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What is an “offset” and why do we need them.. and what does a +33mm or -0mm offset mean? And how does this affect my vehicle? This thread is to help you better understand this straightforward but significant factor when applying a wheel to your car. As an old school hot rod guy, you may be used to it being called backspacing. While this version of offset is easy to understand, it’s missing the details and specifics that offsets provide and can mean the difference between your rims sticking out too far, or rubbing on the inside. If the offset of a wheel is not right for the vehicle, the handling of your car also be adversely affected. When you change the width of the wheel the offset changes as well numerically, and if the offset was to remain the same while if we added width, the added width would be split evenly between the inside and outside of the wheel. Below are the basics of what offset stands for you can see in the diagram below to put it in perspective to make your wheel purchase a little easier to understand. Offsets are not an exact number, meaning you do not have to put the same offset that's on your car now because some styles are different they may need the extra space to clear your brakes and or suspension it's more of a range between 5mm to 10mm perhaps more!
To make this easy. The offset is the distance from the hub mounting pad to the middle of the wheel. The purpose of the offset is to know how many millimeters the wheel will stick out or in from the mounting pad. It’s important to understand even a couple of millimeters can cause a wheel to rub somewhere and can affect ride quality, the stance and performance of your vehicle and most of all your safety. If you are here, you have most likely have spent some time trying to figure out how to get the look you want while at the same time maintaining or enhancing your vehicle's performance. Usually, the OEM wheel on your car is at a high offset, somewhere in the high +32mm to +55mm range. So the wheel tends to sit in under the vehicle fender well more, maintaining the center of gravity under the cockpit reducing the overall vehicle's width. Because of this high offset, most designs are limited in style. The deep lip look is hard to accomplish in one piece wheels with high offsets. However, two and three piece wheels can achieve these concaved and convexed designs but at a higher price due to slower production runs because time is money. The opposite is applied when it comes to lifted trucks and SUV’s Reverse offsets like a -35mm and you now have plenty of room to design to meet aggressive spokes and supper deep lips.
The hub mounting pad is flush with the centerline of the wheel (dotted line to left).
The hub mounting pad is closer to the front or face side” of the wheel. Positive offset wheels are typically used on FWD vehicles and newer RWD cars. An example of a High et wheel is the newer Chevrolet Corvette, the higher offsets restrict design and leave the wheel looking flat, with little to no depth to the lip of the wheel.
The hub mounting pad is closer to the back or inner barrel of the wheel. These wheels are typically a negative offset and are used for old school muscle cars and lifted trucks and SUV’s.
The best way to measure backspace is to lay the wheel face down on the ground, be sure to lay a towel or some cloth first down fist to prevent damage to the wheels face, so the backside of the rim is facing up. Take a straight edge like a yardstick or straight 2x4 longer than the diameter of the wheel because if you do not the straight edge will just fall into the barrel of the wheel. Lay it diagonally across the inboard flange of the wheel (like the photo below). Now take a tape measure and measure the distance from where the straight edge contacts the inboard flange to the hub mounting pad of the wheel. This measurement is called the backspace. Backspacing, similar to offset, is the distance from the hub mounting surface to the inside lip of the wheel (measured in inches).
Wheel backspace (see above)
Wheel centerline (outboard flange to inboard flange measurement, divided by 2)
ONCE YOU HAVE THESE MEASUREMENTS, JUST SUBTRACT THE WHEELS CENTERLINE FROM WHEELS BACKSPACE TO GET THE OFFSET.
If backspace is less than the wheel centerline, the offset is negative
if backspace is higher than the wheel centerline, the offset is positive
To convert inches to millimeters, multiply inches by 25.4
To convert millimeters to inches, divide mm by 25.4