Motorcycle Fuel Economy - Fuel For Thought

Drag
There's always a hair in the soup isn't there? While it's true that motorcycles outshine cars as far as rolling resistance and power to weight ratios are concerned, there's one area where cars hold a clear cut, and substantial advantage, and that's when drag, or more accurately aerodynamic drag rears its ugly head. Despite being much narrower than any car I can think of, including Formula one and Indy cars, motorcycles have coefficients of drag that are on par with your average cross town bus, and that is exactly what puts the bricks to us when it comes to fuel economy.

Lest you think I'm exaggerating the average cruiser, standard, or touring type motorcycle has a drag coefficient of .6 to .7, which is exactly the same as an intercity bus, and only slightly better than a heavy trucks rating of 0.8. On the other hand modern cars typically have drag numbers that hover right around 0.35. Essentially your average motorcycle has all the aerodynamic attributes of a barn door, and when it comes to fuel efficiency, especially at the speeds where most of us live, it's drag that does the dirty work.

How drag affects mileage
At low to moderate speeds, during stop and go driving for instance or while you're motoring around town the affect of drag is negligible, don't confuse this to mean non-existent, any serious bicycle racer can tell you it isn't, but as far as motor vehicles go, as long as we stay under about 50 mph it accounts for only about 5% of a vehicles total resistance to movement. Unfortunately once we reach highway speeds, roughly 60% percent of a vehicle's energy is required just to defeat air resistance, and that grows exponentially as speed, and with it air velocity, increases. If you've ever fought a stiff headwind you know exactly what I'm talking about. Riding into the wind you have to twist the throttle up like an alarm clock spring just to maintain headway. With the wind at your back, you can reach the same speed on half the throttle. Think of drag as a never ending headwind, and you won't be far off the mark.

For the sake of interest a motorcycle with 100 horsepower will see a 7 mph difference in top speed when the aerodynamic drag differs between 0.30 and 0.26. That's basically the difference between sitting bolt upright and laying down on the tank, and if nothing else explains why the legendary Rollie Free, rode his Vincent into the record books laying flat on the bike and wearing nothing but a bathing suit and shower slippers. In a nutshell that's also why small bikes get much better mileage than their larger siblings, they're lightweight and have very efficient engines so they perform really well at low speeds, and by the same token aren't powerful enough to reach the speeds where aerodynamic drag becomes a factor.

Why are bikes so inefficient? Unfortunately the problem of drag is fundamental to the motorcycles design, and by "the" I mean just about all motorcycles including the latest, slipperiest sport bikes, which aren't nearly so slippery as you might think, especially when their riders are wearing shorts and tee shirts instead of leathers. Consider for a moment your typical cruiser. The headlight is flat, the mirrors and turn signals stick out like sore thumbs, the rider's legs are splayed and he's sitting bolt upright. If you factor in things like saddlebags and a traditional (maybe not so well designed) windshield the picture is even worse, especially if the rider is wearing loose fitting or bulky riding gear. A motorcycle's wheels are especially problematic. Rotating wheels create an enormous amount of turbulence, especially if they're constructed with spokes, and have exposed brake rotors and calipers, as most motorcycles do. All of those things and more, act to create resistance and disrupt air flow across the bikes surface, so rather than flowing smoothly, the air is turbulent, particularly where it spills off the back of the bike and around the wheels. That creates a lot of drag and unfortunately, overcoming it wastes a lot of power, and fuel.

On the other hand cars have become increasingly more aerodynamically efficient over the years. As a point of interest this is one reason why they've all come to look alike, if you want low drag coefficient numbers there is only one optimum shape. With their aerodynamically efficient bodies, and fully enclosed wheels cars just work better at slicing through the air than motorcycles do, hence once they're up to speed the use proportionally less power, and fuel to stay there.

So what's the solution?
While the problem is evident, the solution is less so. There are mechanical solutions, low viscosity energy-saving oils are one answer, but those oils are incompatible with the wet clutches used in many motorcycles. Lens shaped tires as used on high performance bicycles are a potential fuel saver, but I'm pretty sure they'd create handling issues especially if they were built in the widths the custom guys like. The car guys already use an energy saving tire, but the jury is still out on whether or not they really work all that well, and again their handling characteristics might leave a lot to be desired if they were used on bikes.

Fully streamlining the bike would help the situation, at least to some degree but that also creates problems. Fully streamlined bikes are adversely affected by cross winds, and when the fairing is tilted during fast cornering, it turns into an airfoil and generates lift, which can and has caused the wheels to come clean off the ground with the predictable unpleasant ending. This is one reason why full "dust bin" style fairings that enclosed both wheels and rider were banned from Grand Prix bikes back in the 50's. And there are also serious styling concerns, I like the Victory Vision, but the styling is polarizing, and that's mild compared to the type of bodywork you'd need to seriously affect fuel mileage.

Bottom line? I don't see any radical styling changes taking place in the near future; at least not the kind that will substantially change the way a bike looks just to improve fuel mileage. Motorcycle riders tend to be a conservative lot, and if you're like me you want your bike to look like or at very least resemble what we consider a traditional motorcycle, and the manufactures have carved those sentiments in stone.

Likewise I don't for see any giant leaps in engine technology either, at least not for the sole purpose of reducing fuel consumption, most of the OEM's have gone down that road, anyone remember the Honda Orbit, and found it a lonely one. Besides I've never been convinced that the majority or even a significant minority of us are the least bit concerned with their motorcycles mileage. If they were the 250 market would be a lot larger.