Channel 4 – Scrapheap Challenge 2004 – Challenges – Sand Racers

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Science

The designs

General science

The sand racers

Pros and cons

The Anoraks' six-wheeler

The Hoselliers' Baja-buggy

The designs

The Anoraks

The Anoraks opted for a wacky six-wheel-drive affair to get around the no four-wheel-drive rule. This had potential as it would have huge traction and scraptacular sand-racing ability – but it was going to be an incredibly intricate assembly.

The Hoselliers

The Hoselliers opted for the classic dune-bashing Baja-buggy style design. It was a rear-wheel drive, rear-engined machine with huge suspension travel and a light-weight rail style box-frame chassis. It was a more simple build but had less traction and more potential for sandy setbacks.

Four-wheel drive

Ground clearance is essential

General science

These speedy sand racers would need to traverse a treacherous terrain – the Scrapheap sand pit. To do so, they would need special abilities to successfully complete the circuit.

Diffs
The differential has three jobs:
- To aim the engine power at the wheels
- To act as the final gear reduction in the vehicle, slowing the rotational speed of the transmission one final time before it hits the wheels
- To transmit the power to the wheels while allowing them to rotate at different (hence differential) speeds – this is essential for turning!

Four-wheel drive
The benefits of four-wheel drive are obvious. If you are driving with four wheels rather than two, you have the potential to double the amount of longitudinal force applied to the ground. For the purpose of this show (and to make things more interesting!) four-wheel drive was banned.

The Anoraks cunningly subverted this ruling by using six-wheel drive.

Ground clearance
Ground clearance is very important for obvious reasons. The higher a vehicle is off the ground, the less resistance and drag the bodywork produces against the mud. It lowers the chances of a vehicle becoming grounded on peaks in the terrain and it allows for more suspension travel.

Tyres
Tyres are a very important consideration on sand. If you are using normal road tyres, it's best to use 75% worn tyres that are deflated – the idea is to obtain the largest possible 'footprint'. A normally inflated tyre has a smaller footprint than a half-inflated one by some 20%. If you let even more air out, you can almost double the footprint and thus half the ground pressure of your vehicle.

Momentum
Maintaining momentum is vital for travelling over sand – the moment you stop you are stuck and will find it very difficult to get going again.

Torque
Torque is the twisting force that the engine produces. The torque from the engine is what moves your car. The various gears in the transmission and differential multiply the torque and split it up between the wheels. More torque can be sent to the wheels in first gear than in fifth gear because first gear has a larger gear ratio by which to multiply the torque. The interesting thing about torque is that in low-traction situations, such as sand driving, it's the amount of traction you have that dictates how much torque you can have, not your engine. A huge gut-snorter of an engine is pointless unless you have plenty of traction.

Six-wheel drive

The sand racers

Six on Wheels

The Anoraks opted for a monstrous three-axle, six-wheel-drive dune thrasher. One large engine powered it all, which created a tricky situation of how to get power to all three axles. These axles had to be identical or the gearing could have been wrong and the engine power would have torn it apart.

It was steered by a braking system which was specific to one side of the machine, much like on tracked vehicles. It didn't have any suspension but instead had large tyres with quite a low pressure. This increased surface area and thus traction but provided quite a rough ride for the driver.

Olivia

The Hoselliers used a front-wheel-drive automatic car as a donor. They built a lightweight, rear-wheel drive, speedy sand-rail. Its low weight was an advantage but also limited their power considerably. They used coil springs in a double wishbone formation to maximise suspension travel. This increased the chances of all four wheels being on the ground at any one time; vital for traversing rough terrain. Its downside was the fragility of such a system.

Overstressed axles

The Anoraks faced a tough challenge in creating a six-wheeler. Wheels turn in relation to the engine revolutions at a rate that is generally 2:1 but is specific to each type of car. The Anoraks needed to make sure their axles were similar or it could have overstressed their transfer boxes and led to huge inefficiency (ie wheels turning at different speeds) or the destruction of their deviously difficult transfer system.

Baja-style buggy

The Hoselliers took the front-mounted engine from a Honda, removed the steering and put it and its auto box to the rear of their scrappy creation. They intended to add a homemade box-frame chassis to create a classic Baja-style dune buggy.

Double-wishbone suspension

The Hoselliers made a complicated suspension system called a double wishbone – a modern system used on many cars. They consist of two wishbones which hold the wheel hub and allow it to move up and down and a diagonal segment which is attached to the shock absorbers. The system isolates the bumps on the terrain the vehicle is passing over and keeps the wheel in touch with the ground over a bumpy surface. The downside of these is that if the Hoselliers didn't make them strong enough, they ran the risk of having sand kicked in their faces.

Transfer boxes

A transfer box transfers the movement of a shaft in a different direction. To create their six-wheel drive, The Anoraks used a normal four-wheel-drive transfer box to drive two axles but used an additional two cases to shift the power back around the engine to drive the front axle, a brain-boiler for even the best bodgers.

Light and agile

Pros and cons

The Hoselliers
Pros:

Light, agile and manoeuvrable with good suspension setup
Relatively easy to build

Cons:

Lacks power
Two-wheel drive lacks traction

The Anoraks
Pros:

six-wheel drive means huge traction
Skid steer allows tiny turning circle

Cons:

Fiendishly difficult to build
Three transfer boxes could prove unreliable