Reflections on the potential of human power for transportation

Friday, December 6, 2013

The Technical History of the Bicycle Part 2: The Wheel Gets Cranks and Pedals

As was discussed in the first part one of this series, the Draisienne seated scooter, many of the aspects of the modern bicycle were realized with one quantum leap of inventive genius. The obvious shortcoming of the design was that the vehicle could move forward no faster than the rider could move his leg backward. The running-style leg kick propulsion needed to be replaced.

What may not have been realized by the would-be inventors of the time was a mechanism was needed that would reduce the speed of limb motion and allow use of greater limb force. The assumption here is that the arms, the legs of both would be used to propel the vehicle.

In hindsight, a crank attached to a wheel is such an obvious and simple solution, one wonders why it took almost 50 years after the Draisienne to be incorporated. As is the case with many simple solutions to problems, they are derived from more complex predecessors. The crank was a piece of a more complex drive mechanism.

The first recorded attempt to add non-running propulsion to a Draisienne was by Lewis Gompertz in 1821.

Mr. Gompertz added a rocking crank with a gear sector which drove a pinion gear attached to the front wheel by a one-way clutch. Although this approach as a technical dead end, it is interesting for two reasons. It was the first case where the rider had to propel and steer a bicycle with the same mechanism. This would become a recurrent theme for inventors trying to add arm power to the bicycle. The one-way clutch the drive used is also of interest since the stresses associated with ratchet-type devices are very high due to the small size pawls reacting large forces.

The next attempt at adding non-running power to the bicycle was by Kirkpatrick McMillan in 1839. While McMillan’s invention was also a dead end, a similar drive system would be the direct antecedent of the crank drive. The drive would also be resurrected numerous times by would-be bicycle innovators.

Both Gompertz and McMillan’s approaches reflect the thinking that what was needed was a mechanism to convert linear motion to rotary motion. The arms or legs moved in large arcs which approximated linear motion and the bicycle’s wheel rotated. This thinking overlooked the fact that arms and also legs could comfortably move through rotary motions.

The classic approach for converting linear to rotary motion is known as a crank-slider mechanism, which was used in steam engines and later internal-combustion engines.

Since a stable sliding link associated with this mechanism is difficult to build without incurring undesirable frictional losses, a long rocker link is often substituted for the slider when pure linear motion is not required. (The kinematics guru at the U. of Wisconsin, John J. Uicker Jr., often said “Never use a slider if you can use a pivoting link”.)

 McMillan used a crank-rocker mechanism to drive his bicycle. McMillan mounted his pedals on the rocker link. Often, when a crank-rocker mechanism is used for human-powered propulsion, the pedals are attached the connecting rod. Now various pedal paths can be obtained with mounting the pedal on this link. If the pedal is mounted in position A, the pedals go in a circle. If the pedals are mounted in position B, the pedals go in an arc. If pedals are mounted in a location between A and B the pedal path is an asymmetric tear-drop shape, a hybrid of the arc and the circle.

In a typical bicycle-development scenario, the next and final step in bicycle propulsion occurs in 1861. One story relates that Pierre Michaux, a builder of perambulators, invalid carriages and three-wheel velocipedes, added pedal cranks to a broken Draisienne he was repairing.

During the period between McMillan’s bicycle and Michaux’s bicycle there were other wheeled human-powered vehicles being built. Four wheel vehicles called velocipedes (not to be confused with the term applied to the later Michaux-style bicycle) were being produced.

One such velocipede, made by Willard Sawyer, may have provided Michaux with the idea of mounting pedal cranks directly to the front wheel of a Draisienne.

Sawyer used crank-rocker mechanisms to drive his velocipedes. The cranks were double-sided allowing the connecting rods to be mounted between the wheels. In some cases the cranks drove the rear wheels and the rocker links were in front. In these cases the straps to hold the feet were near the rocker links and consequently the feet moved essentially back and forth along with the rocker link. In other cases, like the vehicle in the photo above, the cranks drove the front wheels and the rocker links were located in the rear. The foot straps were located close enough to the cranks that the feet essentially moved in near-circular paths. In fact, with drive mechanisms oriented in this fashion, the connecting rod acted like a pedal, providing a pivot between the foot and the crank.

Since Michaux worked on velocipedes of this type, it is not a stretch to assume he saw this type of mechanism with the double-sided cranks attached to the two drive wheels and extrapolated the idea of a single-sided cranks attached to a single wheel with pedals replacing the connecting rods. 
And that, as they say, is history.

Pedal cranks are simpler and more efficient than the crank-rocker mechanism. Nevertheless, the crank-rocker mechanism will reappear in the final stage of bicycle evolution, the development of the safety bicycle.

Before we leave this second part of the technical history of the bicycle, let’s take a parting look at the crank rocker mechanism but with a few substitutions. We will reuse the pedal cranks attached to the wheel. In the role of the rocker link we will use the rider’s upper leg and in the role of the connecting rod we will use the combination of the rider’s lower leg and foot. (Since there is often not much motion between the foot and lower leg, for the purposed of this exercise we will consider the pair a single link.) So the conventional bicycle is still propelled by a crank-rocker mechanism, but with the rider providing two of the links.

I mentioned the efficiency of the pedal-crank-drive approach when compared to other approaches. The efficiency stems from two effects.

First, the thigh, shin and crank mechanism has a relatively constant kinetic energy associated with it. I am referring to the energy stored in the moving thigh and shin. In mid-stroke the thighs are rotating fastest while the shins are rotating slowest. Conversely, at the top and bottom of the stroke where the thighs reached their extremes of travel and have stopped rotating, the shins are rotating their fastest. This near-constant kinetic energy of this system allows for extremely high pedaling cadences. Cadences of 300rpm have been recorded, five revolutions per sec., by Reg Harris, the famous British track cyclist. Harris had an amazingly muscular lower body and, as a result, a lot of kinetic energy stored in this spinning legs.

The second effect is that the pedal-crank mechanism allows for power production to be pulsatile as opposed to continuous. Each leg produces power over approximately 25% of the stroke (this assumes the rider is only pushing down on the pedals and not pulling up, but even so, these two motions utilize different muscle groups.) I have mentioned in previous posts that power production is more efficient with a work-rest cycle instead of a constant work cycle. The is probably due to the fact that the rest period allows for the aerobic energy sources in the muscle to be replaced easier than if the muscle has no rest period. (More on the biomechanics and bioenergetics of human power production in an upcoming post…)

So the crank-pedal method of bicycle propulsion has endured not only because of its simplicity but also because of its efficiency.


1 comment:

  1. Thanks for sharing some historical facts about bike. Hope it will help to understand the mechanism and to get more info.