The wheel offset is a very important parameter. It determines whether the wheel will scratch the wheel’s eyebrows, suspension, calipers, and other vehicle parts. A suitable ET allows the wheel to be flush with the fenders, or make the wheel have a max concave look which many people pursue their wheel. However, the offset is a relatively difficult parameter to determine for ordinary vehicle owners even for some tuning guys.

So here is a question if the wheel size or J value changes, what should the offset value be? I’m sure many people can’t give an exact answer.

Today let’s explain to you step by step.

1.Know the offset (ET):

Offset can also be called offset or ET, which is the distance from the center line of the wheel to the mounting surface of the wheel, in mm

Offset classification:

Positive Offset: Take the centerline of the wheel as the boundary, the mounting surface of the wheel is on the side closer to the front of the wheel (closer to the outside of the body)

Zero Offset: the wheel mounting surface coincides with the centerline.

Negative Offset : Take the center line of the wheel as the boundary, and the mounting surface of the wheel is on the side closer to the inner side of the body.

2. Influence of offset on wheel installation

The offset value determines the lateral position of the wheel on the axle and the distance between the two coaxial wheels. Simply put, the wheel is concave to the inside of the body or protruding to the outside of the body. The larger the ET value of the wheel is: the more the wheel is concave to the inside of the car, and the wheel spacing between the two wheels becomes smaller; the smaller the ET value of the wheel is: the more the wheel protrudes toward the exterior of the car, the gap between the two wheels becomes larger.

For example, as shown in the figure, when the ET value is 30mm, the wheel is basically flush with the fender. When the ET value is 50mm, the wheel is recessed toward the inner fender of the body. When the ET value is 10mm, the wheel protrudes to the outer fender of the body.

3. Influence of unsuitable offset on wheel installation

In the actual installation process, the ET value has always been our important focus. If the ET value of the wheel is too large, the wheel will easily scrape with the suspension. If the ET value is too small, it is easy to protrude from the body and scratch the fender.

4. How to get the perfect ET value

First of all, the ideal installation effect of the wheel is to be flush with the fender (wheel eyebrow). So let’s look at two important installation data, clearance X / clearance Y :

Clearance X: The distance between the outside of the wheel and the fender (requires actual measurement)

Clearance Y: The distance between the inner side of the wheel and the suspension (generally the original car>15mm)

For example, the front wheel of the original car is ET45mm, and the J value is 7J (1J=25.4mm). If the wheel needs to be changed at this time, the width of the new wheel is 8J. Under the condition that the offset remains unchanged, the distances on both sides of clearance X and clearance Y are reduced by 12.7mm. If ET40mm is selected (the wheel protrudes outward by 5mm at this time), the clearance X is reduced by 17.7mm (12.7 mm+5 mm), and the clearance Y is reduced by 7.7mm (12.7 mm-5 mm). At this time, there is basically no problem with scraping the body parts when installing the wheel on the vehicle.

When the clearance X>0, the clearance Y>0, the wheel installation is OK,

Ideal installation effect: clearance X=0, clearance Y>0.

Change of wheel J value or ET value will affect clearance X, clearance Y dimension:

If J value becomes larger, the clearance X and the clearance Y both become smaller;

If ET becomes larger, the clearance Y becomes smaller, and the clearance X becomes larger;

If ET becomes smaller, the clearance Y becomes larger, and the clearance X becomes smaller.

Finally, let’s look at an actual tuning case：

Mazda3 Axela change to 18*8J wheels

Original wheel specs: 16*6.5J ET50. Measured clearance X: 27mm

So how much ET can achieve the effect that the wheel is flush with the fender?

After tuning to the 18*8J wheel, the changes of the clearance X and the clearance Y are as follows:

The clearance X and Y are both reduced: (J value of tuning wheel – J value of original wheel)/2*25.4mm=(8-6.5)/2*25.4=19.05mm(Note: 1J=25.4mm). Since the clearance X before tuning is measured as 27mm, the X clearance at this time is 27-19.05≈8mm. If the wheel is flush with the fenders, it needs to be moved an additional 8mm to the outside. This 8mm needs to be achieved by reducing the ET. So the final ET should be 50-8=42mm.

As a result,18X8J ET42 can make the vehicle’s wheel flush to the fender.