Brakes Part 1: Overview
Following on from the updates relating to wheels, gearboxes, differentials and headlights, we thought it would prepare a series of articles on the brake system we are running on the car and how we selected the system. So sit back and keep following the website over the next few weeks for regular updates.
Except for high end supercars, stock brakes are capable of a single stop from maximum speed at or near the limit of tyre grip, but not repeated stops from high speed. Most stock brake systems lack sufficient thermal capacity – the system’s ability to absorb and transfer heat into the air or surrounding structure during severe driving.
The stock calipers and their mountings are usually not structurally stiff enough at higher line pressures thus reducing the effectiveness of the brakes and most OEM brake pads are also not designed for severe use, since cold stopping performance and quiet operation typically are considered more important to new car buyers.
There are a few basic facts that must always be kept in mind when discussing brake systems:
1) The brakes don’t stop the vehicle – the tyres do. Thus the type of tyre being used has to be taken into account (we have to use tyres set by the relevant event).
2) The brakes function by converting the kinetic energy of the car into thermal energy during deceleration – producing heat, lots of heat – which must then be transferred into the surroundings and into the air stream.
3) Repeated hard stops require both effective heat transfer and adequate thermal storage capacity within the disc. The more disc surface area per unit mass and the greater and more efficient the mass air flow over and through the disc, the faster the heat will be dissipated and the more efficient the entire system will be. At the same time, the brake discs must have enough thermal storage capacity to prevent distortion and/or cracking from thermal stress until the heat can be dissipated. This is not particularly important in a single stop but it is crucial in the case of repeated stops from high speed.
4) Control and balance are at least as important as ultimate stopping power. The objective of the braking system is to utilise the grip capacity of all of the tyres to the maximum practical extent without locking a tyre. The required pedal pressure, pedal travel and pedal firmness must allow efficient modulation by the driver.
To summarise, braking performance is about more than just brakes. In order for even the best braking systems to function effectively, tyres, suspension and driving techniques must be optimised.
To go further it is necessary to understand some of the physics involved, and that requires some definitions.
- Mechanical pedal ratio: The mechanical pedal ratio is the distance from the pedal pivot point to the effective center of the footpad divided by the distance from the pivot point to the master cylinder push rod. The larger the ratio, the greater the force multiplication, but the longer the pedal travel.
- Brake line pressure: Brake line pressure is the hydraulic force that actuates the braking system when the pedal is pushed. It is the force applied to the brake pedal multiplied by the pedal ratio divided by the area of the master cylinder. For the same amount of force, the smaller the master cylinder, the greater the brake line pressure.
- Clamping force: The clamping force of a caliper is the force exerted on the disc by the caliper pistons. It is the product of brake line pressure, in psi, multiplied by the total piston area of the caliper. Increasing the pad area will not increase the clamping force.
A few things are now obvious:
1) Line pressure can only be increased by either increasing the mechanical pedal ratio or by decreasing the master cylinder diameter. In either case the pedal travel will be increased.
2) Clamping force can only be increased either by increasing the line pressure or by increasing the diameter of the caliper pistons. Any increase in caliper piston area alone will be accompanied by an increase in pedal travel. The effectiveness of a caliper is also affected by the stiffness of the caliper body and its mountings. It is therefore possible to reduce piston size while increasing caliper stiffness and realize a net increase in clamping force applied. This would typically improve pedal feel.
3) Only increasing the effective radius of the disc, the caliper piston area, the line pressure, or the coefficient of friction can increase brake torque. Increasing the pad area will decrease pad wear and improve the fade characteristics of the pads but it will not increase the brake torque.
1) In order to brake effectively, the tyres must comply with and grip on the road. Your braking system is no better than your tyres and suspension. The best money that you can spend is on really good tyres (we race on spec tyres so this is not a consideration) and really good shocks (the team has excellent AST shocks).
2) Proper corner weight is crucial for effective straight line braking (our car goes to the best set-up garage in the UAE, Motorsport Wheels). Optimum corner weight for braking is when the cross corner pairs are equal. That is to say the total of the left front and right rear equals the total of the right front and left rear. The team uses the excellent Motorsport Wheels for this.
4) Use at least a 550 degree non-silicone brake fluid and make sure that your brakes are bled properly and, when used hard, often. The team uses Motul Brake fluid. Brake fluid is hygroscopic in nature – given any chance at all it absorbs water. A fraction of one percent of entrapped water lowers the boiling point of any brake fluid dramatically – and causes corrosion within the system.
Road car brakes are fine for daily driving but provide insufficient continuous stopping power on the race track. Endurance racing takes the braking requirement a step further as brake disc and pad life has to be much longer than required for 20 minute sprint racing.
So the next few articles will delve into the considerations that have to be taken into account when selecting race brake systems.