Spring bank holiday 2008 - A few small wheels from an old pushchair, one larger wheel and some scrap wood - it was time to build a downhill kart. Two small wheels at the front, steered by foot, and a third lager wheel immediately behind the driver gave us a neat looking tricycle configuration. A pivoting shaft in the centre of the vehicle acted as a brake by rubbing along the ground when deployed by the driver.
The steering mechanism is a traditional type for downhill go-karts, where the whole steering control shaft forms a rigid system which is rotated about its centre - the so-called bogie-steering-system.
The main requirement for a good steering system is to ensure geometrically precise rolling of the wheels (with no slip) when travelling in bends. In this respect, the bogey system is geometrically satisfactory for three and four-wheeled vehicles. The drawback of this system is that the stability of the vehicle decreases as the front axle and wheels swing further round.
There was a further problem with the design, this time concerning the brake. As the brake is operated, it tends to lift the vehicle and further destabilize it - exactly when the driver requires enhanced stability. The kart went through a few modifications to improve its handling.
Mk.2 - stabilizers added to the rear - like on a child's bike. These did not touch the ground, but supported the vehicle if it started to tilt.
Mk.3 - large wheel removed to produce 4-wheel vehicle. This felt much more secure but stability was still an issue.
It was decided to sacrifice a little performance to improve the stability situation. Therefore, the Mk.4 version used toe-in for the front wheels. An exaggerated illustration of this is shown in the adjacent diagram. This really improved stability.
Mk.5 - (basically the Mk 2 with toe-in). Tested on 4th April 2009, it was the fastest configuration so far - just within the limit of controllability - nice.
Mk.6 - Improved driver-comfort by incorporating a rounded edge to the seat-back - such luxury.
As with all vehicles power provides performance. Gravity-powered vehicles have the potential for sparkling acceleration, with a theoretical minimum 0 to 60 mph time of 2.7 seconds
This of course requires the absence of rolling resistance, no aerodynamic drag and a perfectly vertical track - orientated directly towards the earth's core.
Instead we used the hill over Elmdon Heath - a wonderful, gently sweeping left - right course that has proved to be lots of fun for all the family.
A very basic CAD model of the kart (Mks 1 to 6) is available via the link below:
Rhino software is available free from McNeel. There is no time limit on the software, but the user is limited to 25 saves.
