With big wheels becoming more and more popular, the obvious question to ask is if bigger wheels also are better wheels. As always, when you have questions about cars, there is no simple answer to be found. It all depends on what you want from your wheels.
The best answer probably is that the wheels should be sized in proportion to the vehicle, and to the loads they have to carry. The wheels on an 18-wheeler are big because they are designed to carry very heavy loads. The wheels on a Smart are small, because there is no room and no need for big wheels. If you want big wheels because you think they look good - just go for it. There is no doubt that a well-designed wheel looks better than a tire wall. To avoid disappointment, you should however be aware of a couple of things:
- Bigger wheels are heavier wheels.
- Very big wheels will raise the center of gravity of you vehicle
If the wheels are for show, this doesn't matter. If they are meant for your commuter vehicle, extremely big wheels will make the vehicle more difficult to control and less enjoyable to drive.
Lightweight wheels are essential to vehicle performance. The reason is fairly simple: A rotating wheel acts like a flywheel and wants to keep going at the same speed and in the same direction it is already going. The heavier it is, the harder it is to control.The big problem with big wheels is that the perimeter gets heavier and heavier as the wheel diameter increases.
If you take a look at flywheels or gyroscopes, they are designed with a very light center and a thick and heavy perimeter to make them rotate at a stable speed in a stable direction no matter what happens. This is not what you want in a road wheel. A wheel with a heavy perimeter is difficult to steer, and when a heavy wheel bounces on the road surface, the shock absorber has to work hard to keep it down on the road.
Good road-holding is achieved by keeping the contact surface of the tires in constant contact with the road road surface. If the wheels bounce or if the tires don't stay flat on the surface, the vehicle loses its grip. Keeping the rubber flat on the road is a daunting task when the speed goes up, the turns are sharp, and the road surface is uneven.
The more weight the springs and shock absorbers have to handle, the harder they have to work, to keep the wheels from bouncing. Racers and manufacturers of performance cars therefore make light unsprung weight a priority when they try to improve the road-holding. In other words: Big wheels are not necessarily better than smaller diameter wheels from a performance point of view, just because big wheels weigh more than small wheels.
The performance reason for using bigger rims than standard, is that they make it possible to fit tires with lower and stiffer sidewalls without changing the wheel perimeter and the vehicles ground clearance. Low and stiff tire sidewalls of these low profile tires keep the tire's tread flatter on the road than high and flexing tire walls. Low profile tires are also wider than higher profile tires, as the air cushion has to have a certain volume to carry the load of the vehicle. Thus, a low profile tire has a bigger contact patch with the road, and this big patch stays flat on the road. Up to a point, the improvement in road-holding from the low profile tire more than compensates the drawbacks of a bigger and heavier wheel. Today's supercars feature 18"- 20" wheel sizes and tire profiles down to 30 (tire sidewall height 30% of tread width). These wheels are always forged, to keep them as light as possible. Bigger wheels than this are more for show than for go, as they weigh too much to perform well.
Steel wheel barrels get too heavy at around 16" diameter. For everyday drivers, cast lightweight alloy wheels are a good choice up to 20" wheels. For performance cars lightweight forged wheels are superior, even necessary, when the wheels reach 18" diameter.
Forged wheels can be made lighter than cast wheels, as the forging process changes the molecular structure of the metal, providing more tensile strength and more resistance to cracking and to bending forces. A forged wheel can be made with thinner metal profiles while still meeting the strength requirements.
Most important is that forged barrels can be made much lighter than cast barrels. The barrels are most critical to wheel performance. The gyroscopic effect comes from the peripheral weight of the wheel, i.e. the barrel. The barrel is also the wheel component that is in the most vulnerable position, taking hits from potholes, curbs etc.
As forging of wheels is a very expensive process, wheel manufacturers have come up with ways to combine cast centers with forged barrels, either in multipiece wheels or by "spinning" the barrel of a cast wheel. This provides most of the advantages of forged wheels at a comparatively limited cost.Forging of wheel barrels is done in rotary forging machines, where the cast "blank" is heated, rotated at high speed and pressed against a template by rollers. This process produces a very strong barrel, with a runout that keeps within very tight tolerances.
To sum it up: The most important parameter for vehicle performance is the weight of the wheel. Replacing a standard size steel wheel with the same size of cast light alloy wheel improves performance and road-holding considerably, without any reduction in comfort. Further improvement in road-holding comes with increased wheel size, combined with a corresponding reduction in the height of the tire walls, a procedure called "Plus-Sizing". The improved performance comes with a reduction in comfort, and the effectiveness of plus-sizing is reduced by uneven road surfaces. Big wheels often look better, but have to be chosen carefully to be better for vehicle performance. If you want wheels that are bigger than 18", you should look for forged wheels or wheels with forged (spun) barrels.