Wheel offset and backspacing are quite confusing topics. Although there are many controversies, these are extremely important concepts that all mechanics must be knowledgeable about before they start to work on or modify cars.
Wheel Offset and How to Measure
Wheel offset is how a wheel will sit in comparison to the suspension and body components of the car.
It will vary hugely from car to car as each car has a different design and a different sized wheel well. Wheel offset can have a significant effect on the performance of a car.
This also has a distinct way of being measured.
Wheel offset is measured in millimeters. It can either be positive or negative. A measurement of zero millimeters of wheel offset means that the hub inside the wheel sits exactly in the middle of the wheel.
On the other hand, a wheel offset which is positive means that the hub of the wheel sits on the outside of the wheel. When the wheel offset is negative, it means that the hub sits on the inside of the wheel well.
Almost every single car has a different wheel offset. Typically front-wheel drive vehicles will use a much more positive wheel offset than most rear-wheel drive vehicles even though most newer vehicles use a positive wheel offset.
The eraser for this is that a positive offset will typically offer a smaller turning radius while a negative offset will allow for drifting but not for speed and control over a vehicle.
Negative wheel offset will typically be found on older sports and muscle cars. A major reason that vehicles nowadays will use a positive wheel offset is to prevent the tire from rubbing on any other parts of the car.
Using a negative wheel offset requires a large enough wheel well to accommodate a “deep dish” look on the car.
If the wheel well is not large enough for a negative wheel offset and one is used, the wheel can end up rubbing up against the underside of the fenders which will destroy both the tire and the fender.
Many people have done the mistake of putting wheels with a negative offset on cars that are not able to accommodate a negative offset just because they prefer how it looks and ended up costing themselves thousands in repairs.
Wheel Backspacing and How to Measure
Wheel backspacing is the distance from the rear of the wheel well to the back end of the wheel. This is usually measured in inches.
So a car with more backspacing is a car with wheels that are closer to the front of the vehicle. Similar to wheel offset, the wheel rubbing against the metal of the car is a major determinate of the amount of backspace a car has.
No car will be able to function with the tire rubbing against the rear of the vehicle.Having a larger backspace will often improve the suspension of the vehicle.
Typically, larger cars like trucks will have more backspace than your typical sedan, because of the much larger wheel well in the bigger vehicles.
Off-road vehicles also have large amounts of backspace to take advantage of the effect of the improved suspension.
A reason why a person may want to have less backspace could be because they do not like the aesthetic of there being a gap between the rear of the wheel well and the tire. Or, because a smaller backspace may provide more control over a vehicle.
Simple and Short-cut
To measure offset, you must measure the width of the rims and then measure the distance from the hub to the outside of the rims.
You then subtract the distance from the hub to half the width of the rims.
To measure backspace, you can just use a ruler or measuring tape to find the distance from the rear of the wheel well to the tire.
Wrapping up, these are fundamental concepts in the industry that drivers, mechanics, and even engineers can be confused about.
Being able to properly find the right amount of offset and backspace is a sign of an expert mechanic.
An even more expert mechanic will understand that each and every car has a different offset and backspace for optimal performance.