The idea for the subject of this month’s “Tech Matters” stemmed from a shameful lack of understanding on my part about wheel offset, what it means and what it does, among other things. To fill in the gaps, so to speak, we caught up with Ryan Dettling, president of Jongbloed Racing Wheels in Morgan Hill, Calif.
Dettling explained that wheel offset is explained in terms of negative and positive. Positive offset puts the hub flange closer to the outside of the wheel. Negative offset puts the hub flange closer to the inside of the wheel. A wheel with zero offset has the hub flange directly in the centerline of the wheel.
“Offset is the relation of the wheel center to the rim half, where the center is in relation to the centerline of the wheel,” Dettling said.
Simple, right? Well, it is until you start digging into the ins and outs of wheel offset, and how to get the right wheels for your car. Choosing the right wheel relates to the suspension design of the car you’re working on. Typically, OEMs fit their cars with wheels to achieve a zero scrub radius on the front suspension.
Scrub radius is a bit tangential to our subject, but it is the distance on the road surface between the kingpin axis and the center of the contact patch. Most racecars don’t have kingpins, so the steering axis runs through the upper and lower ball joints on a car equipped with upper and lower control arms. On McPherson strut- equipped cars, the steering axis runs from the upper strut pivot point through the lower ball joint.
Positive scrub radius is when the steering axis lies to the inside of the centerline of the wheel and tire. Negative scrub radius occurs when the steering axis likes outside the wheel center line. When you go with a different offset, you change the scrub radius, which causes greater steering effort.
Typically, racers want a larger contact patch and increased track width, so when they’re increasing the width of their wheels, they need to pay attention to the offset that will get them the widest track width possible. In some racing classes, that means sizing up without having the wheels and tires protrude beyond the vehicle’s sheet metal.
Differing offsets can affect the strength of a wheel, Dettling said. If you have a wheel with a high positive offset, most of the force is being put on the inner rim half, or the inner part of the wheel. All that stress also is being put on the hub center itself as it loads. So, usually, if you have a lower offset wheel, or one with no offset, it will distribute the load better.
“When you’re talking about one-piece wheels, you can thicken up the inner rim half on the barrel of wheel, but then again that adds weight,” Dettling explained. “You can add strength to the center, which will in turn add weight to the wheel. It’s a combination of material and design that you minimize the flex on the wheel.”
Wheel flex is a subject unto its own, but it raises an important point about wheels themselves. Jongbloed’s multipiece wheels are made of 6061 and heat treated to T6, which allows the material to give and return to its original shape. With one-piece cast wheels, they’re not as forgiving to repeated flexing. Regardless of which material you have, racing wheels should be treated as a wear item, and should be replaced at the appropriate intervals.
“All suspension components, including wheels, are subject to fatigue, especially in a racing environment,” Dettling said. “Every wheel company has cracked wheels, has broken wheels. I mean that’s just the nature of the beast. You try to minimize it, but every wheel will fail at one point or another.”
In short, getting the right racing wheels isn’t as simple as finding something cool on a website and ordering it. Work with the vendor or manufacturer, tell them what you’re looking for and what kind of racing you’ll be doing.
Factors such as clearance for brakes, weight, materials and whether the wheels are hub centric all are important. Dettling said it’s better to achieve the appropriate offset with hub centric wheels rather than using spacers. Even spoke design is a factor. For example, a wheel with spokes tapered on the outside help facilitate better brake cooling.
“There isn’t really a good rule of thumb,” Dettling said. “You try to get the widest track width you can and that is usually done with the highest offset. But it all depends on the feel you want. If you’re looking for the best handling, usually you want the widest or the highest offset you can get.”