Image 3 – CAD render of the BAC Mono CSiC Brake Disc Assembly
As the project aim was to replace an existing cast iron disc with a CSiC disc, the basic dimensions; outer diameter, inner diameter and thickness were defined by the original disc. However the design of the cooling vents
was not fixed and as such it was decided to produce two different designs, the first with 8mm radial cooling vents and a second with larger 12mm vents. Table 1 shows a comparison of the key dimensions between the different designs.
Table 1 – Comparison of key dimensions and weight
Both CSiC discs were manufactured by Surface Transforms at their UK plant using continuous fibre material and assembled with aluminium bells using a fully-floating fixing system of ST’s own design.
Disc Mass Verification
Surface Transforms has developed a brake disc sizing tool through dynamometer and vehicle testing which is used to verify the mass
of ceramic brake discs for specific applications. The principle of the calculation is based on the temperature rise due to transferring the kinetic energy of a vehicle into thermal energy in the disc using its heat capacity.
There are two aspects to the thermal calculation;
1. A single stop from the vehicle maximum velocity. This takes no account of any cooling effects or heat loss in the system.
2. A set of multiple stops from maximum velocity with no cool-down time between stops. This makes some assumptions about heat loss in the system, determined using dynamometer test results which remain conservative.
Each disc design was assessed using this technique to verify that it has sufficient thermal mass for the application without exceeding a recommended maximum operating temperature of 650°C.
The predicted temperatures can be found in Table 2.
Table 2 – Predicted brake temperatures
The maximum temperature limit calculated using this tool is 500°C to allow for a margin of safety below the 650°C material limit. The above figures showed that the discs were sized correctly for the application. It is important to state that these calculations do not take into account the cooling vent design.
Disc Strength Verification
A structural analysis was performed on the bell and the disc to confirm that the assembly design was strong enough to safely handle the
expected operating loads.
The force experienced by each mounting bolt hole of the disc when stopping from maximum velocity was calculated. This load was then used in a FEA simulation.
Graph 1 – Fade test comparison between cast iron and carbon-ceramic brake discs
Image 4 – FEA simulation of the brake disc design
The strength of the bell was then investigated using the same process.
Image 5 – FEA simulation of the bell design
The predicted stresses in both components were demonstrated to be well below the yield strength of the material.
A comparison of the relative brake performance between the cast iron disc, CSiC disc with 8mm radial cooling vents and CSiC disc with 12mm radial cooling vents was performed on ST’s brake dynamometer based at Birmingham City University. The following braking attributes were tested after an initial bedding-in procedure had been performed:
2. Pressure sensitivity
3. High velocity performance
Typical AK master fade tests resulted in similar peak temperature between all three discs, ranging from 463°C to 470°C. Both CSiC discs reached significantly lower temperatures between braking operations than the cast iron disc, demonstrating improved cooling performance. This improvement was most significant with the CSiC disc with 12mm radial cooling vents (see Graph 1).
During pressure sensitivity testing all discs behaved in a similar manor, showing minor variations in coefficient of friction values at various brake line pressures. The cast iron disc had a noticeably greater noise and vibration at low pressures. Comparable to the fade test, all discs achieve a similar peak
temperature but the ceramic discs dropped to lower temperatures between stops.
In the high speed stops it is possible to see once again that the ceramic brake discs cool down faster between stops to reach a lower temperature.
Dynamometer testing demonstrated that a CSiC brake disc with continuous fibre construction can achieve the same thermal performance as a cast iron disc allowing for a like-for-like replacement whilst achieving a weight reduction in excess of 50%.
Testing also demonstrated that careful cooling vent design can allow for a reduction in mass without compromising the thermal performance of the CSiC disc. A potential reduction in mass of CSiC brake discs used for other applications can also be explored with far greater confidence as a result of this testing.
It is envisaged that the results of this project, along with additional testing, can be used to develop a revised brake disc sizing tool which also accounts for cooling vent design and can therefore support the production of CSiC discs with further increased efficiency.
Image 6 – BAC Mono CSiC continuous fibre brake disc, weighing just 1.7kg