avarage pad travel

[eran]

hi guys

I've been looking for information about brake pad travel to the disc in order to calculate MC displacement. we are using Wilwood Dynpro calipers but I haven't succeeded in finding the numbers.
can anyone suggest an average travel of the pads?

[Luniz]

If I were in your position, i'd rather calculate my ratio and hence MC diameter according to Forces... Don't worry about pedal travel!

[eran]

we are using 5/8" MC and 4 1.38" caliper pistons, the forces are fine but in afraid the MC stroke would be too short for the oil displacement needed

[eran]

and the pedal travel would be too long as well

[Dash]

Well if you are making your own brake rotors it shouldn't be a problem eh?

[eran]

no matter the width of the rotor, there is always a small clearance between the pad and the disc isn't there?!
I'm looking to find that small distance

[Claude Rouelle]

This question seems basic but it is a very good one. Please help me to put you on track. After braking the brake pad comes back towards the caliper. Hopefully.... otherwise you will be dragging the brake pad on the brake disc in the straight away after the braking zone.
1. What does make the brake pad and the caliper piston (that is not the same!) move away from the brake disc, back into the caliper?
2. Have you measured this distance? In the shop, push on the brake pedal, release the brake pedal, what is now the gap between the pad and the disc? Is that distance the same for each pad / each disc? The same on top, bottom, inside, outside at each 4 corners of each brake pad? If not, why not? Is there a space created between the rear face of the brake pad and the piston? These numbers would be a good indication but it is not the ultimate one. In a test like this, in a workshop, there is not brake disc or brake bad thermal expansion and no shocks or vibration. Still worth to make this measurement. Use the same tool that you use to measure the spark plug gap.

Eran, I am Claude. And you are....?

[MCoach]

eran, using "something" inline with our braking system, we have eliminated the distance from pad to disc to 0.
Our pistons aren't seized, but it helps keep the drivers happy when they go to stand on the brake pedal, expecting it to be a brick wall.

[Claude Rouelle]

A 5 psi brake pressure "retainer" (I don't remember how to call this), good piston seals and antiknock spring (example http://www.apracing.com/Info.a...foID=110&ProductID=7 (http://www.apracing.com/Info.aspx?InfoID=110&ProductID=7)) will do the job.

You want to keep the brake disc - brake pad gap close to zero but not touching either. I have seen a car 7 kn/m slower at the end of a straight. That was the equivalent of 40 HP. The team looked for weeks on engine (They changed it; no difference), transmission, aero, until (to make the story short) they started to notice higher brake temperature then other cars, started to make serious measurement on a chassis dyno, "assaulted" their brake supplier with tons of question, brake supplier who finally admitted that there was a bad batch of brake caliper piston seals which had a huge hysteresis which kept the brake pad dragging on the brake discs.

Minimizing upright, caliper attachment, bearing and hub compliance will help too. If F1, Brembo engineers will tell you which team has the best upright: that is the one using the less play for their floating brake discs. If there was mo mechanical or thermal compliance, no play would be necessary. Less need for play = more rigid parts = better brake response to driver brake pedal input. The same philosophy is valid for steering ans suspension systems. Less compliance = more response, more control, more driving accuracy, more confidence.

Bleeding the brakes always help too! http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Warpspeed]

Claude is quite right, the square section seal in the caliper housing is supposed to flex in shear, then pull the piston back when brake line pressure is released.

This does not always work as expected, especially in freshly assembled brakes that have not had a chance for the caliper seals to work and settle into their final position.

Another thing is that absolute zero disc runout may be less desirable than a thou or two to positively push the pistons back and eliminate drag.

You will undoubtedly find that the master cylinder travel ends up being a lot more than you expect if you just calculate it from expected caliper piston stroke and fluid displacement. The problem is due to a combination of caliper flex, brake hose swelling and often flexing of the master cylinder and pedal mounting points. Nothing is totally rigid.

In the end you may need to go up a step in master cylinder bore size, but only some serious track testing will settle the question for sure.

[MCoach]

Claude, it's actually a 2 psi check valve that we use. http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Claude Rouelle]

I use 5 Psi but these are for real race cars and real drivers

http://fsae.com/groupee_common/emoticons/icon_smile.gif http://fsae.com/groupee_common/emoticons/icon_smile.gif http://fsae.com/groupee_common/emoticons/icon_smile.gif

ooouuuu....

[eran]

Thank you claude

I will certainly try to take all of the above into account. I am a member of "Tel Aviv Uni' Race car Design" team from Israel, I'm in charge of brake system and a part of the "drive train" team as well.
We are a team of 13 students from Tel aviv Uni' and this will be our University's first time in a Formula SAE competition. true pioneers....
we will surely try to make a good first impression.

[eran]

by the way from my calculations we are currently on a 1 to 3 ratio between the volume that is required for the master cylinder to displace to close the clearance of the pads and the stroke volume. which feels a little bit on the risky side for me. we are currently designing for 5/8" MC and 1.38" wilwood dynalite single.


Eran Hoffer
Brakes and drivetrain teams
Tel Aviv Uni' Racecar Design Team

[Claude Rouelle]

Eran,


1 to 3 ratio between the volume that is required for the master cylinder to displace to close the clearance of the pads and the stroke volume

Yep... and be careful; you have 2 master cylinders and the pedal stroke will be limited by the master cylinder corresponding to the calipers where the brake pads touche the brake discs first.

I have seen it in endurance racing: pit stop, front and rear brake pad change. Engineers ask the driver to pump the brake pedal. The brake pedal effectively becomes harder but in fact only the rear brake pads did come in contact with the rear brake discs. The driver did feel the brake stiff but it could have been even stiffer: he should have pumped a few more times until the front brake pads would have come in contact with the front brake discs. a must do lost in the pit stop action. The driver was still P1 when he got out of the pit lane thinking the brakes were fully functional, accelerate in the first straight... braking zone ... no signification deceleration.. brakes harder...locks the rear wheel.. spun.... gravel trap. That was it. They lost the championship right there. From hero to zero in 2 seconds.

Tel Aviv University...? Welcome to Formula Student craziness! Which competition(s) do you intend to participate in this year?

Eran, You should use a post signature

[Z]

Originally posted by eran:
we will surely try to make a good first impression.
Eran,

As a first-year team you will make a good impression if you can BUILD A CAR THAT CAN TRAVEL 30 KMS, at an average speed of 50 kph, without breaking down.

History has shown that inexperienced teams that try to "optimise" every part of their car, such as minimising brake pedal travel, usually do NOT achieve the above (very simple!) goal...

Z