|
Author: Alex Bell, Leeds Formula Student Race Team
 Cost control and reduction is an important element in the Formula Student Team challenge – and for motor manufacturing in general - and at Leeds a small part of this has been achieved through the development of a bespoke caliper that has been designed and manufactured in-house. For the Formula Student Team at Leeds this has proven to be the most cost effective solution for the design of the foundation brake.
The Leeds Team comprises a mix of third and fourth year undergraduate students, drawn from the Mechanical and Automotive Engineering programmes and supported, in part, through long established brake system research in the School of Mechanical Engineering. In the most recent UK Government Research Assessment Exercise the School achieved an impressive 75% of research activity rated internationally excellent or world leading. This strong research base is supporting the exploration of novel design solutions and provides team members with valuable broad based product engineering experience from which to draw upon. The current Leeds car, F12, utilises three fixed four pot calipers, two acting on the front wheels and one inboard rear caliper acting on the differential unit. A fixed caliper and floating disc form the heart of the foundation brake assembly. Together they minimise the likelihood of any problems linked to insufficient pad retraction in the off brake condition.
The caliper halves are manufactured from a solid billet of aluminium (7075 – T6) within the Faculty of Engineering’s state of the art CNC workshop. Aluminium was chosen to keep weight to a minimum, whilst at the same time the 7075 alloy is easy to machine and produces little swarf. The automation of the manufacturing process allows for the design to be changed without large costs or time penalties.
Using the School’s strong links with HiSpec Motorsport Ltd, the caliper halves are sent away to be hard anodised. From the final dimensions of the caliper bores, HiSpec then manufacture the pistons and provide suitable rubber seals. Other bought in components include the bleed nipples, tube nuts and inlet connectors.
During the design of the brake calipers, a four piston per caliper arrangement was chosen. This enabled a greater and more evenly distributed force to be applied to the brake pads. The groove for the piston seal is designed so that the outer diameter of the groove is less than the outer diameter of the rubber seal, ensuring that the seal is squeezed against the caliper piston when assembled. This enables the seal to retract the piston when the brake is disengaged. However, should the piston travel too far due to brake pad wear, the seal preload is overcome and the pistons and brake pads are able to move closer to the brake rotor.
Brake pad backing plates are designed to suit the brake caliper and are laser cut from a sheet of S515 steel. They are then sent to one of our sponsors, Federal Mogul Friction Products, who bond the friction material to the backing plate. They provide a range of friction materials under the FERODO brand so tests are carried out by team members to determine the optimal friction pair combination for expected race conditions.
Stress analysis is carried out on a finite element model of the caliper using the worst case scenario, i.e. 1.5 times the maximum force applied to the brake pedal by the 95th percentile male. From these results, potential failure locations can be determined and rectified through small changes to the design.
 The advantages that come with manufacturing the brake calipers in-house are numerous. Through manufacturing the caliper halves with a CNC machine, the design can easily be changed to suit the mounting points. For example, should the front uprights be redesigned, the designer does not need to worry about the mounting points of the caliper, as the caliper can be redesigned to suit, without substantial time or cost implications.
With regards to cost, according to the 2009 Formula Student Cost Report rules, it costs the University $80 to manufacture one brake caliper, whereas the average price of a bought in caliper is $100. The weight of the brake caliper fully assembled is 500g which is significantly lighter than similarly specified bought-in units.
|
Comments
I'm designing a motor cycle powered hill climb car and thinking about the effect of braking via a limited slip diff, just as you have done.
I wonder about the possible scenario of braking hard with one wheel on a more slippery surface than the other.
If the caliper stopped the disc completely with the car still moving, I can imagine the wheel with highest grip still rolling forward at high slip, and the wheel with lesser grip rotating backwards.
Have you ever had this happen ?
Your reply will be greatly appreciated.
Cheers
John