This happens for two reasons….first when a street based car brake system is used for track driving and second when brake fluid of a poor quality, or old fluid is used which causes vapour lock.
The effect of caliper drag is for the pad operating temperature to shoot up several hundred degrees higher than target which causes a spongy brake and brake fade.
This is often confused with pad fade and the brake pad is criticised. Of course the pad IS ACTUALLY fading but not through any fault of its material design. Seals in standard road going calipers are simply not made for race use and may harden and cause drag or the calliper body itself may shrink and tighten on the seals and pistons increasing caliper drag. For this reason calipers used in racing must be in superb condition and seals changed at intervals to prevent hardening or the caliper changed for a unit designed for performance use. Also brake fluid of the highest wet boiling point should be used to avoid vapour lock.
Vapour lock happens when the water content, which all brake fluid has, starts to boil. Water boils at 1 bar atmospheric pressure at around 100 degrees C or 212 degrees F. There are brake fluids around that withstand boiling to well over 160 degrees C or 300 degrees F such as EBC BF307 which is an ideal fluid for performance use.
Remember the higher the fluid spec the faster it will deteriorate, flushing through by changing fluids 2-3 times a year in any performance driving environment is advised.
In Dyno work in the EBC lab in the UK where a target brake torque of 1200 Nm was required during race test cycles, almost 300 Nm was already measured before the brake was even applied due to caliper drag. This was encountered AFTER A FEW WEEKS USE FOR DYNO TESTS using a stock EVO or Impreza four piston caliper (which is a very good performance caliper) during laps 5 and higher of the standard EBC race brake simulation test and this was on ONE SINGLE CALIPER SO YOU CAN DOUBLE THAT FOR A VEHICLE. This of course means the starting temperature of the brake test in later cycles was already several hundred degrees higher than in early brake cycles of the test. There are huge consequences for drivers that do not maintain their calipers properly.
Race drivers have also complained that as races go on, fade develops. This is the issue caused by caliper drag, as the caliper gets hotter drag sets in and later stages of the race, brake fade is experienced.
The conclusion is that this is not so much a pad material fault as a system fault caused by caliper drag.
Another important consideration is how hot your calipers are getting. We hear race drivers reporting heat paint temperatures of 800 degrees C, that is way above what discs and calipers should ever run and clearly shows a lack of ducting getting to the brake itself. This of course is nothing a different pad compound could ever improve upon, consider improving airflow to calipers in any way you can on your street based race car. If you fail to make ducting and airflow improvements a few laps into any race caliper drag will develop and you will witness brake fade. This happens as the caliper gets hotter but also it is worth remembering that pad retraction in any caliper is totally reliant on seal ‘pull-back’. Caliper seals are essentially square in cross section with a very small one or two degree angle on the outer diameter which pulls the piston back and allows the brake pad to become free from the disc. As the seals age or the caliper gets hot the ability of the seals to retract becomes less or zero, hence the pad drag.
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