Life is one big drag

by Simon Hargreaves

The high (and low) pressure world of aerodynamics

A road cone is more aerodynamic blunt end forwards (if you fill the hole in first). Same goes for a bullet. And most cars – if you geared them to suit you'd get better fuel economy driving in reverse.

Intuition says, aerodynamically, a sharp, pointy front end is more important than a sharp, pointy back end. For you Ferrari and Lamborghini owners, sadly, this is wrong. A falling raindrop points us, literally, in the right direction: pointy at the back. It's seven times more aerodynamic than a pointy wedge. If you cut the 'nose' off a raindrop, its drag increases by 6%. If you cut its tail off, it doubles.

To understand why, think about air flowing around an object – say, a motorbike. At speed, on a Hayabusa or a YBR125, almost all the engine's power goes into pushing through the air. As the air strikes the bike's nose, it slows down and pressure increases. Then, as it cascades away from the point of highest pressure, the air accelerates and pressure drops. It's why bikes use ram-air to duct pressurised air through the headstock into an airbox behind it – the faster you go, the higher the pressure, the better the charging, And also why fairing panels have holes in them, for cooling – the low pressure pulls air through radiators and away from the engine.

So far so good (although smooth edges are better than sharp angles; KTM's RC8 looks sleek but, technically, it's an aerodynamic disaster). The tricky bit is when the fairing stops and the rider, handlebars, footrests, seat, tail sections, rear wheel and luggage gubbins start. The airflow, accelerating over smooth bodywork, loses stability as it meets the lower speed, lower pressure air behind fairing screens, boots, indicators, gloves etc. The result is swirling, or turbulence. The faster air swirls, the lower its pressure and the greater the difference to high pressure. This differential is called pressure drag (there are others, but on a bike this is the biggie).

Pressure drag is a force generated by the change in momentum of air. It has a direction (opposite to the direction of travel), and quantity (the size of pressure differential). So it's not the shape or size of your fairing that matters as much as what the air does once it's passed over it – hence the road cone and raindrop.

The reason it matters is fuel consumption. You need less power (fuel) to maintain speed on a more aerodynamic bike. Tests on a Fireblade showed, at 80mph, a head-down riding position and tight clothing affects fuel consumption more than throttle position and/or gear position, increasing mpg by up to 30% (worth knowing if you're running out).

Unfortunately, while flaring the seat unit as wide as the rider's bum and filling in the space behind with a seat hump is easy, it's not always practical. And there's a rider to squeeze in too. So it's hard to see how manufacturers can improve bike aerodynamics without radically altering their shape. But maybe radical is what's needed: a bike shaped like a road cone. The wrong way round.