Modern disc brakes are so good you don’t usually think about them very much, unless (or until) stopping comes down to a matter of inches and seconds. Safety and security in a panic stop almost always makes you think! Yes, once they’ve saved your neck (or other, more tender parts of your anatomy) you’ll never forget them!
Effective brakes make all the difference. They grant you the precious inches needed to avoid contacting that Buick’s rear bumper, or the split second required to scrub off speed and avoid an unwanted ejection into the shrubbery alongside the road. No one will argue that discs brakes are efficient. But there’s a difference between effective and efficient. The question is, what can you do to make them even more effective—and what with the option of ABS technology and all—why bother?
The job of a brake is to convert kinetic energy into heat, then dissipate that heat into the surrounding air. Nothing else. No matter how exotic the pad or disc material, or how weird the design, a given input of kinetic energy will always give up the exact same amount of heat energy, period! The corollary to that sad fact is, heat energy will always flow from a hot place to a colder one (including your fingers, if you’re silly enough to touch a disc after a hard stop) but never the other way around! Sounds simple, but when you think about it, that means to work well, a brake system has to get real hot, then shed the heat real fast.
The coolest thing about disc brakes is the disc.
As stupid as that sounds, it’s the truth. It’s a simple shape providing lots of swept area and when it heats up it usually just gets bigger with minimum distortion. Neat innit? Discs shed heat bodaciously better than drums ever did (or could) too! The magic here is to come up with the “correct” combination of pad material and shape, and the right size and “heat control” properties. Stabilized airflow in the name of heat control is one of the main reasons for the (nowadays) virtually universal practice of putting the caliper behind the fork leg. (The other two are the reduced polar moment of inertia and to cut down on the “debris” that’s liable to get stuffed in the works.) As for size, well—you need look no further than the general trend towards larger rotors on all motorcycles and the simply enormous “perimeter” rotor favored by Buell to see that size does matter! So does caliper design and leverage ratio and a host of details involved in getting it right. The point is—decades into disc brake design—they’ve got stoppers so nearly perfected that the biggest problem is not exceeding the traction available from the tires. (That “exceeding traction” bit is the reason for the development and implementation of so-called Anti-lock Brake Systems, by the way.)
So, if this is so great what’s the big deal you say?
Controlled effectiveness, the second biggest problem, that’s what! See, it’s not how much friction is created (efficiently), but how it is controlled from one minute to the next (effectively), that really matters. All other things being equal, the hotter the brakes the more energy they can transfer—until they get so hot they can’t. This is where the problem starts… and effectiveness just “fades” away!
Speed vs. Heat
Just for fun, let’s have a closer look at the equations for the conversion of kinetic energy (KE) into heat (Q):
KE = (1/2) x (vehicle weight) x (speed of the vehicle)2
Q = (rotor weight) x (rotor material constant) x (temperature rise)
Since energy can be neither created nor destroyed, all of the KE from the scoot, must be completely turned into Q.
KE (before) = Q (after).
- the weight of the vehicle stays constant with use;
- the weight of the rotor remains constant with use;
- Newton was right;
Speaking of “cycling”…
Just winding down a steep, snaky mountain road at legal speeds, besides the risk of fade, you can easily encounter another “element” of bike control that makes you damned uncomfortable. It’s a little thing called ‘un-sprung” weight, which on motorcycles, is a real mess anyway.
Here’s the deal; if you hit a bump in your car and one of its 80-pound wheels bounces the force back through two tons of chassis, you likely won’t even notice. The ratio of total vehicle weight to un-sprung weight is about 50:1. Try that same stunt on your Harley, and its 50-pound wheel bangs the entire 900-pound load of you and the rest of the chassis so hard your eyeballs rattle. That’s because there’s a huge difference in the weight of the Harley wheel/tire/brake package, relative to the weight of the entire machine and rider. Something more like an 18:1 ratio! No brake works—at all—when the wheel is in mid-air either! These are the clues that anything you can do to improve the “horsepower” and cooling of your brakes, not to mention reducing the mass in all that heavy metal bolted to the wheels, is worth doing—period.
Two approaches to the same solution. On the left, you see a pair of basic Lyndall Racing Brakes (LRB) CMC rotors on my ol’ FXR. On the right, a late-model Buell, sporting its wheel-sized disc. Pads are available for both calipers by the way, and either way, you can’t get ’em hot enough to fade.
