Whether your allegiance is to the classic Pontiac muscle of the early '60s or the newest generation of GTO and G8, you have likely noticed the trend of mounting larger wheels and tires on just about anything that rolls. For most considering such an option, one universal question always pops up: How do I determine the largest wheel-and-tire package that will fit?
Often owners rely on the experience of others with a similar model to help them, but this is not foolproof. And what if you have a non-traditional Pontiac and you don't know anyone who has stuffed big wheels and tires under one like yours?
Percy's High Performance has addressed the problem by offering an affordable measuring device, the "Wheelrite." The innovative tool was a past winner of the prestigious SEMA International Product Award, and is quickly becoming the standard tool of professionals in the wheel-fitment industry.
The Percy's Wheelrite (PN...
The Percy's Wheelrite (PN 01201) is available directly from the company for $99. Capable of measuring wheel diameters between 15 and 30 inches for four and five-lug applications, it can support wheel-width measurements of 6-11-plus inches, and identify the proper backspacing to allow an enthusiast to confidently order custom wheels.
According to Ryan Alder of Percy's, "We developed this product to address the issues an enthusiast faces when he tries to figure out what wheel-and-tire package will fit on his car. The inspiration came from projects where we made costly mistakes because we couldn't accurately determine how the proposed wheels and tires would fit into the front and rear wheelwells without actually buying and mounting them.
"The market has really responded well to the Wheelrite, and we now offer a plastic version for the enthusiast and a metal version for the professional wheel-and-tire service provider."
Before you begin measuring, determine your build goals. Is the wheel/tire package just cosmetic, or are you including suspension and brake upgrades to maximize performance? If not, then the job is considerably easier, as the variables of whether the upgraded brake package will fit inside the wheel becomes a non-factor. Keep in mind that aftermarket suspension systems typically improve upon the front suspension geometry, and sometimes provide additional clearance for larger wheels.
To simulate the tire size...
To simulate the tire size that you desire, either visit a local tire dealer that will allow you to bend the tire simulator wire over a tire, or find a candidate with a similar tire profile. Here David DuSold of DuSold Enterprises is bending the wire over the rear tire of an '09 Ford Shelby GT500, which runs 9.5-inch wheels and 285/40ZR18 Goodyear Eagle F1 Supercar tires in the rear (shown) and 255/45ZR18s in the front.
Follow along as we utilize a Percy's Wheelrite to take you through the procedure to measure and simulate a wheel-and-tire package that is being engineered for aggressive street driving and road-racing with a '77 Can Am.
The wheel of choice is the YearOne Billet Snowflake in an 18-inch diameter. This size was required to ensure adequate front-caliper clearance for a KORE3 Big Brake Kit, which is based on C6 Corvette Z51 Performance Package production parts. The suspension is a G-Plus System from Global West.
Stories on these products will follow, but they are important to note for this article because affect the decision-making process, as compared to a Pontiac fitted with stock suspension and brakes.
The Wheelrite is packaged...
The Wheelrite is packaged with everything necessary to measure wheel width and backspacing. It includes a mount for the bolt pattern, a telescoping arm to lock in the wheel diameter, a set of horizontal bars, and a simulator wire. Instructions make it a snap to assemble and adjust. Note the wheel-width gauge (top), and the hub and wheel diameter assembly (bottom).
Wheel And Tire Measurements
Provide the parameters numbered 1-4 below to your retailer or wheel manufacturer. For example, our YearOne Billet aluminum Snowflake rear wheels measure 18x10 inches with 6 inches of backspacing for the rear, as shown, while the fronts are 18x10 with 5.5 inches of backspacing.
1 Backspacing - This is measured from the hub-mounting flange to the lip of the rim. It's easer to measure without a tire mounted, using a straight edge across the rim. Note here that the 18x10-inch YearOne billet-aluminum Snowflake for the rear was made with 6 inches of backspacing. An original Pontiac 15x8-inch Snowflake has 4.5 inches of backspacing.
2 Rim Diameter - This is the measurement of the diameter of the wheel from where the tire bead sits if you are viewing the wheel from the front. Rim dimensions are stated generally in inches (such as 16, 17, and 18) except in certain special applications.
To better simulate the vehicle's...
To better simulate the vehicle's suspension position on our subject '77 Can Am, a set of 15x8 Snowflake wheels shod with 255/70R15 Goodyear Eagle HR tires were borrowed from Bob Cook. Measurements are taken from the ramp floor of the four-post lift to the centerline of the hub, and to the fender edge closest to the tire (door side) on the passenger side. This is repeated on the driver's side. When weight was applied to the rear end to simulate load, there were 5 inches of clearance from the top of the tire to the top of the wheelwell (29.05-inch tire), and 1 inch of clearance at the narrowest point on the fender lip to the sidewall of the tire and inside to the frame.
A power jack is positioned...
A power jack is positioned under the rear end of the car and lifted so that the left rear wheel can be removed. The Wheelrite is installed with the 285mm tire-simulator wire over the rear hub with five lug nuts, hand-tightened so as not to distort the plastic tool and throw off the measurements. It is critical for accuracy that the Wheelrite is set up at the same suspension height as the car was when measured on a level surface with both wheels.
The distance from the centerline...
The distance from the centerline of the hub to the fender edge is recorded, as well as the distance from the floor to the hub center. It was quickly determined that the rear end of the car wasn't centered on the jack, as the two measurements didn't match those previously recorded. The Wheelrite was removed and the setup process was repeated.