Have you ever wondered how some people can fill the fenders of their cars with rubber without severely hacking up the sheetmetal or committing it to major reconstructive surgery (i.e., tubs, narrowed frame rails or narrower-than-stock rear axles)? The first step might include massaging the inner wheelwell, but a more refined method is to determine what rear-wheel offset or backspacing can be used in conjunction with a given rear axle width—without creating interference on the inside of the wheelwell.

Wheels are available in a wide array of dimensions. For example, a 15x8 wheel in a given style might have backspacing that range from two to six inches. This means that the tire can either be tucked up under the stock fender, or it can hang out in the breeze. The current trend favors the "tucked up" examples (thankfully). Because of this, it makes sense to check and double-check dimensions before buying custom wheels.

Hit & Miss
Rear-wheel dimensions can be determined in a few ways. One popular method is like tossing darts at a board. If you hit and miss, you are out, but if you are on target, you score some points. This method simply involves bolting the new rolling stock to the rear axle housing, lowering the car over the wheels and tires and praying that everything clears.

Backspacing
Another method is slightly more complex: It involves taking careful measurements coupled with some rough sketches to put all of the dimensions into perspective. Selecting the right wheel backspacing (or "offset") is critical for two reasons: sliding the biggest wheels and tires into the smallest space, and saving the most time and money. Stuffing fenders might not be a big problem for race cars with mondo tubs, but if you "campaign" your street machine on the weekend, clearance concerns can provide more than their fair share of headaches—especially if the car is wheelwell-limited. In this case, one mistake on the backspace dimension will mean that the wheels and tires do not fit (or the car isn't legal).
Luckily, lightweight race wheels are available in myriad dimensions. Believe it or not, many OE wheels are also available in a plethora of backspaces, so it only makes sense to check and double-check the dimensions. In order to measure backspace, try one of the two methods that follow:

OE Measuring
Flip a mounted OE wheel and tire over so that the brake disc/drum pad mounting flange is facing up. Place a straight edge across the face of the tire. Using a second straight edge, measure down to the mounting pad. This figure is the OE backspace with the tire "bulge" included. The dimension without the tire mounted is the true wheel backspace. This figure should be used as a reference point when purchasing new wheels. The difference between the "mounted" and "dismounted" numbers will give you an indication of the clearance required for the tire sidewall "bulge." Using this method, you can easily purchase an aftermarket wheel that is wider than stock, but has an actual tire contact patch that's exactly the same—or close enough to—the factory dimensions.

If you don't have a factory wheel to measure or if the car has a narrower- or wider-than-stock rear axle, you'll have to use the following method to determine wheel offset. It involves some careful measurements to put all of the dimensions into perspective. By using this method, you can calculate the backspace dimension and also the required rear axle width if the housing has been narrowed.

Custom Measuring
1. Create a "plumb bob" from string and a weight (a bolt will work as a weight). Tape the plumb bob to the inner lip of the fender. This is your major point of reference.
2. Measure the distance between the plumb bob to the inside of the fenderwell or to any suspension component that "intrudes" into the well. Due to the fenderwell configuration, there can several different dimensions. Use the smallest figure. You can make a small drawing of the wheelwell, showing the narrowest possible inside dimension. Call this dimension A.
3. Measure the distance between the plumb bob to the face of the rear brake drum (the area where the wheel bolts to). Include this dimension in your drawing. Call it dimension B.
4. Subtract dimension B from dimension A. Subtract a minimum of one inch as a clearance dimension and call it dimension C. This is the largest possible overall rear-backspace figure that will fit into your existing fender.
5. Subtract a minimum of one inch from dimension B. Call it figure D. This is the largest possible curbside or face width of the wheel in question. (Subtracting an inch from steps 4 and 5 above allows ample clearance in case the tire moves or "wrinkles." A smaller fudge factor than this might allow rubber to rub against sheetmetal or suspension components.)
6. Figure D added to figure C is the maximum wheel-tire width possible in your application. Keep in mind the tire width with the "bulge" included or "section width" is what you are striving to obtain—not simply the overall wheel width.
7. Flip over a new wheel so the brake drum pad mounting flange is facing up. Place a straight edge across the face of the wheel and using a second straight edge, measure down to the mounting pad. This figure is the backspace with the wheel without the tire "bulge" included. The dimension without the tire mounted is the true wheel backspace. This figure should be used as a reference point when purchasing new wheels. Earlier, we looked at "mounted wheel backspace." The difference between mounted and dismounted backspace is basically the tire bulge. The difference between the "mounted" and "dismounted" numbers will give you an indication of the clearance required for the tire sidewall "bulge." Just remember the tire will "bulge" on both the curb and the brake side of the wheel.

If you have decided to build a corner-burner with fat front rubber, the basic process is the same but with a few more variables. The steering must be turned to full lock on both sides, and measurements should be taken to the closest sheetmetal, suspension, and/or steering components. Typically, the tire will rub first on the inner wheelwell. Additionally, the car should be "rocked" on the suspension—which is tough to do if the car is heavily sprung—to ensure that suspension movement doesn't create more interference.

In most situations that include wider-than-stock wheels, increasing the backspace by at least one inch from the OEM combination is possible. Keep this in mind when purchasing aftermarket wheels. Armed with the dimensions gained from the above guidelines, you can make rational decisions regarding rolling stock. Just remember to measure the wheel backspace dimensions before you lay down the cash. Aside from wheelwell clearance (sheetmetal interference), the backspace dimension is the most critical of all wheel dimensions.

In many cases, surgery such as this is not required at the rear of the outer wheelwell. Once the welding process is complete, a small amount of plastic filler is used to smooth the area. In the end, the modifications are very subtle and it takes a very well-trained eye to determine that the wheelwells are actually reworked. If you follow these steps, you can fit a 29.5"-tall drag slick under a "tight quartered" car without wheel tubs.

Valvoline.com

 

Good write-up matchframe.
Thanks for taking the time with a very informative post.

 

I wish I wrote it!!

I just copied and pasted it from the Valvoline site:
Valvoline.com

I thought it was good information and wanted to share it!




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