Break It Down
The wheel dates back more than 6,000 years when artisans stuck wooden discs on top of shafts and spun them around, making fancy clay pots and all sorts of modern-day museum fodder. Mesopotamians later took a couple of wheel-shaped discs, slapped a log in between, and created the world’s first axle. They were no TE37s, but those tenacious Mesopotamians have got to be given credit for predating whatever later sat underneath chariots, wheelbarrows or your Toyota. Despite the wheel’s existence since 4,000 B.C., though, none of that’s made figuring out what’ll fit underneath your S2000 without banging up against important suspension members or body parts any less complicated.
As it turns out, there’s more to you getting a set of rims to fit than besides whether or not they bolt up and don’t perch your hatchback off the ground by its fenders. Easy stuff, like the wheel’s overall width and diameter, as well as more complicated considerations, like its centerbore and offset have all got to be figured out before ordering and installing whatever it is you’ve been eyeballing.
The bolt pattern is the most obvious consideration when determining whether or not a wheel will fit. Lucky for you, figuring yours out is easy. Picture an imaginary circle that passes through the center of each lug. Give yourself a hand; you’ve just located your car’s bolt pattern. Bolt patterns are expressed using two figures: the number of lugs and the distance between them. For example, a 4x100 bolt pattern is made up of four lugs that are spaced 100mm apart, measured between any two non-adjacent lugs’ center points. For five-lug wheels, measure the distance from the center of any hole to the farthest edge of the hole, two spots over.
If whatever it is you’re driving is new enough to be sold with seatbelts and a cup-holder, chances are you’ve got hub-centric wheels. Hub-centric wheels feature machined holes on the rim’s hub-mounting surface that slip over extruded sections located on the hubs. The design positions the wheel exactly where it needs to be and eliminates vibrations and vertical stress that the mounting studs would otherwise be subjected to. Most aftermarket wheels are made with centerbores that are larger than what you need, though, to remain versatile with other makes and cost-effective. Here, ring adapters must be used to locate the wheel appropriately, and avoid vibrations and excess stress placed upon the studs. Whatever ring adapter you use has got to have an internal diameter that slips over the hub and an external diameter that fits inside of the wheel.
When it comes to wheel diameter, smaller is almost always better. Smaller diameter wheels are lighter, allow for better acceleration and handling, and can even improve ride quality. Generally, stick to an overall wheel diameter that’s close to whatever your car was originally equipped with and that allows you to clear whatever braking equipment you’ve got. Wheel width is a different story, though. Here, wider is better. Wider rims increase stability and handling and allow for more responsive steering. Wider is always better. More on wider rims in a bit.
Wheel weight doesn’t just affect the overall weight of the vehicle, which nobody has to tell you is a good thing in the case of lightweight wheels, but also the car’s unsprung weight. Your car’s unsprung weight is the weight of all its parts not supported by its springs, which includes its wheels, tires and brakes. Less unsprung weight means better handling and reduced rotational inertia. Just like reciprocating engine components, reduced rotational inertial leads to better acceleration but also better braking. The less rotational mass there is, the easier it is to stop. Before settling on any wheels, be sure to consider how heavy they are.
Offset: The Good, The Bad, The Ugly
If there’s anything the Internet’s got muddled up about wheels, it’s offset. Offset is nothing more than the distance between a wheel’s centerline and its hub-mounting surface, and determines the wheel’s lateral placement in relation to the car. Alter that distance and it can result in all sorts of goofy-looking scenarios, making an otherwise properly sized wheel fit and perform nothing at all like it should. A wheel’s offset is generally expressed in millimeters and can be classified three ways: positive, negative or zero.
Positive-offset wheels, like you’ll find on most FWD cars, mean the hub-mounting surface lies closer to the wheel’s outboard side. Numerically increasing a wheel’s offset transitions its mass toward the vehicle. Most manufacturers design their FWD platforms with positive offsets to allow for a negative scrub radius up front, which is the distance between the point where the steering axis intersects the ground and the center of the tire’s contact patch. A negative scrub radius is what partially allows for traction variances between both front wheels. When a single wheel loses grip, the opposing wheel toes out, which helps retain the car’s direction. You don’t need to know what exactly the scrub radius is to know that you shouldn’t go around messing with it. Upsetting it can lead to traction woes while accelerating and braking and an overall unstable feeling in your stomach while cornering. (For obvious reasons, the scrub radius only applies up front, but manufacturers typically apply the same offset in the rear to reduce costs.)
