Wheel style is usually the first consideration when selecting new aftermarket wheels because most wheels are purchased for their different-than-factory styling which helps personalize the vehicle's looks to match its driver's tastes. After that comes the functional requirements of selecting a wheel diameter, width and offset that are right for the vehicle and appropriate for the desired tire size.
While all wheel styles evolve, there are some classic wheel designs that are almost timeless, including some of the basic five-spoke, six-spoke and cross-spoke designs. And since the look of many of these classic designs has evolved from racing wheels, their form follows function. This means that their basic design helps the engineer balance style, strength and weight. On the other hand, there are also fashion wheels which, like clothing, tend to change their look with the seasons. And with many of the fashion wheel designs, their function follows fashion. This means that their designer (not engineer) places emphasis on style over weight. Additionally, because most of the fashion wheels are expected to have relatively short life cycles, the design the engineer receives to put into production places emphasis on the ability to manufacture them economically in small volumes. Unfortunately, this normally precludes using the most recent high tech manufacturing methods.
However, not so obvious is the role that wheel weight plays in determining the vehicle's overall ride, performance and feel. This is why so many new vehicles have replaced the less expensive rolled, stamped and welded steel wheels with cast aluminum alloy wheels.
Ever since the development of the original mag wheels, which actually used a magnesium alloy to reduce wheel weight on the race cars of the 1950s, lightweight wheels, where they are allowed by the rules, have been used to enhance vehicle performance in almost all forms of racing. And while the generic mag name is still used today, most magnesium alloy wheels are too fragile for daily street use and are very limited in their finish options allowing only painting or polishing (and frequent polishing of magnesium wheels is required due to magnesium's willingness to oxidize). In modern times, the most common uses for magnesium wheels are on Champ Cars, Indy, Formula 1 and GT cars where the race team's continuous monitoring and testing can confirm their strength and durability between events. Mag wheels used on modern race cars are often forged or use some other high tech manufacturing process and special magnesium alloys to reduce the weight while they retain the ability to handle the high loads. Fortunately for the rest of us, the wheel manufacturers have also developed lightweight aluminum alloy wheels that provide the appropriate street durability and numerous finish options while they reduce weight.
A vehicle's total weight is the sum of all of its parts and affects its ability to accelerate, brake and corner. Reducing the total weight will enhance the vehicle's performance because less weight needs to be controlled and therefore, less energy is required. Unsprung weight is the weight under the springs which moves up and down as the vehicle rides over uneven roads and leans in the corners. Reducing unsprung weight allows the springs and shock absorbers to be more effective in controlling the suspension's movement. Additionally, a vehicle's rotational weight includes all parts that spin including everything in the vehicle's driveline from the engine's crankshaft to its wheels and tires. This affects the energy required to change speed as the vehicle accelerates and brakes. As you would guess, reducing the weight of any of these rotating components will enhance the vehicle's performance because less energy will be required to increase or decrease their speed.
In order to get a better understanding of the effects of reductions or increases in wheel and tire weight on performance, comfort and feel, the Tire Rack Team conducted a Ride & Drive comparing a BMW 3 Series equipped with its Original Equipment (16") wheels and tires in order to establish a comfort and handling baseline, to a second BMW 3 Series equipped with Plus One (17") wheels & tires using lightweight aluminum alloy wheels, and a third BMW 3 Series equipped with the same size Plus One application, but this time using heavyweight aluminum alloy wheels.
Original Equipment (OE) BMW 3 Series Alloy Wheels and Tires
16" x 7" sized OE BMW wheels and 205/55R16 91H-sized Michelin Energy MXV4 Plus tires
The aluminum alloy wheels which came as Original Equipment on our 3 Series test car features cross-spoke styling and weigh an estimated 20.5 pounds each.
The Energy MXV4 Plus tires used as Original Equipment on our BMW 3 Series are Grand Touring tires which blend some of a performance tire's looks and handling with a standard passenger tire's longer life and more comfortable ride. The Energy MXV4 Plus incorporates an advanced version of Michelin's Radial XSE Technology (which consists of a Smart Tread Compound, Optimized Casing Shape and Optimized Mass Distribution). The OE Michelin Energy MXV4 Plus tires weigh 22.5 pounds each.
When combined, the weight of the Original Equipment Energy MXV4 Plus tires mounted on the BMW alloy wheels results in a 42.5 pound Tire & Wheel Package.
On the road, the OE BMW alloy wheels and Energy MXV4 Plus tires were praised for their good ride, low noise, real world handling and steering feel. The car felt balanced and generated a relatively light steering feel that felt appropriate for the car. This combination made it easy to understand the combination of real world ride, noise and handling qualities that BMW engineers wanted for their 3 Series sedan. On the track, this combination was rated as responsive and predictable while providing good braking, cornering traction and handling. However, as expected, this car turned in the slowest lap times of the three combinations tested.
