HOW TO MEASURE BACKSPACING FOR MRW WHEELS

1. Place the wheel (without tire) on a flat surface like a kitchen table or flat work bench with the valve stem up toward the ceiling.
2. Put a ruler down through the center hub hole.
3. With the ruler touching the table notice where the bottom side of the center touches the rule- That's what we call the wheel backspacing or offset. 

4. HOWEVER   If you plan to purchase a wheel with more or less backspacing or are changing to a different size or width wheel or tire  take special care to make certain the inside diameter of the bell of the wheel clears  brake caliper (#3 diagram) or brake shoe, and also when you turn full left or right the tire or wheel will not rub the vehicles body, tie rods, struts, or other suspension parts at maximum spring compression or no compression.

A. INSIDE RIM LIP

B. DISHED CENTER

C. INSIDE VALLEY EDGE

D. CENTER WELD POINT

E. OUTSIDE VALLEY EDGE

F. OUTSIDE RIM LIP

RIM SHELL DIMENSIONS

"G" = RIM ID DROP OR VALLEY

13”

11.248”

15”

12.620”

16”

13.479”

16.5"

13.479”

17”

15.470”

18”

16.250”

20” = 18.375”

20” = 17.375” MILITARY

"X" = BELL DEPTH

3”

3 ½”  AND 5”

3 3/4"

3 ½"

4”

3”

2”

HOW TO CONVERT OFFSET TO BACKSPACE 

Zero (0) offset means the dead center of the wheel . 

Plus (+) offset is how far towards the outside of the wheel from dead center = (more backspace) . And Minus (-) is how far towards the inside the wheel from dead center = (less backspace) .

OFFSET TO BACKSPACE EXAMPLE 

1" inch plus offset on an 8" inch wheel Converts to approx. 5 1\4" inch backspace ---------- as you will need to include the width of the lip of the wheel ---------

 1" inch minus offset on an 8" inch wheel Converts to approx. 3 1\4" inch backspace ---------- as you will need to include the width of the lip of the wheel ---------

lYOUR BEST BET

When you're ready to buy a new set of wheels, measure the backspacing of the wheels that you know work - the ones on the car-. 

The the backspace (sometimes called offset) is the distance from the inside surface of the dished center (B) to the inside rim lip (A). (technologically the offset of a wheel is determined by measuring plus or minus from the center line of a rims width,  So this dimension changes for each rims width. So for simplicity MRW will use the tried and true backspace measuring terminology).  

On a 15x8" wheel this backspacing dimension can be approximately anywhere from 1" to 5" for modifieds or 1" to 4" for stock cars, This holds true for 15X8,  15x10" , 15X12" wheels etc. because in most cases  the only dimension that changes is the width of the 'valley.'( NOTE  in 7"width wheels and under (X) is decreased ).  

While it may appear that the backspacing range should increase with the width of the valley, actually the critical dimension is the distance between the inside rim lip (A) and the inside valley edge (C). In the illustration above you will see that it is labeled (X) and has an approximate dimension of 4". Regardless of the width of the wheel  8"and above, X  will  be approximately  4" and Z will be approximately  1.5"  . (There is an exception to this in our 'Wide Back Bell' shells. In this case X is increased to approximately  5".)  

-Notice when measuring,-  (H) & the inside depth of the bell must clear the  brake calipers (3) or drum , rim lip (A) and the bulge of the sidewall and the outside diameter of the tire must clear the vehicles body, steering and suspension assembly when turned to the extreme left or right at full spring compression or zero weight no  compression  .

MAXIMUM BACKSPACING

If (I) x2 (times 2) is greater than 12.125" (G), obviously your maximum backspacing must be somewhere under 4". Otherwise the caliper will hit the valley of the wheel.

MINIMUM BACKSPACING

The minimum backspacing is determined by the 'dish' of the center and the Back Bell Depth (X). While the typical center is 1-1/4" deep, part of that is the weld surface, leaving only a part of the actual center depth to work with. Also, the actual flat weldable part of the valley is shortened by the curve of the valley wall.

Placing the center at the extreme usable edge of the valley gives only an approximate  3"?  backspacing.

SO HOW DO WE GET LESS THAN 3" BACKSPACING?

By turning the wheel around and putting the Front Bell on the inside. This time when we put the center at the extreme weldable surface of the valley, we achieve and backspacing of approximately 1".

Normally, you don't need to worry about how we do it when you order wheels from us. We've been doing it a long time. We automatically orient the wheel to get the specified backspacing. If it's from 1" to 5", or more.

COMPLICATIONS

While we fabricated our shells (rims) and centers to exact specifications, the actual 'dish' and 'weld surface' of the centers can be different from the figures given here. For example, the dish of a Wide 5 bolt pattern is less than a 5 on 5 pattern. Some centers are only available in a flat configuration - with no dish at all! A flat center changes the backspacing considerably "

Another complication is the matter of other support and steering members installed on the vehicle. It is possible to build a wheel that clears the hub and brake calipers but still won't work because it bangs into the steering assembly, suspension, or vehicles body when the front wheels are turned. Not good. (See above)

YOUR BEST BET

When you're ready to buy a new set of wheels, measure the backspacing of the wheels  that you know work - the ones on the vehicle- .

HOW TO MEASURE BACKSPACING

1. Place the wheel (without tire) on a flat surface with the outside up (hub side down).

2. Put a ruler down through the center hub hole.

3. note where the bottom side of the center plate strikes the ruler - That's the backspace.

THERE IS NO SUCH THING AS A 'STANDARD' BACKSPACING OR OFFSET!

There are too many variations between car and axle manufacturers. 2", 3", and 4" backspacings are popular and common, but not standard. We depend upon you to give us accurate backspacing measurements. If you don't, you may have just purchased a very expensive set of paperweights.