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TKART magazine Tech Talk | Brakes liquid
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Gianluca Covini
14 May 2022
This is the fluid that transforms the force on the brake pedal into the force that compresses the pads on the disc to brake the kart. For this it is essential to make sure that it keeps its characteristics unchanged for as long as possible
Brake fluid is a hydraulic oil that transmits the braking force on the brake pedal to the wheel calipers. The oil is pressurized by the brake pump, which can impart a variable pressure, from a few bars up to over 100 bars. There are two distinct families of brake fluids: those based on glycol and those based on silicone. The former, glycol-based, are characterized by their corrosivity and the fact that they are hygroscopic, that is, they tend to absorb the moisture present in the air through the tubes and seals.
Humidity is a factor that compromises the efficiency of the liquid, altering its physical characteristics. The higher the boiling point of the brake fluid, the greater its hygroscopic property. Silicone liquids, on the other hand, are neither hygroscopic nor corrosive, but are more compressible than glyconic ones. The most common type of these products is DOT 4, which has excellent resistance to temperatures and low hygroscopicity.
“Glycol-based brake fluid is hygroscopic, meaning it absorbs the moisture present in the air”
The main characteristic that a brake oil must have is that it must not be very compressible, otherwise the stroke of the brake pedal lengthens, with the sensation of elasticity of the control.
The graph shows the trend of the wet boiling point, over the years of use, for a glycol-based liquid. Due to the absorption of water vapor, the boiling temperature is lowered over time. Depending on the type of oil, the trend will be more or less sharp.
Compressibility is the physical property by which the volume varies according to the variation of the pressure. It is often said that liquids, such as water or oil, are incompressible: in reality they are, even if in a very limited way compared to gases. Brake fluids undergo a volume decrease ΔV which depends on the volume present in the system and on the pressure variation ΔP, caused by the force exerted by the brake pedal and therefore by the brake pump. Hence the above indicated equation, in which the proportionality coefficient α is called compressibility.
Viscosity is a physical quantity that measures the resistance of a fluid to flow. The higher the viscosity, the greater the resistance will be opposed to the flow of the liquid. Viscosity has a directly proportional effect on the pressure drops distributed in the system. This means that with a higher viscosity the system pressure will be reduced and therefore the efficiency, that is the ability to transform the braking force impressed on the brake pedal into braking torque on the wheels.
To best perform its function, brake fluid must have multiple characteristics. First of all, it must be not very compressible, even at high pressures and temperatures, to minimise elasticity and the stroke on the brake pedal. Secondly, it must have a high boiling point (or minimum boiling temperature) in order to avoid the phenomenon of "vapour lock", that is, the formation of air bubbles in the circuit, with consequent lengthening of the pedal stroke. In particular, this phenomenon occurs when the water, present in the form of humidity in the braking circuit, boils due to the excessively high temperatures reached by the brake fluid (due to the heat generated by the pads, discs and brake calipers when braking on the track) producing compressible air bubbles. The pedal therefore becomes elastic, spongy, and the braking force exerted by the foot does not transfer completely and directly from the pump to the brake caliper, causing the system to lose efficiency. The brake fluid must also have a low viscosity value, to ensure minimum pressure drops (pressure losses) even in the narrowest passages of the braking circuit, such as inside the pump or brake caliper, where the tubes also have diameters of a few millimetres.
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