Modern Tire Dealer

Handbook 2019

Magazine for the professional tire industry

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Page 17 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 16 W h e e l O f f s e t customers to understand, and can be an enormously difficult concept for service professionals to explain. 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. is article is about the basics of under- standing and measuring wheel offset. FROM THE CONTACT PATCH PERSPECTIVE Offset, scrub radius and contact patch are the only three technical terms we 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 flange 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 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, page 14). 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. at's because on the front suspension, 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 le 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 le and right, the tip doesn't move in a straight line, it moves through an arc. e 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 to put that arc on the car. Looking at the car head- on, imagine a line drawn from the upper strut mount or upper ball joint and down through the lower ball joint, and then on down to the road. e distance between the center of the tire's contact patch and the point where that line touches the road is the scrub radius. If the line touches the road inboard of the contact patch, scrub radius is positive. If the line touches the road outboard of the contact patch, scrub radius is negative (see Figure 2). at's the technical explanation, but here's an easier way to remember it. e point where that line meets the road is the steering pivot point, same as where you held one end of the pen. As the wheel is steered, the point at the center of the contact patch moves through an arc around that pivot point. e arc can be inboard (negative scrub radius) or outboard (positive scrub radius) of the pivot point, and the distance between the arc and the point might be just a few millimeters, but that small distance makes a big difference in what the driver feels in the steering wheel. HOW TO MEASURE OFFSET Sometimes the offset isn't printed on the wheel, or it might be printed in digits that are difficult to read or impossible to decode. No worries; it's easy to measure offset yourself. Technically, offset is the distance from the hub mounting flange 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 find offset by measuring from the tire sidewall. 1. Lay the wheel/tire assembly on the floor and place a straight edge across the tire. 2. Measure the distance from the floor to the straight edge and divide by two. at calculates the centerline of the wheel. If the centerline number is smaller than the hub measurement, offset is positive; if it is larger, offset is negative. COMPENSATING FOR WHEEL OFFSET If you have a service information system or an alignment machine with a built-in database, look up "steering angle incli- nation," sometimes called "steering axis inclination" or "king pin angle." On a vehicle where it's adjustable, chang- ing the angle of that line will adjust scrub radius to compensate for changes in wheel offset. Some OEMs use this adjustment to set scrub radius on vehicles that are sold with different wheel size options. This is a 45 mm offset. It is often, but not always, accompanied by an "ET" marking. 'WHEEL OFFSET IS THE DISTANCE BETWEEN THE CENTERLINE OF THE WHEEL AND THE FACE OF THE WHEEL'S MOUNTING FLANGE WHERE IT TOUCHES THE HUB.' COURTESY SEAN PHILLIPS

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