I want bigger wheels. How big can I go?
That depends on your car and the amount of work you're prepared to put in. Generally, most cars will be able to handle an increase in wheel size of up to four inches, with no need for any modifications. However, this degree of upsizing should be coupled with a lower profile tyre, to make sure the overall radius of the wheel/tyre is the same as the car's original equipment specifications. This means that the car's overall gearing will be unchanged and the speedometer will still be accurate.
What options give the same rolling radius?
As a rough rule of thumb, the following combinations give approximately the same rolling radius for the overall wheel/tyre combination:
195/65R15 Rolling diameter 645mm
205/55R16 Rolling diameter 642mm
225/45R17 Rolling diameter 642mm
225/40R18 Rolling diameter 645mm
225/35R18 Rolling diameter 647mm
Should I go bigger on the size or the width?
Keeping the same width of the wheel and tyre package while increasing the diameter of the wheel, with an accompanying lower-profile tyre to compensate, means that the car will look better and you would see the benefits of lower profile tyres, in terms of grip and steering response. It also means that generally, since the overall wheel/tyre package is the same size as the original equipment, there wouldn't be any real problems with tyres hitting the wheelarches or changes in your car's handling.
What about if I want really big wheels?
Then you may need to look at modifying your wheel arches to prevent contact, particularly when turning the steering. You can usually get a bodyshop to either roll or cut the inner lip of the wheelarch away to give more clearance - rolling tends to keep inherent strength in the arch, while cutting tends to lose it.
What is the offset of a wheel?
The offset is the distance from the mounting face of the wheel, where it mounts against the wheel hub, and the centerline of the wheel's width. It will be expressed as a distance, usually in mm and called ET.
The offset can be one of three types:
The hub mounting surface is even with the centerline of the wheel.
The hub mounting surface is toward the front or wheel side of the wheel. Positive offset wheels are generally found on front wheel drive cars and newer rear drive cars.
The hub mounting surface is toward the back or brake side of the wheels centerline. "Deep dish" wheels are typically a negative offset.
If the offset of the wheel is not correct for the car, the handling can be adversely affected. When the width of the wheel changes, the offset also changes numerically. If the offset were to stay the same while you added width, the additional width would be split evenly between the inside and outside. For most cars, this won't work correctly. We have test fitted thousands of different vehicles for proper fitment. Our extensive database allows our sales staff to offer you the perfect fit for your vehicle.
To determine wheel offset:
* Position the wheel on a flat surface and measure its overall width.
* Divide the overall width by two, then subtract this result from the backspace value.
Formula to determine the offset:
Offset = Backspace - (Rim Width ÷ 2)
Why is offset important?
When a car is designed, the suspension is configured in a certain way, which uses wheels with a certain offset. If this offset changes, then the distance between the mounting face and the centre of the wheel changes and this can affect the handling of the car. One situation is tram-lining, where the car wanders, following imperfections in the road surface, rather than running straight. Another possible situation is torque steer, where the steering pulls from one side to the other under acceleration.
Wheel Offset more info:
If you put a wheel on the same hub with a bigger offset, the wheel will sit further inside the wheel arch because the wheels bolting face will have to move further in to meet the hub.
Alternatively if it has a smaller offset it will sit further out from the wheel arch.
For sports purposes grip would be improved by increasing the width of the wheel(and tyre) and using a smaller offset to allow for the extra width on the wheel in the wheel arch.
Vehicle wheels are usually described with a designation of something like '13-5.5J' and this is decoded as follows.
The 13 is the diameter of the wheel in inches from edge to edge, measured inside the flange with sizes ranging from 10 inch on the original mini's to 23 inch on 4x4 vehicles.
The second number is the width of the rim, again measured in inches between the flanges. The original mini used 4.5 wheels and sizes range all the way up to 12 as used on some supercars.
The letter, in this case J refers to the shape of the rim. Rim contours are standardized by the Tire and Rim Association, so that tires will fit.
Another important thing is the wheel offset, which is the distance between the middle line of the centre of the wheel width and the mounting surface of where the wheel bolts to the hub on the vehicle. The offset is described by the term ET, which is from the German word 'Einpresstiefe' translated as 'insertion depth'. Most wheels have a positive offset which means the mounting surface is further outboard than the wheels centre. If you reduce the amount of positive ET on the wheel the the vehicles track is widened and vice versa if the ET number is reduced. Straying too far from the original offset can be detremental as fouling to the suspension and bodywork can occur as well as eccess strain on wheel bolts or studs
What is the PCD?
PCD stands for Pitch Circle Diameter and is a measure of the wheel stud/bolt hole locations. The PCD is a circle with its centre at the centre of the face of the wheel. The centres of the mounting holes are equally spaced around this circle. So for example, a 4x100mm PCD means four mounting holes, equally spaced around a 100mm diameter circle form the centre of the wheel.
What does a Spigot Ring do?
This is a plastic ring that sits inside the alloy wheel, locating it centrally on the car's wheel hub. The spigot doesn't form part of the mechanical join between the car and the wheel, that's done by the wheel nuts/studs. The Spigot simply makes sure that the wheel is located centrally on the hub. If it wasn't, the wheel nuts/studs would have to centre the wheel and if the wheel wasn't perfectly central on the hub, there could be possible vibration and balance problems.
If I've fitted new wheels and tyres, what pressure should I run?
The general rule is to keep running the same pressure as the manufacturer recommends for the original equipment, as it's this which supports the weight of the car.
High Performance Wheels
If you're opting for high-performance tires, you'll probably want to include some new wheels to enhance both appearance and performance. To assure the right fit, you'll need to make sure that you've got the proper replacement wheel size, dimension and load-carrying capacity.
Increase your vehicle's stability, steering response and cornering capability. A Yokohama quick tip—use a rim width which is 90% as wide as the tread width of a performance tire for street driving. This will provide a good balance between performance and ride quality.
Result in an improvement in ride quality, but may sacrifice some of the tire's ultimate performance capability.
Mid-Range Rim Widths
Provide a balance between handling capabilities and ride quality. The wheel's width influences handling and ride quality. Always choose a rim width within the range of the tire's acceptable rim width specification.
Never attempt to mix millimetric wheels and tires with standard inch rim wheels and tires. Always replace a tire on a rim with another tire of exactly the same rim diameter designation and suffix letters. For example: A 16" tire goes with a 16" rim. Never mount a 16" size diameter tire on a 16.5" rim. While it is possible to pass a 16" diameter tire over the lip of flange of a 16.5" size diameter rim, it can't be inflated enough to position itself against the rim flange. If an attempt is made to seat the tire bead by inflating, the tire bead will break with explosive force and could cause serious injury or death.
Wheel backspace and offset are two measurements which play an important role in determining proper fitment between the tire and wheel.
This is the distance from the back edge of the wheel to the hub mounting surface. To determine the wheel backspace:
* Position the wheel face down.
* Lay a straight-edge across the back of the wheel.
* Measure the distance from the straight-edge to the wheel's hub mounting surface.