offset 101 for dummies

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offset 101 for dummies

Post  nissan~master on Thu May 19, 2011 6:20 pm



RIM MARKINGS
If you look inside almost any wheel you’ll find markings that provide you with information on the wheel. Typically, they’ve been marked on the rim and it would be marked something like “22x7JET38”. “22” represents the wheel diameter, and the “7” is the width of the wheel and is measured from the distance between the flanges that support the bead. The “J” represents the shape of the flange, and the “ET” is for the German term, einpress tief, which translates into offset. Lastly the “38”, represents the offset measurement.



WHEEL (RIM) TYPES
Wheels can be broken down according to their cross-sectional shape. Passenger car tires usually have a drop-centric rim. Below are a few of the main types of rims.

Drop-Center Rim
These rims are used on passenger car tires, light truck tires, and the tubeless type of truck and bus tires.

Divided Rim
The divided rims are seen on light truck tires, industry vehicle tires, and agricultural vehicle tires.

Wide-Base, Flat-Base Rim
They are only used truck and bus tires (tube type)

Semi-Drop Center Rim
Used on light truck tires.

CENTERING
Hub centricity is one of the bigger elements that go into whether or not a wheel can fit on a car. Hub centric wheels have a hole at their center that fits closely over a round feature on the hub, which centers the wheel on the axis of the spindle, as well as bear the vertical weight of the vehicle. The bolts then only have to secure the wheel onto the hub and are loaded only in tension where they are strong. Factory wheels are all manufactured to fit their specific application exactly, and even some aftermarket wheels as well.

Though that may be the case with some aftermarket wheels, the majority rely on centering rings. This means that the manufacture produces all the wheels to one size and then inserts a centering ring to allow the wheels to fit on car. This allows for easier inventory and is less costly. Centering rings are designed to fit snuggly in the wheels. Not every application will need hubcentric rings though. If you would like to inquire on which applications would necessitate the hubcentric rings please call one of the WheelsNEXT.com Wheel + Tire Experts.

OFFSET
Just because the wheel centers correctly doesn’t mean that it fits correctly. This means that the offset could be wrong. The offset is the location of the flat mounting surface of a wheel relative to the wheel’s centerline. A negative offset indicates that the mounting surface is toward the center of the car, and a positive offset that it is toward the outside of the car. It will appear that the wheel is pulled in toward the center. Offset will lead to more problems than just making the wheel appear to be sticking out of the fender. It can lead to rubbing problems when the suspension is compressed or the wheel is cut to turn. Offset also affects the steering geometry’s scrub radius, possibly leading to problems with the torque steer or the self-centering characteristics.

Offset can also affect the suspension’s motion ratio, which determines directly the effective spring and damper rates. In a heavily loaded vehicle, offset can potentially affect wheel bearing life. This problem though is seen more in trucks than in smaller cars. This is why using the proper offset wheels are essential.

Brake caliper clearance also comes into play when discussing offset. When choosing a set of wheels, it is important to make sure that the ones you choose do fit over the calipers. Spacers are available to alleviate this issue, but it is recommended to purchase a set of wheels that do cover the calipers and meet the offset specifications. You can be sure of this by contacting your wheel and brake manufacturers. Some aftermarket brake companies will provide you with a template of their brakes for you to check against your prospective wheels.



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