Motorcycle aerodynamics - How do they affect a bike?

Welcome to our Home Schooling series. The idea is to present motorcycling technology in a simple form everyone can understand, even children. If you’re a British Superbike technician or a professional rider, stop reading and go make a brew. For the rest of us, pin your ears back, get a note pad and pen and read on. Unlike school, there won’t be any homework, there isn’t a test at the end and because you’re at home nobody knows if you’re picking your nose or eating your lunch early. Now concentrate on today’s lesson...

How aerodynamics affect a motorcycle

From the shape of a bike or what you wear to your size or the luggage you carry, it all influences the motorbike’s stability

Lesson 1 - How aerodynamics affects a motorcycle

Basics: Aerodynamics sounds complicated, but it isn’t, not at its most basic level. We can simplify aerodynamics to make it extremely easy to understand (even my 6-year-old son has worked it out).

In essence, we are looking to improve the air-flow around an object. Let’s look at an example: a London double-decker bus, which presents a flat, rectangular shape with a large surface area to the air. Assuming it could float, it would make for a terrible boat because it would be ineffective at cutting through water. You instinctively know that. A floating sports car, however, would be far more aerodynamic because it’s more streamlined (pointy), with a smaller surface area. Square bad, pointy good. If you dive into a pool you try to create a small surface area, the opposite of a belly-flop. It’s natural. 

On to the motorbike: In a car, it doesn’t matter how big or small you are, or what you’re wearing, you don’t affect the aerodynamics – but on a bike you do. It doesn’t matter if you ride a sports bike, a naked or an adventure bike, your physical presence and the shape you make in a sitting position affect the aerodynamics. 

Worst case scenario: You’re riding a naked bike at speed, let’s say 70mph. You are a large rider, wearing bulky kit because it’s winter. The wind is hitting the you, so you’re pulling on the bars, and putting pressure onto your lower back and into the seat. Over time this will cause discomfort. Increase the speed, and the rider holds on tighter, pulls on the bars harder and the pressure on the seat increase, which in extreme cases causes the rear suspension to squat. 

What does this do to the bike? Because the rider is holding on tightly to prevent himself being pushed back by the airflow, he is also extending the forks and raising the front end slightly. At the same time, wind pressure hitting the unprotected rider is causing the rear suspension to sit slightly. Raising the front and lowering the rear causes instability, which is why some nakeds feel awful at very high speeds, especially if you are still sitting upright. 

How do we resolve this? We need to think back to the double-decker bus; we need to be more aerodynamic and reduce the pull on the bars. Simply tucking in and reducing our frontal area can improve stability as we won’t be pulling on the bars as much, but that’s not practical on a normal ride. We can also improve the bike's aerodynamics by fitting extra bodywork: a fly screen or, on some bikes, simply replacing the standard screen for a larger item. 

Does it really make a difference? Yes, the difference in top speed between tucked in, chin-on-the-tank, and sitting bolt-upright is huge, between 15mph and 20mph, sometimes more depending on the bike and shape. Even a differently built riders can have a noticeable effect on top speed: a small rider who can get tucked in is usually a few mph faster than a taller, larger rider. 

More drag, more power: As we increase drag, we need to increase power to maintain the same speed. In racing, larger riders will need more power to maintain and achieve the same speed, and again this depends on the power of the bike. In the small-capacity Moto3 class the size and shape of the rider dramatically affects top speed, and getting in the slipstream (tucking behind a rider in front to reduce drag further) is hugely important. 

So how does this affect me on the road? If we need to increase power when we create more drag to maintain the same speed, we must, therefore, use more fuel. If we ride a bike solo, in relatively tight leathers and stay tucked in, we will need to use less power, therefore we will use less fuel, and increase our mpg and tank range. If we now add bulky kit, a rucksack, or even panniers, we are increasing our surface area, we are causing more drag, and using more fuel, which means our tank range has reduced. In extreme examples, we can reduce our fuel consumption by 10mpg or our range by 30-40 miles, which is significant, especially if we are embarking on a long touring holiday.  Now before I get a million emails, we all know when a bike is fully kitted with panniers and pillion, it will use more fuel, and this isn’t just because of the larger surface area, it’s also down to weight. But drag does play a huge part.

Interestingly electric bike designers are looking at aerodynamics evermore closely because, while an electric bike’s weight is always the same (fuel levels don’t go up or down), the easiest way to improve range without advancing battery technology is to make the bike more aerodynamic. When I raced in the TT Zero race at the TT, we didn’t have any more power, so to improve range we made the bike as aerodynamic as possible – changing the shape and the size of the screen made a significant difference. 

Let’s talk luggage: So, we know an increase in the surface area or decreasing the aerodynamics of our bike can decrease our mpg, which reduces our tank range. We must also increase power to maintain the same speed, and in some situations this can reduce stability as the rider is pulling on the bars, and pushing down on the rear. We need to remember this when adding luggage or accessories to our bikes. Carrying a large rucksack which protrudes higher than the rider causes huge drag, will increase the fuel consumption and discomfort. The wind pressure on the bag will make the rucksack ‘feel’ heavier.

Instead of going for one large bag, distribute your luggage with a tank bag, tail pack and a small rucksack. And, finally, when you’re adding accessories, think about the wind pressure. If you’re fitting wide panniers, loaded with camping gear on top, are those extra straps going to be strong enough? If you’re doing 70mph into a 30mph heading wind that’s 100mph of pressure. When you’re driving your car, put your hand out of the window near the mirror at 50mph. The pressure is strong. Now try to imagine the wind pressure on your pannier at 100mph, which has a much larger surface area. It makes you think, doesn’t it?