Rotor selection based on weight.

DaveR · #1 (**permalink**) 09-13-2003, 12:33 PM

I'm building a daily driver that will go to Willow springs 2 or 3 times a year. I want major stopping power for the track so I bought Wilwood 13 X 1 3/8 inch curved vane rotors @ 15.2 lbs each. Now that I have them, they seem like overkill. Should I trade them back in for 12.19 X 1.25 inch curved vane rotors @ 10.1 lbs each, a weight savings of 5.1 lbs of rotational mass? It's a question of weight versus stopping power/warp resistance/heat disipation. Also, same question for calipers, 3.75 lbs superlights with 8.6" pad surface area, or 2.5 lb dynalights with 5.9" pad surface area?

Bob Putnam · #2 (**permalink**) 09-13-2003, 03:10 PM

We use the smaller rotor (12.19" x 1.25"T) with the Sierra/Superlite and Superlite II caliper. It seems to be adequate for hard track use with 550+ bhp. Also, the 12.19" rotor fits into a 15" wheel.

The Dynalite is very marginal for the front brakes if you have any substantial horsepower, or want to go fast for more than one lap at a time.

Jack21 · #3 (**permalink**) 09-13-2003, 03:13 PM

Weight difference is minimal. Put the biggest brakes you can find on the car. You'll just be stressing them less with street driving, so keep what you've got. Just make sure your wheels will fit.

DaveR · #4 (**permalink**) 09-14-2003, 11:02 AM

Jack21 - I plan on using 17 inch wheels, so I dont' think clearance is an issue. Thanks for the input on weight, 5 lbs seems like a lot when you read race literature, but maybe it isn't for an amateur like myself. I thought the up and down motion of the front wheel on a bumpy track in a turn might impact handling. Warping is a major reality, and I failed to mention that most of the weight difference is due to the wall thickness between 13" GT48 rotor and 12.19" Ultralight (thin wall) 32 vane rotor.

Bob,

Thanks for the caliper advice, I was thinking along the same lines since it isn't rotating mass. I definately want to go fast for more than one lap at a time. My original question was in regard to front brakes, but does superlight versus dynalight also apply to the rears?

Excaliber · #5 (**permalink**) 09-14-2003, 11:13 AM

I heard once that 1 pound of rotating weight is the same as 3 pounds of static weight.

Ernie

DaveR · #6 (**permalink**) 09-14-2003, 01:10 PM

Excaliber,

You have a good memory, 3 to 1 it is. Herb Adams thought this was important enough to include a chapter (chapter 15, "Rotating Inertia") in his book "Chassis Engineering". He has a chart showing eqivalent horsepower and speed at the end of 1/4 mile with 15 pounds of static weight versus 15 pounds of rotating weight. He goes on to say that weight reduction of rotating weight at axle speed is 3 to 1, but weight reduction of rotating weight at engine speed (anywhere in the driveline) is 15 to 1, amazing! Reading his book put me in a quandry and brought me to the board for input from the experts so I could reconcile race theory for people trying to shave tenths, from Cobra reality.

So, a ten pound difference in the 2 front rotors is like having a 30 pound weight on the hood. Seems like it offsets the advantage of a set of aluminum heads. I'm still not sure what to do, however, I did notice that the rotors on the race car pictures in his book are HUGE!

Bob Putnam · #7 (**permalink**) 09-14-2003, 03:13 PM

DaveR,

Another compromise option is to use 13" x 1.25" rotors.

The Dynalites should be fine for the rear. The front brakes absorb roughly twice the energy of the rears.

Because of that force differential, keep the piston area of the Dynalites close to 1/2 that of the front calipers so that you don't have to create a major pressure reduction in the rear.