• Welcome to engineeringclicks.com
• # How do you decide the diameter of the rotor in a disc brake??

Discussion in 'Calculations' started by Suhael, Apr 20, 2013.

1. ### SuhaelMember

Joined:
Apr 2013
Posts:
7
0
Hello People..
I do the brakes designing for our Mini Baja Vehicle. When I went through the calculations there was nothing like a rotor diameter mentioned anywhere but it looks like when you want to purchase a rotor from a standard company for your vehicle they expect only the "Braking Torqe"... What I would like to know is how is it that they're able to decide the rotor diameter with the help of just braking torque??

Please guys help me in this..

And the blogs' jus awesome and the guy who created this forum.. "YOU ARE AWESOME!!!"

2.
3. ### mhjones12Well-Known Member

Joined:
Feb 2012
Posts:
78
1
The rotor diameter would be related to the brake torque by:

T = r*u*F

where:
T = braking torque
u = coefficient of friction between the rotor and the brake pad
F = piston force on rotor

If they only require the T, they must be assuming a value for F. Also, I imagine there must be some assumption of duty cycle for the energy dissipation.

4. ### SuhaelMember

Joined:
Apr 2013
Posts:
7
0
thanks a lot.. do you have any reference which I can add??.. cuz we have to show the technical inspectors from where we get these equations..

5. ### ErichWell-Known Member

Joined:
Feb 2012
Posts:
221
2
You don't need a reference, draw a free body diagram of the disc and caliper and apply Newtons laws.

You can come at the problem from a different direction.

1. You want to be able to skid the wheels on dry pavement.
that constraint along with knowledge of your cars weight, wheelbase and center of gravity will allow you to calculate brake torque required to skid

2. Generally you want your hydraulic brakes to operate around 1000 psi.
This info and knowledge of your brake caliper piston diameter will allow you to calculate brake clamping force.

6. ### mhjones12Well-Known Member

Joined:
Feb 2012
Posts:
78
1
As Erich mentions, the relationship I showed was derived from first principles based on a free body diagram of the system. I ignored the diameter of the caliper in the relationship. If you want a reference anyways, I found a paper called "The Disk Brake Design and Performance Analysis." They give the relationship in a more general way:

T = r*u*P*A*n

and

r = D/2 - (d/2+1) //I'm not sure where the "+1" term comes from. Anyone have any idea??

where:
r = effective braking radius (distance from piston center to brake rotor center)
u = coefficient of friction
P = piston pressure
A = piston area
n = number of pistons
D = rotor diameter //This is what your solving for
d = piston diameter

7. ### CarolRoblesGuest

What about the rotor diameter how can we choose this to make above equation proper ??

8. ### SuhaelMember

Joined:
Apr 2013
Posts:
7
0
Very clear.. thanks again

I need to be making the heat transfer calculations also right??

9. ### SuhaelMember

Joined:
Apr 2013
Posts:
7
0
Very clear guys thanks a lot..!!

Joined:
Apr 2013
Posts:
7