Channel 4 – Scrapheap Challenge 2004 – Challenges – Off Road Rescue

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Science

General science

The designs

The Puddings' winch with a 4:1 gear ratio, effectively quadrupling the power of their motorcycle engine

The Riders use two long rams, taking advantage of their tractor's purpose-built hydraulic pump

General science

Getting through mud, like any loose surface, is dependent on momentum. If you keep moving, you won't get stuck. A low-range gearbox is good because it allows the driver a lot more control over the vehicle, so you can keep moving without having to go too quickly. Tyres are vital, because they're your only contact with the ground. Mud tyres have an aggressive pattern with a deep tread, designed to cut through the mud to find something solid to grip on underneath – like paddles on a paddle steamer.

Three forces worked against our teams as they rescued (or extracted) the vehicle:

the weight of the vehicle
the weight of the mud
the vacuum caused by the watery slop

The teams couldn't work against these forces by using a clever design (suspending the vehicle before pulling it out, for example), so they had to use winches. Systems of gears increase the number of revolutions the handle or motor has to turn, to wrap a length of cable around the winch drum. Therefore, each turn of the handle requires less work for that revolution. The teams also used hydraulics – and we all know how much fun they can be! Centre of gravity was critical in this case, as it changed as soon as the teams picked up and rescued the vehicle.

The Bakewells plan to use a winch to pull the car onto a pivoting flatbed

The Riders want to use hydraulic rams to power a large lifting frame, similar to a skiploader

The designs

Bakewell Puddings

The Bakewells' design was as innovative as it was ambitious. They wanted to convert a dumper truck into a flat-bed truck, so it would basically be a car-carrying lorry – in reverse! They crafted a winch out of scrap and an old motorcycle engine to drag it up onto the flatbed.

The Pink Pudding winch relied on a 4:1 gearing ratio. A small cog ran from the motorbike engine to a cog that was four times larger. Every time the small cog turned four times, the big cog turned once. This reduced the speed of the output but gave the winch a much needed boost of power.

The Puddings' machine was a simple design although it lacked suspension, meaning the team would have a bumpy ride. Also, having a car in front of them gave them a couple of issues with visibility on the return journey.

Rough Riders

The Rough Riders opted for a crazy chimera that was part tractor, part skip loader and part trailer. The front end was a tractor with the front axle hacked off. There was a trailer behind with two hydraulic arms to lift the vehicle into place. They were an advantage as a tractor is pretty much a ready-built off-roader, and it was well-balanced once the recovered car was on the trailer. But it was a big old build and the pivot had to be strong.

The Rough Riders' pivot was an ingenious bit of bodging. To cope with humps and bumps, it pivoted horizontally – as one wheel went up, the other was free to go down because the pole hitch was sleeved in a tube and therefore moved independently of the tractor front. As part of the steering there was also a vertical pivot in the form of a pin. This allowed the back trailer to move left and right, although Barry's ingenious welding needed to be magnificent to prevent a terminal sheer.

The Riders powered their hydraulics from the tractor engine and they used the high-pressure oil system to lift their car out of the swamp. The Rough Riders tractor also steeredvia a hydraulic ram placed between the trailer and the tractor. With so much riding on the machine's hydraulics, they had to pray for no burst hoses or leaky pipes!