Brake data logging yields surprising results

[parsonsj]

I've recently added data logging to my Z06, including brake line pressure. I have two different brake kits for the car, neither of which are stock. I did a controlled experiment of slowing down from 60 mph and logged the brake line pressure and longitudinal G forces (how fast the car slows down).



Here's brake kit 1:

• psi Long-G
• 985 -1.19
• 1097 -1.26
• 1200 -1.16
• 1249 -1.16
• 1057 -1.11

Here's brake kit 2:

• psi Long-G
• 538 -1.11
• 603 -1.21
• 599 -1.11
• 462 -1.09
• 502 -1.02
• 597 -1.07
• 488 -0.88

The difference is remarkable. Both kits achieve roughly the same amount of negative G forces, which largely shows the tire's mechanical grip. The slightly lower negative G numbers in the second set are probably just due to slightly damp roads I think. Kit 1 takes approximately twice the line pressure to achieve the same braking as kit 2.

The car is exactly the same other than the brakes. I logged data for kit 1, then put the car on the rack and swapped the brakes, bled them, and logged the data for kit 2 the next evening.

[T-Bone]

John, those are quite interesting data. Do the front brake caliper areas correspond (inversely) to the pressures?

[parsonsj]

Kit 2 has more piston area, but kit 1 has more pad swept area.

Kit1: 4.04 square inches of piston, 10,879 square mm of pad.
Kit2: 4.85 square inches of piston, 9765 square mm of pad.

Both are 6 piston calipers.

Two other key differences:

Kit1: 14.25" carbon ceramic rotor
Kit2: 14.8" steel rotor

[alocker]

Thanks for posting this but I am curious as to how were the tests performed? Wouldn't the carbon ceramic rotor on kit 1 require higher temps to perform optimally?

[parsonsj]

Wouldn't the carbon ceramic rotor on kit 1 require higher temps to perform optimally?

Maybe so. I did 10 60-5 mph stops at max braking before logging data. I'd think that would be enough to get heat into the brakes. That's about what I'd consider enough for warmup on the track.

I tested on a straight section of road with u-turns about 1/8 of a mile apart. Blast to 60, shift to neutral, stop to 5 mph, u-turn, do it again. These data were collected after 10 stops.

[dontlifttoshift]

That's a significant difference in longitudinal G forces between the two tests.

Have you done the math to see if clamping force even comes out close?

[raustinss]

Would have thought the carbon ceramic would have been stickier... maybe a different brake pad compound

[parsonsj]

That's a significant difference in longitudinal G forces between the two tests.

Have you done the math to see if clamping force even comes out close?

Do you mean the difference between 1.26 (max of first test) and 1.21 (max of second test)? I'm attributing that difference to wet roads. It rained between the two tests and I waited for the road to dry, but there were still some damp patches. As for the math, no. The pad areas and piston sizes seem close enough that the only significant difference I can see is mu (coefficient of friction for folks unfamiliar with the term).

[parsonsj]

Would have thought the carbon ceramic would have been stickier... maybe a different brake pad compound

Me too... and this is the 3rd compound supplied by the manufacturer. The first two were even less sticky.

[dontlifttoshift]

Stupid Florida and "Ill rain whenever I want"....didn't Mother Nature know you were in the middle of a scientific test.

I threw out the high and low numbers, actually the two bottom numbers for test 2.. Average looks like 1.17 for test 1 and 1.095 for test 2.

A 20% increase in piston area explains the drop in pressure, I am assuming the pressure number was plucked from the graph at the same point in time as the max g occurred.

Given the information, despite the damp roads, I would say kit 1 would be faster in the long run.

How did the pedal feel between the two kits? Were you just standing on the pedal and letting the ABS do its thing?

On a related note, what are you logging data with? Did you you buy the brake pressure sensor from that same supplier?.....them *******s are expensive.

Data driven discussions are the best. Thanks for posting your findings.

[parsonsj]

This data comes from a Stack MFR (multi-function recorder), with their brake pressure sensor. And you're right: it is expensive.

Yes, the data is plucked around points of max G, and I tried to threshold on the edge of ABS, but it was difficult with kit2. Here's a response from the manufacturer of kit1 (Wilwood... from this morning):

You're absolutely correct, the coefficient of friction on a C/SiC rotor simply isn't there. Our experience is that there is no pad that can create a Mu that matches a cast iron rotor. The problem is that C/SiC is slippery. That's one reason why all of the manufacturers that put C/SiC rotors on their cars from the factory use a 15" disc. They're really trying to create brake torque by increasing the lever length. All of these reasons are why people don't usually race on C/SiC rotors. The reduced weight is really their only benefit. If you're looking for ultimate brake torque, it's hard to beat a cast iron rotor with our Polymatrix A compound pad squeezing it.

All that being said, as you can see the C/SiC gets the car stopped with the same level of deceleration, it just takes more PSI in the caliper to get there.

[Chad-1stGen]

Great test John. Thanks for sharing.

[alocker]

Maybe so. I did 10 60-5 mph stops at max braking before logging data. I'd think that would be enough to get heat into the brakes. That's about what I'd consider enough for warmup on the track.

I tested on a straight section of road with u-turns about 1/8 of a mile apart. Blast to 60, shift to neutral, stop to 5 mph, u-turn, do it again. These data were collected after 10 stops.

I would say that is a fair warm up.

Who makes Kit 2 and what kind of car is this on?

[parsonsj]

The car is a 2007 Z06, and kit 2 is a Brembo GT kit with 380mm front rotors.

[raustinss]

I wonder how this would compare to the ZR1 carbon ceramic set up