Motorcycle Fuel Economy - Fuel For Thought

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?