Negative-offset wheels mean the hub-mounting surface lies closer to the wheel’s inboard side. Numerically decreasing a wheel’s offset transitions its mass away from the vehicle. Most RWD applications are based upon low or negative-offset wheels since the scrub radius is less of a concern.
Finally, zero-offset wheels simply mean the hub-mounting surface aligns with the wheel’s centerline.
Because offset affects a wheel’s lateral position, it also determines your car’s track width. Reducing offset increases track width while increasing it does the opposite. Increased track widths are good; the results can improve cornering speeds as well as loads placed on the outside of the tires. Besides adding spacers, fudging a wheel’s offset is one of the easiest ways to alter track width. A wheel’s width won’t change it, which makes playing with low-offset wheels so tempting.
Offset also affects suspension stiffness by means of the system’s motion ratio. As offset decreases, so does the effective wheel rate since the wheel’s mass is moving outboard. This isn’t entirely unfixable, though; stiffer springs can typically overcome all of this, but the leverage change can prematurely wear wheel bearings and the hubs themselves.
If you can use a tape measure and perform mundane mathematical feats like subtraction, then figuring out a wheel’s offset isn’t hard. Interpreting the results into something meaningful and then not bolting on a set of wheels that’ll deliberately upset your car’s handling requires exponentially more restraint, though.
By now you should know that offset matters. It determines whether or not there’ll be enough room for struts, suspension members and brake calipers as well as how well your car will do important things, like go around corners. Whoever manufactured your car did their homework; altering its offset significantly more than however it was designed will almost always end in regret—affecting your car’s scrub radius, increasing torque steer and making for all-around poor handling. Calculate it wrong and the consequences vary from rubbing up against a brake caliper to you landing in a ditch.
Most of the time though, minor changes have got to be made to account for wider wheels, which are almost always never a bad thing. Here, offset must be reduced, which means the hub-mounting surface must move inward, placing the bulk of the wheel farther outward. If offset were to remain the same, then half of the wheel’s extra width would be shoved underneath the fender, most likely interfering with something it shouldn’t. You can plan for all of this ahead of time by performing some basic calculations using your existing rims.
You’ll need to know your wheel’s backspacing, centerline and width to calculate its offset. Start by placing the wheel face down on the ground. Lay a straight edge across its surface and measure the distance between the wheel’s hub-mounting surface and the straight edge. The result is the wheel’s backspace. The wheel’s centerline is simply half its width. To arrive at the wheel’s offset, subtract its centerline from its backspace. You can pat yourself on the back now because you’ve just determined your wheel’s offset for yourself without having to speculate as to whether or not what Honda-Tech.com’s VTaKBoyeee4lif tells you is indeed correct.
Once you know your existing wheel’s offset you can calculate what else might fit. Measure the distance between the inner sidewall of the tire and whatever it comes closest to, like an upper control arm or strut. Meanwhile, bear in mind that half of the new wheel’s additional width has got to fill that space. If there isn’t any extra space, offset must be reduced in order to position the wheel outward and away from, well, those struts and stuff. For example, if you’ve got one inch of clearance between the existing tire and the upper control arm, it’ll take a two-inch-wider wheel to contact the control arm. To allow for enough clearance, either decrease the new wheel’s width, decrease its offset or position spacers in between the hubs and the wheels. Also, keep in mind that not all tires are created equal, despite their sharing the same advertised size. Be sure to allow for at least an eighth-inch additional clearance to account for potential tire size irregularities.
Bolt patterns, offsets and widths are absolute, but figuring out what will and won’t fit underneath your fenders is as much art as it is science. The variables are many and are every bit as dynamic as you think: camber, caster and toe changes as suspensions travel, which means clearances aren’t fixed. Tire sidewalls can also flex and react differently depending on driving conditions. Your only hope is to measure properly, go with your gut and give yourself an extra quarter-inch for good measure. The rest is a crapshoot.