From:
http://www.worldoneperformance.com/blog/2009/07/22/how-do-wheel-offsets-work/
While the first thing most people do to their car is upgrade the power, often times when it comes to wheels, many people seem to be at a loss. Often people don’t know what wheels fit their car, let alone what’s a good quality brand. However, before we get into wheel manufacturing technology (which we’ll go over in a future blog entry), let’s start with the basic wheel feature that affects how wheels will both fit and look on a car: wheel offset.
Whenever your look up wheel specs, they usually have a listing of offsets, such as +53, +45, +29, and so on. These offsets represent the distance in millimeters that the wheel hub (where you actually mount the wheels onto the car) is from the centerline of the wheel. I created this diagram from a Volk TE37 for Porsche wheel to help illustrate:
As you can see, I’ve placed a line the goes down the center of the wheel, and I’ve shown that the right side of the wheel is on the outside toward the fender, whereas the left side of the wheel is on the inside toward the suspension. As the diagram shows, the wheel hub is directly on the center line, which means it has a zero offset. When the wheel hub is pushed out toward the outside of the wheel, then the wheel has positive offset; when the hub is pushed in toward the inside of the wheel, then the wheel has negative offset. Makes sense, right? Perhaps it does, but how does this affect wheel fitment?
Herein lies the problem with offset. Wheels have to clear 3 main things in order for them not only to fit on the car, but also roll freely: fenders, suspension, and brakes. Since ever car is different, not every wheel offset will work. Some cars, such as the 350Z and M3, have plenty of room between the outside fender and the suspension. Other cars, such as the 93-01 Impreza, have nowhere near as much space. Still other cars, such as the S2000 have an average amount of room in the front of the car, but barely any room in the rear. On top of all this, wheel offsets are also affected by the width of the wheel. Thus, a wheel that is only 7.5 inches wide fits differently than a wheel that is 10.5 inches wide with the same exact offset. Remember, the offset is measured from the centerline of the wheel, so the wider the wheel, the distance available from the centerline. This can very much come into play when there’s a big brake kit on the car, since you’ll need to choose an offset that will clear the brake calipers.
Despite all the possible complications, here are a few generalizations that will help with your wheel offset decisions:
High offsets (such as +50 for example) make the wheel fit inward toward the suspension. This helps the wheel clear the outside fender, but since the wheel goes in more, it could pose problems clearing brakes or contacting the suspension.
Low offsets (such as +25 or anything in the negative for example) make the wheel fit outward toward the fender. This helps the wheel clear a big brake kit, but it could pose problems with your wheels and tires rubbing against your fenders.
Since every car is different, certain offsets (typically lower offsets) combined with a certain wheel bolt pattern can cause undo stress on your wheel bearings, causing them to wear out prematurely. For this reason, many wheel manufacturers simply do not manufacture wheels in certain fitments.
So how do you decide what’s the best wheel offset for your car? First off, do your research: find out what your stock wheel offset is first and foremost, and then you can even measure yourself to determine how different of an offset will suit your needs. Also find out what other people with your same car are running and find out if they have any wheel fitment problems. After you’ve done your research and you’re still unsure, please don’t hesitate to contact us, since we are familiar with a great many different wheel options and fitments for various vehicles. If you find out that the size and offset you need aren’t readily available, we can always special order you a set and, depending on the brand, even custom order a set of wheels to your exact specifications.
