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The following information and diagrams are to try and help you understand offsets and PCDs.
“The offset of a wheel is the measurement the wheel is offset from the centre line of the overall width of the wheel.”
A wheel which sits further under the wheel arch with a small outer rim is normally a negative offset as per Diagram A below, and shown as 'ET' offset. A wheel which sits further out and has either a larger outer rim or a small outer rim and a large block which takes the hub mounting face beyond the centre line is normally a positive offset and shown as an 'ET' offset.
An example a 6x15 wheel with a ET49 has a negative offset and has a flat outer rim and the ET-06 has a positive offset with a dished front face and a large outer rim.
To work out the offset the wheel must be measured accurately using the following steps and using Diagram A
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Diagram A
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Measure the overall width of the wheel - (ii)
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Measure the ‘T’ dimension, which is the measurement from the back of the bolt up face to the outside of the inner rim - (i)
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Divide the overall width by two which gives you the centre line.
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Minus the ‘T’ dimension from the centre line.
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A negative answer the wheel would be ET.
A positive answer the wheel would be ET–.
An example - AT (AT1355) with an offset of ET35 has an overall width of 159.7mm. Therefore the centreline would be 79.85mm and the ‘T’ dimension should be 114.85mm.
So 114.85 - 79.85 = 35mm.
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“The Pitch Circle Diameter is the diameter of the circumference of the centre line of the drilling holes.”
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Diagram B
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The PCD on the FTO is 114.3mm
This can be four, five or six stud and can be measured as shown in Diagram B The easiest way of measuring four or six lug wheels is to take two diagonal holes and measure from the outer edge of one hole to the inner edge of the second hole.
With five lug wheels this is not possible as there is not two diagonal holes, therefore the simplified way would be to measure the outer circumference and inner circumference of the drilling holes. The difference
should be divided by two added onto the smaller circumference or deduced from the larger circumference.
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“The centre bore is the location hole machined in the centre of the wheel.”
This varies from one vehicle to another. It is essential, the wheels purchased have the correct centre bore for the vehicle they are to suit, otherwise you may experience balancing problems.
Most manufacturers of wheels are universal fitting and are drilled to fit a variety of vehicles. Therefore to obtain the correct fit for the vehicle the correct spigot rings must be inserted into the rear of the wheel (iii).
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It goes without saying if you have just spent a load of money on a nice new set of wheels, you should invest in a set of lockers, also most insurance companies will want them fitted.
McGard Locking wheel nuts are the almost the worlds best and at £30 per set they should be, for most aftermarket alloy wheels Part No 24157 (M12 x 1.5) will be the ones to go for.
Check out the McGard website here
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Aspect Ratio
Aspect ratio is the ratio of a tyre's width to it's height. A 40 series tyre, therefore, is a tyre whose height is equal to 40% of its width. Higher Profile Tyres hence have higher series numbers.
Low Profile Tyres - Performance Characteristics
Compared to conventional passenger car tyres, low profile car tyres have greater width ratio to cope with the demands of higher performance vehicles.
The key performance advantages are:
- Improved handling and grip
- More traction and braking power
Disadvantages are:
- Increased road noise
- A harsher ride
- Less resistance to aquaplaning due to the amount of rubber on the road
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