Motorcycle Fuel Economy - Fuel For Thought

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Dear Motorcycle Cruiser Editor,
In "Best Buys" (Motorcycle Cruiser, February 2009) you state "a motorcycle is an inherently efficient vehicle..." If you compare the absolute miles per gallon of the average bike to that of the average car, I guess you're right. But when you consider rolling resistance, weight, and drag, how efficient are motorcycles, really? With my current ride, a Honda Shadow ACE 750, I can break 50 mpg if I'm lucky. In my commuting car, a Honda Fit, I'm averaging 37.4 mpg. Considering that small as it is, the car still weighs more than four times as much as the bike (with much greater frontal area) shouldn't the bike should blow it away? Granted, the Fit is a slowpoke off the line, but so is my Shadow ACE. I would expect my bike's fuel economy to be at least double that of my car. Please explain.
John Maher
Marshalls Creek, PA

An excellent question Maher, and one that demands a fuller explanation than we'd have room for in our regular Tech Q&A; section, so let me attempt to clear things up here.

Whether or not "motorcycles are inherently efficient vehicles," is a debatable, and complex issue. While it certainly merits discussion, this is neither the time nor place for it, so we'll let it pass for the moment and confine our conversation to the subject at hand, namely why aren't motorcycles more fuel efficient than they are?

Actually, when compared to the average car, motorcycles do fairly well when it comes to fuel economy, and this is particularly true when we're discussing lightweight/small displacement bikes, many of which sip their petrol in a downright miserly fashion. In John's case, his ACE 750 gets roughly 33% better mileage than his car, which all things being equal, would be considered a stellar improvement if we were comparing apples to apples. But as he points out, the car weighs considerably more than the bike, and I'd add that it can also accommodate four people, so it's no surprise he's questioning his bikes fuel efficiency, and he's by no means alone in that.

Rolling Resistance and Weight
Like that Newton fellow once said "nothing much happens till you beat inertia." Okay so maybe that wasn't exactly what he said but it makes the point. Everything has inertia, and it must be overcome before we can make any headway. When we're talking about cars and motorcycles, or for that matter anything that rolls, be it a rubber ball or a locomotive its opposition to movement is called rolling resistance and there are two things we need to remember about it. First, and somewhat obviously, rolling resistance affects the vehicle as a whole, and secondly, the moving components that make up the vehicles drive train, for example the engines crankshaft and pistons as well as certain accessory devices like the alternator or even a mechanically driven speedometer or tachometer all contribute some amount of friction or resistance, no matter how marginal, which is added to the total.

Weight is a large (no pun intended) and unwelcome component of rolling resistance. Plain and simple, heavy objects take a lot more effort to get rolling and require a lot more energy to keep them in motion than light ones. This is one of the reasons why you see so much effort expended by manufactures of everything from automobiles to airplanes to reduce weight, all things being equal, a lighter vehicle simply needs less power to accomplish the same amount of work as a heavy one.

Because they're built with smaller, lighter parts, motorcycle engines, transmissions and final drives are more efficient, that is they have lower frictional losses than their automotive counterparts. If this seems doubtful just do the math, for example, a typical V-twin engine's bottom end has two main bearings and two rod bearings. A four cylinder car engine has at least four (sometimes more) main bearings and four rod bearings all of which add friction to the engine, this can be extrapolated throughout the entire drive line, particularly when you factor in six or eight cylinder engines, (complete with air conditioning compressors and large alternators) automatic transmissions and all wheel drive, which leads us nicely into a very salient point.

A fair amount of a vehicles rolling resistance, in fact somewhere between 15 and 25%, depending on the circumstances, is created by the tires. So if we reduce the number of tires, we automatically reduce the vehicles rolling resistance, last time I looked most bikes were still using just two tires so there's a built in advantage right there. Furthermore because a motorcycle banks to turn the tires are designed with a rounded profile, this makes the contact patch proportionally smaller for a given weight and amount of power than a cars, further reducing the rolling resistance.

As far as weight goes there's no contest. Even the heaviest motorcycle, weighs substantially less than the lightest car. The Daimler Smart Car, reportedly the lightest DOT legal automobile on the market tips the scales at 1600 pounds, compared to it, the heaviest popular motorcycles, the Honda Gold Wing weighs in at a relatively svelte 926. So the bottom line here is that when it comes to weight and rolling resistance motorcycles are clearly more efficient than cars, but like they say, "Wait, there's more."

The Power to Weight Factor
One way to contrast the efficiency of two vehicles is by comparing their power to weight ratios. But we need to be careful. While a good power to weight ratio offers us some insight into a vehicles performance, especially where acceleration is concerned and provides some inkling has to how efficient the vehicle is, it can be misleading. For instance top fuel dragsters have incredible power to weight ratios, they generally weigh in at about 2300 pounds, and make between 7500 and 8000 horsepower depending on the set up, yet they are by no means fuel-efficient. Typically they burn 22 gallons of fuel to cover the quarter, which is 88 gallons per mile, and that my friends is poor mileage no matter how you slice it.

In a more mundane example John's ACE 750 makes roughly 45 horsepower (at the crankshaft) and weighs 524 pounds or so ready to ride. Skipping the math it means that the Ace has one horsepower for every 11.6 pounds of weight. On the other hand his car, weighs 2500 pounds and pumps out 109 horsepower, which is only one horsepower for every 22.9 pounds of weight, no wonder he described the thing as a slowpoke. This is a theme that's repeated in nearly every comparison between cars and motorcycles, and is especially prevalent where sport bikes are concerned.

Again I'd caution you that power to weight ratios aren't necessarily a good indicator of fuel economy, but they do provide a rule of thumb way to evaluate a vehicle level of efficiency. In this particular case the results speak for themselves, the lighter vehicle has proportionally more horsepower, therefore its engine doesn't have to work as hard to overcome rolling resistance and propel it down the road. In this instance the motorcycle accelerates harder, and uses less fuel to accomplish the same amount of work as the car. However its power to weight advantage is nearly two to one, and it is nearly five times lighter overall than the car, so shouldn't its fuel mileage be as Mr. Maher asks, closer to 50% better?

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.

Fully exposed rotating wheels create an enormous amount of drag, especially when they're in contact with the ground. Pull a wheelie (figA) and drag diminishes, good news for stunt riders.
Everything, from the rider to the mirrors contributes to drag, remove the fairing and the situation becomes much worse.
A properly designed windshield offers better protection and reduces drag.