Creating a pedal vehicle

Creating a pedal vehicle
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Multi pedal vehicles
This thread discussed the reasoning behind the choice of a multi pedal vehicle.

The following thread explores the construction of such a device.

An important physical reality to be kept in mind is that the locus of a circle is the equal movement of two points oscillating along perpendicular axes, for fixed and equal distances.

Applied to pedal movement, this means that in order to emulate the motion of a wheel, two identical linear actuators need to be fixed perpendicular to each other, and to move in perfect tandem.

However, in the real world, once the choice of legs is made, there is no longer any reason to actually emulate a wheel, that is, to describe a full circle. Instead, an elongated oblong will do the needful, to provide a horizontal platform the ability to move level with a horizontal surface.

This means that the horizontal actuator needs to describe the full cycle, while the vertical actuator needs only to move the minimum distance that will ensure that the horizontal platform (the purpose for which this device is being described, and for which the prototype will be built) will not collide with the maximum unevenness of the ground over which the vehicle will move.

Modern sedan cars are designed for a chassis clearance of 17cm, while consumer off-road vehicles are designed for 20-25 cm. This is not a rule, but is certainly useful as a guide to designing a practical vehicle.

There is a second practical phenomenon that comes in useful for designing such a vehicle. This is the delivery of a comfort factor, for a passenger in such a vehicle. This comfort factor, the equivalent of the rolling movement of a wheel, is ensured by combining the action of 3 to 4 legs moving equal arcs that, in combination, add up to 360°. With 3 legs, the motion of each one is 120°, and with 4, each one is 90°.

In the earlier discussion, the minimum number of such assemblies needed to create useful vehicles is 4, in order not to have to take extraordinary care in locating the load that will be borne by the vehicle, whether it is just a single person, several persons, or one or more persons carrying baggage. Naturally, this is a minimum, as more such assemblies will continue to add stability and to decrease the energy load of each assembly, and the individual components of each assembly.

However, I’m suggesting here that the first prototype should have just 4 assemblies, to minimise the probable cost of the devices. To some extent, in fact, using many multiple assemblies will lower the cost of each one, but this will probably not happen to any productive extent, until the design is transformed into production of standardised units.

The next element of design for consideration is the prime mover.

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Prime movers: there are two choices for creating a prototype vehicle, human and aided. Aided movers either supplement or supplant the energy that a human being can generate alone.

The simplest device is therefore a human powered vehicle, in which no external energy is used, so that there is no need to provide for external fuel and external energy conversion devices.

The simplest human energy convertors take the ability to move the hands and feet and convert these motions into the motions needed for the vehicle to move.

As described in the previous post, the motion needed is linear, but in two perpendicular directions. Which is actually really easy to do, with pivoted levers. If the pivots are made movable, the throw and energy exerted becomes variable, which is the equivalent of a geared drive to an axle.

Steering is also quite simple. It is only necessary to vary the relative speeds of the ‘inner’ and ‘outer’ sets of assemblies in order to turn the platform, which is also done by moving the pivots. Depending on where the driver’s seat is located, it may be necessary to vary the fore and aft speeds at the same time.

The simplest such vehicle will only have two sets of assemblies, but the rider will then have to balance in motion. Since there is little or no gyroscopic effect in the absence of wheels, it may have to be added. The advantage of adding a gyroscope is that it also acts as an energy storage device, like a capacitor, so that the driver/rider can rest while the vehicle is in motion. It may also help by providing some relief while ascending or descending slopes, in fact, this is quite likely. It might therefore be useful in the prototype, whether it is built with 2 or 4 assemblies.

As was earlier defined, the assemblies have been christened, in the course of our discussions at the Maker Labs, as rewheels, while the vehicle is called a chalopede. The variants described here are the multi pedal equivalents of the familiar cart and bicycle.

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This is my attempt at drawing the basic mechanism of the vehicle, using helical springs wound on metal rods (pistons)

Each set of 4 piston assemblies represents one rewheel. They are offset from each other by 90°, so one is fully extended, in contact with the ground, and one fully retracted, the other two in between. The other 3 rewheels are in the same state. At any given time, with this arrangement, 4 pistons will always be in contact with the ground.

As can be seen in the drawing, the individual pistons are also offset horizontally, also 90° from each other in sequence. Thus, the piston in contact with the ground is shown at one end of the horizontal slot in the platform (chassis), and is about to be retracted and returned to the other end of its slot. The other 3 are in varying positions, again, offset in a sequence of 90° of separation from each other.

To achieve this is, as remarked in an earlier post, fairly easy, using foot or hand pedals. In addition, the mechanism should drive a wheel, fairly heavy and, if only two rewheels are used instead of 4, of reasonably large diameter, to act as a gyroscopic stabiliser and, dual purpose, a flywheel/capacitor. If 4 rewheels are used, only a small heavy wheel will be necessary, to act as a flywheel/capacitor.

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Since, in this simplified arrangement, 4 pistons are in contact with the ground, the load bearing capacity of each spring needs to be about 25kg, in order to handle a total loaded weight of 100 kg in this first prototype. If the rider is taller and heavier, the spring capacity may need to be around 30kg.

This spring calculator informs that such springs can be made of standard spring metal, ASTM 227, with around 30 coils of 9 gauge, to handle a throw of 25 cm. This is not precise, and there is a simple reason, that the prototype is intended to be built from recycled shock absorber parts acquired in the local spare/scrap market in Kurla. This has not yet been explored in detail, but obviously, using some scrapped shock from the market saves the trouble and expense of engineering a theoretically ideal device. Standard shocks also have handy readymade casings that should make it easier to fix the other parts of the drive mechanism.

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Having said which, in an arrangement that draws the spring upwards with human power, and releases it down to touch the ground using the energy stored in the helical coil, there is no particular reason to have only one piston in each set of four as the active one. It should be fairly simple to rig the levers to keep 2 or even 3 in constant contact with the ground.

Just thinking aloud, this reduces the strength needed to compress the unused springs to a third, while tripling the load bearing capacity of the rewheel in motion.

That means each shocker need have an effective load capacity from 8 to 10kg only, making them far lighter.

And, hopefully, cheaper.

And, another bonus, the ride will surely be even smoother.

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This animated clip describes graphically how a full circle is created from pairs of perpendicularly moving points. In a real world pedal vehicle, of course, one saves an enormous amount of energy by limiting the locus to a reasonably flattened oval.

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