Lightning, 0.94 was assumed for overall mechanical efficiency. The other constants peculiar to the type of bicycle and rider are tabulated on the following page. The estimates of drag coefficient and frontal area were based on coast-down tests and accelerometer measurements of effective frontal area performed by experimenters other than myself. Because of the population sample variation in most of the constants in the table, they should be considered ball park representative estimates, but numerous speed comparisons performed by me indicate that they are reasonably accurate for the class of practical vehicles (I don’t have access to a fully streamlined record HPV).

For more information on the Lightning F-40, contact

Lightning Cycle Dynamics, Inc., Lompoc, CA 93436, USA, (805) 736-0700

PWatts = Av³ + Bv for Vm/sec

To calculate v directly as a function of P, A, and B:

v = (X + Y)^1/3 + (X - Y)^1/3

where

X = P/2A

Y = [X² + (B/3A)³]^½

Gerald Pease is a 51-year-old satellite-orbit-determination analyst at the Aerospace Corporation in El Segundo, California, who is finally fulfilling his 25-year quest for a practical bicycle fast enough to allow him to stay in front of any pack of racers he is likely to encounter. -ed.