Brake Rotor Tips

New Rotor Prep
The first thing you’ll want to do to your new rotors is turn them down to the minimum thickness.  The rules allow the rotors to be as thin as 10.5 mm.  If you closely inspect your rotors you’ll find a minimum thickness spec stamped or cast in the rotor.   Regardless of the rules, I would not turn the rotors thinner than the manufacturer’s minimum spec.  The picture here shows the new “06 90 B” BRACO rotors I got with chassis #630.  As you can see, the spec is stamped on the edge as 10.5 mm.

The reasons for turning the rotors down are:
1) Reduce unsprung weight (going from the 12.1 mm stock thickness to 11mm, removes about 1/2 pound per rotor).
2) Reduce rotational inertia.  You have to accelerate and stop those rotors from spinning.  It’s the same as more HP or better brakes.

Break-In
With the Performance Friction brake pads we run, you must get the rotor coated with carbon material for the pad to work correctly.  This takes awhile.  Running your freshly turned rotors at the Pro street race is NOT the time to break-in the rotors (can you say “Hello, Mr. Concrete”?).  I strongly recommend a test day to do this.  Get the brakes hot (several VERY hard stops), and then let the brakes cool off (a cool-off lap followed by parking the car).  Repeat this process a couple of times.   The rotor braking surfaces should acquire a reddish-brown color when they are properly coated.

Seating Surface
Many people experience significant brake drag.  The most common reason is that the caliper pins are bound up.  (See the article “Brake Maintenance” for info)  Another common reason is that the rotor is not running true -- that it is out-of-round or wobbling. This out-of-round behavior causes the rotor to drag against the pads for part of the rotation.   The most common cause of this that I’ve found is dirt on the back mounting surface of the rotor.  Referring to the picture, you want to make sure that the rotor mounting surface is clean.  If any dirt gets between this mounting surface and the hub, the rotor will run out-of-round.  Whenever you remove your rotors, or if you have big brake drag, clean this surface.

Warped Rotors
Sometimes, rotors warp.  Although I’ve never had this on an SRF, I sure have had it on street cars.  If the rotors warp, they’ll drag.  If you’ve got brake drag, and you’ve cleaned the caliper pins and rotor seating surfaces, you should check your rotors for warping.  Big time warp can be felt in the brake pedal as pulsing.   But I’ve found that even 7-10 thousandths of run-out is more drag than I’d like.   To check this, tighten the rotor to the hub (without the wheel on, obviously), and use a dial indicator to measure the run-out.  Ours run between 1-3 thousandths.   The fix, typically, is to turn the rotors to true them back up.  Of course, if you’re already at the minimum, it’s time to buy new rotors.  Check to be sure that your wheel bearings are good, before you make the final decision, because bad bearings will also show run-out.

Rev. 7/4/98

Barry