What is wheel offset, ET and PCD? We translate the jargon

When it comes to driving, one of the most important parts of driving safely is the car's ability to grip the road. The tyres on your car, their material, set-up and specification is of the upmost of importance.

In this article we'll focus on one of the more confusing topics when discussing tyres. What is wheel offset? You may have heard the term but why you should care? Keep reading for some helpful tips on what to look out for and when to speak to your local service centre.

So, what is wheel offset?

Wheel offset is explained as the distance from where the wheel mounts to the car, to the centre of the wheel. In plain English, it is whether the tyre is in the correct position after attaching the wheel to the car. If the tyre isn't in the correct place this creates wheel offset. Wheel offset is split into three types: positive, negative or zero. The type depends on how much the wheel protrudes from the mounting surface.

Positive offset

Positive offset is when the distance from the face of the wheel and wheel mount is closer.

Negative offset

Negative offset is the opposite. It is when the mounting surface is closer to the back of the wheel.

Zero offset

A zero offset is when the wheel mount is in line with the centre of the wheel.

Needless to say, having too much of either a positive or negative offset can cause problems. If your wheel is close to the chassis of your vehicle, it can rub against the body or suspension when you turn. Incorrect wheel offset can result in a number of issues, including:

• Poor handling and stability.
• Ability to brake effectively.
• Steering wheel kickback.
• Premature tyre wear.

How to find out the offset of a wheel

All wheels include the offset they should be set at. This is marked on the wheel as the ET value, and is usually stamped on the wheel itself. ET is German for 'Einpresstiefe' which translates in English as 'offset'. This value is the millimetre distance between the hub mounting surface on the axle to the wheel's centreline.

How ET impacts the wheel

A decreased ET will create a negative offset. This will result in making the wheel protrude as indicated in the image above. Inversely, an increased ET will result in positive offset. This will mean the wheel is further in the wheel arch.

Although a negative ET can cause a wheel to extend beyond the wheel arch, this isn't the only cause. A wider wheel with the same ET can stick out past the boundaries of the vehicle body.

It is important to understand how different wheels can impact ET and offset. Original Equipment Manufacturer (OEM) wheels are supplied with the correct offset for the wheel. An aftermarket third-party wheel may not be the correct offset you require.

A manufacturer typically allows for a 2% change in the vehicle's wheelbase. If the car's wheelbase is 180cm, this would mean a change of up to 3.6cm. This would fall into the manufacturer's guideline limit. This means that the offset could be increased to up to 18mm each side. Keep in mind that this is assuming the wheel and tyre sizes do not change.

Is there a difference between ET on alloy and steel wheels?

If a driver wants to change the steel wheels that came with their car as standard, do they need to consider offset?

If the alloy wheel and the steel wheel size is not exactly the same, you should consider the offset. The material the wheel is made of does not impact the ET of a wheel.

That said, it is not uncommon for a steel wheel's width to be less than an alloy wheel. This means adjustments when fitting will ensure the wheel is safe to drive on.

Adjustments for wider wheels requires knowing the correct new ET. Don't panic though, there is a simple formula to work out what the new ET number and offset should to be.

New wheel ET = ((new wheel width - steel wheel width)*25)/2) + steel wheel ET))

What does PCD mean on wheels?

PCD stands for Pitch Circle Diameter. Imagine a circle on the wheel that would pass through the centre of each of the stud holes. The diameter of this circle is the PCD. This number increases or decreases depending on whether the wheel has four, five or six stud holes. To calculate this measurement, you simply need a tape measure. Four stud wheels are the easiest to calculate the PCS for. Measure from the centre of one stud hole to the stud hole opposite to determine the PCD.

When it comes to five and six hole wheels, you need to measure the distance between two studs, this measurement is 'S'. If there are no holes directly opposite, you just need to use the below formula:

• 4 Stud PCD = S / 0.7071
• 5 Stud PCD = S / 0.5878
• 6 Stud PCD = S / 0.5

You can use the calculator below to provide the PCD for you.

At Arnold Clark we have a large team of experienced servicing technicians. If you are unsure about changing your wheels, our team of experts are happy to help. Reach out to our car servicing team or your local Arnold Clark branch for more information.