S.S. Wilson

The Pedal Rover: Transport for the Third World

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S.S. Wilson
Engineering Laboratory,
Parks Road, Oxford.
22 March, 1974.

Excerpt from: Bicycle Reference Manual for Developing Countries. Edited by Barbara Gruehl Kipke, April 1991.

In many parts of the Third World the sandy conditions make the use of a conventional bicycle difficult. Particularly is this so if the soil is largely wind-blown sand having polished, spherical grains "loess", but there are other situations, eg. mud, where bicycles are difficult to use, yet the need for wheeled transport for people and goods remains.

Possible bicycle modifications might include the use of fatter tyres and a lower gear ratio - the normal bicycle has a 46-tooth chain wheel, an 18-tooth rear sprocket and a 26" diameter wheel with a tyre 1 3/8" radial width (approx. the same across the tread). This gives a gear ratio equivalent to a wheel of 66 1/2" diameter i.e. equivalent to a larger wheel than even the biggest penny farthing bicycles. Carrier bicycles have slightly fatter tyres, 1 3/4" and 'Chopper' rear wheels are 2 3/8" but only 20" diameter.

Test results for wheels of different diameter and width in soft soils show that in general the large diameter wheel is to be preferred to one of small diameter. Since a lower gear ratio than 66 1/2" is also needed for soft going and rough conditions a simple solution might be to use a direct-driven wheel of 45" diameter (the low gear with a normal Sturmy-Archer 3-speed hub is equivalent to about 50"). Such a wheel of diameter nearly twice the normal bicycle wheel should have an appreciably lower rolling resistance and with a rim of about 2" width should perform well in soft soil - a wider rim up to 3" could be used if necessary. The use of direct crank drive would eliminate the chain, a considerable advantage under sandy or primitive conditions. The bearings could be fully-sealed ball bearings; the nears should keep out sand or water and retain the grease lubricant.

Such a wheel poses certain problems of design if it is to have adequate lateral strength but avoid too great a hub length and consequent excessive distance apart of the pedals. A possible method of construction is shown on the attached drawing, PR1; the construction is essentially similar to that of the larger-diameter wheel for a wheelbarrow shown in drawing W1, i.e. concave steel rim, wire spokes threaded both ends bent to a U-shape and fitted with standard bicycle spoke nipples at the outer ends with the inner bends fitted into semicircular grooves in the hub flanges. However, to give adequate lateral strength with a short hub a sheet metal collar is proposed at a radius midway between hub and rim, alternate spokes running over the edge of this collar and then to opposite hub flanges. This should give adequate lateral triangulation of the spokes; the collar might well be termed a 'bracegirdle'. The axle is 17 mm. in diameter and is locked to the hub flanges by standard cotter pins; standard bicycle cranks and pedals are fitted outboard of the ball bearings, the shaft being reduced to 5/8" diameter.

The ball bearings are clamped in the fork ends and double tubular members form braced forks fixed either to a steering head (for front wheels) or to a similar clamped tube for rear wheels. The complete assembly may be regarded as a standard driving unit which can be incorporated in a variety of vehicles to suit particular purposes. For example, a single unit could be used to form the front wheel of a penny-farthing - less extreme than the original machines, and of smaller diameter, with the rear wheel being perhaps rather larger, so the whole would be more like the even older Michaux 'Velocipede'. It is worth noting that the first man to cycle round the world did so on a penny-farthing typo of machine, so it is clear that such machines could cope with rough going.

A tandem version might be made from two units, though it is not altogether clear what procedure would be best for mounting and dismounting - possibly the rear rider could hold the machine steady while the front man mounts, then jump on behind as the machine starts; similarly the rear rider might dismount first and hold the machine while the front man dismounts.

More practical perhaps, and more suitable for the carriage of goods, could be a tricycle version, with central steerable wheel in the front, triangular chassis behind forming a load platform and two driving units at the rear. Being inherently stable, no problem of dismounting and mounting arises, while for cornering a differential action at the back wheels is obviously possible. A two-man version of the tricycle is possible by fitting the front wheel - of any suitable diameter - with a steering mechanism for use by one or both rear riders; a simple tiller with curved cross member might be the simplest arrangement.

