THE ROLE OF TIRES IN FUEL EFFICIENCY
Inertia and friction
All rotating parts of a vehicle, and tires are no exception, have to overcome rotational inertia in order to accelerate
Excluding pistons and valves, everything in a car rotates: from the drive shaft – moved by the straight motion of the pistons - onwards everything rotates including gears, pulleys and bearings, ending with the wheels, the only part of the car that actually touches the road. All this "spinning" generates friction all the time, but during acceleration, the ever-present friction is also joined by the natural inertia of the car, including the resistance that all these rotating parts naturally oppose to changes in velocity. These additional mechanical resistances to speed changes must be added to that of the car resulting in the overall "effective mass" of the vehicle. A former Firestone executive said that one of Detroit's Big Three companies has challenged its suppliers, in order to comply with CAFE regulations (Corporate Average Fuel Economy) on fuel consumption, offering a 50 cents / pound (per car) bonus for weight reduction on a series of vehicle components. The prize doubled, significantly, if the weight reduction applied to a rotating part. This award was offered, coincidentally, at the start of a downsizing trend for both vehicles and components in the US, in order to improve scores in the CAFE standards.
Less weight, more money
The doubling of the prize earlier mentioned must be attributed to the implicit acknowledgment that a weight reduction on a moving part, such as a tire for example, is more important in the light of fuel economy than a static part. And that is even more critical for tires; if a spare wheel lodged in the trunk just adds mass, the same wheel on the road dissipates energy due to rolling resistance, and adds rotational inertia, to the point that the motto "a pound on a tire is like two on the frame" was coined. Engineers of course know that the resistance of a wheel to variations in speed depends not only on its size and mass but also on the mass distribution of the same (in the calculation also the rim and the brake drum or the brake discs are included) with respect to its rotation axis. The physical property that governs the response of a rotating body is the moment of inertia and increases rapidly as the mass grows away from the rotation axis.
In 2010, a popular American car magazine tested a Volkswagen Golf equipped with tires of the same make and model but with sizes ranging from 195 / 65R15 to 235 / 35R19 (the total height was about the same). The results were quite different from what was expected: the tires perceived as sportier - i.e. low profiles with an aspect ratio / 35 – were slower by three tenths of a second in the classic 0-60 mph test.
The rotational inertia of the tire-brake-wheel unit
Consumption too increased by almost 10% compared to the / 65s. In other words, the 15-inch wheels out-performed the much more "trendy" 19 inchers in two important results; it was interesting to note that this trend was evident in each of the tires tested - 5 different sizes from /65 to /35.
The different distribution of the tire-wheel mass affected the results (the braking system remained the same course): the low profiles had "more" rim and rubber externally, and this increased the moment of inertia . The wider tires, therefore, failed to compensate with a higher grip (which, incidentally, increased fuel consumption due to the higher friction) a higher moment of inertia.
A university team has recently worked in Goodyear’s R & D department, using the manufacturer’s state of the art equipment for inertial measurement. The calculations were based on theoretical principles and perfected through experimental data, and concerned various alloy wheels and steel radial tires for cars, trucks, SUVs and even airplanes. Among the results the team reported that the aforementioned effective vehicle mass increases by 2.6% with 195/65 R15 and 4.2% with 235 / 35R17 as a result of the moments of inertia of the whole tire/wheel unit; the latter is equivalent to a 59 kg increase! Even more unexpected was the fact that a decrease in mass due to tread wear is more important, in reducing the moment of inertia, than, say, substituting a steel wheel with an alloy wheel: inertia, in fact, depends mainly from the distance of the mass from the rotation axis.
Furthermore, the rotational inertia of the tire-brake-wheel unit greatly affects fuel consumption, especially in stop-and-go driving in a city center, while at a constant speed, such as on motorways, this effect is notably reduced, unlike rolling resistance, which is always present. This also explains why electric vehicles, which are called on to make the most of their battery power, generally have narrow tires with fairly generous diameters: larger diameters in fact promote low rolling resistance.
The visual impact of a low profile tire on a large wheel is undoubtedly remarkable, as well as the handling it provides, since the reduced height of the sidewalls enhances driving precision. The downside is an unexpected drop in acceleration, due to the high concentration of the tire-wheel mass away from the rotation axis, especially noticeable when accelerating and braking continuously: one more reason to drive smoothly!