Magazine for the professional tire industry
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suspension are more likely to have a positive scrub radius. Chassis engineers use scrub radius to create the feedback the driver feels through the steering wheel. If it were zero (steering pivots exactly on the center of the contact patch), the steering would feel loose, especially during acceleration and braking, because the front tires would tend to squirm like the wheels on a shopping cart. So now we've seen how changing wheel offset moves the center of the contact patch inboard or outboard, which changes the scrub radius, which changes the way the car feels and handles during acceleration and braking. Before installing wheels with a different offset, you need to figure out if those wheels will increase or decrease the scrub radius so you can let the customer know how the car's handling might change. Wheel selection One of the more common reasons people buy aftermarket wheels, at least up North, is to make it easier to switch over to winter tires in the winter. If the car has cast wheels, customers often buy steel wheels for their winter tires because they (usually) cost less and, more importantly, they can avoid exposing their cast wheels to salt and potholes. It's not always easy to fnd steel wheels that match the size of the OEM cast wheels, but it shouldn't be hard to match the offset. Customers who want after- market wheels for their appear- ance often want something bigger than the OEM wheels. If they ask your advice, do a little homework frst. Search the Web forums to find out if anyone else with the same vehicle wrote about their expe- rience with bigger wheels. Did they need spacers to make the wheels clear the brake caliper? Did they note any handling or tire wear issues? Did they use different alignment settings? Even if you don't fnd much information, the questions alone could help your customer recognize the risks that come with installing oversize wheels. If your customer already owns the wheels and asks you to install them, you'll need to check the offset yourself. Most alloy wheels have the size cast into the inside of a spoke or the mounting fange, and the last digits in the string of letters and numbers defnes the offset. For example, size 7 J x 1 7 E T 4 5 t r a n s l a t e s a s follows: • "17" is the wheel diameter. • "7" is the wheel width between the bead seats. • "J" is a classifcation of the rim shape. • "ET45" is the offset; in this example it's 45 mm positive. You may already know that BMW and Mini Cooper wheels are not like everybody else's. In the wheel shown bottom left, the last three digits in the size specifcation indicate the wheel is made for run-fat tires. The offset appears below it: "IS48" means the offset is 48 mm positive. How to measure offset Sometimes the offset isn't printed on the wheel, or it might be printed in digits that are diff- cult to read or impossible to decode. No worries; it's easy to measure offset yourself. Technically, offset is the distance from the hub mounting fange to the centerline of the wheel between the mounting beads. Since you can't measure between the beads with the tire on the wheel, here's an easy way to fnd offset by measuring from the tire sidewall. Lay the wheel/tire assembly on the foor and place a straight edge across the tire (see photo on page 12). Measure the distance from the foor to the straight edge and write down that number. Divide that number by two to calculate the centerline of the wheel. Now measure from the hub fange to the straight edge (see photo on page 14). Subtract the smaller number from the larger number. That gives you the offset of the wheel. If the centerline number is smaller than the hub measure- ment, offset is positive. On most cars, changing offset by just 5 mm is enough to notice a change in the car's handling because it changes scrub radius Above: On this Mini Cooper wheel, "EH2" at the end of the size spec means the wheel is de- signed for run-fat tires. The "IS48" below the size indicates offset. Below: The size on this Jeep wheel shows the offset is 56.4 mil- limeters. Using the technique described, our measurements were within 1.2 mm, close enough for our purposes. 10 Feature