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.
Don't get us wrong, the low...
Don't get us wrong, the low offsets look cool, like on this EP3 Civic, but don't expect your front-wheel drive to handle as well as it did if you were to stick with something closer to what Honda intended.
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.
Although not related directly...
Although not related directly to wheel offset, another dangerous trend adopted from the drifting world is tire stretching. Undersized, stretched tires make for great slides on the track but a dangerous ride on the street. Do this one at your own risk.
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.