Helping Your Customer Understand Why Wheel Offset Matters
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.
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 performance, if any, can be difficult to predict.
That doesn’t mean, however, they are difficult 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 influence 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 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 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 acceleration and braking. That’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 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 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. The 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).
That’s the technical explanation, but here’s an easier way to remember it. The 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. The 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.
So now we know that changing the wheel offset will move the tire contact patch inboard or outboard, and doing that will also change the scrub radius. The question is, how will that affect the way the car handles? The answer depends partly on whether the car is front- or rear-wheel-drive.
First we’ll look at a front-drive car with negative scrub radius because that’s more common. When scrub radius is negative (center of the contact patch is inboard of the pivot point), then under acceleration the front wheels will try to toe out as they pull the car forward. The length of the scrub radius influences how much toe-out force is generated. It’s just like pushing on a lever: The longer the lever, the more force is generated. A front-drive car with a 100 horsepower engine won’t generate much toe-out force, but imagine what happens if scrub radius is increased. If one front tire has a bit more traction than the other, it will feel like the steering wheel is being pulled out of the driver’s hands.
The same thing happens under braking, except the front wheels try to toe in. In that case, it’s not horsepower but the car’s weight that pushes on the lever. A larger scrub radius amplifies the toe-in affect, which might actually reduce stopping distance just a bit, but only if he driver can hold the steering wheel straight.
On a rear-drive car, acceleration and braking forces both have the same effect on the front wheels. If scrub radius is positive (contact patch arc outboard of the pivot point), the wheels will try to toe out.
Cars with strut suspension are more likely to have a negative scrub radius, and cars with double A-arm suspension are more likely to have a positive scrub radius. Chassis engineers use scrub radius to create the feedback the driver feels through the steering wheel. If it were zero (steering pivots exactly on the center of the contact patch), the steering would feel loose, especially during acceleration and braking, because the front tires would tend to squirm like the wheels on a shopping cart.
So now we’ve seen how changing wheel offset moves the center of the contact patch inboard or outboard, which changes the scrub radius, which changes the way the car feels and handles during acceleration and braking. Before installing wheels with a different offset, you need to figure out if those wheels will increase or decrease the scrub radius so you can let the customer know how the car’s handling might change.
One of the more common reasons people buy aftermarket wheels, at least up North, is to make it easier to switch over to winter tires in the winter. If the car has cast wheels, customers often buy steel wheels for their winter tires because they (usually) cost less and, more importantly, they can avoid exposing their cast wheels to salt and potholes. It’s not always easy to find steel wheels that match the size of the OEM cast wheels, but it shouldn’t be hard to match the offset.
Customers who want aftermarket wheels for their appearance often want something bigger than the OEM wheels. If they ask your advice, do a little homework first. Search the Web forums to find out if anyone else with the same vehicle wrote about their experience with bigger wheels. Did they need spacers to make the wheels clear the brake caliper? Did they note any handling or tire wear issues? Did they use different alignment settings?
Even if you don’t find much information, the questions alone could help your customer recognize the risks that come with installing oversize wheels.
If your customer already owns the wheels and asks you to install them, you’ll need to check the offset yourself.
Most alloy wheels have the size cast into the inside of a spoke or the mounting flange, and the last digits in the string of letters and numbers defines the offset. For example, size 7Jx17 ET45 translates as follows:
- “17” is the wheel diameter.
- “7” is the wheel width between the bead seats.
- “J” is a classification of the rim shape.
- “ET45” is the offset; in this example it’s 45 mm positive.
You may already know that BMW and Mini Cooper wheels are not like everybody else’s. In the wheel shown bottom left, the last three digits in the size specification indicate the wheel is made for run-flat tires. The offset appears below it: “IS48” means the offset is 48 mm positive.
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.
Lay the wheel/tire assembly on the floor and place a straight edge across the tire.
Measure the distance from the floor to the straight edge and write down that number.
Divide that number by two to calculate the centerline of the wheel.
Now measure from the hub flange to the straight edge.
Subtract the smaller number from the larger number. That gives you the offset of the wheel. If the centerline number is smaller than the hub measurement, offset is positive.
On most cars, changing offset by just 5 mm is enough to notice a change in the car’s handling because it changes scrub radius by the same amount, and that might already be only a few millimeters. If the OEM wheels have a positive offset, you can safely install wheels with even more positive offset and use spacers to move the contact patch out to its original position, keeping scrub radius the same. Just make sure the new wheels and tires fit inside the fender without rubbing anything.
If the new wheels have negative offset, there’s nothing you can do to move the contact patch in toward the center of the car. The only way to know for sure how much the new offset will affect scrub radius is to put the car on an alignment rack.
A few final details
Even if you’re confident the wheels have been properly selected for the vehicle, it’s up to your technician to make sure everything fits properly. It’s a good idea to test-fit the wheel onto the front and rear hubs before mounting the tire. The first thing to check is brake caliper clearance: On OEM wheels it might be as small as 1/8 of an inch (4 mm). This can be hard to see, but it’s easy to simply install the bare wheel and turn it by hand. If clearance is that tight, the tech must take extra care when placing the wheel weights.
Mounting a tire on an aftermarket wheel versus an OEM wheel may be different; it depends on the location of the wheel’s drop center. Usually the drop center is toward the outside of the wheel, so the wheel is placed outside-up on the tire changer. But some aftermarket wheels have the narrowest part of the drop center toward the inside of the wheel. These must be placed on the tire machine inside-up. If not, the machine will try to stretch the bead — which can’t stretch — and that can damage the wheel, the tire, the tire machine or the technician.
Finally, make sure your techs tighten the lug nuts with a properly calibrated torque wrench every single time! Uneven lug nut torque is almost guaranteed to cause warped brake rotors within a few thousand miles. Finally, remind your customers to come back after five to 50 miles to get re-torqued. ?
Jacques Gordon has worked in the automotive industry for 40 years as a service technician, lab technician, trainer and technical writer. He began his writing career writing service manuals at Chilton Book Co., and writes for Modern Tire Dealer’s sister publication, Auto Service Professional. He currently holds ASE Master Technician and L1 certifications and has participated in ASE test writing workshops.
Compensating for wheel offset
If you have a service information system or an alignment machine with a built-in database, look up “steering angle inclination,” sometimes called “steering axis inclination” or “king pin angle.”
On a vehicle where it’s adjustable, changing 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.