We do not expect that this guide will turn you into an expert overnight. We have made every attempt to be accurate and easy to read. But we cannot impart the gifts of skill, experience, and common sense. If after reading these pages you feel inclined to carry out alterations to the braking system of a bike we will not accept responsibility for what happens next. You are responsible for your actions, and this guide has been placed online to only offer an introduction into the braking system and to hopefully give you a more significant background into how things work, what effects changes may have and what different parts of the system do. We will obviously be happy to answer any questions.
Fact is that the most powerful part of your motorcycle is not the engine – it’s the brakes. For example, a GSXR1000 can do a standing quarter mile in 10.8 seconds with a terminal speed of 138mph. Yet, the brakes can do the same amount of work, but in reverse, in just 6.8 seconds.
The name hydraulics comes from the Greek word ‘hydro’ meaning water and ‘aulos’ meaning pipe. The basics of any hydraulic braking system are as follows. For a hydraulic system to function it must be closed and full of fluid and leak-free. No fluid can be allowed to leave, and no air can be allowed to enter.
In a closed, sealed hydraulic system as outlined above the following laws are true Fluid cannot be compressed to a lesser volume, no matter how high the pressure Pressure is equal over all surfaces of the containing system The things that concern us most with brake hydraulics are the following :
It is a constant in the hydraulic system. If you put ten psi into a system hydraulic laws, state that this ten psi will act on all surfaces within the system equally. Pressure In = Pressure Out.
A 1 square inch master cylinder with 10 lbs. of force applied to it will produce 10 lbs. of force per square inch or 10 psi. If the calipers have a surface area of 10 square inches, then the force here will be 10 x 10 lb which will mean the calipers produce 100 lb of force. The pressure acting on the system is the same, in this case, ten psi, but the force can be altered by changing the surface area of the calipers.
If our one square inch master cylinder travels forward 1″ then the amount of fluid it displaces will be one cubic inch. If this fluid is then spread over the 10 square inches of the caliper, the calipers will only be able to move 1/10th of an inch. You will have a huge force (100 lbs.), but the caliper pistons may not move forward enough to grip the disc tightly and so give a weaker brake. This can be seen to happen on some motorcycles which have a small master cylinder piston and two pot calipers. If the two pots are exchanged for six-pot calipers, then the surface area increases and the force increases at the caliper, BUT the small master cylinder only moves a small amount of fluid which in turn equates to a lower movement of the caliper pistons.
Lever Pivot Point
Any effort you apply with your hand should push the pads against the disc. But your hand isn’t that strong. To make the point grab hold of something and squeeze it hard – do you think that’s enough pressure to stop 270kgs of bike and rider from 180mph? Initially, the brake lever itself gives you an advantage by multiplying the force at your hand through the lever. For instance, the distance from the lever pivot point to the middle of the lever (where your fingers are) might be 130mm. And yet the distance from the pivot point to the point where the lever acts on the master cylinder might only be 20mm. So, roughly speaking any force applied to the lever will be 6.5 times greater at the master cylinder (130mm/20mm = 6.5).
QUESTION – Can you use any fluid in the braking system?
No, you cannot. The fluid in the braking system must be a designated brake fluid which meets with certain specifications – DOT3, DOT4, DOT5 or DOT 5.1 are the most common and your owners manual will inform you of the fluid used as a standard in the system. Don’t use aircraft hydraulic fluid (even if you can get hold of it) – it isn’t the brake fluid equivalent of Avgas and do not use the mineral based fluid formulated for Citroens either
QUESTION – What types of brake fluid are available?
There are two types of fluid: glycol-based and silicone-based. You can use one or the other but NEVER mix the two fluids. If you want to use a different fluid than the one in your system ( glycol to silicone/silicone to glycol), then you will need to flush the system before changing. Brake fluid is available in different specifications to meet American Department Of Transport (DOT) requirements. These DOT regulations are occasionally updated but primarily the higher the DOT rating, the higher quality, higher boiling point and more expensive the fluid. DOT3 is the basic brake fluid. DOT 4 is a higher standard, and both of these fluids absorb water – they are hygroscopic, but DOT4 fluid contains additives which prevent water affecting the performance of the fluid to the same degree. DOT 5 was originally formulated for silicone based brake fluid although there is now a new DOT5.1 standard for glycol-based fluid which has some of the desirable properties of silicone fluids. Both types – glycol based and silicone have different pros and cons though.
QUESTION – What are the boiling points of brake fluids?