An advantage of the tricycle is its avoidance of racking strain on the chassis even on rough ground, but its disadvantage is that on rough roads with two wheel-tracks the centre wheel would be working in the central, unused part of the track. For such conditions a four wheel version might be preferred, in which case both front wheels must steer and be linked together by a track rod. This could easily be done by using that part of the triangulated bracing on the front forks which protrudes in front of the steering head. By suitable choice of the point of attachment of the track rod the essential geometry for Ackerman steering may be incorporated, making the inner front wheel during a turn incline more than the outer.

A greatly-increased load platform of rectangular plan would be provided, and such a vehicle, having all the good traction features of four-wheel drive, could well be termed a 'Pedal Rover' and prove a viable alternative to the ubiquitous Land Rover or its deluxe version the Range Rover.

Even further additions may be desirable in particular circumstances and the best method of extending the vehicle might be the addition behind a 4-wheeler of 2-wheel units having a chassis and load platform similar to the tricycle but fitted with a universal swivel instead of a front wheel. The swivel would be attached to the mid-point of the rear edge of the load platform of the 4-wheeler, so the 2-wheeler would automatically follow in the wheel tracks of the leading vehicle. There is no limit to the length of train that could be built up in this manner, and the longer it is the less should be the resistance to motion compared to the driving effort available, since much of the power loss is due to the initial formation of the wheel tracks. Also, since the various pedals would normally be out of phase with each other, the total tractive effort should be smoother than with only one or two riders; this is important in hill climbing or in overcoming any 'little local difficulty'. Of course, there will always be the problem of the slacker, but this may be offset by the sense of team effort, while there is no reason why in a sizeable team each rider in turn may not have a rest.

One general point to bear in mind is the exceptionally high ground clearance of these vehicles - all except the pedals will be at a height of 45" or more above the ground, so minor obstructions such as small bushes can be avoided and pools or streams could be crossed without getting wet - even deeper streams could be forded by dismounting and pushing the vehicle while the load remains dry.

There is, of course, a problem in stability against overturning due to the high centre of gravity; this can be combated by having an adequate width between wheels ('track'), at least 5 feet, possibly more, but a limit may be set by the normal width of trackways as well as of gateways and openings.

The wheel design requirements differ from those for the wheelbarrow wheel in two respects; tile necessity to transmit the driving torque from the hub to the rim and the need for the rim to achieve traction in the soil. The normal method of transmitting driving or braking torque with a spoked wheel is by the use of tangential spokes i.e. alternate spokes are sloped forward and backwards to meet the hub flanges tangentially. The proposed design may have sufficient frictional forces between the spokes and the semi-circular grooves in the hub flanges to transmit the necessary torque (normally the spokes are threaded through holes of small diameter, so little frictional torque can be developed). If this is not sufficient the design could be modified to permit some degree of overlapping i.e. triangulation, of the spokes.

To achieve traction at the rim some form of tyre may be needed. In the absence of any suitable pneumatic tyre with tread a solid rubber tyre might be contemplated, but a possible solution might be the use of a helical steel spring joined to form a complete circle of unstretched diameter of only 1/2 to 1/3 of the rim diameter. When stretched into place in the concave rim the coils would open out to from an effective ribbed tyre which should not easily be dislodged. However, there may be a tendency for the coil to slip round the rim, in which case means must be provided to prevent such movement, though a gradual movement, especially rotation about the axis of the helix, might be welcome in order to even-up wear of the spring. Two possible means of preventing circumferential slipping of the spring are to allow the outer ends of the spokes to protrude slightly from their nipples to engage the turns of the helix. This implies that the number of turns of the helix is an exact multiple of the number of spokes, in order that the torque is shared as evenly as possible between the spokes. Alternatively the rim could be provided with a number of extra holes to enable a lacing wire to be threaded in and out of the holes and turns of the spring. Further measures to prevent slip might be taken by adjusting the radius of curvature of the rim section relative to the spring, and its length of arc, in order to provide a wedging action between the lips of the rim. Such wedging could be further helped by forming dents of a controlled pattern in the rim edges. Which of these methods or combinations of methods would prove most effective would be a matter of experiment.

The potential of this type of locomotion for Third World uses is such that prototype wheels should be constructed and tested. This may be possible under the 'Pedal Power Project' supported by OXFAM at the Oxford University Engineering Laboratory, especially if it is further supported by S.R.C. money, but if any other method of proceeding is possible, in view of the existing work load on limited resources, such an alternative should be considered.

Missing picture: Blueprint of the Hub
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