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Offset
Offset is the location of the flat mounting surface of a wheel relative to the wheel's centerline. Negative offset means the mounting surface is toward the center of the car, positive offset means it is toward the outside of the car, or the wheel is "pulled in" toward the center. Offset affects many things other than just whether the wheel has the appearance of "sticking out" past the fender. The wrong offset can cause rubbing problems when the suspension is compressed or the wheel is turned. Offset affects the steering geometry's scrub radius, possibly leading to problems with torque steer or self-centering characteristics. Offset also affects the suspension's motion ratio, which directly determines the effective spring and damper rates. Potentially, in a very heavily loaded vehicle, or with extreme changes in offsets, wheel bearing life can be affected, but this is more often talked about by truck people than by small car enthusiasts. It is very, very important that the proper offset wheels be used.
The formula to find offset located above the photo is incorrect. The correct formula is: Offset = Backspace - (width/2)
While not directly a matter of offset, brake caliper clearance is a related issue. If you have, or plan to have big brakes on your car, be sure that your wheels, or the wheels you are going to use, will fit over the calipers. Spacers are available to solve the problem if they don't, but it is best to get a wheel with enough dish to meet your offset specs and still fit your brakes. Consulting the wheel and brake manufacturers ahead of time is wise. Many aftermarket brake companies even have templates of their brakes available that you can easily check against any wheel.
Mounting
Wheel mounting surfaces can vary in thickness, which means that longer or shorter wheel studs or bolts may be required. Fortunately, those items are standard parts and are available in a variety of lengths to fit most cars. The main concern is that there is adequate engagement between the lug nuts and studs, or wheel bolts and the hub. If there is not, the fasteners could seem tight, and even appear to torque down properly, but cause problems down the line. Inadequate engagement could lead to threads stretching or stripping, loosening, and the wheel coming off. We consulted a variety of sources for a recommendation on how much thread engagement is enough. Across the board, there was agreement that it's hard to have too much, but recommendations for a minimum varied. H&R Special Springs said that with their Trak+ wheel spacers (see sidebar) they follow a German standard requiring 6.4 threads to be engaged on 1.5mm pitch steel fasteners, for a total of 9.6mm. Robert Wood, of Wheel Enhancement, said he likes to see at least one diameter of engagement. For example, a 12mm-diameter fastener would have 12mm of threads engaged. The Southern California Timing Association, which governs the racing at Bonneville, requires at least 5/8 in. of thread engagement. It also prohibits the use of closed-end lug nuts, presumably to allow measurement, but also encouraging full engagement. If you have a center cap covering the lug nuts, they'll clear a stud that protrudes from the nut by one thread. It's likely that if you compete, your sanctioning body's rulebook will have something in it about wheel fasteners.
Tire Sizing
We can't talk about wheel fitment without talking about tires, since wheels are mostly just there to connect the tire to the suspension. Overall tire diameter affects speedometer accuracy, as well as acceleration, braking, and suspension geometry. If tires are too tall, they may rub fenders. Overall diameter, if not available from a tire's manufacturer, can be calculated from the sizing information by the following formula:
Diameter (in.) =
section width (mm)
•
aspect ratio
• 2 + wheel diameter
25.4
100
It is best to keep this within a few percent of the original tire's diameter. Don't obsess over this number however, as a tire may wear through 1/4 in. of tread in its lifetime, reducing overall diameter by 1/2 in., roughly 2 percent on a 25-in.-diameter tire. As well, tire manufacturers differ, and two tires of the same listed size may not actually be exactly the same size. Section width leads to many fit problems, when people choose a tire that is too wide. Tires that fill a car's wheelwells almost always look better. Certainly, wider tires make more grip in theory, and usually in practice as well. A wider tire has a shorter contact patch, less prone to distortion, and should operate at a lower slip angle for a given side load, a characteristic also influenced, perhaps to an even greater degree, by sidewall stiffness. A wider tire has more material to absorb the heat of cornering, and more surface area to carry the loads, so it may run cooler when driven hard. Wider tires also have greater rolling resistance and aerodynamic drag, so underpowered cars may want to strike a balance with slightly less than monster rubber on a fast track. Wider tires hydroplane easier with water on the road. If your suspension doesn't control camber adequately under cornering loads, the extra width may be wasted as the high edge does little work. Again, different tires are manufactured with different shapes. Some have very square shoulders, some are more rounded, so one tire may fit fine while another tire of the same nominal size may rub. There are different criteria for choosing the width of wheel/tire combinations. One expert we asked says he likes to see wheel width between 85 and 90 percent of section width, while another tells us he likes the section width to be about 1/2 in. wider than the wheel width. Everybody seems to agree that having the flange of the wheel extend past the sidewall is bad, both for tire performance, and because that means the wheel hits a curb before the tire.
Load Rating
While there are DOT standards for wheels that apply to the original automobile manufacturers, and each manufacturer has its own standards in addition, there are none for the aftermarket. There are some industry norms, and the better aftermarket wheel manufacturers follow them, or have some variation they prefer. Kinesis, for example, puts its wheels through fatigue cycle tests twice, rather than just once.
There are no industry-wide standards for aftermarket wheel load rating, but you should definately follow manufacturers' recommendations.
BLUE_CP9A wrote:Useful link
http://www.retro-rides.com/wheels.asp
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