BBK Comparison thread: Brembo vs. StopTech vs. AP Racing

[AP - Chris_B]

Originally Posted by gary_c

Regardless of the type of usage, track or street, all metals expands with heat. Because of the fact that cast iron and aluminum also expand at different rates, it's important to allow the disc to have room to grow so that it doesn't stress the mounting points or hardware between the different materials. There are issues that affect the rate of fatigue when you join dissimilar materials and expose them to high stress applications. This is the exact reason that racing applications lead to the development of floating rotors, and away from fixed/bolted applications.

Gary and I agree that for racing applications, floating disc assemblies are preferred for the many advantages they offer. Brembo has done a good job over the years mitigating the side effects for the street, with the GT-R being one notable exception. Others have used floating discs in an attempt to hide poor metallurgy, sub-standard engineering or lack of testing. To modify an old phrase: If you don't know, float it!

But for most mortals that keep bulk disc temperatures below 1150°F or so, we have found that floating discs are not required or even desired in many cases. This applies to almost all street guys and most beginner to intermediate track guys. In fact for many kits, the downside outweighs the benefits since the advantages don't even come in to play at lower temps. Note: I am only saying this under the assumption that high quality materials and workmanship are used throughout. Brake systems that spend time over 1150°F would benefit from floating discs more than the drawbacks. These are the faster intermediate and advanced drivers or those that, for whatever reason, are just harder on their equipment than most. Anyone who tracks their car should be at least using temperature paints on the OD of the disc so they know what temps their brakes are seeing. This is imperative for pad selection and the common discovery that better cooling might be a good idea.

Originally Posted by gary_c

Cast iron by nature doesn't have the ductility to deform and return to it's original state which is why there is a higher chance of warpage and/or cracking when it is not allowed proper room for expansion. I personally do not recognize the "maintenance-free nature" of a fixed/bolted disc unless you are fully aware that you are not introducing the components to temperatures/expansion that exceed the natural elasticity of the materials used. Some companies simply recommend that you regularly inspect and re-torque the disc assemblies, others suggest that you safety wire each piece of hardware.

Cast iron is a less ductile material than aluminum, for sure, but it certainly expands and contracts during thermal cycles. The trouble begins if it is operated beyond its plasticity region. It doesn't much matter if the disc is bolted or floating, once taken beyond the plastic part of the stress-strain curve, the material is screwed. Floating disc assemblies do hide problems better as they decouple the disc from the hat, but the problem is still there if it ever starts.

The most common mode of failure for bolted disc assemblies is coning, or the out-of-plane distortion that occurs when a very hot iron disc is trying to expand due to temperature and high rotational speed, but is held on only one side by the hat. While the aluminum hat does give a bit for the iron, at a certain point, a combination of temperature and speed, it is not enough. This is the primary issue that floating discs address (among a few others). Yet it only occurs under extreme use and extreme temperatures, approaching race usage. Again, full-on racing brake systems are different for a variety of reasons. Not all of the solutions found on the track could or should be used in brake systems primarily meant for the street, but a lot of advances to come from racing. Anyone who has tried race pads and full-floating rotors on the street (and lived to tell about it) knows exactly what I'm talking about!

This is where I usually hear, "Great Chris, just cut to the damn chase and tell us if floating discs are better than bolted!" OK, here's my answer, "Yes. And no. And sometimes. It depends on what they are being used for." Now isn't THAT helpful?

Since we're on the topic, I do not agree at all with those companies (not Gary's) that tout they race on the same exact equipment they sell to the street guys. This leads to either an under-performing race setup or an overly compromised street system. When comparing both applications, there are certain requirements that are diametrically opposed and that is why both AP Racing and Brembo use different equipment for each. If you compare a professional racing brake system to even the highest-priced street systems, you will find a significant price difference (sometimes 2-3x!). Some of the most advanced systems cost over $30k per car, not including pads, lines, fluid, hardware, etc. That should tell you a lot about the differences.

And for those that might be reading this and wondering, we would NEVER, EVER recommend re-torquing an AP Racing disc assembly. These are carefully torqued during assembly to a prescribed value that takes thermal expansion into account. The #1 cause of broken hardware is an over-eager, but well-meaning, mechanic torquing the nuts to a value that might be considered "normal" for a 1/4-28 steel fastener (usually 85-90% of yield strength). Then when everything gets hot and starts to expand, the bolt is stretched beyond its limit and either loosens up or fails outright. Either of which is a recipe for disaster!

Chris