What is Fitment?

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What is Fitment?

INTRODUCTION

If you’re reading this, you’re probably curious as to what “Fitment” is. Instead of scrolling through all this text and guessing your Fitment, we highly encourage you give this a read.

Seriously.

Don’t skip this anything!

WHEEL OFFSET AND BACK - SPACING

You’ve FINALLY saved up the cash to get the perfect set of wheels for your car; now what? Your obvious first start is Google; right? You learn about your bolt pattern and spacing before heading off to Ebay to order some wheels. HOLD ON, Ebay? No way?

But did you know there’s a hell of a lot more to it than that? Offset, back-spacing, clearance, overall diameter, and oh so much more. So let’s drop some knowledge bombs for you.

– Wheel offset is the distance (in mm) that the hub of the wheel, (where you bolt the wheel to the car) is from the center of the wheel.

Example 1: +25 mm offset = The center of the wheel is 25 mm closer to the outside, or curbside, of the wheel.
(+) offset pushes the wheel farther into the caliper/brake. Extreme positive offset can result in poor overall fitment due to the wheel tucking, or even worse, suspension rubbing!

Example 2: =25mm offset = The center of the wheel is 25 mm closer to the inside, or brake/caliper, of the wheel. This is often seen on concave wheels or large lipped wheels. This pushes the tire out toward the fender making it flush on your favorite static car.


HOW TO MEASURE BACKSPACING (BACK OF WHEEL)


Backspacing
The easiest way to do it? Read the back of your wheel!

Wheel specifications, 18" with 8 width and 55 offset
This would be an 18" diameter with 8" width and +55 offset

BASIC CALCULATORS

Ready for some math? No? Ok.

Example 1: 205/60-15
Tyre width = 205 mm
Aspect Ratio = 60
Sidewall height is 60% of tire width) | 0.6 * 205 = 123 mm side wall
Wheel rim diameter = 15 Inches
Determining Diameter:
Tire: 205/60-15
= (2*205*0.6/25.4) + 15 = 24.69 inches diameter is same as “height”
Note: 25.4MM = 1 Inch
Formula from mm and ratio to inches:
(2 * tire width mm * tire ratio*) / (25.4) + rim Diam. (In inches) = Tire Height (in inches)
Width Tyre (Inches)
Tyre Width MM/25.4 = Width in Inches
Convert from inches to mm
Example 1.
Tire width = 8″
Tire height = 25″
wheel dia. 15″

FORMULA

1.) (Tyre Width (In.) * 25.4) = Width in mm
2.) Round to 5mm or 8 * 25.4 = 203.2 or 205mm
((Tyre height – rim diameter) / 2) / (tire width in * 25.4)

FORMULA FROM INCLES TO RATIO IN %

((Tyre height inches – rim diameter inches) / 2) / (tire width inches)= % again round to nearest 5 (60 or 65 using our previous numbers)

FOR WIDTH TYRE

width inches * 25.4 = width mm

SO WHAT NOW?

As the offset is moved toward the Curb Side (+ offset, remember?) – toward the fender – it has the opposite effect on fitment; moving it in toward the caliper which creates a larger gap between the outer edge of the tire and the outer edge of the fender. As the offset moves toward the caliper ( – offset) the wheel and tyre are moved out toward the fender and with a large enough negative offset the tire will stick out past the fender.

CAUTION! CAUTION! CAUTION!

Note: The following diagrams assume the wheel is the same width in both examples. Remember if the wheel width changes this will affect how much the offset will move the wheel.

Example: a 7.5″ wide wheel with a +25 offset will be “tucked” into the fender while a 10″ wide wheel with +25 wheel offset could be flush. How? You should account for the 2.5″ of extra wheel width. That means 1.25″ on each half of the wheel from the center line. Since every inch is worth 25.4 mm that means you have added 31.75mm of wheel toward the fender and another 31.75mm toward the caliper, even though the offset was the same.

LET'S GET BACK TO IT

Assuming you are keeping the same width wheel, this is what offset would do as you change from positive offset such as +25 to less positive such as +12 or past the center line of the wheel to negative wheel offset such as -12. First an example looking from the rear of a vehicle with a stock or positive offset. The suckin or tucked, factory look.

Wheel Fitment

Wheel Fitment Negative camber

This second diagram looking from the rear of a vehicle with a negative offset (same width wheel). This setup will give the flush or with a larger negative offset one can achieve the aggressive look as you move outside the fender.

BACKSPACING

So now that you know exactly what wheel offset and fitment is we will move on to the other word used by the custom wheel world. Backspacing. Not all that different however to keep you on your toes it changes where we measure from. We are still looking for the distance from the mounting surface of the wheel, however rather than measuring to the ‘center line’ of the wheel, backspacing is measured from the back edge of the wheel to the mounting surface. Like above.

Hopefully, this has cleared the air for you and you now know what fitment is.

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