Let's talk about secs

2y ago


I don’t know who were the first people to quote a 0-60mph or 0-100kmh time for a car, but it must have been a long time ago. These figures have been around for as long as I’ve been reading about cars, and that’s almost half the lifetime of the car as a credible idea. Anyway; I wish they hadn’t.

As beardy archaeologists say when they unearth half a Viking pot or whatever, ‘What does it mean?’ Not as much as you might think, actually, and without wishing to get too bogged down in amateur physics, here’s why.

Acceleration, as a real physicist will tell you if you’re unlucky enough to be cornered by one, is defined as ‘the rate of change of velocity’. What this means in normal English is how quickly something goes more quickly.

Let’s take the example of the most universally understood and experienced accelerative force on the planet, which is gravity. In round figures, gravity will accelerate a falling thing at the rate of 10 metres per second per second; that is, if you come over all Galileo and drop a metal ball from a tall building, it will be going 10 metres per second faster for every second that passes. (This sort of thing, by the way, ignores all the annoying stuff that stops physics working properly, such as air resistance. Also known as reality. But let’s not worry about it.)

'What this means in normal english is how quickly something goes more quickly.'

Sir Ernest Gowers

Here’s the first problem with 0-60mph times: ‘4.8 seconds’ or ‘8.6 seconds’ are not measures of acceleration, they are simply measures of time. I know this sounds a bit pedantic, but to express acceleration properly we need speed and time, to show how quickly the speed is increasing. Rate of change of velocity, see?

The second problem is that cars don’t accelerate like a Renaissance boffin falling off a wonky tower in Pisa. Gravity makes things accelerate at a constant rate. Cars rarely do that.

Throw away the stopwatch an trust your guts. They never lie.

Take two examples from my own motoring history. My current electric BMW i3 does 0-60mph in 7.2 seconds. The Ford Fiesta RS1800 I owned way back in the ‘90s did it in 8.3 seconds. So the BMW is faster. But it doesn’t actually accelerate as fast as the Fiesta did. Wait for it…

The BMW, being electric, has a very flat torque curve and accelerates in a very linear way. The crudely turbocharged Fiesta went off the line in the rather sluggish manner of any other hatchback, but when the turbo finally kicked in at about 4000rpm the view through the screen went all squiffy like it did on the Starship Enterprise at warp speed, and my eyeballs changed shape. Only briefly, mind.

The point is this. As car enthusiasts interested in cheap thrills, the rate of acceleration is what excites us. It’s what presses us into the seat and makes us snigger. The BMW arrives at 60mph before the Fiesta, but 60mph is merely a speed. The peak metres per second per second figure in the Ford was higher.

And that’s what car makers should be quoting.

Credits: Getty Images & Africa Studio / Shutterstock.com