Introduction
In this continuation of our Brake Tech series articles we'll be taking a look at brake fluid and the importance of selecting the right one for your intended use. If you haven't already be sure to read our Brake Pad Selection article.
In a racing or even track day environment, every component of a vehicle is pushed to its limits. Brake fluid plays a critical role in ensuring that a car's braking system performs optimally under the most extreme conditions. Unlike regular brake fluid used in everyday vehicles, racing brake fluid is specifically formulated to handle the high temperatures and stresses encountered on the track. This article covers the key aspects of racing brake fluid, different types available and overview of key characteristics including temperature ranges, viscosity, and maintenance best practices.
Brake Fluid Types (DOT)
In North America brake fluids are classified based on their chemical composition and performance characteristics.
The main classifications are DOT 3, DOT 4 and DOT 5.1 which are all glycol ether (DOT3) and borate ester based (DOT 4 and DOT 5.1). DOT 5 is a silicone based fluid. For the purpose and scope of this article we are only interested in DOT 4 and 5.1 fluids.
Fluid Temperature Ranges
One of the primary distinctions of racing brake fluid is its ability to withstand extremely high temperatures without boiling. The boiling point of brake fluid is crucial because when the fluid boils, it creates vapor bubbles, leading to a spongy brake pedal or complete brake failure. Racing brake fluids have higher dry and wet boiling points compared to standard brake fluids.
- Dry Boiling Point: This is the boiling point of new, uncontaminated brake fluid. Racing brake fluids often have dry boiling points exceeding 300°C (572°F).
- Wet Boiling Point: Over time, brake fluid absorbs moisture from the environment, which lowers its boiling point. The wet boiling point is the temperature at which the fluid will boil after it has absorbed a certain amount of moisture. Racing brake fluids aim to maintain a high wet boiling point, often above 200°C (392°F).
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| Figure 1 |
Bleeding and Flushing Frequency
Brake fluid is hygroscopic, meaning it absorbs moisture from the environment over time. This absorption can reduce the boiling point of the fluid, affecting its performance (see figure 1). The absorption rate is not linear and is affected by weather conditions. More water is absorbed during hot humid summer weather and rainy race conditions.
- Moisture Management: Use brake fluid from sealed containers and avoid opening the reservoir cap unnecessarily. Store brake fluid in a cool, dry place.
- Flushing: Flushing involves completely replacing the old brake fluid with new fluid. Due to the high demands of motorsports, it is recommended to flush the brake system more frequently than in regular driving conditions. Depending on the racing schedule, this could mean flushing after every few races or more often if the conditions are particularly harsh. As a general rule, we recommend replacing/flushing the brake fluid at lease once a year at the start of the racing season. For optimal performance consider flushing every few race weekends. Most cars can be flushed with top spec racing fluid for under $100, a relatively minor expense in the grand scheme of racing costs that offers some peace of mind.
- Bleeding: This is the process of removing air from the brake lines. In motorsports applications, it's common to bleed the brakes before each event or even between sessions during a race day. Air in the brake lines can compress, leading to a soft or spongy brake pedal. Regular bleeding also helps maintain a high boiling point by cycling old moisture laden fluid from the system.
Fluid Viscosity
Viscosity, or the thickness of the fluid, affects how well it can flow through the brake system, especially in varying temperatures. Racing brake fluids are designed to maintain consistent viscosity across a wide temperature range, ensuring reliable and consistent brake performance whether the brakes are cold at the start of a race or extremely hot after several laps.
With the advent of modern anti lock brakes and electronic stability control systems, some automobile manufacturers are specifying low viscosity brake fluids for a faster response of ABS and ESC systems. DOT 5.1 fluid typically meets these requirements (viscosity at -40C of ≤900 mm2/s). There are also low viscosity DOT 4 fluids that meet the specs. Racing DOT 4 fluids can have viscosities as high as 1900 mm2/s at -40C which can cause ABS/ESC system activation delay in winter driving for regular street cars, something to consider you also drive your track car in the winter months.
I the racing world, the same viscosity consideration can come into play in very cold winter rally conditions. If you are experiencing slow ABS pump activation in this use scenario, consider switching to a DOT 5.1 or DOT 4 LV fluid.
Fluid Selection
Not all racing brake fluids are created equal. Different brands and formulations offer varying performance characteristics.
- Reputable Brands: Choose fluids from reputable brands known for their quality and performance in racing environments. Brands like Castrol, Motul, Brembo, Red Line and Hawk are well-regarded in the racing community.
- Formulation Types: Different formulations (DOT 3, DOT 4, DOT 5.1) have specific characteristics. For example, DOT 4 Racing fluids generally offer the highest boiling points suitable for racing.
Top Tier Racing Brake Fluids:
Mid Tier Racing Brake Fluids:
Street Performance Brake Fluids
- Motul DOT 5.1
- Motul DOT 4 LV (low viscosity)
- Hawk HP-520
Conclusion
Racing brake fluid is a specialized product designed to perform under the most extreme conditions of motorsport. With regular maintenance through bleeding and flushing, you can ensure that the braking system delivers optimal performance when it's needed most. For racers, understanding these aspects and adhering to a rigorous maintenance schedule can make a significant difference in both safety and performance on the track.