Offset
Offset (ET) Explained:
This is important if you are thinking of purchasing aftermarket alloy wheels. The offset is the distance in mm between the centre line of the wheel rim, and the line through the fixing (hub) face. You can have positive (+), negative (-) or neither (0). The first important reason offset is so important is that it is ultimately responsible for the position of the wheel under the wheel arch. Get it wrong and your wheels can scrub against the bodywork, suspension or at worst not turn at all! Secondly it determines how the suspension and self-centering steering behave. The most obvious problem that will occur if you get it wrong is that the steering will either become so heavy that you can't turn the steering wheel, or so light that you need to spend all your time keeping it in a straight line.
Example:
A Citroen Saxo and a Ford Fiesta have the same stud pattern, so in theory a Fiesta wheel will fit a Saxo wheel and vice versa right? Wrong. Although both cars have a positive offset the Saxo’s is very low at around ET15 and the Fiesta’s is higher at around ET38. This means that the hub mounting face on the Saxo’s wheel is closer to the centerline of the wheel and the Fiesta’s is closer to the outside of the wheel (roadside).
In a nutshell you need an offset as close to the vehicles original wheel offset as possible.
In the above diagram, the black centre line of the wheel is equal to an offset of 0. This means that if the hub is positioned directly in the centre of the wheel, the offset is 0. Every other offset is then measured from this line. If the hub position moves toward the car or the 'hub side' according to the diagram i.e where the green line is, the offset will be negative.(This is not common) However if the hub moves in the direction of the outer rim or the road side according to the diagram i.e where the red line is, the offset will be positive. It is far more likeley that the offset will be positive and for most road cars between 15 and 50.
So how do I know what my Wheels Offset is?
Below is an diagram of the back of an alloy wheel. Enlarged is the information that you need to find on the wheel. You will notice that this one has marked "7J x 17 H2 ET35". The "7J x 17" part of that is the size of the wheel rim - in this case it has a depth of 7 inches and a diameter of 17 inches. The "J" represents the shape of the tyre bead profile (you don’t need to worry about this). The "H2" means that this wheel rim is designed to take "H" speed-rated tyres. The "ET35" figure below it the information we are after. This shows that these wheels have a positive offset of 35mm. In other words, the hub mounting face is 35mm from the centre line of the wheel towards the roadside. This information is fairly easy to read but on many aftermarket wheels, they don't want to ruin the style of the outside of the wheel so it's likely to be found inside the rim or at the back, or on one of the inner mounting surfaces.
that might be helpfull
alloys? how big?
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