Some brake questions

[Ali]

Hi,

Anyone knows the formula for "effective rotor radius"? Can it be approximated to the distance from the center of the caliper piston to the center of the disc?

Also, how important is the stroke of the master cylinder? A pedal ratio of about 3 is required to get enough braking force, but the pushrod travel is only about 2cm, much less than the stated stroke of the M/C, which is about 4cm.

Thanks

[Ali]

Hi,

Anyone knows the formula for "effective rotor radius"? Can it be approximated to the distance from the center of the caliper piston to the center of the disc?

Also, how important is the stroke of the master cylinder? A pedal ratio of about 3 is required to get enough braking force, but the pushrod travel is only about 2cm, much less than the stated stroke of the M/C, which is about 4cm.

Thanks

[Denny Trimble]

Ali,
We've used the center of the pad for effective radius. I can see how the center of force may not be exactly there, but I think that's a good enough assumption for us. You can always test it (but that would be difficult).

Master cylinder stroke used would be zero if everything in the system were infinitely stiff. So, if you want to calculate pedal or MC travel, you need to know the system stiffness.

When you refer to "pushrod travel is only 2cm" do you mean measured on the car? If so, that would be a good thing to use less than the maximum stroke of the MC http://fsae.com/groupee_common/emoticons/icon_smile.gif

University of Washington Formula SAE ('98, '99, '03)

[Ali]

Denny,

The pedal initially starts of at an incline, leaning towards your foot. The 2cm that I mentioned is the amount that the pedal is able to move the pushrod, until the point that the pedal reaches 90 degrees. After that, the pedal ratio starts to decrease again.

Including the pushrod travel required to move the pads to the disc, will 2 cm of pushrod travel be enough? I'm thinking maybe the initial 1 cm of travel is used to engage the pads, and the remaining 1 cm to overcome flex in the system.

[Denny Trimble]

Most calipers don't have pad retracting springs (except for mountain bike discs). So, the pads will normally ride right on the rotors, with a slight amount of drag.

The "free travel" you feel is the port in the MC being covered up. Normally, with your foot off the pedal, this port is open, so the brake system is open to the atmosphere. Once the piston in the MC travels a small distance and covers up that port, the pressure can start to increase.

I can't say if 1cm will be enough... you'll have to test it out. You could change the incline of your pedal to allow more travel before 90deg if you run into trouble.

What school are you from by the way?

University of Washington Formula SAE ('98, '99, '03)

[Jarrod]

the actual travel of the pedal doesn't matter too much, so long as there is some freeplay so the brakes don't drag. The driver will feel the pedal by force, not distance. Our pedal last year had very little travel at all, it almost felt solid by hand, but the feel was excellent when driving.

[madman]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Anyone knows the formula for "effective rotor radius"? Can it be approximated to the distance from the center of the caliper piston to the center of the disc?<HR></BLOCKQUOTE>

I've used the centre of the piston for all my calcs. I can see how this is an assumption and the real value will be affected by pad shape, piston position, pad stiffness etc.. But I don't see it making a difference (IMHO).

For most curved pads, the centre of piston assumption should be a conservative estimate anyhow.

Regarding system stiffness and travel, remember that the stiffness of your seat, the brake foot heel rest, and your pedal box mount can have a large affect on the system stiffness. Almost all the travel in one of our previous cars was due to pedal box compliance.

Most modern calipers don't have retracting springs, but use the elasticity of the square edged piston seals to provide the piston retraction.

[Charlie Monash OZ]

I am entirely unsure about how much brake travel is best for a driver to judge his/her braking.

I was under the impression that braking was best with more modulation of the pedal, enabling the driver to bring the brake as close as possible to the point of locking. That view was confirmed by Claude Rouelle at his last melbourne seminar. However, nearly everyone else I have spoken with prefers less travel. I would have thought that with more travel comes better modulation of pressure, therefore better feedback to the driver.

Has anyone done any conclusive tests with regard to this? or have any thoughts

[Marc Jaxa-Rozen]

I agree with Jarrod- IMO a shorter-travel pedal presenting more resistance offers better feedback as it allows the driver to modulate braking with the force applied on the pedal, rather than by its displacement.

[Charlie]

I agree, do not confuse travel with modulation. It is much easier to control pressure than displacement.

-Charlie Ping
Auburn University FSAE (http://eng.auburn.edu/organizations/SAE/AUFSAE)
5th Overall Detroit 2003
? Overall Aussie 2003. http://fsae.com/groupee_common/emoticons/icon_smile.gif