So as part of the th anniversary of Modern Tire Dealer, we collected our wheel offset stories from and combined them into a shortened primer for measuring wheel offset. If it leaves you wanting more, we can accommodate you. Just follow these three simple steps: 1 Go to www.
It never hurts to be more informed about a topic so important to the handling and safety of a vehicle. With this in mind, there are three factors that consumers and service professionals must take into account when choosing and installing a different size of wheel. Plus sizing. When you are changing the size of the wheels, the overall diameter of the tire and wheel assembly must remain the same to keep the proper speedometer and odometer settings.
In this example, the process is called Plus-One sizing. The same theory applies to mounting wheels one-inch less than the originals; in that case, the process would be called Minus-One sizing. Proper wheel size. The wheel must be the correct and approved width for the size of tire that it will carry. Wheel offset. Perhaps the most important factor in choosing and installing new wheels is the proper handling of wheel offset.
Offset also can be one of the most difficult factors to get right when replacing wheels. However, it is important to get it right, because getting the offset wrong risks badly damaging the replacement wheels and tires as well as suspension components, and can put the driver at serious risk of a blowout.
Offset, scrub radius and contact patch are the only three technical terms we need to understand. However, offset might be easier to visualize with a few simple drawings that focus on the tire contact patch. When the surface of the mounting flange is exactly aligned with the center of the tire contact patch, offset is zero. If the flange is inside the center of the contact patch closer to the center of the car , offset is negative.
If the flange is outside the center of the contact patch closer to the curb , offset is positive see Figure 1. On most OEM wheels, offset is positive contact patch slightly inboard , and it can have a major effect on steering feel and stability during acceleration and braking. To visualize this, lay a pen on the table and hold the top end still.
Many factory rims have this offset type. Backspacing is an older system of measurement to determine how deep the mounting pad is located in the wheel.
Correct backspacing allows enough room for the suspension, brake, and steering systems to operate without interference from the wheel. Positive offset creates more backspace, while negative offset reduces backspace. The example shown here is 6 inches wide with a 3 inch backspacing. Backspacing is, therefore, the combination of the offset and the wheel width.
This comes into play if the new wheels are wider than the old, as the offset may need to change to compensate for the greater width. If you get the offset or backspacing wrong when you are procuring and installing new wheels for a customer, you run the very real risk of having the wheel sit too far inside the wheel well, causing the inner edge of the wheel and tire to rub against the suspension.
Tire rub is even worse, as it will very quickly rip up the inside sidewall of the tire, an area where the damage is very hard to detect until the tire blows out. What you do need to keep in mind is that more negative offset, i. Usually, this will not cause a problem, although it might result in a pretty funky look. In contrast, too much positive offset is a danger, as it will push the wheel farther in toward the suspension. But going from a 20 mm to a 45 mm offset not only may destroy the tires, but also has the potential of affecting vehicle control and injuring the customer — not an optimal customer service outcome!
Finding the actual offset designed into the car is key to the whole process of matching new wheels. Even when your customer is looking to put OEM wheels from a different model by the same maker onto his or her car, you cannot count on the offset being the same.
BMW in particular is notorious for having a dizzying variety of different offsets across its model lines. Some cars will even have different offsets for front and rear axles; BMW again being notorious for this. The site also boasts an enormously useful Match Calculator, which will give a list of other possible wheel fitments for any given model.
Most wheels will have their offset embossed on the back of the wheel, either on the mounting plate itself or on the back of one of the spokes. If the offset is not printed, as with unmarked aftermarket wheels whose history is unknown, measure the offset yourself. Place the wheel on its flanges on a flat surface, and put a paperclip on the string to make it hang straight.
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