disk brake rotor size

[DADA]

Iam a beginner, and iam in team for supra sae(SIMILAR TO FSAE, BY SAE INDIA). i am working on brake. we have done calculations of centre of gravity, forces on front and rear wheels and torque on them. now i want to that how i should decide the appropiate size of rotor to be used. plz help me...

[Owen Thomas]

First you need to decide exactly how much braking force you're going to need (enough to lock up all 4 wheels). From there you have to do the moment calculations for torque, etc. for all kinds of different rotor sizes and pad/caliper piston sizes and so on (you need adequate tire data for this too). Since it would appear you're on a first year team, I'll give you some recommendations to make your life easier.

Seeing as you're in Supra SAE and you have to use that monstrously heavy engine package, I would suspect that you will need as much braking force as you can get. Since it's better to overbuild brakes than to under build them, you should find the largest rotor that will fit into your wheel. Keep in mind what calipers you will be using and make sure the whole package fits inside the rim (WITH plumbing/fittings).

If you are confident in the assumptions you can make and the calculations that you have already done and wish to make a more refined braking system, then you are probably confident enough to finish the calculations from there. Here's a nifty page I found on Google (first hit searching 'brake calculations') that should help in justifying your decision. http://www.engineeringinspirat...o.uk/brakecalcs.html (http://www.engineeringinspiration.co.uk/brakecalcs.html) .
From my experience, the general process for finding exactly how much of something you need depends on what has already been chosen on the car. Basically you pick one part of the system (the one you have) and work your way out to the rest of the components. Sounds like you've almost found how much braking force you need, so now all you gotta do is a little bit of what we call Engineering.
ALL of the components in the system (from the master cylinders to the rotors) are dependent on the others in respects to what size you need. This means that to decide what size rotor you need is an iterative process based on your specific setup, and there is no set formula.