Quick Tips for Maintaining Your Bike Brake Fluid: The Confusion Stops Here!
In this installment of "Tech Time" we will be looking at the different types of brake fluids used in motorcycle hydraulic brake and clutch systems. Let's first look at the function of brake fluid in a hydraulic brake system. Fluid that is free of air or gas bubbles is basically non-compressible. The master cylinder pumps (displaces) brake fluid through the brake lines to the caliper (slave cylinder) and forces the brake piston(s) outward where it applies force to the brake pads. The brake pads contact the brake disc, and slow the vehicle through sliding friction. This friction creates heat and that heat must also be dissipated by all the brake system parts and the brake fluid.
Next, let's look at the different types of brake fluid. The first type is DOT 3 fluid. This fluid is very common on automobiles and is a polyglycol ether based fluid. This fluid has a minimum dry boiling point of 401 degrees Fahrenheit and a minimum wet boiling point of 284 degrees F. This fluid is also hydroscopic (absorbs water). The second type is DOT 4 fluid. This is the most common fluid used in motorcycles and is also a polyglycol ether-based fluid, but it has borate ethers added to raise the boiling point of the fluid. DOT 4 has a minimum dry boiling point of 446 degrees F and a minimum wet boiling point of 311 degrees F, and is also hydroscopic and is compatible with Dot 3 fluid.
The third type of fluid is DOT 5 fluid. This fluid is silicone based and has a minimum dry boiling point of 500 degrees F and a minimum wet boiling point of 356 degrees F. This fluid is not hydroscopic (does not absorb water) and is not compatible with any polyglycol ether based fluids. The fourth type of fluid is relatively new and is called DOT 5.1. This is a polyglycol ether based fluid with synthetic additives and has a minimum dry boiling point of 518 degrees F and has a minimum wet boiling point of 375 degrees F. This fluid is commonly referred to as "Super DOT 4" fluid in automotive circles and is also hydroscopic. This fluid is compatible with DOT 3 and DOT 4 fluids, but is not compatible with DOT 5 fluid.
We will now look at wet and dry boiling points. The dry boiling point is at what temperature fluid that contains no water boils at. The wet boiling point is the temperature at which fluid with 3% water content boils. Polyglycol ether based fluids absorb water and keep it in suspension to protect metal brake system parts. As the dry/wet boiling point numbers indicate, water saturated fluid loses way over 100 degrees F of boiling point! Silicone fluid can't absorb water so as water collects in the system it stays separate and attacks metal brake system parts. This water can cause major braking performance issues because water boils at 212 degrees F! It is for this reason that silicone fluid needs to be replaced often. Polyglycol ether based fluids should be replaced every 2 years.
If brake fluid reaches its boiling point gas bubbles form in the fluid making it very compressible. It is at this point that brake feel becomes spongy or is completely lost, the results of which can be catastrophic! Hopefully this information clears up some of the confusion about brake fluids. Until next time, ride safe!
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Dan Cunningham has over 26 years of technical experience, including 19 years as service shop foreman at Honda East in Maumee, Ohio.
