For many car enthusiasts, one of the first modifications to their cars is changing wheels. Wheel preferences vary and no one has the right to judge other people's choice of wheels. However, when it comes to wheel sizes, our government – in particular the Land Transportation Authority (LTO) – has recently found itself at the center of an online backlash due to the enforcement of a law that restricts what we do with our vehicles. You can read our report here.
Either way, now is a good time to talk about the technical aspects of changing wheels in light of this recent brawl between government regulators and motorists.
Wheel diameter
Most people change wheels to give the car a sportier look and better handling. But be aware that while larger wheels may look good, a larger rim size can come with downsides such as extra weight and a stiffer ride.
In many cases, the larger the wheel and tire combination, the heavier it is. And while a larger size may look better, being heavier is definitely a downside, the most significant downside being a less comfortable ride. In addition, fitting a larger wheel may result in the purchase of tires with a lower profile. A lower profile tire also has less damping effect and again contributes to a stiffer ride. If you value ride comfort, don't buy wheels that are too heavy or tires that are too low profile.
Despite the ongoing failure of LTO, I can safely assume that there is an acceptable range when it comes to changing wheel sizes. My guess is that we can still change the rim dimensions if the resulting rolling circumference is between 2% and 3% of the car's OEM specs. Such deviation from the OEM specifications is permitted by the vehicle's design and will not have any adverse effects on the vehicle's systems such as speedometer calibration, ABS, traction control and stability control.
Thus, when increasing or decreasing the diameter of the wheel, you should check the final result of the change by contacting the tire dealer. Better yet, do your own research by consulting online tire calculators and checking the size and resulting rolling circumference of the wheel/tire you are about to buy against your vehicle's OEM specifications. To make sure you're still within the law, make sure you're within 2-3% of these requirements. So it can still be argued that you haven't changed the size of the wheels effectively.
Wheel offset
The wheel offset is the distance between the wheel hub and the wheel center line. The OEM specification for your wheel offset is often printed on the wheel itself and is prefixed with "ET" followed by a number expressed in millimeters, such as "ET+45".
The offset determines how the wheel fits into the vehicle's wheel arch and how far the inside edge of the wheel extends towards the suspension. Setting the offset incorrectly has consequences. But the most important misalignment concepts to keep in mind are this: if you put more emphasis on the positives of your OEM offset specification, then your wheel is going deeper into your fender and closer to rubbing against the suspension. When driving in the negative direction of the specified wheel offset, the wheel is pushed out of the wheel housing. The farther you go, the more stress will be put on the wheel bearings, and as your tire goes out of the fenders, the dirtier your car becomes due to the dirt your tires kick up.
Just in case, you want your new wheel offset to be within 5mm of the offset specified by the manufacturer.
The above is the simplest way to explain the impact of OEM wheel diameter and offset differences. This is by no means an exhaustive discussion. And the advice I gave is very conservative and not everyone will agree. But it should help you safely determine the specifications of your new wheel and stay within the law. I hope this wasn't too technical and hope it helps when you go shopping for new wheels.
