What is wheel offset and why is it important?

Wheel offset explanation

What is wheel offset?

Wheel offset is the distance from the centerline of the wheel to the mounting face of the hub (the face touching your rotor).  Traditionally this measurement is measured in millimeters.

What are the three types of wheel offset?

Wheel offset comes in three types: zero, positive and negative.  

Zero offset means the hub mounting surface is directly even with the center of the wheel.  

Positive offset is when the hub mounting surface is located towards the face of the wheel in relation to the center line.  Positive offset is the most common offset found on cars today.  

Negative offset is when the hub mounting surface is located closer to the inside wheel flange in relation to the center line of the wheel.  Negative offset wheels are mostly found on trucks and off road vehicles where the wheels stick out past the fenders.

How can I find the offset of my wheels?

Most all OEM BMW wheels are marked with an offset marking ET followed by a number.  ET is a German abbreviation for "Einpresstiefe" or "insertion depth".  The ET is followed by a number stating the offset in millimeters.  A marking of ET35 has a positive offset of 35mm.  See image below for clarification.

Why is wheel offset important to understand?

Wheel offset is of no use to you if you are running stock wheels.  Knowing and understanding wheel offset comes into play when you are in the market for new wheels.  Most people just buy wheels based off of looks and stud pattern but often overlook offset as a key factor in installation fit.  

Too low of an offset and your wheels will hit your fenders, too high of an offset and your wheels will hit your inner suspension components.  It is important to note that if you are going for a wider wheel but keeping the same offset, you are already moving the wheel face closer to your fender.  If changing wheel width you have to consider offset for a proper fit.  Consult with a proper technician or do your research on changing offset before buying an expensive set of wheels.

Can I change wheel offset without changing wheels?

Companies make wheel spacers with different offset sizes to allow you to change the offset of your wheel.  The wheel spacers essentially reduce the distance from the center of the wheel to the hub, thus reducing positive offset.  The lower number positive offset push your wheels out towards the fender or sometimes beyond the fender.  

If you add spacers to your wheels, the stock bolts will not thread all the way in and your wheels are at risk of loosening and falling off.  It is critical to get longer bolts to accommodate the spacer or you can change from the bolt design to a stud design using a stud conversion kit.

How will too much positive offset negatively affect my car?

Too much positive offset (the wheel sits too far in towards the car) can cause damage to inner suspension and brake components from the inside lip.  This can lead to poor handling making the car unstable at speed.  Sometimes the rubbing will happen on the thin inner sidewall of the wheel causing a rupture of the tire.  

How will too much negative offset negatively affect my car?

Too much negative offset (the wheel sits too far outwards from the car) can also contribute to poor handling due to additional stresses on the suspension components.  The steering wheel can flick back in hard cornering causing unstable handling and a possible accident.

Why would I want to change my wheel offset?

One of the most popular reasons is it allows for a more aggressive look, giving the car a wider stance.  You would be surprised on how much a 10mm wheel spacer can change the look of a car.

If you lower your car a substantial amount the car's camber will change.  Likely, the front struts will rub against the inside of your wheel.  You need to install wheel spacers to push the wheel outward and provide proper clearance.  Make sure you are able to move your steering wheel to full lock without any signs of rub.  

image source of wheel offset: lesschwab.com

German Wheel Offset marking ET
BBS Wheel offset marking ET
Wheel spacers
BMW F30 M4 wheel offset spacers
BMW F30 M4 wheel offset spacers