Gbboenda

OK, guys, in an earlier thread I mentioned upgrading to OEM PCCB on my 987.2 Spyder and fellow Rennlisters asked if I happened to have weight details on disk and calipers for comparison; which I do. Starter thread here https://rennlist.com/forums/987-981-...fter-dyno.html

Background was that I fell over a 'good' deal on an almost new set of PCCB while traveling in Germany and had it shipped over to good ol' Georgia. Part numbers which I read of the disks are

- 997.351.031.00
- 997.351.032.00
- 987.352.031.02
- 987.352.032.02

Disk diameter is 350 mm on all four corners. Disks are gripped by six-piston mono-bloc aluminum calipers at the front and four-piston units at the rear.

Installing PCCB was a bit more involved than I had anticipated...I changed brake master cylinder from standard MC to GT3 (yes, I chose GT3 MC over the PCCB version which also exist for the 987 for the slightly shorter pedal travel). Curve ball was that due to the much larger front calipers, they did no longer line up with the existing mounting points on the front spindles - they had to be replaced for a spindle version for PCCB. Hard earned was rolling down the hill from that point on To complete build, I added stainless steel brake hoses.


As for the weight of the components, here we go,

Front:
- PCCB calipers/pads (set): 11.9 kg
- Red calipers/pads (set): 9.1 kg
- PCCB disks (set): 11.4 kg
- Red disks (set): 18.4 kg

Rear:
- PCCB caliper/pads (set): 9.2 kg
- Red caliper/pads (set): 7.6 kg
- PCCB disks (set): 11.0 kg
- Red disks (set): 14.0 kg

Total weight - caliper/pads + disks
- PCCB: 43.5 kg
- Red: 49.1 kg

So, from an unsprung weight point of view, PCCB saves 5.6 kg total for all four corners. The PCCB front disks only weigh 62% of the front steel disks, but much of the saving is offset by the larger/heavier 6 pot calipers that comes with PCCB. Overall, PCCB hardly offers a significant unsprung weight saving on its own; bigger savings can be had a lot cheaper by focusing on reducing weight of tires and rims, I suppose. It is icing on the cake when all other weight saving options have been pursued - including reducing amount of bacon around the waist

Another aspect is the difference in rotating inertia from the different weight and diameter of the disks. I will do some 'backside of envelope' math on the torque impact from the inertia to see if it has a real world impact. Will upload over the weekend.

So, on paper it seems upgrading to PCCB wasn't of much practical use. However, I am still very pleased with lack of dust, improved pedal feel - and of cause the aesthetics of the big, yellow anchors behind the rims But for all practical purposes, PCCB might be overkill on a light car like the 987.2 Spyder which makes utilizing the PCCB brake fade resistance less likely.

(But I am keeping the PCCB - LOVE 'em. I will be posting the red brake kit on eBay over the weekend - disks, calipers, spindles and MC)

Gbboenda

Inertia of disk and rim/tire is approximated with that of a ring, e.g. mass is between R1 and R2 while mass at R<R1 is zero



In above post I assessed difference in unsprung weight between PCCB and Red brakes on my 987.2 Spyder. Outstanding was to find difference in inertia between PCCB and Red - logic is that it is easier to spin up a light disk than a heavy, assuming same diameter. But does its magnitude matter in the big scheme of things?

Well, stuck with jet lag this morning, I got up and found my old physics books and calculated inertia difference. My interpretation is,

- Front axle: PCCB decreases inertia with 31% which is a good thing, in particular in front where we have acceleration as well as steering input. Makes front end more responsive
- Rear axle: There virtually isn't any inertia difference. Yes, PCCB is lighter but advantage over Red steel disks is offset by the increase in PCCB disk diameter. So, no inertia upside

So, in isolation PCCB has a 31% inertia advantage over Red on front axle. However, to put into perspective I also calculated inertia from tire and rim for two popular tire/rim combinations - Michelin PSS and PS4 on OEM rim. It shows the following,

- Inertia from rim/tire is a magnitude of ~19 times bigger than that of the brake disks. So, there is a lot to gain by focusing on light weight rims + tire
- Inertia difference introduced from choice between two popular tires has an inertia impact 2.3 times greater than advantage added from PCCB over Red due to weight differences between tire models.
- Relatively speaking, choice of rim/tire has much bigger impact on inertia than whether car has PCCB or not

My conclusions from above is,

- PCCB has inertia advantage over Red - but only on front axle
- There is a total unspring weight advantage of 5.6 kg from PCCB
- Bad choice of rim and tires can easily undermine these advantages
- From a strict performance perspective, PCCB on 987.2 only makes sense as icing on cake once other upsides has been harvested (tire, rim, others)
- Main reason for PCCB would be its brake fade resistance...an advantage most of us are unlikely to see from street driving in such a light 2900 lbs car. Hard track use might justify the higher fade threshold
- So, big reason for getting PCCB on a 987.2 would be 'softer' advantages...like no dust on rims, aesthetics and more. If they are worth it from a $$ perspective, well, that is a personal choice

But I stil LOVE my PCCBs Call me irrational...

Marine Blue

Thank you very much for the two very detailed and thought out posts. Combining the two really puts the engineering behind the advantages of unsprung weight. I admit it does make me feel better that I didn't option the PCCB's on my car.

You brought up differences in tires and I personally noticed a difference in how the car handled when switching to Michelin's which weighed 3 - 4 lbs less per tire as compared to the Bridgestone. They biggest difference is the way the car handles bumps, it seems the suspension does a better job keeping the tire connected to the road. If anything the ride isn't as smooth as it was with the Bridgestone but the overall connection is better. The Michelin's seem to have a softer sidewall so downside is the responsiveness is slightly reduced.

Cman21

Did you change the brake bias from standard brakes to the pccb set up ?

Cman21

Gbboenda

I thought brake bias would be set automatically by the combination of PCCB calipers and GT3 MC? And since these are OEM, my take was that bias would be set correctly by default. I am not aware of a valve or the like to manually adjust bias.


On the road, I didn't notice any unusual bias between front and rear on hard braking. In a world of ABS, I guess it is compensated in real time.


Or did I miss something regarding bias adjustment?

gearFX

OP, those were the best and most informative 13th and 14th posts I've ever seen on this forum... (or at least that I can remember). Nice work!

orthojoe

Fantastic post. Somebody sticky this thread so it can be referred to every single time a pccb zealot starts spewing their usual garbage.

Thanks for confirming objectively what Walter Rohl said subjectively years ago.

orthojoe

The one single advantage of the pccb setup in the caymans is the larger calipers and rotors prevent overheat that will occur with the standard brakes and rotors. If you are going to a track like laguna and run lap times below 1:43, the standard brakes will overheat. Your options are to get pccb from the factory and swap out the rotors or get an aftermarket bbk. Factory is always more elegant.

Gbboenda

From geek to geek, I am honoured

Surge74