Fancier still, is this “lug-drive” adaptation of an LRB rotor to a 2009 ABS-equipped Screamin’ Eagle carrier. As installed by James at California Harley, on Rick Meile’s FL, this Touring model can stop every one of it’s 110 cubic inches and extra 70lbs of GVWR—all the time, every time. So—what’s stopping you?
Here’s a shot of those special Carbon/Kevlar pads (in either “red” or “gold” varieties) required to make the ‘system’ work. You see, the pads transfer material to the ultra hard rotor surface and from then on, braking friction is created material-to-material. Result—superior stopping, excellent feel and no marks or striations in the rotor itself.
As (not exactly) seen on Bob Robinson’s 2007 FLHX, equipped with W8LESS brand rotors – CMC discs are individually serial numbered—and guaranteed. Really guaranteed—like a Zippo! True of both purveyors of CMC brakes (LRB and W8LESS). You could say both companies stand behind stopping!
That is precisely where Ceramic Matrix Composite brakes come into the picture! In a nutshell, these “packages” of rotor and matching pads, address the issues of un-sprung weight and sheer stopping performance with 21st Century technology and clever engineering. CMC rotors—the latest evolution in brake technology—utilize production techniques and materials derived from the Aerospace industry (notably, the B-2 Stealth Bomber). The CMC rotor “matrix” is a carbon-reinforced silicon carbide that is 10 times tougher than traditional ceramics. (Take that, Porsche!) These materials are superior to conventional metal brake discs in every way: half the weight of OEM, with a far longer lifespan due to less distortion and wear, better wet weather performance, and not the least bit heat sensitive! The only unique requirement of CMC brake rotors is the use of special Carbon/Kevlar compound pads—which are readily available in replacement styles for virtually any Harley OE application and most all popular aftermarket types.
Here’s the “brake”-down of major CMC advantages:
Cools five times faster than steel or cast iron
This is the quality of CMC rotors that pays off best! Virtually eliminating fade (the occasional burnt finger) and any inconsistency in stopping performance, better than any other brake package available anywhere.
It’s no secret that standard rotors, used hard, are actually easy to bend or warp. The thermal “indifference” of ceramic matrix rotors puts warping into the “can’t really happen” category on any Harley street bike.
With something like 10 times the noise dampening capabilities of standard metal rotors, that means the end of squealing, squeaky stops.
Extended service life
CMC rotor materials provide an ultra-stable, ultra-smooth friction surface once the “transfer” of the special pad material is complete. That bodes well indeed for brakes that will last and perform far longer than any conventional set up.
Disc dimples reduce fade
Drilled rotors are actually very “Eighties” and virtually no car company does it any more. Drilled holes can cause tiny stress cracks, warping and fracturing. Today the trend is toward slotted and/or dimpled surface treatments, because that helps control so-called “out-gassing” just as much, with no downside.
A true “full-floating” composite rotor
Floating rotors allow just enough movement to align the full contact area of the pads with the full contact area of the rotor—so the maximum “wipe” is achieved every time you apply your stoppers. Hey—all this and they don’t rust either!
Up to 60 percent weight savings
(1.9 lbs. vs. 4.9 lbs. for an O.E.M. brake rotor) Put another way: Two of these have less un-sprung mass than one stocker!
Keeping the wheels (and tires) in contact with the tarmac better means better handling. Better, more consistent “feel” at the brake lever makes a big difference too!
Because the un-sprung mass is less, the wheel can react faster and more gently, so the ride is both more comfortable and safer on less than great road surfaces.
Easier high speed steering
It’s the physics of the so-called “gyro” effect of the bike’s wheel assembly—it increases at the square of the speed! So the faster you’re moving the better the bike responds to your steering and handling inputs.
Another law of physics—inertia in heavy wheel mass takes a lot more horsepower than you’d think to get rolling. Some say a ten-pound weight reduction is worth one horsepower, free!
Offsetting that impressive list of advantages is the one (and only) disadvantage—CMC discs are not cheap! (Although the special pads are no more expensive than factory pads.) Still, people spend a lot more on dumber things for Harleys—and what price safety after all?