Motocross Stopping Power: The Science Behind Brake Fluid's High Boiling Point

Motocross Stopping Power: The Science Behind Brake Fluid's High Boiling Point

Brake fluid for motorcycles and dirt bikes is designed to withstand the high temperatures generated during braking, thanks to its high boiling point. However, it gradually absorbs moisture from the air over time due to its hygroscopic nature.

Fresh, clean brake fluid can boil up to 230°C (446°F), depending on the fluid (see the chart below). In contrast, water boils at just 100°C (212°F). The more water the brake fluid absorbs, the lower its boiling point gets. Hard braking generates significant heat, further elevaing the temperature of the brake fluid. (See Chart Below).

If your dirt bike's brake fluid has too much water, it may boil and make steam. Unlike brake fluid, steam can compress. Consequently, when hitting the brakes, the steam may compress instead of applying sufficient pressure on the brake pads. This can make your brakes work worse or even fail, as the steam prevents the appropriate amount of force from reaching the brakes.

Dry Boiling Point                 Wet Boiling Point
DOT 3 205°C/401°F            140°C/284°F
DOT 4 230°C/446°F            155°C/311°F
DOT 5.1 260°C/500°F          180°C/356°F

Modern motocross bikes across the 50cc, 65cc, 85cc classes, and full-size models have small rear brake reservoirs. This can lead to the fluid heating up rapidly due to the limited volume.

Additionally, motocross riders frequently apply more force to the rear brake for air control and tight maneuvers. This heavy usage further speeds up the temperature rise, increasing the likelihood of brake fluid boiling.

To prevent these issues, riders and mechanics often use higher-grade brake fluids with higher boiling points. Regular maintenance and checks are crucial to ensure that the brake fluid is in good condition and hasn't absorbed too much water, which would further reduce its boiling point and braking efficiency.

Flushing your brake system is straightforward and doesn't require special tools—just brake fluid.

Don't wait until the brake fluid darkens in color or the brakes feel spongy; by then, it's already too late.

For preventative maintenance:

• Street Bikes: Flush brake system every 10,000 miles or 2 years.
• Dirt Bikes, ATVs, UTVs: Flush system every 40 hours of use.

A simple upgrade you can make to your bikes is to switch to DOT 5.1 brake fluid.

Switching to DOT 5.1 brake fluid from DOT 4 is straightforward since they blend easily, both being glycol-based. However, it's crucial not to confuse DOT 5.1 with DOT 5; DOT 5 is silicone-based and incompatible with other fluids, posing a risk of brake failure.

The high boiling points of DOT 5.1 fluid (DOT 5.1 260°C/500°F) ensure improved braking performance, even under extreme heat or during racing conditions. Therefore, when making the transition to DOT 5.1, ensure it is the correct type for enhanced brake safety and functionality.

This tool: Phoenix V-5 Bleeder Brake & Clutch simplifies and speeds up bleeding brakes and clutches. Its standout Reverse Bleed Technology excels at air removal. It's perfect for refilling and air-purging completely dry systems, in-vehicle or bench setups. Works with DOT 3, 4, and 5.1 fluids and is incredibly user-friendly.