Power To Weight

If you’ve read my previous article on horsepower and torque, then you’ll understand why the whole top-trumps argument is completely irrelevant when actually choosing which motorcycle to buy. Those numbers tell you very, very little about what any given bike will actually be like to ride. If you’ve not read my article…well, you’re welcome to. But if anything I discuss doesn’t make sense, don’t say I didn’t warn you! : )

Now, as much as I criticise the unscientific nature of quoting peak numbers, power-to-weight ratios are victims of these same issues. Because when we talk about a vehicle’s power:weight ratio, the “power” segment of the equation is, you guessed it, peak power, that most pointless of measurements.

But…it’s all we’ve got. And while it’s still not as good as the ultimate performance measurement, the dyno chart, I’ll let it slide for now. Because if all you’ve got is the numbers sheet, it can be a very revealing metric.

Jealous of your friends’ litre sports bikes? Let’s just say your little bike might be faster than you think…

I started using p:w ratios to explain to people why motorcycles were awesome back when I first started riding. In principle, a 1-litre/1000cc Volkswagen Up! should be able to destroy a 0.125-litre/125cc Honda CG 125. I mean, the engine is eight times bigger, right?

Same size engine as a FireBlade! It must be fast!

Same size engine as a FireBlade! It must be fast!

Well, with 60 and 11 horsepower each, the odds certainly seem stacked in the car’s favour. And yet even a very small city car like the Up! weighs around 940kg, whereas the diminutive Honda weighs just 120kg. That’s far less weight that needs to be hauled up to speed. I’ll put it another way: which one would you rather push up a hill?

We’re going to have to dip into physics again here, I’m afraid. You may have come across the mathematic expression F = MxA, or force = mass x acceleration. If you can recall your algebra, that also means that acceleration = force / mass. The greater your force, or the lower your mass, the more rapidly an object will accelerate. The power:weight ratio of the Up! is 60/940 = 64 horsepower per tonne; the Honda… 11/120 = 92 hp/tonne.

To say it another way, in order to keep up with the tiny little learner bike, the £8,000 car would need an engine almost 50% more powerful (~87bhp). Kinda makes you feel better about your 125, doesn’t it? : )

Low power…low weight

Low power…low weight

The numbers only get better when you scale up. For example, a Porsche 911 Turbo costs £120,000 and makes 520bhp. That should be a very, very fast car. But it also comes in at 1.6 tonnes, which is a lot of weight to be carrying around. I considered my Bandit to be a heavy bike, with a moderate amount of power, never able to keep up with my brother’s VTR. It weighed 250kg and made around 90bhp by the time I was done with it.

The Porsche screams away with 520/1600 = 325bhp/tonne. My slow, lardy Bandit limped off the line with… 90/250 = 360bhp/tonne. And that thing cost just £6,000 new, less than what it would probably cost to insure me on the Porsche!

A lot heavier than your bike…

A lot heavier than your bike…

Now I’m not here to rail on cars – they have their uses; I even bought one earlier this year because I decided that -13C was no fun on a bike, even with a heated vest. The reason my car weighs 4 times as much as my Bandit did is because it has to carry twice as many wheels, a roof, several windows, heaters, air conditioning, dozens of air bags and four seats, so I’ll forgive it.

No, I’m here to suggest that p:w ratios are a great way to compare different motorcycles. 0-60 times are meaningless on bikes, because their light weight and short wheelbases mean that if you try to get to 60 in under 3 seconds you’ll either lift the front wheel or spin the rear. There’s no beating physics here, folks.

But once you’re already moving, weight makes a big difference. Slowing down, speeding up, changing direction – you’re always scrubbing off or trying to gain speed in one or more directions at once. You need to lose or gain momentum, and the less weight (or more power!) you have, the faster you can do this.

My Bandit had the same brakes a GSXR would have had just a few years ago, but those brakes suddenly had to deal with a good 60kg more than they were really designed for – hence they never worked all that well. The less momentum you have, the quicker you can stop, the easier it is for your front tyre to drag your bike around in a turn.

When I was shopping for my Bandit’s replacement, I never once worried about getting a bike with more power – the KTM Duke 690 was on my shortlist, and that tops out at 69bhp. But it also weighs a full 80kg less than the Bandit, giving it a power to weight ratio of around 400bhp/tonne. The Street Triple I ended up buying manages a very respectable 580bhp/tonne, which is why I can open the throttle and blow the doors off pretty much anything on the road.

So remind me – what was the price of that Porsche again? : )

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