Magazine for the professional tire industry
Issue link: https://mtd.epubxp.com/i/664171
Helping your customer understand why it matters By Jacques Gordon A car's chassis is more than the sum of its parts. Everything from the shock absorber's top bush- ing right down to the tire contact patch is designed to work to- gether 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. The part that has the most impact on ride and handling is the tire. Of course you already know that, and when your customers replace their OEM tires with something different, they rely on you to tell them what to expect from their new tires. Uniform Tire Quality Grading (UTQG) standards provide some very specific information in a format that's easy to explain. But when your customer wants you to install non-OEM wheels, the resulting changes to the vehicle's perfor- mance, if any, can be diffcult to predict. That doesn't mean, however, they are diffcult to explain. Here's a review, with simple illustrations, of wheel and suspension technology that you can use to help your customers understand how the size of a car's wheels infuence ride and handling, and why they will be happier with wheels that match the original offset. From the contact patch perspective Offset, scrub radius and contact patch are the only t h r e e t e c h n i c a l t e r m s w e need to understand. You've probably read about "wheel offset," typically described as the distance between the centerline of the wheel and the face of the wheel's mounting fange where it touches the hub. 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 fange is inside the center of the contact patch (closer to the center of the car), offset is negative. If the fange is outside the center of the contact patch (closer to the curb), offset is positive (see Figure 1 below.) On most OEM wheels, offset is positive (contact patch slightly inboard), and it can have a major effect on steering feel and stability during accel- eration and braking. That's because on the front suspen- sion, wheel offset determines "scrub radius." We can describe that by looking at how the contact patch moves in a turn. When the front wheels are steered left or right, the center of the contact patch doesn't just pivot around a single point; it swings through a slight arc. To visualize this, lay a pen on the table and hold the top end still. As you swing the other end left and right, the tip doesn't move in a straight line, it moves through an arc. The radius of that arc is the distance between the pivot point that you are holding and the tip that moves through the arc. Now make another drawing Figure 1 7 Feature