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Brake Tech #04

Standard diameter brake upgrades for fast road cars

Brake system basics

The tuning scene faces the potentially dangerous situation in that we have tuners all over the world making cars go faster and yet there is often very little focus on how to bring your brake system up to a level that matches your driving style and the modifications to your car.

The following summary should give you a basic understanding of how your brake system works and allow you to make qualified decisions regarding standard-diameter brake upgrades that are available to the owners of modified road cars.

The brake system on your car works by converting the momentum of the car (dynamic energy) into thermal energy (heat). For brake component manufacturers, the challenge is to manage how much heat is being produced by a specific vehicle’s brakes and how best to deal with that heat.

The amount of heat that is produced by your brake system is a direct result of three factors:

  1. The weight of your car.
  2. The speed that you are braking from.
  3. How often you apply the brakes (as well as the amount of cooling time you allow between brake applications).

The real catch is that the temperature increases do not follow a linear curve – they are exponential. In other words – If you double the weight of a car, the brakes will generate twice as much heat. If you double the speed from which you are braking, the brakes will generate four times as much heat. If you double the weight and speed, your brakes will produce eight times as much heat. Of course this is an extreme example but it illustrates just how much extra weight (think heavy ICE installs) and extra speed (think about all those engine mods) influence brake temperatures.

A practical example

Let’s assume that you drive a compact hot-hatch weighing in at around 1100kg’s with driver on board. For our example we will assume that the car is fitted with standard 239mm (9.4 inches) front ventilated brake discs weighing 3.7kg’s (8.14 pounds) each. By running the above scenario through Powerbrake’s custom written software, we can calculate that one emergency stop from a speed of 120 kph (75 mph) to standstill will result in a rise in front disc temperatures of approximately 164 deg C (327 deg F).

Now let’s add 250 kg’s (550 pounds) of extra weight into the car in the form of a heavy ICE install and one extra passenger. The same emergency stop from 120kph (75 mph) would result in a rise in front disc temperatures of approx. 205 deg C (401 deg F).

Next, let’s up the speed a bit and assume that you perform the same emergency stop but this time from 160 kph (100 mph). This single stop would result in a rise in front disc temperatures of approx. 376 deg C (709 deg F). That’s 56% more heat being generated in a single stop by adding just one extra passenger, a sound install and 40 kph (25 mph) to your traveling speed!

The real food for thought though is that we used the phrase “rise in disc temperature” in the above examples. That’s to remind you that if you are having a spirited drive and applying the brakes frequently in succession, your discs won’t have enough time to cool down sufficiently between brake applications. This leads to a compounding effect with regards to disc temperatures. For example: Assume that you were having a spirited drive through a mountain pass. You apply the brakes hard on the entry to corner number one and we assume that the disc temperatures go from 0 to 150 deg C (302 deg F) (You are not braking all the way to a standstill in this case).

When you come off the brakes the discs will start to cool, as they are designed to. However, if the corners are relatively close together on the pass, by the time you reach corner number two, we assume that the discs are still sitting at 100 deg C (212 deg F). The disc temperatures then rise by the same amount as they did into corner one, so your disc temperatures now climb to 250 deg C (482 deg F). Again, the discs will cool a bit before the next turn and let’s say they are at 180 deg C (356 deg F) when you brake hard into turn three. When you exit turn three your discs are sitting at 320 deg C (608 deg F).

The point is that by turn six or seven your disc temps can easily have reached temperatures in excess of 550 deg C (1022 deg F) at which point there is a strong possibility that you will be experiencing serious brake fade with standard discs, pads and brake fluid. This same logic applies to fast driving from intersection to intersection in urban areas.

Results of extreme brake temperatures

Excessive brake temperatures are the enemy in braking terms. If you run standard discs, pads and brake fluid over their maximum operating temperatures (MOT) you will experience brake fade (which can have serious consequences) and very high wear rates on your discs and pads (which can be expensive to keep replacing).

There are essentially two types of brake fade. The first and more common type is referred to as “pad fade”. This occurs when your disc and pad temperatures simply exceed the max operating temperature of the pad compound that you are using. Your brake pedal will remain relatively firm but it will feel as if the car is just not slowing down quickly enough, requiring you to press the brake pedal harder and harder.

The second type of brake fade is referred to as “fluid fade”. This occurs when the brake fluid behind the pistons in your calipers reaches its boiling point and evaporates, turning from a liquid into gas bubbles. Gas is far more compressible than liquid, so your brake pedal gets longer and longer, eventually going straight to the floor. In the brake industry, this is referred to as vapor lock.

The really scary thing about vapor lock is that when you’re actually on the brakes, you are pressurising your brake fluid, which temporarily raises the boiling point. The fluid is most likely to boil in the instant that you come off the brakes (when the fluid is extremely hot and the fluid pressure is reduced, hereby lowering it to its normal boiling point). You will only realise that your fluid has boiled when you get to the next corner and hit the brakes only to discover that your pedal goes straight to the floor with very little brake effect. Anyone who has experienced vapor lock will tell you, it is not something you want to experience first hand!

Driving style

Firstly, small changes to your driving style can make huge differences to disc temperatures. The most important point is to try and allow enough cooling time between brake applications. If you are having a spirited drive and need to perform a number of hard brake applications in quick succession, try to limit these to five or less before driving on for a while without excessive braking, hereby giving the discs time to cool down. Avoid stopping your car when the discs are extremely hot. Discs act as impellers – pumping air through the cooling vanes in order to cool down. When you stop, there is no airflow, which is a killer for discs.

People often say to us that “they only drive their car on the road and never take part in track events but they are still overheating their brakes”.

Bear in mind that the distances between corners on the average race track are usually far longer than the distance between traffic lights on the street. That allows cooling time and airflow. For those readers that like to speed from intersection to intersection, be aware there is nothing harder on your brakes than hard traffic light to traffic light abuse. The distances between intersections are too short for sufficient cooling. Also, sooner or later you end up having to come to a complete stop if you catch a red light, which means that your discs are extremely hot and there is zero airflow to cool them. Remember that race cars never stop with their discs at peak temperatures. The drivers normally do one or two cool-down laps after a race, during which they hardly touch the brakes and there is plenty of airflow to cool the discs prior to stopping in the pits.

Upgraded disc and pads

If you find that your brakes are overheating, leading to brake fade and accelerated disc/pad wear, you essentially have three options. The first option is to to change your driving style. The second option is to fit discs and pads that are designed to operate effectively at the temperatures that your brakes are reaching. The third option is to fit a complete Big Brake Kit that will lower disc and pad temps substantially (we will cover option three in another article).

The challenge you face is deciding which pad compound to match to which disc design in order to squeeze the maximum performance and durability from the brake system on your car. There is a real misconception in the market that you can simply take any performance brake pad, run it on any disc design/material and you will be upgrading your brakes. It simply doesn’t work that way. The secret to optimising brake performance is matching a disc design / alloy and pad compound that have been developed and tested to perform optimally at the disc temperatures generated by a particular vehicle and driver.

As a performance and race disc manufacturer, Powerbrake spends an enormous amount of time testing different disc and pad combinations using state of the art brake dyno’s as well as conducting continuous on-car testing using sophisticated data-logging equipment. Powerbrake stock a range of different high-performance street and race pad compounds, which are sourced from some of the world’s leading pad manufacturers.

Also available from Powerbrake and their distributors, is a range of high-temperature brake fluids with boiling points of up to 325 deg C (617 deg F) - compared to standard DOT 4 fluids, which will boil at around 235 deg C (455 deg F).

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