Often we get asked ‘what’s the difference between one and two-piece brake discs?’ Or ‘how will this effect my braking performance?’
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. This can be managed this by having a single piece discs design featuring a large enough mass of material (most commonly high-grade 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. It’s worth noting here that the use of any form of floating disc should be avoided on armoured brake applications, as the mounting bobbins are likely to become clogged over time causing noise and vibration.
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. Alcon Chief Engineer Special Vehicles Michael Jones 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.
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 to 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.
For armoured vehicles these are typically not important considerations, and the larger mass of a correctly designed single piece discs will likely be preferable, with two-piece discs being specified for low-volume production runs, or new applications prior to mass production, which require time and larger investment in bespoke foundry tooling.