Brakes!

[Vector006]

Hey Guys,

Just wondering if anyone has any tips on how to perform calculations to determine suitable rotor sizes front to rear regarding: load transfer, tire friction coefficient, deceleration G's, rotor coefficients, clamping force/master cylinder sizes etc etc.....

Wilwood tech support seemed to be baffeled when i asked them what friction coefficient i should assume with their pads. But I finally got a Cf=0.38-0.42 from the head tech support dude. Does this seam reasonable?

Also, we have been concidering braking on the sprocket which we intend to make from anodized aluminum. Has anyone had success with aluminum rotors? Tech support at Wilwood was trying to sway us away from braking on aluminum because they argue that the aluminum absorbs the heat faster than it can let it off. But then I'd think aluminum should disipate it faster than steel too. So im kinda stumped... http://fsae.com/groupee_common/emoticons/icon_confused.gif

And one more thing before i forget, what steel has most people been using for their rotors? We laser cut some mystery metal last year, and kept having a glazing problem, which held us back from competing last year.

THANKS in advance for any insight.

-Jeremy Keller
University of Windsor
"school of the unstoppable car of 2003" http://fsae.com/groupee_common/emoticons/icon_mad.gif

Jeremy V. Keller
FSAE04 Technical Head
Roush Industries/UofW CO-OP
vector006@hotmail.com

[Mi_Ko]

This is what I think off braking on the sprocket:
It's wasting your time and resources.

A few contras:

- You'll have to design your own caliper!
- Don't you use lube for your chain?
- The jury likes 4 brakes!!
- No real weight advantage, because the caliper has to be bigger.

2002/03 University of MARIBOR - Team Member

[Vector006]

Hmm, ive heard many different things about what judges think about single inboard braking(see thread "sprocket/rotor combo"). Aslong as you can explain to the judges why you did it, i think itd be ok.
We have selected a lube free chain and plan on using the Wilwood dynalite single, and modify the pad.
But, anyone have any commments on the calculations of rotor size. Is there a good book somewhere? i was surprised to see that race car vehicle dynamics didnt mention anything about these calculations.

Thanks!

-Jer

Jeremy V. Keller
FSAE04 Technical Head
Roush Industries/UofW CO-OP
vector006@hotmail.com

[PatClarke]

Hello all, I am interested to hear the opinion that the 'Jury likes 4 brakes'. That is not so! What the Jury likes is that the team understand why they use 3 or 4 brakes, and when a team can fully explain their decision to the satisfaction of the judges, then there is no penalty applied.
The judges will play 'Devil's Advocate'. If you have inboard brakes, they will want to know why you don't mount them outboard (To help with heating the tyres), if they are mounted outboard, they will ask about unsprung weight and feeding the brake torque into the chassis via the suspension arms. If you have a single brake they will ask about the effects of braking across the differential. If you brake on the sprocket you need to understand a whole lot of answers, especially about the effects reversing the torque input to one side of the diff.

Like so many other facets of designing a car, there is no one correct answer, and a Judge's opinion on how he would design a car is of no importance.
What the Judges want to see is that the team have really researched and understand their design decisions. Not just in the brakes, but in every aspect of the design and construction of the car.
Thats why it's called 'Design Judging'
Regards
PDR

I thought I made an error once, but I was mistaken

[Dick Golembiewski]

Echoing Pat's comments:

We all have opinions on how to build a FSAE car. That doesn't mean that we necessarily JUDGE you in design based on those opinions. We DO ask you to defend your design, and we DO quiz you re: your basic understanding of vehicle dynamics and basic engineering principles.

Now if you ask us what we personally like and and don't like about your design, you're likely to get candid answers, but understand that there is a difference between those answers and how you're judged.

- Dick

[Mechanicaldan]

Pat,

Could you expand a little bit "about the effects reversing the torque input to one side of the diff?"

What would the situation be if the differential is centered and all the axles and shafts are equal lenghts on both sides?

