One-piece vs. two-piece brake discs

Often we get asked 'what's the difference between one and two piece brake discs?' or 'how will this effect my braking performance?'. Our Chief Engineer - Special Vehicles, Michael Jones talks us through this in more detail below.

Look through the wheels of most passenger cars that have disc brakes and you will probably see a one-piece brake disc. This means that the disc friction ring and the bell (or hat) have been machined from a single piece casting.

A lot of energy is required to stop a vehicle from high speed. As the disc gets hotter, it will expand. But the bell will remain ‘cold’ and will restrict movement. Too great a level of expansion will cause the disc to cone or crack. You can manage this by having a large enough mass of material (most commonly cast iron) to absorb the heat, restricting expansion to a safe level.

The form or shape of the disc at the point where the friction ring joins the bell is critical. You will often see deep undercuts, relatively thin sections, ‘swan necks’ and ‘back-vented’ designs that are in place to allow some expansion and movement under temperature.

However, some vehicles are equipped with two-piece discs, where the disc assembly is made from two separate components. In these discs, a friction ring is attached to the bell using various fixing methods.

On high-performance vehicles, the weight of disc required to repeatedly safely stop the vehicle without temperature related issues, may be much higher than you want. Mike has mentioned in a previous post how speed is a bigger driver than weight with regards disc temperatures. Two-piece disc concepts allow the friction ring to radially expand without being constrained by the cold bell.

There are many different concepts for floating disc, from motorsport type bobbins to high-volume pin-drive systems.

Having two-piece discs also allows you to use a different, often lighter in weight material for the bell. An aluminium bell may be used with an iron disc for example, this will often allow for better heat dissipation also. Additionally, this can mean a significant reduction in unsprung mass, which will then have an effect on the handling and suspension performance of the vehicle.

Another reason for two-piece discs is where disc manufacturers may have a variety of flat disc friction ring castings that can be bolted to bells to make a disc assembly, allowing them to be manufactured without investing in bespoke foundry tooling. This keeps introduction costs down.

Keep an eye out for future technical posts where we will cover more on the science and technology behind braking systems and components.

For more information, please visit: https://www.alcon.co.uk/