To measure the available room in the wheel well, you have to create a "plumb bob." As you can see, it doesn't have to be fancy: A piece of string with a weight on one end will work (a low tech lug nut works for a weight too). A dropped plumb bob affixed to the inner wheel well lip works as a reference point for taking measurements. Just be sure to steady the thing before jotting down critical dimensions.
Two points of possible interference is the leaf spring or a trailing arm. Typically, many manufacturers "splay" leaf springs and suspension members. That means they don't necessarily run straight back and forth. Usually, the springs angle out toward the rear. The same applies to cars with coil springs and lower trailing arms. Because of this, some cars with large backspace dimensions might encounter interference problems with the trailing arm or spring before anything else comes in contact with the tire.
The distance between the plumb line to the face of the rear brake drum or disc hub (the area where the wheel bolts to) should be measured next. Basically, this dimension determines the amount of room for the wheel (and tire) "face." Subtract a half-inch as a clearance figure. The figure will give you the maximum "face" or curb side wheel width your car can handle.
Measure the distance between the plumb line and the inside of the factory wheel well or to any suspension component that "intrudes" into the well. In this case, the shock was the closest component. Due to the OE wheel well configuration, there can very well be several different dimensions. Use the smallest figure. Subtract 1.0-inch from the total (for overall clearance). This is the maximum wheel and tire width your car can accept.
Place a straight edge across the wheel mount flange (disc brake hub or brake drum face where the wheel mounts to). Measure back to the closest suspension member or inner fender sheet metal or frame section and subtract 1/2-inch. This is the maximum wheel backspace your car can accept. Be sure to double-check everything for clearance (see next photo).
|Measure Before Purchase|
Virtually all cars (trucks too) have different wheel well dimensions from one side of the car to the other. Our Buick was no exception. In addition, some cars have staggered shocks (one ahead of the axle, one behind it). This can sometimes create some real clearance headaches. Measure both sides of the car before buying wheels and tires.
|Full Lock Clearance|
The process is exactly the same at the front, however you have to keep in mind the front of the car is more dynamic than the back. The wheels not only go up and down, they turn. As result, it’s a good idea to turn the steering to full lock and check the clearance available between the wheel/tire combination and the frame or other sheet metal or suspension hardware. In addition, it’s also a good idea to bounce the car (on the suspension) to determine if there are any clearance issues when the suspension is compressed.
|About Back and Face|
Backspace and wheel face dimensions are measured this way. Flip a mounted 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 and measure down to the mounting pad. This figure is the wheel backspace with the tire "bulge" included. The dimension without the tire mounted is the true wheel backspace. If you take a measurement from the wheel rim flange down to the disc/drum-mounting flange in the wheel center, you’ll get the actual wheel backspace (without the tire bulge). Turn it around and take the same measurements at the face. Don’t forget to take the actual thickness of the wheel center into consideration when performing your calculations.
|If you use a stock wheel for these measurements and couple them with your wheel well dimensions, you can determine what aftermarket wheels and tires will fit and what won’t. Don’t forget to take the actual thickness of the wheel center into consideration when performing your calculations.|