Cyclone Racing
www.cyclone-racing.com (http://www.cyclone-racing.com)
Iowa State University
Project Director

[PatClarke]

Nope Dan =] You have to research that.
Hint: It could be that differentials have a different slip coefficient in either direction....and then again, they might not http://fsae.com/groupee_common/emoticons/icon_smile.gif

The real point is that you need to know BEFORE the judge asks you.
Regards
Pat

I thought I made an error once, but I was mistaken

[Arvin]

Jeremy,

A good book for brake calculations is "The Brake Handbook" by Fred Puhn. He pretty much goes through most of the important brake calculations step by step.

I am personaly against Aluminum Rotors, and I would guess most of the judges are too. I can remember Caroll Smith's talk during the 2002 competition where he said he didn't like aluminum rotors unless they are MMC. I also don't see the point of anodizing the rotors I would think the anodize would wear off pretty quickly in that application.

my $0.02

[MattG]

Has anyone had a chance to compare Puhn's book with Mavrigians "Brake Systems" (or any other braking texts). Puhn is out of print and could be trickier to get in this corner of the world. However, if it's the one to have we'll find a way to get it.

[jack]

back to the original question:
vector006, there are lots of calculations about all of this stuff everywere. the reason it isnt in most books is beccause it doesnt need to be, most of that stuff can be measured or calculated using simple physics.

Ask the suspension guy on your team about load transfer. you should at least know your weight distrabution, and from there you can get an idea.

companies supply tire Cf's. use what they claim, allthough you will most likely never see what they claim, use it as your bench mark, because you sure would rather have your tires break loose, before you fail your brake system.

With the above, you can figure out deceleration G's.

rotor Cf, or more specifically Cf between rotor and pad are supplied by companies, for the same reason as above, use what they give you, allthough FSAE brakes wont get that hot. if your fealling saucy, you can easily be measure this. just ask your physics prof how. make sure you bake the rotor/pad to get it near a good temp.

If you have all this figured out and you know the diameter of your tires and rotor, and the length of your pedal (which can be changed of course), and how hard you can push on the pedal (we measured the guys on our team, and we got about 100lbs sustained, 450lbs max), you can figure out the MC/SC ratio. now you get a catalog and see who makes MC and pistons that get close to the ratio you want.

...and before you know it you have a brake system that (should) stop the car. if you need more help, carrol smith talks about this stuff in tune to win, and people in your school's ME or physics dept. better be able to do this.

now for alluminum rotors. we all know that aluminum melts with less heat than iron (cast iron is said to be the best), so how do we solve that problem? well, big ass rotors for one, but if you are braking on the sprocket that is constrained (case for four brakes...) smaller pads will keep the rotors cooler too, and this function is independant of pressure, so the brakes will still work just as well, just wear quiker. personaly, i think sprotors are not a good solution for FSAE. brakes aren't powerfull enough, put heat in the wrong places, cant cool off, and frankly sprotors were invented for show motorcycles--which dont really do much stoping.

well thats about it for brakes...if you want to learn about brakes--design your cars calipers http://fsae.com/groupee_common/emoticons/icon_wink.gif

jack @ WWU
http://www.etec.wwu.edu/

[ben]

One other thing to be wary of is pad material with aluminium/MMC. We got criticised last year for using standard pad material with MMC.

We get a decent deal out of AP Racing so we asked them about pads for MMC and they said Ferodo DS3000 or a PFC compound. This would have been too expensive so we're switching to cast iron discs at the front and stainless steel at the rear so we can use the standard pads that come with the APs

Ben

University of Birmingham
www.ubracing.co.uk (http://www.ubracing.co.uk)

[Mechanicaldan]

Well, here's a few items to think about:

The second place car at last year's FSAE competiton had a sprotor.

Wilwood makes pads meant specifically for aluminum.

There is a smaller amount of heat to disipate on our cars because they weigh 600-650 pounds and are having to reduce speed from 65 MPH to 25 MPH. That's not much braking compared to other racecars.

If the speeds were higher, the cars weighed more, the races were longer, sure, I'd agree aluminum would be a bad choice.

Cyclone Racing
www.cyclone-racing.com (http://www.cyclone-racing.com)
Iowa State University
Project Director

[Mechanicaldan]

Pat,

That's a fair answer. One should always try by asking. http://fsae.com/groupee_common/emoticons/icon_smile.gif

Cyclone Racing
www.cyclone-racing.com (http://www.cyclone-racing.com)
Iowa State University
Project Director