Mind blown - Stoptech 355 BBK vs stock rotor size

B8 S4
#1

Longish post. Hear me out though!

TLDR; 355mm rotors found in a stoptech bbk offer negligible advantage over a proper rotor in the stock size–they have the same thermal mass.

Some ppl will know that I had recently sold my ST60 355mm brakes. The reason I did this is that I did not see a huge beit in going to these brakes. I was cooking fluid on stock brakes, and I was cooking fluid on the 355’s. It wasn’t a fluid problem (used SRF). I admit it was a pad problem (read: not using race pads).

Anyway, I decided to go back to the stock system and re-assess the situation. Pads for the stock calipers are less expensive than pads for the Stoptechs. Same with rotors and everything else. If I move up to an appropriate pad (planning on Pagid RS19s), I may alleviate the fluid boiling problem. Logically, one would argue that going down to a stock size is blasphemy. You lose thermal capacity. That’s the whole point of a BBK! Not to mention, stock brakes don’t vent well, and one side doesn’t even vent properly!!! Boo-hiss! But my logic is this - test out the stock brakes on a race pad. Only if it doesn’t work then move up to a BBK. Otherwise, I’m dumping 4k for no purpose.

So here’s what I did. I emailed Girodisc to inquire about their 2 piece rotors. Yes, they are spendy, but they vent properly, the air gap between the 2 sides of the rotor is larger, there are more rotor veins internally and the veins are curved AND the metallurgy in the Girodisc rotor is the same as those found on motorsport rotors. This is not your stock rotor. But wait. More boo-hissing. Why is boro spending so much on crappy stock size rotors???
What about thermal capacity? Afterall, stock is 345x30 and my old BBK was 355x32!!!

Mike at Girodisc did me a solid. He dug up the rotor rings from his inventory and compared them. Stoptech 355x32 ring replacements weigh 16.4lbs and the B8 345x30 ring replacements weigh 16.2lbs!!! Read that again. The difference in mass of a stoptech 355mm BBK and a proper 345mm rotor ring is only 0.2lbs – 90g!!! But wait!! How is that possible? The stock rotor is so much smaller! It’s because the pad on the stock caliper is deeper(thicker… more annulus) . So the stock rotor ring is thicker than the ring on the 355 stoptech–this is where it gains it’s thermal mass back.

So here we have a properly venting rotor–the same design as that found in a BBK. It’s 345mm yes, it’s thinner too. but it trades virtually nothing in thermal mass. So arguably, moving to a 355mm BBK actually does you absolutely nothing other than improved pedal feel and less caliper flex. The 355mm kit for 2.5k or whatever just buys you pad selection, stiffer caliper (better pedal feel)…and that’s it. The running costs of maintaining a BBK and replacing consumables is far greater than the stock system. For those of us who track our 4k lbs pigs, you understand that consumables on this car suck and are eaten quickly. Reduce cost where you can, spend it where it matters (more track time :smiley: :smiley: :D)

FWIW the Girodisc rotors are actually the same casting as the Stoptech rotors. So design wise for air gap, # of veins etc, it is the same.

So the real word on BBK is…if you’re going to do it on this car, do 370/380mm size. 355 offers advantages, but knowing this info, it’s more bling than anything else.

#2

So you’re trying to say that the stock setup with upgraded disks and pads is just as good as the st60’s ???

While I’m not a brake aficiando as some of you are here, are these things not advantages:

  • 6 piston vs 2 piston
  • Unsprung mass - the stock calipers are pigs themselves
  • Caliper flex = bad no?
  • Larger rotors, more surface area = quicker cooling, and bigger pads for harder braking?

What am I not getting here.

#3

Yes to all points on the caliper. However, surface area on the 345 is actually greater due to the larger annulus–this is why the rotor mass is nearly identical despite stock being 10mm smaller in diameter and 2mm thinner. The stock pad is bigger than the st60 pad.

Caliper flex is going to affect pad tapering…but if you swap pads for track anyways, u can rotate pads after each day or run stainless steel slider pins and brass bushings.

I am not saying st60 is inferior. Infact its superior. But the point i am trying to make is that a st60 with 355 is not going to be as big of an advantage as many people think. If we agree that the primary issue with the car is dealing with brake heat, then we can also agree that a 355 bbk isnt going to be a big difference. Save your 2.5k and either move up to a 380 size or stay stock with a proper proper pad.

Lastly, if one cares about unspring mass and caliper flex, q5 brembos solve that. Even cheaper pads and equal pad selection. Same pad shape as sti, evo, mustang. Q5 calipers and 2 piece stock rotors will yield the least unsprung mass of any brake option.

#4

Stock front brake is single piston floating caliper - not two pistons.

I can’t remember if pad tapering was an issue with the stock caliper. That and I think the brake modulation on a fixed caliper is better than a floating caliper.

Mike@Girodisc is awesome.

And a 380 size is great for the B8/B8.5.

#5

My brembo BBK weighs about a pound less than stock. The cast sliding caliper is something over 20 pounds. I weighed it and posted early in my GT-R thread. Not all thermal mass is created equal. BBK use much better metallurgy and alloys. Giro disc has good forum reputation with the street guys but the track guys know they crack. Look over on Rennlist on the GT3 and Turbo car forums.

#6

To be fair, all rotors crack. Also to be more fair, Stoptech rotors aren’t the best in my experience.
They don’t last very long, and Scott here found better luck with his Girodisc setup than his old Stoptech BBK setup. Many race teams use the Girodisc rotor with that casting. While I hear you that one can always get better iron/alloy mix in a rotor, we’re talking about an expensive consumable here.

For instance, a girodisc front ring replacement is 300 each including hardware. An AP racing equivalent is 500 each. Is the AP racing iron really that much better than the Girodisc iron to warrant nearly 2x the cost? Not sure. In the end, any of the above mentioned solutions is superior to the stock setup. Having wicket awesome best ever metallurgy for a car that sees time attacks (3 lap races!!) or max 30 minute track sessions (read: not 24hr racing), we’re splitting hairs here and the difference isn’t a big deal.