Brake system

(Archived from August 2, 2007 5:35 pm)

Brake system

What is brake coefficient ?


Coefficient of Friction (µ  - pronounced “mew”) - A number measuring the “grip” of a material used in brake pads.  Coefficient of friction can vary depending on the type of material used for the brake rotor.  Typically service brakes are concerned with dynamic coefficient of friction, or the coefficient of friction measured while the vehicle is moving.  Below are a few of the main characteristics.  Depending on the desired performance, the characteristics can be minimized or maximized.

1. Speed Sensitive - Coefficient of friction typically drops as the speed of the vehicle increases.
2. Pressure Sensitive - Coefficient of friction typically drops as more clamp force is generated.
3. Temperature Sensitive - Coefficient of friction typically drops as the temperature of the brake system increases.

Engineers measure friction by its coefficient, which is calculated by dividing the force required to slide an object over a surface by the weight of the object. For example, if it takes 100 pounds of force to slide a 100-pound block of iron over a concrete floor, the coefficient of friction between the two materials is 1.0. If it takes only 2 pounds of force to slide a 100-pound block of ice over the same floor, the coefficient is only .02.

The coefficient of friction also effected from cleanliness of the pad surface and surface roughness.


Brake Fade


Brake fade is define as a decrease or loss of brake power and typically occurs in 3 ways :

i) Pad fade (Friction Material Fade)
All friction material (brake pad stuff) has a coefficient of friction curve over tempreture (as mention above). When pads reach high temperatures they can sometime can redeposit themselves on the surface of the brake pad, thus decrease of brake power. This is also described as classic glazed pad.

ii) Green fade
Green fade always directed to new brake pads. The reduction of friction also can be caused by the gasification of organic materials, it will create a gas cushion formed between pad and disk, or by the melting of the binding resins in the pad then act as a lubricant. Green fade can be overcome or prevented by doing brake bedding or bed-in. This is to boil off the resins, this I will explain later.
iii) Fluid fade
This type of fade happenes when the brake fluids inside the hydraulic caliper boils. This also produces bubles in the master brake pump container. Since bubbles compressible, it will make a spongy pedal. This kind of fade can be avoided by using DOT 4 or high grade brake fluid, even frequent changed of the brake fluid. I’ve read some page mentioned, that they spray a thermal barrier coating at the back of the brake pads, so it will isolate the heat, but i never tried it before. Would you like to give a try…?


Brake Squeal

Brake squeal happened when brake pad, brake rotor or caliper piston creates an extremely high frequency vibration between each other. There’s are a few factor that causes brake squels :
i) dust in the drums - as brake shoe wears off, dust were form in the brake drum thus make it squeals during braking - solution, open the drums, sandpaper the drum (200 grit sandpaper shouldbe fine) and then blow the dust off.
ii) cheap lining - cheap low cost lining doesn’t withstand the heat generated. The lost of friction causes it to squel and decreasing brake efficiency .
iii) hard lining - this is more common for  buses, trucks, taxis and others that use cheap, hard, long lasting brake. Due to the hard compound, lining don’t  have cushioning during braking, thus create the squeal. - solution, change the pad to better quality
iv) others - include bad brake pads, bent disk plate or drum, weak caliper piston / drum’s spring or saturated linig causes by leaking cylinder / piston kit.


Solving brake squeal

Copper-based grease Copper-based grease, this grease is extremely resistant to pressure and heat, so putting it at the back of the brake pad may help to solve it, thus it also prevent brake fade. Here same sample of product that can help noisefree and molyslip.

Brake pad shim Installing shim at the back of brake pad, this will eliminate the vibration as the shim absorb the vibration created.

before skimming brake after skimming brake Skimming your rotor for smoother surface and improve brake efficiency. Don’t forget to run bedding proceed to bed-in the brake pads in the newly resurface disk.

More link on brake squeal , link1 and link2


Brake bleeding


Definition: This is the process of removing air bubbles from the brake system by pumping fluid through the lines. Air bubbles are bad because they compress when pressure is applied resulting in a low or spongy feeling pedal. The correct procedure for bleeding the brakes on most RWD vehicles is to start at the furthest wheel. Do the right rear then left rear brake, followed by the right front and left front brakes. On a FWD vehicle with a diagonally-split brake system, do the right rear then left front brake, followed by the left rear and right front brake. - Source : Auto Repair
Just like the mechanic do when you changed the brake pad, they said…"Pump…Press…" ("pump…tekannnn" - malay words) a few times.

Brake bed-in


Most of us don’t know what the hack is bed-in, brake bed-in is to maximize brake performance. This normally done on the new set of brake pads, to avoid green fade (as mention earlier in brake pad section).
Normally this is how it works.

Step 1 : Make 10 stops from 50km/h down to 15km/h using moderate braking pressure and allowing approximately 30 seconds between stops for cooling. Don’t drag your pads during these stops. After 10 stops, allow 15 minutes for your braking system to cool down.

Step 2 : Make 5 consecutive stops from 80km/h down to 20km/h. After 5th stop, allow your braking system to cool for approximately 30 minutes. This complete the break-in of your pads to the rotor surface.

During those 2 steps, brake odor will happened, this is normal and it’s part of process your pads must go through to achieve ultimate level of performance. Your front disk will look a little blueish color and smelly abit and please don’t act clever and touch the extremely hot disk, wait for it to cool off.

Full seating of your new brake pads normally occurs within 1,600 km.

**Please refer to brake pad manufacture for more details and the correct way for your brake bed-in / bedding.