Have you ever wondered how some people can fill the fenders of their Chevys full of rubber (particularly late-models with all sorts of "intrusions" inside the factory wheelwell), without resorting to major re-constructive surgery (i.e.: tubs, narrowed frame rails, narrower-than-stock rear axles, etc.)? Sure, you can carve up the wheelwell, but the first thing to do is to figure out just how much "tire" can actually fit inside the existing well-without creating interference on the inside.
If you haven't gathered it by now, selecting the right wheel backspace (or "offset") for wheels is critical on two counts: (1) Sliding the biggest wheels and tires into the smallest space and (2) saving you the most amount of money. Adding big rubber in a Chevy that isn't tubbed isn't a problem, but it can provide more than its fair share of headaches. In this case, one mistake on the backspace dimension will mean the wheels and tires don't fit. What about jacking the back of the car up? It's usually a waste of time and traction. We should also point out that it's no secret that most shops simply won't accept an exchange if the wheels show any signs of mounting (can you blame them?).
Wheels are available in a wide array of dimensions. For example, you might be able to purchase a 15 X 10-inch wheel with backspaces that range from 2 inches all the way up to 6 inches. This means that the tire can either be tucked-up under the stock fender of your car (good) or it can hang out in the breeze (not so good).
Quite frankly, there are a few ways to determine rear wheel dimensions. Some rely upon hit and miss tactics, but the best method is a bit more complex. It involves taking some careful measurements to put all of the dimensions into perspective. By using this method, you can easily calculate the required backspace dimension, and of course, the maximum size of tire that will fit into your existing wheelwell. Here's the drill:
First, locate a wheel with a backspace/offset dimension that's as close to stock as possible. Flip a mounted OEM wheel and tire over so that the brake disc/drum pad-mounting flange is facing up. Place a straightedge across the face of the tire and 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. The difference between the "mounted" and "dismounted" numbers will give you an indication of the clearance required for the tire sidewall "bulge." Consider this your "baseline" backspace dimension.
You're not done yet. Most late-model Chevys can swallow a bit more backspace than stock (this also works if you're messing with rearend housings that have been narrowed slightly). You need to take some careful measurements to put all of the dimensions into perspective (you can make a few rough sketches to help).
In most Chevrolets with wider-than-stock wheels, you will find that you can increase the backspace by at least 1 inch from the OEM combination. Keep this in mind when purchasing wheels. Another problem you might run into when "stuffing fenders" is the lip that intrudes into the well. The "bulge" of the tire often smacks the 90-degree lip at the top of the well as the suspension goes through its travel. The solution is simple: The upper 90-degree section of the wheelwell lip (found inside the curbside of the fenderwell), has to be cut and bent inward, which in turn creates a smooth transition from outer body sheetmetal to inner wheelwell.
Yet another problem area can be the overall length of the wheelwell. The "length" of the wheelwell is critical, especially if you want to slip tall tires under your car. If you measure wheelwells, you'll find that many aren't really "long" enough to handle most big-diameter tires (including slicks, if that's your weekend cup of high-tech tea). This simply means that tall tires will sometimes rub on either the leading or trailing edges (occasionally both) of the outer wheelwell. The real solution is to create more length at the front or rear of the well. In order to accomplish this (and it's really beyond the scope of this article), the 90-degree lip at the leading (or trailing) edge of the well should be cut out, taking care not to remove the piece entirely. Next, a very small pie-shaped (approximately 11/2 inches wide) cut is made in the sheetmetal from the bottom up. Weld the works back in place, filling the pie-shaped cut.
In many late-models, this surgery isn't required at the rear of the outer well. If everything is welded correctly, only a small amount of plastic filler is needed to smooth the area. This trick is very subtle and it takes a very well-trained eye to spot the rework. If you follow these steps, you can stuff a 29.5- or 30-inch tall tire, (such as a drag slick), under a late-model car without tubs.
Armed with the dimensions gained from the above guidelines, you can then make a rational decision in regard to 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.
The Routine Goes Like This:
* Create a "plumb bob" from string and a weight (a lug nut works). Tape the plumb bob to the inner lip of the fender. This is your major point of reference.
* Measure the distance between the plumb bob to the face of the brake drum (the area where the wheel bolts to). Subtract 1/2- to 1 inch as a clearance figure. The final figure will give you the maximum "face" or curb-side wheel width you can handle.
* 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 shape, there can very well be several different dimensions. Use the smallest figure. This is the maximum backspace your car can handle.
* Take the tire "bulge" calculation you made earlier and subtract it from the above maximum backspace dimension. Subtract another 1/2- to 1 inch as a clearance dimension. This is the largest possible overall rear backspace figure that will fit into your existing fender-wheelwell opening.
* If you add the maximum face or curbside dimension (step 1) to the final figure in step 4, you'll end up with the widest possible wheel that can fit inside your stock wheelwell.
Virtually all built-in-Detroit cars (trucks too) have different wheelwell dimensions from one side of the car to the other. Our late-model F-body was no exception. In addition, some Chevys have staggered shocks (one ahead of the axle, one behind it). This can create some real clearance headaches. Be sure to measure both sides of the car before buying wheels and tires.
Another point of possible interference is the rear trailing arm (such as this) or, in the case of a late-model truck, the front segment of the leaf spring. Typically, GM "splayed" 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 late-model 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.
As mentioned in the text, the leading edge of the wheelwell often creates another interference point for tall rubber. In this case the leading edge of the outer wheelwell has been "pie-cut." This cosmetic surgery creates approximately 1 1/2 to 2 inches of added clearance at the front. A tall tire can easily fit in this wheelwell. If your car has the same clearance problem, you might be forced to perform external metal surgery, too.
At the back of the F-car, the same clearance dilemma entered the picture. This wheelwell segment hasn't been cut yet (in this case, it's a plastic fascia panel). As you can see, the clearance is much tighter here than in the front. What's the solution? We'll cut and paste the plastic (in this case), then stitch it back together. The same process can be done to metal. The text offers some very basic info on the process.
Once you have your new wheels and tires, test fit them (sometimes it's good to test the wheels before the tires are mounted). What you're looking at is a 15X10-inch Monocoque race wheel stuffed into a stock-width, late-model wheelwell. Keep in mind that the application called for a 9-inch wide slick. As a result, the tread width of the tire is actually a bit narrower than the tire bulge (this is a common Stock Eliminator racer trick, and it also applies to most weekend warriors as well).
A bird's eye view shows that the tire bulge is actually inside of the wheelwell on the top. It's close, but it does fit. Remember to check the clearance numbers again with the unmounted wheel. It's possible to return the wheel at this point if you screwed up. Once the tire is mounted, the wheels are probably yours.
This view shows the rear. Since the car was under construction, we blocked everything up to ride height so as to double check all of our measurements (which we took a long time ago). It's not a fancy process, but it works. Copy the idea. It works just as well on other under-construction late-models, no matter what make. As you can see, the tires are tucked up inside the original wheelwells, and yes, these slicks are much larger than the original factory rubber (especially in terms of overall diameter).