In Part I we looked at wheel weights. And while weight is critical, something else is hugely important for wheel performance – wheel offset. The industry is filled with virtually any combination of wheels with specific standard or custom backspace dimensions. So what’s right for your car?
In some cases, there is only one possible backspace dimension for a specific car. But if you’re building a fresh car, then the sky is pretty much the limit when it comes to backspace. Believe it or not, some backspace dimensions can offer distinct advantages over others. But before we get into the differences of backspace dimensions, lets consider the actual wheel combination along with the car.
In order to solve a “tight quarters” wheel dilemma in a car with OEM tubs and wheel wells (a legal NHRA Stocker is a good example), then you’ll have to work diligently on measuring the car to fit the rolling stock. This doesn’t necessarily mean that wheels with the same backspace and overall width as stock are mandatory. Today, you can purchase something like a 15 X 10-inch wheel with backspace dimensions that range from two or three inches all the way up to seven or eight inches. This means that the back tire can either be tucked into the stock wheel well or it can hang out in the breeze.
If you don’t have a set of OEM wheels in your possession to use as a reference or if the car has been tubbed and has a narrower-than-stock rear axle, you’ll have to use the following method to determine wheel backspace. It’s slightly more complex and it involves some careful measurements to put all of the dimensions into perspective (you can make a few rough sketches to help).
Step 1: Create a “plumb bob” from string and a weight (if you don’t have a real plumb bob a good old fashioned nut will work as a weight). Tape the plumb bob to the center of the inner lip of the fender. This is your primary point of reference.
Step 2: Measure the distance between the plumb bob to the inside of the fender well or to any suspension component that “intrudes” into the well. Due to the fender well configuration, there can very well be several different “intrusions” and of course, different dimensions. Use the smallest figure. You can make a small drawing of the wheel well — showing the smallest possible inside dimension. Call this dimension “A.”
Step 3: Next, measure the distance between the plumb bob to the face of the rear brake drum or the disc brake hat (the face area where the wheel bolts to). Include this dimension in your drawing. Call it dimension “B.”
Step 4: Subtract dimension “B” from dimension “A”. Subtract an additional 1.0-inch (minimum) 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.
Step 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.
Note: The subtraction of 1″ (respectively) from Steps 4 and 5 is included so that ample clearance is provided as the tire “wrinkles” or moves during your pass. If you choose to provide for less clearance (say, 1/2″ per side of the tire), then there is a chance of rubbing against sheet metal or suspension components.
Step 6: Dimension “D” added to Dimension “C” is the maximum wheel-tire width combination possible in your application. Keep in mind that the tire width with the “bulge” included or “section width” is what you are striving to obtain — not simply the overall wheel width.
Step 7: Flip over a new, un-mounted wheel so that the brake drum/disc 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 dimension for 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. 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 side wall “bulge”. Just remember that the tire will “bulge” on both the curb and the brake side of the wheel. Alternatively, if you have tires in hand (and the wheels are mounted), you can simply check the overall backspace and not bother with any OEM dimensions.
That’s not the end of it. You can also purchase skinny front drag race wheels with different offsets. For example, Champion wheels (and others), the manufacturer of the wheels shown in the accompanying photos offer a line of front wheels with larger-than-normal backspace dimensions. I’ll get into the reasons for this down the page. When it comes to the front of the car, the measuring process is the same, but there are a few more variables. The steering must be turned to full lock (both sides) and measurements should be taken to the closest sheet metal, suspension (and/or steering) components. Typically, the tire will rub first on the inner wheel well. Additionally, the car should be “rocked” on the suspension to insure that suspension movement doesn’t create more interference.
Aside from the correct fit on a car, there is a way to improve performance on a car by juggling backspace dimensions. Wheels with small backspace dimensions are physically lighter than those with large backspace dimensions, even when all other measurements are identical. In other words, a 15X14-inch wheel with a 4-1/2-inch backspace will be lighter than an identical wheel with a 7-1/2-inch backspace. Typically, this is an across-the-board situation with all wheel manufacturers. It’s no secret a contemporary race wheel/drag slick and combination is very light — much lighter than an axle housing tube and axle. By building a car with a narrow rear axle housing width, then making up the difference with wheel backspace, you can save a considerable amount of unsprung weight. An added bonus is the fact a narrow axle is also a stiffer axle. And speaking of axles, there’s something else to ponder: Wheels with small backspace dimensions are actually easier to remove and replace on a racecar. That’s something to consider when you’re trying to wrestle an eighteen-inch wide wheel and tire over a four-inch axle drive stud on a car with production sheet metal.
Larger-than-stock front wheel backspace dimensions can prove helpful when trying to clear disc brakes on the nose of a race car. Further to this, many modern cars have a relatively wide OE front track dimension. For the most part, Detroit is currently building vehicles with large backspace wheels. Some of these wheels have OEM backspace dimensions approaching 7-inches. This means the front wheel hub (and consequently, the wheel mounting surface) is moved outboard in relation to the curb. In some cases, a skinny 15X3.5-inch drag race wheel combination with a standard 1.75-inch backspace dimension will actually protrude slightly from the front fender when mounted. That’s not good. The solution is to use a special backspace wheel (Champion and others manufacture them). Front wheels are available with either a 1.75-inch backspace (typically standard) or a 2.5-inch backspace (deep backspace). In cars with extremely wide track measurements, the deep backspace front wheels can solve innumerable clearance problems.
Another point to ponder is suspension travel. In many cars (new and old), a deep backspace front wheel will allow the front of the car to move up and down without sheet metal interference. A standard offset or “centered” front wheel might not. Because of this, it’s a good idea to take the vertical clearance of the wheel (during travel) into consideration when figuring out front backspace. Jacking the suspension through its travel and taking measurements can accomplish this. In some cases, this might be easier said than done (as you’re jacking the suspension up, the entire car moves). The solution isn’t pretty: Remove the spring, and then take the measurements.
Armed with the dimensions generated from the above guidelines, you can determine backspace exactly. Just remember it’s easier to measure the wheel backspace dimensions once or twice before you buy your wheels.
Photo Wheel 7: