If there's one thing hard parkers and road racers can agree upon it's that wheels and fenders should fit flush with one another. Perhaps it's because of how good it looks when the wheels and body surface are in the same plane. Or maybe it's because cars tend to handle better with the wheels positioned farther out. Either way, since wheels travel upward as suspensions compress, the farthest outboard they can go is just about flush with the fender. If the car looks and handles better, then despite what we've been told, maybe we can have our cake and eat it too.
Offset: What the heck is it?
All of this brings us to wheel offset, which is simply the distance between the wheel's hub-mounting surface and its center plane. Positive offset means the hub-mounting surface is closer to the wheel's outboard side. Conversely, negative offset means it's closer to the inboard side. If the mounting surface coincides with the wheel's center plane then offset measures in at zero. In other words, offset determines the lateral, or side to side, position of the wheel. As wheel widths change, the offset combined with the new width must be chosen properly so that the wheel and tire have enough space within the wheelwell to avoid rubbing or unwanted contact with other components.
Front-wheel-drive cars are generally equipped with positive offset wheels. Most manufacturers design cars with a negative scrub radius up front that is made possible by positive offsets. The scrub radius simply refers to the distance between the point where the steering axis intersects the pavement and the center of the tire's contact patch. Since the scrub radius has to do with steering geometry, such positive offset requirements only apply up front. However, since factories typically prefer using similar wheels all around to reduce manufacturing costs, the rear wheels typically get the same positive offset wheels as the front. If we're talking about rear-wheel-drive cars, then most OEMs incorporate a minimal scrub radius up front.
Why should you Care?
The ability to understand wheel offset can help solve a variety of wheel fitment issues. Wheel and tire upsizing often requires altered offsets. The wheel supplier will usually have the information necessary in terms of which offsets will and will not work, and, if that doesn't work, there's probably at least one thread somewhere on the Web with at least one guy who's tried the same wheel combo you're considering, but don't count on it. Sometimes the info just isn't there and the only way to know for sure is to measure and calculate. It all begins with your stock wheels and tires.
The minimum clearances on both sides of the OEM wheel/tire package must be accounted for prior to assuming a potential maximum tire width-this includes the space between the nearest suspension component as well as the fender. Keep in mind that oftentimes when dealing with extreme steering positions, minimal, inner wheelwell tire rubbing might occur. Also, some tires measure differently than others. For example, some 225mm-wide tires measure similar to some that are labeled as much as 245mm. It's best to allow an 1/8-inch of play for potentially wider tires.
More about offset
With clearances measured, you're almost ready to look really smart in front of your friends. First: A wider tire on a similarly offset wheel reduces the gap the same amount on both sides but adding 1mm of offset moves the wheel away from the fender-closer to the inner wheelwell-by 1mm. Usually, the inner and outer gaps will be different and this is why new wheels require different offsets. To center the tire so that the inner and outer gaps are equal, take the distance of the outer gap, subtract the inner gap from that figure, and divide this number by two. Add this figure to the old wheel's offset, this gives you the new wheel's offset. For example, imagine a stock 195mm tire and a new 225mm one. Picture inner and outer gaps, 30mm and 21mm, respectively. Give the old wheel a 45mm offset but first be sure the new tire will fit. Since the total gap in this example is 51mm (30mm plus 21mm), and the tire width increased by 30mm, then there is 21mm left over. Even after assuming that the new tire might be an extra 1/4-inch (6.4mm) wider than expected, there's still more than 14mm clearance. In short, 225mm will work. Subtract the inner gap from the outer gap, which is -9mm (21mm minus 30mm), to obtain the new wheel's offset. Take half of this number (-4.5mm or simply round to -5mm), add this figure to the old offset, which was 45mm to net your 40mm offset. A quick check of Volk TE37 and CE28 wheel offsets reveals a +35mm and +42mm offset available in 15x7, both of which will work just fine.
On a related note, you also need to know about backspacing. Specifically, it's the distance between the wheel's inboard outer edge and its hub-mounting surface. Proper backspacing calculations ensure against any unwanted clearance issues between tires and suspension members, especially when larger wheels and tires are used. Enough said.
How to measure it
Offset can be measured with or without the tire installed. When measuring with the tire, offset can still be measured without deflating it, however, it might be more accurate to measure offset on a bare wheel but that doesn't mean you can't get within a millimeter or two of the true figure with tires in place. After all, as long as offset is measured within a 2mm tolerance, things should be fine. Should you go the tire-installed route, it's easiest to measure the overall tire width using a straightedge placed across the tire to a level spot on the ground. Next, measure from the same straightedge location to the wheel's mounting surface. The only tools required to measure offset are a long straightedge, a tape measure, and this simple mathematical formula:
Offset equals 0.5 (tire or wheel width) minus the distance from the mounting surface to the tire or rim edge.
Offset and track widths
Altering offsets also changes track widths. Reduced offsets result in larger track widths. This can improve cornering speeds by reducing lateral load transfer from the inside tires to the outside tires. By keeping the tire loads uniform, tires can generate increased lateral grip. This is why most race car tires are as far outboard as possible. On the other hand, changing the wheel width does not change the track.
Another way to increase track without having to purchase new wheels is to install wheel spacers. Spacers are available in different thicknesses, from about 5mm to 30mm or more, depending on the application. Eibach manufactures one of the nicest sets of spacers on the market. Their spacers are precision machined with tight tolerances, which helps ensure that wheels stay securely attached to their hubs. Eibach manufactures 5mm non-hub centric and 15mm hub centric options, each with longer wheel studs.
Determining the proper offset when upsizing wheels and tires can be challenging. It's difficult to determine how far the tire will extend over the wheel lip for a particular tire and rim combination. Therefore, it's difficult to know beforehand whether the tire will rub on the suspension or the fender. Even with a small tire width change on the same wheel it's possible to estimate how much wider the new tire will be. For example, West Coast Honda Challenge H4 multiple champion Edik Stepanyan somehow shoehorned a Toyo 245/45-16 RA-1 onto a rules-specified, 7-inch-wide wheel and then fitted four of them onto his '93 Integra race car. He used 225/45-15 sized tires previously and with careful clearance measurements on both sides of the tire he determined that the 245's extra width would fit using wheels with a more common offset, and this was on a car that came from the factory with 195/60-14s on 51/2-inch rims. Needless to say, the gaps on both sides of the tire are minimal.
The scrub radius
Altering wheel offsets also affects the scrub radius. Recall that the scrub radius is the ground-level measurement between the kingpin inclination axis (KIA) and the tire contact patch's center. For double-wishbone suspensions, like most pre-'01 Hondas, the KIA is the angle between a vertical axis and the imaginary line through the top and bottom ball joints' centers when viewed from the front of the car. For a strut-based car, the imaginary axis travels from the top bearing mount's center to the lower ball joint's center. If the KIA intercepts the ground outboard of the tire contact patch's center, then the scrub radius is negative. On the other hand, if the KIA intercepts the ground inboard of the tire contact patch's center, then the scrub radius is positive. Front-wheel-drive cars, including Hondas, are usually set up with a negative scrub radius.
A negative scrub radius is preferred for front-wheel-drive cars since it gives a stabilizing effect when traction between the left and right wheels varies. If a single front wheel loses traction during acceleration or braking-like what would occur if one tire goes over a patch of ice-the other front wheel will toe-out a certain degree depending on the amount of steering compliance, which will tend to steer the car in a straight line. At the same time, the driver will feel a certain amount of kickback through the steering wheel.
Spacing wheels out by means of smaller offsets or spacers increases the scrub radius. This makes a negative scrub radius less negative, maybe even positive. This could lead to unequal front-wheel traction depending upon the difference between the old and new offsets.
Offset also affects suspension stiffness through the suspension's motion ratio. With less offset, the wheel's leverage about the inboard lower control arm pivot increases while the suspension spring leverage remains constant. The net effect is a reduction in the effective wheel spring rate. For example, a stock DC2 Integra fitted with wheels with 10mm less offset will have a 5 percent lower wheel rate. Therefore, to recover the lost suspension stiffness, a 5 percent stiffer spring is needed.
Offset and its effects
Production cars are built with wheel offsets that minimize wheel-bearing load. In corners, excessive lateral tire loads add stress to wheel bearings. Altering offsets affect how much load each of the two wheel bearings see both in straight-line driving and during cornering. Production-based race cars like the CRX and Integra that run stickier tires and less positive offset, exhibit somewhat high-bearing loads. Thankfully, Honda wheel bearings are strong and few problems occur.
Even suspension knuckles aren't immune from offset changes. Longtime Honda racer Sam Rothschild has suffered two complete knuckle failures on his CRX. Of course, he runs +35 offset wheels with sticky Toyo 225/50-15 RA-1 tires and a stiff suspension. The added leverage created by this offset causes a much larger bending moment in the knuckle, which over time led to a fatigue fracture on two different knuckles, but this is for a highly stressed race car and is a rather unlikely scenario when concerning the typical Honda street car. Regardless though, offset does do more than just fill or not fill the fenders-it affects handling, steering, traction, even the service life of otherwise unbreakable suspension components like knuckle assemblies. That should be more than enough reason for you to rethink that sick, fat lip, rear-wheel-drive-looking offset next time you go shopping for new shoes.