Wheel Statistics

Bonnie at Bonneville 2005

Wheel Statistics

by Peter Webb

Nothing makes a car look better than aftermarket wheels. It’s the first thing most people change when they buy a car new or used. The problem is, Alfa Romeo’s do not have many wheel options mostly because of strange bolt patterns and offsets.

First a little background on all the wheel statistics you see thrown around:

• Width and Diameter – specified as diameter by width. A 15x7 wheel is 15 inches in diameter and 7 inches wide.

• Bolt Circle – number of lugholes and the spacing. Sometimes called bolt spacing. Common spacing for an Alfa is 4x108 for 105/115 cars, 4x98 for 116 cars and 5x98 for 119 and 164 cars. Simply, this means 5 holes spaced 98mm apart at the pentagram or 4 holes spaced 108mm on the diagonal.

• Offset – the number of millimeters the wheel hub is off the exact centerline of the wheel. A +35 offset means the wheel hub is 35mm closer to the back of the wheel than the front. More offset means the wheel is closer to inner fender.

• Backspacing – the number of inches from the inside wheel rim to the face of the wheel hub. This differs from offset because a wider wheel will have more backspacing from the rear lip. The offset can only be measured from the centerline.

• Center bore – the diameter of the hole in the center of the wheel. This is important because it has to clear the hub diameter and it should fit snug on the hub to make the wheel what’s called hub-centric.

• Hub-Centric – where the wheel bears it’s weight on the hub rather than the lugs themselves. This is a critical safety dimension because the stress of the lugs themselves can yield under hard cornering.

Many people use spacers to obtain the correct offset and tire clearance to get larger wheels and tires to fit. The problem with spacers is the limited thickness you can use and still have enough lug to attach safely. You can use 1⁄2" spacers at most on stock lugs.

The safe alternative is a spacer than has it’s own lugs pressed in. The spacer has holes that bolt to the existing lug nuts safely and the wheel bolts to the lugs pressed into the spacer. The thickness is almost unlimited with this solution. However, these spacers are expensive.

While considering this kind of spacer for my car, I had a radical idea. What if the spacer had a different bolt circle pressed in? No more searching for oddball wheels with specific offsets. I can pick a common bolt circle, in my case 5x100 and set the spacer thickness to calculate a common offset, in my case +35ET.

I contacted Charlie DiMarco at Serpent Autosport in California (http://www.serpentautosport.com/) He was able to make the spacers for me for about $250. Sounds expensive but when you consider a nice 17" wheel for a Milano or GTV6 is over $300 each, they pay for themselves with the first set of wheels.

I opted for 5x100 because of the wide range of wheels and offsets. Common 5x100 applications are Volkswagen at +35ET, Toyota at +40ET and Subaru at +48ET. There are other cars that use these patterns too, most notably the Dodge Neon with a wide range of 15, 16 and 17-inch wheels. Most aftermarket wheels have pockets between the lugholes so any lug that protrudes past the spacer will tuck nicely in the pockets.

I opted for a 22mm spacer. When coupled with a +35ET wheel, it increases the track about 1 3⁄4” for a performance gain. This can be a tricky dimension because wide tires are difficult to fit inside an Alfa wheel well. I picked this number specifically because under the AROC Comp Code, it’s the maximum track increase without incurring point penalties for the modification.

Stock offsets vary from model to model. A typical transaxle car will use a +35ET and 58.4mm centerbore. A 15mm spacer and +40ET wheel will increase the track slightly and still fit in the stock wheel well. Most 105/115 cars use a +15ET.

A note about tire fitment: I have found through experimentation that a transaxle car can squeeze a 215 tire and a maximum of 24-inch diameter. A GTV can accommodate a 225 tire. A Giulia Sedan can only squeeze a 205 tire because of the narrow rear wheel wells. A Spider can take a 205 but you can roll the wheel arches to take a 225. All have a maximum diameter of 25 inches.

When shopping for tires, you should pay attention to the section width more than the tread width. An 8” section width is the absolute maximum for most any Alfa. Also pay attention to the overall diameter. Shoot for 24 inches as a maximum number.

If you are racing and/or time-trialing, pay particular attention to the rules. Modifications to increase the tire size or even the tire type could incur points penalties or even preclude you from a particular class. Track increases are prohibited in some classes as are spacers.