AfriCart – a handcart for Africa

ED302 – design & technology

02.2 Research – Wheels

bicycle_wheel

When it came to choosing wheels for the africart three factors were considered: size, efficiency & availability.

Size & Efficiency – Rolling resistance:

There are a number of variables that determine rolling resistance: tire tread, width, diameter, tire construction, tube type (if applicable), and pressure are all important.

Smaller wheels, all else being equal, have higher rolling resistance (RR) than larger wheels. “Rolling resistance increases in near proportion as wheel diameter is decreased for a given constant inflation pressure. ”

The primary cause of rolling resistance is hysteresis:

A characteristic of a deformable material such that the energy of deformation is greater than the energy of recovery. The rubber compound in a tire exhibits hysteresis. As the tire rotates under the weight of the vehicle, it experiences repeated cycles of deformation and recovery, and it dissipates the hysteresis energy loss as heat. Hysteresis is the main cause of energy loss associated with rolling resistance and is attributed to the viscoelastic characteristics of the rubber.

Rolling resistance also is reduced with increasing tire pressure, although the practical benefits become small at pressures significantly above 120 psi for the average bicycle rider. Thinner bicycle tires are lighter and have less wind resistance than wider tires, however although not particularly relevant here, wider tires offer better traction, comfort and stability.

So from the above we can see that the most desirable wheel is tall with a narrow track (width), which confirms the early settlers’ choice of wheels for their covered wagons etc. The problem now is the availability of such tyres in rural SSA, obviously they cannot be produced on a bespoke basis as this would be prohibitively complex and expensive, they need to be “found” in the local environment.

Availability

One of the advantages of the Chinese influx of mass produced goods into SSA has been the availability of a fairly low cost transport solution known locally as a “Black Mamba” – a heavy duty road bike produced by the Flying Pigeon Company.

PA-02-flyingpigeonnyc.com

Since these bikes are endemic all over SSA their wheels, either from collision damaged or otherwise non-working bikes, can be recycled, thus providing the perfect wheel for the africart: tall, narrow, with industrial “heavy duty” spokes and rims; and with the added benefit of pneumatic tubes and rubber tyres.

Because these wheels use tensioned wire spokes they are a better balance between strength (load carrying capacity) and weight than solid wheels. They also benefit from a metal hub containing the axle and a sealed set of bearings that allow the wheel to run with the minimum of friction.

The most common size of bike wheels are 26 and 29 inch diameter and so the frame of the africart will be made to accomodate either.

Fixing the wheels

The problem with bike wheels is that the front and rear wheels both use different size axles. Due to the gear set the rear wheel uses a slighty longer (7″), slightly thicker (1omm) axle. Because my design depends on the axle being offset and extending “through” the outer boards, rear axles (spindles) were needed – this would allow the spindle to rest completely within the width of the wood plank.

chinese spindles

This offset also required spindles that were threaded for their entire length and not just each side of a centre unthreaded section. For this I need to source second hand spindles from a re-cycled bike shop, however Chinese bike spares, like Chinese bikes, are quite ubiquitous in Africa.

The inner side of the wheel spindle originally sat in the bear wooden box but the box needs protecting to stop it wearing out and so I used some copper boxing.

africart_copper_sleeve

The outer spindle passes through both the inner box wall and the handle side arm – it is then secured with the original bike nut.

africart-008

Written by Mr. Elphick

March 17, 2009 at 4:36 pm

%d bloggers like this: