Rolling Radius

The Rolling Radius: The ‘rolling radius’ is something that needs to be considered when buying new larger alloy wheels. It is the measurement from the centre of a wheel to the outer edge of the tyre tread when it's unladen. The arrow on the image below shows the measurement to consider. There are three key reasons why it is important to maintain the manufactures wheel and tyre diameter. • Over sizing wheels and tyres will cause bodywork clearance problems • The wrong ‘rolling radius’ will change your cars performance and gearing • It will make your speedometer read incorrectly

It is a common misconception that by buying larger alloy wheels, you will ‘fill your arches’. In reality, you need the outer tyre diameter to stay the same size to make sure that you don’t have any of the problems that we have mentioned above. Below is an example of what we want to achieve with the tyres when increasing the wheel size. Essentially if you are increasing your rim, you will need to decrease your tyre profile.

It's worth pointing out that the actual radius the manufacturers use for speedo calculation is the 'dynamic' or the 'laden' section of the wheel at the recommended inflation pressure and 'normal' loading. Obviously though, this value is entirely dependent on the unladen rolling radius.