Sometimes you get a phone call and someone asks you for help: they get you on a good day, at a positive moment and you think “yeah, why not? That sounds like fun”. Later, you realise you’re committed to helping to break a world record — the fastest speed for a gravity powered sled — and people are expecting you to deliver. Worse, you’ve committed your colleagues too and they don’t even know yet. And that they’re going to be on Channel 4. Then there’s the star of the show, Guy Martin: great sideburns, a love of speed and a real engineer.
The first thing the TV production company North One asked was whether we could design a sled to beat the current world record of 100.18 kph set by Rolf Allerdissen in 2010 on the Pitszal Glacier in Austria. The record requires a sled that you lie on, with skis, that goes down a snow slope with only gravity to propel it. As in the bobsled, the key is to maximise the mass** and minimise the forces slowing you down — aerodynamic drag and ice friction.
Stage 1. Modelling
The word “modelling” is the scientific euphemism for “guess”. The first guess using a Newtonian model (Figure 1) simulated a 100 kg Guy/sled combination on Allerdissen’s slope and showed that a speed in excess of 100 kph was possible. Playing with the model showed that the initial parameters to reduce were the frontal area and the drag coefficient (how efficiently air flows over Guy and the sled). Once the sled’s design was right, the key would be to get a long, steep slope with cold icy conditions.
Stage 2. Making a prototype
The first prototype sled built by Terry Senior and Nick Hamilton consisted, more-or-less, of a big block of metal with foam padding attached to some skis. One important design feature was a hand operated lever which dug into the snow to act as a brake: stopping, we assumed, was as important as going. Initial testing on a 12° indoor ski slope in Castelford under the capable instruction of Amy Williams (UK Olympic skeleton gold medallist in 2010) gave us a speed of around 54 kph (34 mph). This was really promising given that the slope was only around 75 m long. All we had to do was double it!
Stage 3. Making the sled
Computational Fluid Dynamics (CFD) was used to look at the aerodynamics of the prototype sled and come up with a new design. This involved scanning Guy with a 3D laser scanner (wearing a less than flattering lycra suit) and creating a full computer model of him with the sled (Figure 2). The images clearly show the vortices tripping from the front of the skis, off Guy’s shoulders and from his body on the prototype. A curved canopy shielding the front of Guy improves the airflow not only across his body but also at the front of the skis. The efficiency of fluid flow is characterised by the drag coefficient ‘Cd’ which was reduced by around 50% (from 0.58 to 0.30). The canopy actually increased the frontal area hitting the oncoming air so the overall effect of the new design was to reduce aerodynamic drag force by about 35%.
Stage 4. Testing: trial and failure
The canopy body was manufactured by EPM Technology, one of UK’s most advanced manufacturers in carbon fibre bodies for elite sport, with Andy Butt at the helm. The sled was assembled at the Centre for Sports Engineering Research — where a parachute was attached to slow it down. It was then shipped out in the back of Guy’s van to Andorra. The destination was the Avet speed-skiing slope in Grandvalira, at an elevation of about 2,300 m. It has the shape of a brachistochrone i.e. steep at the start (about 30°) and flat at at the end (for stopping), just perfect for a world record attempt.
The first run started about 250 m up the slope (about half way) where it was about 15°. The first trial run went without a hitch but Guy went under the official timing gates and no speed was recorded. A second trial about 50 m further up was going really well, the line was straight, Guy applied the brakes gently at first but then quickly ran out of slope. He pull hard on the brake lever and then everything went into slow motion — the sled wobbled, jack-knifed sideways and immediately rolled, flinging Guy off like a rag doll. Then everything was stationary, the broken sled forlornly on its head, Guy standing a little dazed.
Stage 5. Going for the record
Good engineering is as much about practicalities as it is about good science. Terry and Nick had engineered the sled so that in the event of a crash the thing likely to take the brunt – the carbon fibre body – could be replaced. The fatally cracked body was replaced and next day we were back at square one doing a couple of easy trial runs to hone Guy’s skills under Amy’s tutelage. The parachute was tested and opened well from the 300 m mark. The record attempt was to be from around 375 m where the slope reached 22°. The start took inordinately long as ski marshals tentatively pulled a slippy sled up a very slippery slope. Getting the sled into starting position wasn’t something we’d really considered.
The minutes ticked by and then the marshals stepped back and Guy was away with a slight wobble, accelerating rapidly to the timing point. The chute opened, the brake went on and Guy didn’t crash. Behind us, the timing board showed 134.368 kph or 83.49 mph – a new official Guinness World Record by over 34 kph.
We tried another run, adding weights and waxing the skis but by this time the hot sun was directly on the slope and the snow was starting to melt: we achieved 133 kph. We toyed with the idea of going higher but the marshals were against it on safety grounds. Secretly I think we were happy being only too aware of the earlier crash. Guy, I think, might’ve gone from the top and the computer model predicts he might’ve got close to 200 kph.
Next time, eh?
** Those of you who know your physics might puzzle why maximising mass makes you go faster down a slope when all bodies accelerate the same in the earth’s gravitational field. The key is that there is also drag and friction slowing you down. The net force down the slope is dependent upon the ratio of the propulsive force (gravity) to the retarding forces (aerodynamics and ice friction). Increasing the mass increases the propulsive force but not the aerodynamic drag which is independent of mass and hence the sled goes faster.
Speed with Guy Martin can be watched in the on Channel 4 via 4OD.