Disk Brake – Brake Disc/Rotor (Part 1)

As mentioned in one of the previous articles that Drum brake is not sufficient for excessive heat condition, require more effort from rider and needs to be adjusted frequently. To overcome above problems disc brake is used. A disc brake uses hydraulic pressure to stop the vehicle.  It is widely used in motorcycles, cars, trucks, buses, bicycle etc. but here we are focusing only on motorcycle disc brake.

The first caliper-type automobile disc brake was patented by Frederick William in 1902 but it was failed due to use of copper disc. The first motorcycles to use disc brakes were racing machines. The first mass-produced road-going motorcycle to sport a disc-brake was the 1969 Honda CB750 (A Successful motorcycle launched by Honda.

Honda was planning to sell 1500 units a year but the demand was surged to 3000 units a month)

A Disc brake assembly contains brake rotor (Disc), Calipers, brake pad, master cylinder, brake lever or pedal and cable.

Brake Rotor or Brake Disk

The brake rotor or brake disk is the main rotating part of the disk brake assembly. It is the component by which the brake caliper clamps its hold to stop the wheel rotation. It is hard wearing and resists the high temperatures that occur during braking. Its manufacturer who will specify the minimum thickness for the rotor. It is generally made of cast iron. In high performance vehicles composite materials such as ceramics or carbon fiber used to make rotors.  Since some of the rotors have directional vanes they can be fitted to only one side of the vehicle.

Size of rotor/disc:

The size of disc/rotor plays an important role in disc brake. In India we find mostly 240mm front disc. BUELL XB12SX Lightning City X uses 375 mm of front disc brake. The front disc is always larger than rear disc because front brake is more effective in stopping a two wheeler than rear brake.  The larger the rotor/disc is the better the braking is. There are following reasons behind the same.

• A larger rotor provides more surface area compare to a smaller rotor. The brake pads maximum grip is generated while initial bite, after the initial bite the level of grip tends to fall. So if a brake pad provided larger area in the initial bite it will stop the vehicle early.
• When we apply the disc brake, there is lots of heat generated. This makes disc to overheat. In order to avoid overheating condition the disc is needed to cool constantly. A smaller disc has low surface area to disperse the heat while larger disc has more surface area to disperse the heat, which in turns cools the disc more early than the smaller one. Result better braking power.

• The further the contact point of rotor and pad from the rotation of axle the more the braking torque is generated. In the large disc/rotor the contact point is at more distance compare to smaller rotor.  The same principal applies to a door handle which is on opposite side of hinge.  That’s why some bikes have rotor nearing the wheel rim. (See above picture)
• The larger disc/rotor is heavier than the smaller which also helps while stopping the vehicle.

Type of disc/rotors

In motorcycle disk brake assembly rotors can have cross slotted or cross drilled. These both type also known as “ventilated disc. Previously plain disk was used as a brake disk.

Cross Drilled Rotor

In this kind of rotor you will find holes drilled in the disk. This style was first introduced in 1960 in the racing cars.

The main advantages of this type of rotor are:

• The drilled holes will help to disperse the heat which is generated while braking. When brake pads come in contact with the disc lots of heat and gas generated. the cross drills help to disperse this heat and gas.
• The same reason also helps in reducing brake fade under repetitive and hard braking.
• It is widely used. In India we mostly find this type of rotor.
• Some weight saving due to cross drills.
• The disk looks good with cross drills.

There are few disadvantages as well

• These kind of rotor tends to crack down due to two reason first the surface of disk is weakened while drilling holes in the disk and due to large amount of heat and forces. Although this thing rarely happen but it happens.
• Drilled rotors remove more surface area than slotted rotors which reduces instant braking.

Cross Slotted rotors

In the cross slotted rotors, slots are shallow channels machined on the surface of the disc. The slots can be of various depth and width and can be machined in various angles. They are not that much popular as the Cross Drilled rotors. Generally you cans see its applicaiton on Harley Davidson motorcycles.

Its advantages are as under:

• Cross Slotted rotors has double of life compare to cross drilled rotors.
• Slots help to keep the brake pads clear of dust and garbage.
• These rotors are more efficient in rain condition to remove water from disk.
• They have more surface area then Cross drilled rotors which help in effective braking.
• Cross Slotted rotors are considered more effective in dispersing heat compare to Cross drilled rotors

The disadvantages are as below:

• The brake pads worn out very fast in case of Cross Slotted rotor compare to Cross drilled rotor.
• They are not much popular in low capacity bikes.

Cross drilled and slotted rotors

To overcome the problems of Cross drilled and Slotted rotors a new type Cross Drilled and Cross Slotted type rotor is used. It has both the advantages of the Cross Drilled and Cross Slotted while minimizing the disadvantages.

Petal Rotor/Wave Disk

There is another type of rotor/disc which is known as petal disc or wave disk. The outer edge of the rotor is shaped like a wave instead of being a plain circle in petal/wave disc. The petal disc is more effective while braking compare to plain circle disc. It has following advantages:

• As we have seen above that a larger surface area of disc provides better stopping power. A wave of petal disc/wave disk is designed and located in such a way that it constantly provides new surface area for pad to bite on disc while rotation.
• Due to the same reason pads are always at the optimum braking point
• This designing structure of wave also helps to keep the rotor cool while friction of rotor with pad. It also helps to avoid overheating condition of rotor.
• The petal disc weighs lesser than the big rotors which in turns increases acceleration. The motorcycle front wheel has less mass rotating with it which makes acceleration faster even though there is no horsepower gain.
• As lesser weight also improve cornering of the motorcycle as there is less centrifugal force.

There is only one major disadvantage of this kind of disc that it wears out the pads at faster rate.

The rotor can be damaged in one of these three ways 1. warp 2.scar 3. crack

1. The warping caused due to two major reasons one is heat generated during friction between pad and the rotor and another is an overheated disc cooled down immediately, both of which causes it to be reshaped. Improper lug nut settings can also cause warping.
2. Cracking which occurs only in drilled discs. Holes near the edge of the disc develop cracks because of the disc’s uneven rate of expansion when heated.
3. Overheating of disc also causes scaring.

If you find your disc in damaged condition get it changed instead of repairing it.

Shivam-