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Wheels Tech

Offset - what's that?

The offset of a wheel is not the discount one can get by buying a set of four. Rather it is the parameter that tells whether a wheel will fit under a particular wing, whether and by how much the wing should be widened, or whether the roll radius that stabilises the tracking will be held constant if this wheel is fitted.

For the front axle the roll radius is the distance between the right-hand and left-hand average point of contact of the tyre with the ground (the centre of the wheel footprint). If the offset is positive the roll radius reduces, and if the offset is negative or spacer disks are fitted the roll radius increases. The smaller the roll radius, the more pleasantly the car drives and the easier it is to control in difficult situations. The visual effect is of course a matter of taste, and many people like the wide look! To be able to assess what looks OK, what doesn't look OK and what generally makes sense the one thing that must be fully understood is how offsets are calculated.

For tuning enthusiasts offset is therefore a parameter of particular interest. It not only simplifies the correct selection of wheels and bodywork parts, but also shows, for example, whether technically necessary modifications to the chassis (such as can occur for instance when installing non-standard braking systems) can be compensated for by the use of particular wheels.

Each wheel has a surface that fits against the connecting face of the brake drum or the brake disc (only if it is bolted to it as designed, of course!). This surface is called the flange. The second parameter that is important here is the wheel well. This is the region between the wheel rims. The distance between the wheel rims is the width of the well, and this provides a measure for the wheel width (always defined in inches). One inch corresponds to 2.54 cm. A "9-inch wheel", for example, measures approximately 23 cm in width.

The wheel offset defines the distance of the flange from the centre of the wheel mouth in millimetres. If the offset is zero, the flange is located exactly in the centre of the wheel, and the wheel extends exactly as far outwards as inwards. For example, a 9 inch width wheel with an offset of zero extends 11.43 cm outwards and 11.43 cm inwards. A positive offset (e.g. +25) indicates that the wheel extends further inwards (towards the centre of the vehicle) than outwards (towards the wing housing).

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Starting from zero, the track would become narrower by 25 mm. A negative offset (e.g. -25) would mean that the wheel extends outwards and the track would be wider (again by 25 mm starting from zero). If we therefore had two pairs of wheels of equal width to choose from, one having an offset of plus 25 mm and the other minus 25 mm, we could make the track 5 cm wider on each side by fitting the pair with the negative offset rather than the other pair. In total the track would be 10 cm wider than with the "positive offset" wheels.

The running surface of the tyres, which is still a fundamental parameter for the TÜV inspection in accordance with German standards (if not European) does not alter its location at all as a result of any variation in wheel width, but solely as a result of its offset. If therefore there is insufficient running surface cover, no narrower wheel will help, but only one with a greater offset.

At the present time there is an enormous number of wheel types. Only three-component wheels allow exact matching of width and depth, and these wheels also have the advantage that, for example, in the event of kerb damage only the outer ring has to be replaced (cheaper than buying a new wheel or getting the damaged wheel ground and repainted).

Wheel information

Bolt pattern

The bolt pattern is the name given to the centres circle of the wheel bolts that secure the wheel to the brake drum or brake disk. Varying from one manufacturer to the next, various bolt pattern diameters and numbers of holes are on the market. The most common bolt patterns in VW usage are:

5 holes on a 205mm bolt pattern
Production standard bolt pattern for the
Beetle -67, Karmann Ghia -67, Thing, Type 3 -65, Porsche 356AB

4 holes on a 130mm bolt pattern
Production standard bolt pattern for the
Beetle 68-, Karmann Ghia 68-, Type 3 66-, 411, 412, Porsche 914/4

5 holes on a 130mm bolt pattern
Production series bolt pattern for the
Porsche 356C, 911, 912, 914/6

5 holes on a 112mm bolt pattern
Bus 72-, and also used by Mercedes, Audi and Ford

4 holes on a 100mm bolt pattern
This bolt pattern is used by VW, Opel and BMW

From the drawing you can see how the bolt pattern diameter is measured. It's difficult to determine the bolt pattern on a 5-hole wheel since the measurement must be made on an imaginary circle line.

Wheel width

The wheel width is measured between the flanks of the surface on which the tyre is seated. The result is usually expressed in inches.

Wheel diameter

The wheel diameter is similarly referred to the surface on which the tyre is seated. This result is also usually expressed in inches

Offset (ET)

This is measured between the wheel centre-line and the wheel seating face. The offset is positive if the wheel seating face is located further outwards than the wheel centre-line. If the offset is negative the wheel seating face is located further inwards than the wheel centre-line. When the offset = 0 the wheel seating face is located exactly on the wheel centre-line.

Rear Spacing (RS)

This is very easy to determine. The measurement is taken from the wheel inner face to the wheel seating face. From the rear spacing the offset can be calculated.

Offset = RS - ½ wheel width - rim width

Be careful!
When calculating the offset pay particular attention to the units used (e.g.: mm)

ET - calculation programs:

Comment:
The routines supply values independently of wheel design and pitch circle, i.e. it may be that a wheel design with a particular pitch circle is not available with the calculated offset.