Modern Tire Dealer

Handbook 2019

Magazine for the professional tire industry

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Page 15 of 61

P e r f o r m a n c e H a n d b o o k 2 0 1 9 14 A car's chassis is more than the sum of its parts. Everything from the shock absorber's top bushing right down to the tire contact patch is designed to work together as one complete system. Changing the design or material or just the adjustment of any one part can have an impact on the car's handling and braking, not to mention the way the car feels as it rolls down the road. 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. 1. 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. For example, when you mount and balance wheels that are an inch larger than the originals, the profile of the new tires must be an inch smaller in order to keep the same overall tire/wheel diameter. In this example, the process is called Plus-One sizing. e same theory applies to mounting wheels one-inch less than the originals; in that case, the process would be called Minus-One sizing. 2. Proper wheel size. e wheel must be the correct and approved width for the size of tire that it will carry. A chart of approved wheel widths for tire sizes is available in the Ultimate Wheel & Tire Plus Sizing Guide ( and the Tire Guide ( 3. Wheel offset. Perhaps the most impor- tant 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. It's a concept that is oen very difficult for Wheel offset BA S I C S IT'S TIME FOR A REFRESHER ON HOW IMPORTANT THIS MEASUREMENT IS TO BOTH HANDLING AND SAFETY W h e e l O f f s e t ILLUSTRATIONS COURTESY OF JACQUES GORDON After so many years of publishing our Performance Handbook — 26 years and counting! — you might think it would be hard to pick our most popular stories. Surprisingly it isn't. Based on our feedback, when we write about wheel offset, it is a social media sensation for us! And despite the exclama- tion point, that statement is not hyperbole. So as part of the 100th anniversary of Modern Tire Dealer, we collected our wheel offset stories from 2013-2017 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.mod-; 2) Click on our White Paper channel; and 3) download "Helping your customer understand why wheel offset matters" by Jacques Gordon. It never hurts to be more informed about a topic so important to the handling and safety of a vehicle. — Bob Ulrich 3 THINGS TO REMEMBER ABOUT WHEEL OFFSET • If the new wheels are the same width, the new offset should ideally be within 5 mm of the old offset in either direction. If offset must be substantially different, avoid using more positive offset at all costs. • If the new wheels are wider, backspacing must be calculated. The offset on the new wheels may need to be less to counteract the extra width. • Offset must be carefully checked even if you are putting on different OEM wheels from the same maker. 'ON MOST CARS, CHANGING OFFSET BY JUST 5 MM IS ENOUGH TO NOTICE A CHANGE IN THE CAR'S HANDLING.' FIGURE 2 FIGURE 1

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