Brake System Design

[rjh23]

I am a member of the NJIT Formula SAE team. I am working on brake system design and would like some help on equations could be used to calculate braking forces, hydraulic pressure and brake torque.

[rjh23]

I am a member of the NJIT Formula SAE team. I am working on brake system design and would like some help on equations could be used to calculate braking forces, hydraulic pressure and brake torque.

[Composites Guy]

You can do this. Pull out your freshman physics book. Draw a free body diagram of the rotor and caliper, and of the pedal and master cylinder. Start your design with the bias bar set at 50/50 to maximize tunability later on. Estimate car weight, vertical CG height and weight transfer. Assume locked up decelleration around 1.3-1.5g's. Start with a design which causes the front and rear wheels to lock up at the same time/effort. Assume pad coeffient of friction of ~.5, but eventually look up chosen manufacturer specs. Get some idea of what you need caliper and rotor wise then investigate available products.

Estimate pad travels at about .03" a piece. Make sure bias bar doesn't bind throughout travel. Estimate the strength of your leg (i.e. max pedal loading). Make sure you don't exceed 1000-1250 psi in your hydraulic lines.

[Christopher Catto]

uh. did this 2 years ago so not so fresh on it right now. All of what CG said, plus for the maximum pedal effort and bias bar:

i used one of our Longacre weight scales. Put it against one pillar, then i sat on the floor with my back to another pillar. Then pressed on the scale with my foot as hard as I physically could. I pushed about 100kg I think (ok, am no bodybuilder...)

I then calculated that our carbon seat would not be as stiffly installed as a cement pillar! so say you would press 30% of your maximum, since you dont want to get so tired during the enduro. Also try it with different people, maybe put a scale in the footwell and see what force people can comfortably push with their foot

Also, i m sad I cannot find it, but I made an Excel preadsheet with bias bar turns. Basically we went testing and found a good baseline position (so it depends on your master cylinders and say you have the baseline with tyres that have 4 hours use). Then I wrote down the number of half turns clockwise till the bias bar is fully on one side (say full front bias although the bar would in fact bind). This way I would always know how to set it to baseline.

Then in Excel I did some simple lever theory to calculate approx how much the bias would change from some turns of the bar. This is not fully correct due to non linearities in the linkage, since at 50:50 the bar works symmetrically and to either side of this, one master cylinder may get rather more or less force than you geometrically calculate. However, it was enough to give some indication of how to quickly adjust bias.

Final step. Go to the track. Put some bald tyres on the car so that no matter how much you wear them, hey, theyre still crap on grip. Go for some brake tests with bedded-in discs and pads. See where the bias bar needs to be set so that you have mild front lock up compared to rear. Also make a note of setting so that you have mild rear lock-up. This setting is useful for go-kart drivers who like the rear to help them corner.

Put brand new tyres (demould them beforehand) and go for some brake testing. this will be trickier because i have had it happen that the tyres were so grippy that we were bending the mounts of the pedalbox. you may laugh at this but with our chassis having lightweight brackets everywhere and thin steel pedals, when you have hillclimb car compound and relatively small discs locking wheels is not so easy in FSAE.

write down your bias bar setting for front mild lockup and rear mild lockup. you will have to assume that your desired setup is in between. Warning: this is an assumption. http://fsae.com/groupee_common/emoticons/icon_frown.gif

[js10coastr]

F=ma

F=P*A

T=F*L

[Botin]

Hi rjh23
Just a little help from Race Tech Magazine "Braking Model":
http://www.racetechmag.com/user/software/software.asp?menu=4

Remember it doesn't make you job, but it get you going.

[Christopher Catto]

regarding the bias bar

http://www.mitchellsoftware.com/brakes_2.htm