4 replies to this topic

[TRD JZX100]

Found this a while ago and thought it might be of some use to those that don't like the black magic/pot luck approach to buying new wheels.

I've also seen a bolt up gauge type of contraption which can be used to attach to your wheel hubs and then measure exactly what kind of offset/backspacing you need. I'll see if I can find it again.

Essentially though, if you take proper measurements of your wheel wells, hubs, etc and then use the info below you should be able to find out exactly how a particular wheel will sit on your car.

 bakspace.jpg   19.7K   41 downloads

Offset
Offset is measured from the centerline of the barrel of the wheel to the hub mounting surface. If that hub mounting surface were in the exact center of the barrel of the wheel the offset would be �0". If the mounting surface is away from the center of the wheel the offset is measured in millimeters. There are 25.4 millimeters to an inch. So if the mounting surface is 12mm from the centerline of the wheel it has a 12mm offset. That would also mean that it is about � inch. If the mounting surface is moved away from the vehicle that is a positive offset.

The picture shows a wheel with the positive offset. A positive offset will cause the wheel to set in or tuck into the vehicle. Originally you found positive offsets on just front wheel vehicles. Due to that some people refer to positive offset wheels as front wheel drive offsets. Currently there are many rear wheel drive cars and trucks with positive offsets. The higher the positive offset the less it sticks out from the vehicle and it will have a higher Backspace.

If the mounting surface is moved in toward the vehicle past the centerline, that is a negative offset. A high negative offset will produce a lower Backspace. Once again the measurement in millimeters is how far away from the centerline the mounting surface is. A -24mm offset means that the mounting surface is located 24mm or 1 inch from the centerline toward the vehicle. The wheel will come out farther from the vehicle and will have that deep dish look.

Width
The width of a wheel is measured inside the beads which are usually � inches. If you measure on the outside of the beads an 8 inch wheel will measure 9 inches.

Backspace
Back spacing is measured from the inner edge of the wheel to the hub mounting surface. It is a convenient measurement in that, as long as the back spacing remains the same, the clearance to the suspension also remains the same. If you know the width of the wheel and the offset you can compute the backspacing. For example if you have an 8 inch wheel with a +24mm offset. An 8 inch wheel is actually 9 inches wide so if the offset were 0 the mounting surface would be right on the centerline and the backspacing would be 4.5 inches. With a positive offset the mounting surface moves off center 24mm or 1 inch toward the outer edge of the wheel. This will make the inner edge go more inside which will result in a 5.5 inch backspacing.

Centerbore
This is the diameter of the center of the wheel. The centerbore must be equal to or larger than the hub of the vehicle for the wheel to seat. If the wheels centerbore is the same as the hub on the wheel then that wheel is hub-centric and uses the hub to center the wheels. If the centerbore is larger than the hub then the lugs will center the wheel and that is lug-centric. When using a lug-centric wheel be sure and snug the lugs up slowly and tighten them in an opposite or star pattern.

Bolt pattern
A bolt pattern of 6x5.5 means it has 6 lugs and if there were a circle through the center of all the lugs the diameter of that circle would be 5.5 inches. On vehicles with an even number of lugs you measure from the center of one lug to the center of the opposite lug.
With a five-lug pattern, measure from the center of one stud hole to the center of the farthest stud hole, skipping the adjacent hole. The resulting measurement is slightly smaller than the actual bolt-circle diameter. The conversion table below displays the difference

[TRD JZX100]

Also helpful:

Wheel Offset Calculator
Wheel Tire Calculator

[snoopz]

Wow, thats quite helpfull Just calculated from stock to wheels I want on the rear -

Inner clearence - 21mm less
Outter position - EXTEND an extra 81mm

That will be an interesting fit

[Wink]

It's not hard to understand how offset works.

The hard part I find is getting the biggest offset wheels you can for your particular car, as it depends on how much it's lowered, guard rolling/flaring, camber and tyres - all these factors are different from one modded car to another.

[ChauChaser]

Like Wink said, it depends on alot of variables. But there is 1 variable that is constant=rear axle. Found a page that might help if your car or rim are not stock.