Recalibrating the power to weight context for Formula E

When they arrive next year like a field full of Rampant Rabbits on wheels, Formula E racing cars will bring with them a whole new language and set of buzz words to wrap our heads around. Some of the terminology might be dimly remembered from the days of school science lessons; much of it is probably largely forgotten. Even for those of us who can vaguely recall joules, watts, current and voltage, knowing how these translate into the world of racing is another matter altogether.

A set of comparisons with modern motorsports will give ourselves half a chance of understanding what to expect. It’s what the pioneers of the car did, after all, giving us “horsepower”. Any idea how much work a horse can do over a given time or distance? Me neither. But we do know that 750bhp in a lightweight sports car means pants-wetting performance. So that’s where we’ll start.

A preliminary note: electric cars don’t use bhp. They measure power in kilowatts (kW), so we’d better get used to it. As a rough rule of thumb, 1kW is equal to 1.34hp (which is the potential capacity of an internal combustion engine, rather than the more familiar bhp, which is output at the wheels).  

Just like internal combustion engine cars, electric cars come on a sliding scale, from the typical family car through to lightweight sports cars and on to supercars and – at the top of pile – the Formula E single seat speed machine. Just as with ICE cars, the bigger the power to weight ratio, the bigger the thrills.

Three examples

First up: the Nissan Leaf. Think of it as a Focus, a Mondeo, a Vectra, an Astra: middle of the road, to the shops and back, front-wheel drive. The Leaf is rated at 80kW (about 110hp) and weighs 1483kg. Its power to weight ratio is therefore 54kW/tonne.  

Next: the Tesla Roadster. Effectively an electric Lotus Elise after a few too many pies, it develops 215kW against a weight of 1235kg, laid down on the road via the rear wheels. That’s a power to weight ratio of 174kW/tonne. Which sounds better, if slightly anaemic.

Lastly: the AMG SLS Electric Drive, an all-wheel drive monster of a machine that I very badly want in my life. 552kW and 2110kg gives it a power to weight ratio of 262kW/tonne. Break out the confetti.

What does this all actually mean in pub science? When Autocar magazine tested the Leaf, they clocked its acceleration at 10.8s for the 0-62mph sprint. Tesla claims that the Roadster manages that in 3.9s, as does AMG for the fire-breathing volt-spitting SLS.  

On Top Gear’s test track, the Tesla posted a time (albeit on a slightly damp track) of 1min 27s; despite a whopping weight difference, the AMG demolished that by six seconds.

Formula E racer: fastest by far

So, to recalibrate our expectations, something in the region of 250kW/tonne is going to be pretty special to watch wheel to wheel. Enter the Formula E racing car, officially launched next month at the Frankfurt motor show. We’re expecting 200kW max power, delivered through the rear wheels. The car is likely to weigh around 700kg (requirements state that car plus driver must meet a minimum threshold of 800kg; Formula 1 cars usually have to use ballast to meet weight requirements and we can expect similar from their Formula E cousins).

That works out at something like 285kW/tonne. While that’s some way short of Formula 1 power to weight ratios – around a third, in fact – using our (albeit pretty short) sliding scale of road cars above, we can expect the racing car to be pretty quick then. Other factors will come into play – full torque delivery from zero revs, for example; no fuel to burn off; different tyres – but ultimately, these open wheeled machines are going to be no milk floats. Well done Formula E. Bring on the action.

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