Before you purchase a set of aftermarket performance wheels, there's information that you need to know such as diameter, offset (known as ET in the wheel industry) and backspacing. There are several ways to come by this information. The easiest is the internet, check out any of the numerous car clubs on the web or sites dedicated to your particular make or model.
Just remember that without proper offset and backspacing, an otherwise perfectly drivable car will become a torque steering monster and any changes in offset can affect a car's straight line stability and braking characteristics.
Backspacing is simply the distance form a wheel's hub mounting surface to an imaginary line across its diameter. Offset, usually measured in millimeters, is the distance from the hub mounting surface to the cetner line of the wheel. In other words, a wheel's hub mounting surface can be located either inside or outside of its center line. If the surface is outside (toward's the wheel's face) the centerline, then the offset is positive. Most modern front wheel drive cars requires a positive offset. SUVs usually feature a negative or zero offset. Negative offset places the tires outside of the fender well and are are usually used in off road applications where wide off road tires are needed and track width increased.
Other factors that you should consider are the wheel's diameter and bolt pattern. Bolt pattern refers to the number of lug nuts that are used to attach the wheel to the car. Most cars use a 4 x 100 bolt pattern (4 holes, 100mm diameter). Here is a chart of bolt patterns on some of the more common cars:
Along with purchasing new wheels, new tires are always in order. Upgrading to larger wheels and tires is known as plus sizing. The aim of plus sizing is to keep the overall diameter of the wheel and tire package similar. Most cars from the factory with 14 inch wheels. Here's an example:
A car comes from the showroom installed with 14" wheels and 195/70 R14 tires. 195 means that the width of the tire is 195 mm. 70 is the aspect ratio, this means that the height of the tire's sidewall is 70% of the width, or 136.5 mm. R14 means that this tire will fit 14" wheels. Putting it all together we get:
Sidewall Height X 2) / 25.4 + Wheel Diameter (in.) = Overall diameter
(195 x 0.70) x 2 / 25.4 + 14 = 24.75inches
Say you want to switch to 17" wheels for that aggressive look. Naturally you choose a 17" wheel in the correct offset and bolt pattern. Next you have to choose the tire. Not all 17" tires are the same, the same diameter may have different aspect ratios. You can browse over to the Nitto, or Avon Tyre websites to get a complete listing of the 17" tires available. For our wheel example, a tire size of 215/45 R17 will yield a similar overall diameter of 625.3 mm, which is well within the +/- percent allowable variation.
One of the biggest developements in the wheel industry in the last few years is fitting massive sized wheels on to equally massive SUVs. Aside from dramatically improving looks, bigger wheels along with bigger tires yielded improved grip and stability as C! Magazine found out when they tested a Ford Expedition with 22" Concept One Wheels and 305/45/ZR22 tires. At speeds above 70 kph, the Expedition handled like a car half its size, the twitchiness and floating feeling was gone. The Expedition not only looked devastatingly good but it performed well beyond expectation.
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