Plus Size Wheels & Tires Using Lightweight Wheels
SSR Semi-Solid Forged 17" x 8" alloy wheels fitted with 245/45ZR17-sized Pirelli P Zero Asimmetrico tires
The SSR aluminum alloy wheels used in this test feature five-spoke styling and a unique manufacturing process called Semi-Solid Forging (SSF). Using a process originally developed for aircraft and automotive parts which require high strength, low porosity and lightweight, SSF produced wheels use a special aluminum alloy which is heated until it has about the consistency of warm butter, and then it is forced into the mold under high pressure. This results in a strong, lightweight wheel that offers many of the traits of wheels forged in the traditional ways, but costs less to manufacture and therefore is available at a more affordable price. The lightweight 17" x 8" SSR aluminum alloy wheels on our 3 Series test car weighs a scant 17 pounds each.
The Pirelli P Zero Asimmetrico Max Performance tires used in this test feature a lightweight construction and an asymmetric tread design developed to maximize the tire's ability to put the power down whether accelerating, braking or cornering. Additionally, the P Zero Asimmetrico tread compound combines traditional carbon black with a special blend of silica and synthetic polymers to enhance wet traction and reduce rolling resistance. The P Zero Asimmetrico is used as Original Equipment on many performance vehicles, and for that matter, the tires in this test have been used as Original Equipment on the BMW E36 M3. The 245/45ZR17-sized Pirelli P Zero Asimmetrico tires weigh a respectable 21.5 pounds each.
When combined, the weight of the Plus One-sized P Zero Asimmetrico tires mounted on the SSR Integral alloy wheel results in a 38.5 pound Tire & Wheel Package which is actually 3 pounds lighter than.
On the road, this combination was praised for its steering feel and responsiveness, while it generated a small increase in ride harshness on bumpy roads. On the track, this combination was praised for its feel, feedback and responsiveness. It felt light and nimble, ready to transform the driver's input into performance. The car equipped with the lightweight alloy wheels and Pirelli P Zero Asimmetrico tires turned in the fastest lap times of the three combinations tested.
Plus Size Wheels & Tires Using Heavyweight Wheels
17" x 8" alloy wheels (brand name withheld) fitted with 245/45ZR17-sized Pirelli P Zero Asimmetrico tires
The heavyweight aluminum alloy wheels used in this test were selected because they were designed to emphasize a trendy fashion style without regard to overall weight. The heavyweight aluminum alloy wheels on our BMW test car weigh 27 pounds each (ten more pounds per wheel than the lightweight wheels).
When combined, the weight of the Plus One-sized P Zero Asimmetrico tires mounted on the heavyweight alloy wheel results in a 48.5 pound Tire & Wheel Package which is about 7 pounds heavier than the stock combination.
On the road, this combination was only praised for its ride quality. Our experience showed that the heavyweight wheel's reluctance to be moved as the vehicle rode over expansion joints, patches and potholes actually damped the impact harshness transmitted to the suspension, and forced the tires to absorb more of the jolt. However, when it came to steering feel and responsiveness, heavyweight wheels made the tires less communicative and responsive at highway speeds. On the track, this combination found no praise. It felt heavy and lethargic, and actually reduced the feedback the tires normally transmit to the driver, making the vehicle more difficult to drive at the limit. This made it more difficult for the driver to know how much input was required and reduced the vehicle's ability to transform the driver's input into performance. For that matter, many of our team members compared the feeling of running on heavyweight wheels to that of jogging with ankle weights or hiking with field boots. You really feel the difference when you take them off. The car equipped with the heavyweight alloy wheels and Pirelli P Zero Asimmetrico tires benefited from the handling of the tires but couldn't match the performance of the lightweight combination, and its lap times fell between the other two combinations tested.
While we expected to feel the gyroscopic effects of wheel weight at highway speeds (where at 60 mph they revolve about 13 times a second), we were rather surprised at their influence on the car's feel at the lower speeds on our performance test track drive. Additionally, realizing that the impact of heavyweight wheels would increase as top speeds climbed on a race track (where at 150 mph they would revolve about 33 times a second), it left little doubt as to why professional race teams spend thousands of dollars for wheels which minimize weight while retaining strength.
Finally, we monitored our test car's fuel economy using their onboard trip computers during our road rides. Covering a total distance of just over 350 miles, the three vehicle's average speeds were virtually identical (all within 3/10 of 1 mph). During that time the BMW 3 Series equipped with its Original Equipment Grand Touring Michelin Energy MXV4 Plus low rolling resistance tires averaged 22.8 mpg. The vehicle equipped with the Plus Size Pirelli P Zero tires on lightweight wheels averaged 21.5 mpg, while the Plus Size Pirelli P Zero tires on heavyweight wheels dropped to a 21.1 mpg average.
© 2012 Tire Rack