High Disc Brake torque Value

[4fsae]

Hi,

I was designing brakes for my (450kg 35:65) car and for a single disc brake i got a torque value of 1800+ Nm. I am designing for a 4 piston 31.8 mm dia caliper, with a master cylinder of 5/8" bore,a pedal ratio 6:1and a rotor size of 9". For pedal force, i took 75lbs i.e around 333N and a coefficient of friction of 0.5 . I used formula,
Brake torque = Line pressure*Caliper Area*Friction Coefficient * Rotor Radius
Line Pressure = (Force on Pedal * Pedal Ratio)/Area of Master Cylinder
I couldnt find where i went wrong, need some help, Thankyou.

[4fsae]

Hi,

I was designing brakes for my (450kg 35:65) car and for a single disc brake i got a torque value of 1800+ Nm. I am designing for a 4 piston 31.8 mm dia caliper, with a master cylinder of 5/8" bore,a pedal ratio 6:1and a rotor size of 9". For pedal force, i took 75lbs i.e around 333N and a coefficient of friction of 0.5 . I used formula,
Brake torque = Line pressure*Caliper Area*Friction Coefficient * Rotor Radius
Line Pressure = (Force on Pedal * Pedal Ratio)/Area of Master Cylinder
I couldnt find where i went wrong, need some help, Thankyou.

[Francis Gagné]

The equations you have posted here seems ok, but with the value you furbished my quick calculations where less lower than 1800Nm.

In fact it seems too low to lock the wheels on such a car (Assuming 13" rims). I'd suggest you double check your unit conversions and your piston area calculations.

On the other end, you might be taking this problem the wrong way, ask yourself what torque you need, and then what brake sizes from there and not what torque some brake gives you. If you use two master cylinders think about how the forces are transmitted from the foot to those cylinders.

Also be prepared to justify your choice in design judging regarding the choice of pedal force, pedal ratio, etc.

Regards,

[4fsae]

Thanks for your info. You are right, we should be trying a brakes for the torques we need. These parts were available to us, i was trying if they will match our needs.
1)About calculations we have a 12" wheels, and for piston area its just (3.14*d*d/4)*number of pistons right ?
2)What is the typical range of brake torque values?

[Francis Gagné]

Piston Area calc is ok,

As for the typical range of brake torque there is not really one for FSAE, as it depends wildly on Car Weight, CG Height, Mass distribution, Tire diameter and Tire coefficient of friction.

To find the torque you need with these factors you should be able to make a simple DCL of the car and wheel. You can get much of the explanations for making this in any good chassis design or vehicle dynamics book (Such as Race Car Vehicle Dynamics by Milliken and Milliken.)

Regards,

[Cardriverx]

Start from the tire/road and work your way from there using free body diagrams.