Stopping Fast! – The importance of brake fluid
Contributed by: Daniel Owsiany (Service Director – High Road Vancouver & Langley)
The brake system, specifically the brake fluid in your motorcycle plays a critical but often overlooked role. We riders expect a firm pull on the lever to decrease our speed in less time and distance than it took to generate it. What’s amazing is that it does just that. Take BMW’s S1000RR for example. From a standstill it accelerates to 100 mph in 5.1 seconds and well under 500 feet. Yet it slows to a complete stop in just under five seconds. This impressive braking system will slow the S1000RR from 250-0 km/h in 750 feet. That sort of performance is due to the engineering, power and resilience of the modern hydraulic brake system. In this article we will discuss the important role brake fluid plays in the complex brake system.
Brake fluid is a type of hydraulic fluid used in motorcycle brake systems. It is used to transfer force into pressure. It works because this fluid is not appreciably compressible – so the force applied to the brake lever or pedal will transfer to the brake caliper. Most brake fluids used today are glycol-ether based. Brake fluids must meet certain requirements as defined by various standards – most brake fluids are classified as “DOT 3” and “DOT 4”.
Brake fluids must have certain characteristics and meet certain quality standards for the braking system to work properly.
Brake fluid can be subjected to very high temperatures, especially at the brake calipers. It must have a high boiling point to reduce the possibility of brake fade under severe use. DOT standards refer to a brake fluid’s “dry” and “wet” boiling points. The wet boiling point (containing 3.7 percent water by volume), which is usually much lower (although above most normal service temperatures), refers to the fluid’s boiling point after absorbing moisture as is common after a year or so of regular use. Glycol-ether (DOT 3, 4) brake fluids are hygroscopic (water absorbing), which means they absorb moisture from the atmosphere under operating conditions.
Brake fluids must not corrode the various materials and metals used inside components such as calipers, master cylinders and ABS control valves. They must also protect against corrosion as moisture enters the system. Additives (corrosion inhibitors) are added to the base fluid to accomplish this.
Brake fluids must maintain stable viscosity, even with varying temperatures to accommodate different environmental conditions. As compressibility increases, more brake lever travel is necessary for the same amount of brake caliper piston force. Consistent lever feel and travel is crucial to proper brake function.
Brake fluid must be formulated to lubricate all moving parts in the system – seals and cylinders in the master cylinder and brake calipers.
Service and maintenance
Given that brake fluid is hygroscopic, moisture diffuses into the fluid through brake hoses and rubber seals and, eventually, the fluid will have to be flushed when the moisture content becomes too high. The corrosion inhibitors also degrade over time. Degraded inhibitors cause corrosion in the braking system. Lubricating properties of the fluid also degrade – this causes premature wear of the rubber seals.
Brake fluid level in the master cylinder will drop as the brake pads wear and the calipers extend further to compensate. Brake fluid level may also be low because of a leak, which could result in a loss of hydraulic pressure and consequently a significant loss of braking ability. If the brake fluid level consistently drops, the cause should be investigated and repaired.
BMW and Ducati recommend flushing brake fluid every 24 months. This is a general guideline and can vary depending on the specific use of the motorcycle. Brake fluids should be flushed more frequently when used under severe conditions. (Race, Track day, areas affected by heavy rainfall and year-round riding etc.)
Remember – Brake fluid is toxic and combustible and can damage painted surfaces.
Above: Crystalized brake fluid due to poor maintenance. Above: Fresh clear brake fluid