Brake fluid works in a harsh environment being close to the heat generated by the braking system the fluid heats up and under extreme conditions such as racing this can lead to the fluid boiling. These manufacturers found that by increasing the boiling point, the fluid performed better in use as when fluid boils it leads to cavitation within the system and loss of braking pressure. The boiling points are as follows :
|Minimum Specification||Good Brand Min Spec|
|DOT 3||205 Deg C||140 Deg C||220 Deg C||150 Deg C|
|DOT 4||230 Deg C||155 Deg C||260 Deg C||170 Deg C|
|DOT 5||260 Deg C||180 Deg C||270 Deg C||190 Deg C|
|DOT 5.1||260 Deg C||180 Deg C||270 Deg C||190 Deg C|
Wet figures above show the drop in performance when the fluid has a specific water content (less than 3%). For example, when DOT 5.1 is contaminated with 3% water (which is considered a lot) performs almost as well as basic DOT 3 in prime condition. The most important thing to do is regularly change your fluid – we suggest every three months, but manufacturers suggest at least once every twelve months. After six months use a typical DOT 3 fluid may have had it’s boiling point lowered from 205 degree’s C to about 165 degree’s C
QUESTION – What are the pros and cons of fluids?
DOT 3 and DOT 4 are hydroscopic – they allow water to be absorbed into the fluid. This means that when heated up a fluid with water in it will boil more quickly (water boils at a lower temperature than brake fluid) which will lead to cavitation. DOT 5 does not absorb water and will not strip paintwork but as the water is not absorbed it merely sinks to the bottom of the system which is usually the caliper where operating temperatures are very high. Water in the caliper is bad as it can cause corrosion within the caliper although silicone fluids contain corrosion inhibitors and as water has a lower boiling point than fluid (100 Degrees C), there is more chance of boiling under relatively light braking. Glycol based fluids can strip paintwork whereas silicone based do not. Silicone fluids also have a very long life which is why it is used in military vehicles which may be stored for years without use but be expected to be ready for action at a moments notice.
QUESTION – Do -02 lines give more pressure than -03 lines?
No – the fitting of the smaller dash two hoses sold by some manufacturers makes NO difference to the pressure produced at the caliper as is commonly believed. There are no substantial benefits associated with the use of dash two hoses except that the manufacturer can charge you more for them. The only way to increase pressure coming out of the system is to increase the pressure going into the system. 500 psi in equals 500 psi out – Fluid cannot be compressed to a lesser volume, no matter how high the pressure and Pressure is equal over all surfaces of the containing system.
QUESTION – What is the difference in performance between two full-length front lines and an over-the-mudguard line kit?
Nothing except for the way the system looks. Both systems operate in the same way and give you the same performance. As the system is a closed hydraulic system without air in it when the fluid moves at one end the same movement takes place at the other end. Because they have different lengths doesn’t mean that the system will work slower at one end than the other. Imagine pushing two sticks away from you which are both 6″ long – the ends both move at the same time. Now imagine pushing two sticks – one is 6″ and the other is 36″ – the ends still both move at the same time. So the over the mudguard gives the same performance as the two full length – it’s just which you prefer.
QUESTION – What’s better – three front lines using the OEM splitter, two full-length fronts or 2 fronts with one over-the-mudguard?
As we have said above the three systems, give the same performance, but the three-line front has an additional two possible leak points in the system, is heavier and harder to bleed than two front lines. It is easier to fit though as you follow the OEM setup. Another thing to remember is to do you race or use the bike on track days? If so go for the two full-length front as in the UK one of the ACU rules state that you are not allowed an over the mudguard system unless it is an original system.
QUESTION – Friction
Friction is resistance to sliding. Any two objects in contact with and trying to move relative to each other have friction. This can be low or high depending on the types of surface in contact. If two surfaces in contact are sliding, the friction produces heat. In braking systems, friction is used to produce heat.
The process of creating this heat stops the motorcycle. The amount of friction between two surfaces depends on the materials and their roughness.
The amount of friction is described by a number called the coefficient of friction. This number is obtained by Friction Force being divided by the Perpendicular Force. To make matters more complicated, the coefficient of friction has two different values. It is higher when there is no sliding, but as soon as the surfaces start to move relative to each other, the friction coefficient drops to a lower number.
This is why it is harder to start something sliding than it is to keep it sliding. After sliding starts, it is called dynamic coefficient of friction. The higher the number, the higher the amount of friction and the lower the number, the lower the amount of friction.