Tech talk: Know your brake fluid
FB: What is the purpose of brake fluid?
TM: To stop you! Brake fluid is what transfers the action from your brake lever to the discs via fluid pressure. When you pull the brake lever, you’re moving a piston in the brake master cylinder. This pushes the brake fluid all the way down towards the wheels, and at the wheels are calipers. These calipers also have pistons that the fluid now presses on and squeezes the brake pads tight against the brake discs.
FB: What is it made from?
TM: All DOT fluids (with the exception of DOT 5) are made up of a polyglycol base. Glycol-based fluids have a mixture of ingredients with as many as 10 separate substances making up the final product. These substances can be broken down into four key components: 20-40 per cent is a lubricant – like polythene or polypropylene, to keep parts moving freely. 50-80 per cent is a solvent diluent – usually glycol ether that determines the fluid's boiling point and viscosity. Then the small remainder is a made of a modifier-coupler, which changes the amount of swelling of exposed rubber parts, and inhibitors – to prevent corrosion and oxidisation. DOT brake fluid is required to meet strict standards set by the Society of Automotive Engineers and the Department of Transportation (DOT), which is why it’s called DOT. These standards centre around maintaining brake fluid performance in a range of temperatures (high and low) and specify the minimum boiling temperatures which the manufacturers must adhere to.
FB: Does brake fluid have an expiry date?
TM: The shelf life of glycol-based DOT brake fluid is very poor due to its hygroscopic properties, which is why every bottle should come with an airtight foil liner covering the opening. As soon as this airtight seal is broken, it will start absorbing moisture from the environment and the boiling point will begin to drop. In our opinion it would be unwise to use DOT fluid from a bottle which has been open for more than a few months. Mineral brake fluid, however, is hydrophobic and does not absorb water from the environment, and one of the great advantages it has over DOT fluid is that once opened, it can be stored indefinitely.
FB: And what about a performance life?
TM: Performance life of your brake fluid can be determined by many varying factors, such as the bike you’re riding, where you are riding, how you are riding, but a good brake fluid that's properly maintained can be just the thing that saves your bike – or even your life. A brake fluid flush should therefore be one of your rituals. Suck the old fluid from the reservoir (careful, it's corrosive). Top off with fresh stuff – not from an opened bottle that's been absorbing water on a garage shelf – as you bleed each caliper, starting with the one furthest from the master cylinder. Since you're down there anyway, inspect for uneven pad wear, pitted discs, and frayed or cracked lines.
FB: How often should brake fluid be changed?
TM: Regardless of which brake fluid you are using (DOT 3/4/5.1) the majority of manufacturers, and indeed motorcycle workshops, will always advise that your brake fluid be changed every two years, regardless of use, but more is always better. Especially when racing!
FB: How good is standard brake fluid?
TM: With DOT 4 and DOT 5.1 both being a glycol-based fluid, and having to meet the strict Department of Transportation standards, the brake fluid which comes as standard in your motorcycle will be as good as any of the ‘aftermarket’ brake fluids. What you will find as well is that wherever you buy your motorcycle from, be it a main dealer or smaller independent shop, they will have branded oils/lubricants in their workshop so you know your bike is receiving the best products. The only real step up from standard brake fluid would be a track-based product such as MOTUL’s RBF660, which is specially designed to resist the high temperatures of racing-actuated brake (steel/carbon) and clutch systems. RBF660 brake fluid is particularly suitable for the demands of racing and for those requiring a very high performance brake fluid. RBF 660 is a 100 per cent synthetic brake fluid from poly-glycol bases.
FB: So what’s the difference between the different ‘Dots’?
TM: DOT 4 and DOT 5.1 are both glycol-based brake fluids and are used widely in the automotive and cycle industry. The main difference between these two brake fluids is in their boiling points. Part of the standards that need to be met by the manufacturers of DOT fluids are the minimum dry and wet boiling points. These are the minimum temperatures that the brake fluid must perform at before the brake fluid starts to boil, which can lead to complete brake failure.
FB: When should DOT 5.1 be used?
TM: DOT 5.1 is one of several designations of motorcycle brake fluid, denoting a particular mixture of chemicals imparting specified ranges of boiling point. DOT 5.1, like DOT 3 and DOT 4, is a polyethylene glycol-based fluid. DOT 5.1 is the non-silicone version of DOT 5. A DOT 5.1 should be used when a higher boiling point is required, such as track day use, or racing.
FB: What’s the best fluid to use on the road?
TM: DOT 5.1 will perform better at the very extremes of use, but at the level at which your motorcycle is going to perform on a track day then a good quality DOT 4 brake fluid is going to perform to just the same level. It’s like the semi vs fully synthetic oil debate, a fully is going to give you slight benefits in performance, but a semi synthetic is more than good enough for the job. Neither is ‘better’ and both make fine performance brake fluids, but one option simply makes more sense for track riders.
FB: Can you mix different fluids?
TM: It’s funny actually, as DOT 5 is the odd one out and is not compatible with any other DOT brake fluid. By mixing DOT 3, 4 and 5.1 brake fluids, assuming it is fresh fluid, the worst thing that can happen is a drop in the boiling point of the entire fluid volume. Some brake manufactures pre-fill their brakes with DOT 4 brake fluid from the factory. Others choose to use DOT 5.1. Many riders with DOT 4 in their brakes will opt to bleed with DOT 5.1 to benefit from the higher boiling point and improved heat resistance.
Five fast facts!
1) MOTUL was founded in 1853.
2) The MOTUL 300v was born in 1971, inspired by the brand’s 300 victories in all competitions.
3) In 1966, Century 2100 appeared on the market. It was the first semi-synthetic car lubricant.
4) In 1971, MOTUL innovated again with the Century 300V, the first 100 per cent synthetic car oil.
5) You can visit MOTUL’s oil selector website to check whether you are running the correct fluids in your bike.
Brake fuild compatibility chart:
Source: Fast Bikes UK