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Motorhead Memo: Just stop it—breaking down braking

By Kip Woodring

Motorhead_Memo

 

Let’s get this out of the way up front, ’cause it isn’t really what I want to address in this particular Chautauqua.

Anti-lock brakes (or ABS to us lazy folks) are first and foremost the biggest advance in motorcycle safety since disc brakes themselves. Secondly, they get better with each generation and some machines are on Gen 9… if I’m not mistaken. The inclusion of algorithms that include bank angle “readings” and traction measurement at over one hundred times a second means that there aren’t one in 1,000 (maybe 10,000) riders that have the faintest hope of out-performing the “system” in an emergency with any amount of training and/or practice. The simple fact is ABS can save you from a crash better than you can.

Debates center around the notion that you don’t want to use ABS any more than you want to use the airbags in your car—both are safety measures designed for unforeseen situations. Perhaps the biggest problem lies in the rider being unwilling or unused to grabbing a handful of brake lever and trusting all the electronics will do the rest. It is seriously counterintuitive to us old-timers, but there you have it! There’s also room for intelligent disagreement when it comes to stopping distance. On dry pavement, under optimum conditions, it could just be that a skilled human can still best ABS. On lesser surfaces under poor conditions, most conclude that ABS is way less effective, taking perhaps as much as 60-percent more room to stop. Mind you, falling off and sliding as a result of a panic stop gone wrong takes even more time and distance… so the point might be moot. It might also not be the point at all!

As you have read here many a time over the years, tires are what stop your motorcycle. Want to decrease stopping distance? Look first to sticky tires of proper tread depth/pattern and carcass design, correctly inflated—period! Brakes, whether conventional or ABS, are basically a means of converting kinetic energy into heat… through friction.

Ah, now we might be getting at something. Friction is required for brakes and tires to do their thing. Too little can cause… uh… “performance anxiety,” and too much can cause excessive heat and failure in the form of squirming tires and/or brake fade. Friction is the key. Whether in the form of tire traction or rubbing resistance between brake pads and rotors, proper mastery of this in all forms, all weathers and conditions—day in/day out—still falls to the rider. We’re not talking about emergency “on the edge” stuff. Rather it’s a matter of using and/or improving the hardware the designers provide until you can safely, confidently and constantly use your stopping power right up to the threshold.

On the threshold

“Threshold braking” amounts to an acquired skill that allows braking right to the limits of tire traction… straight up. (Beyond that is where ABS comes in.) Not to be confused with another skill of virtually no use on Harleys on the street, called “trail braking.” (That one is more a race track technique to “set” the chassis in a turn, while leaned over.)

While we’re on the subject of tire traction, it might be a good time to reiterate that no tire can be all things to all riders. Tire choice should be about primary usage. Therefore a bike that spends its time running rifle-barrel roads at supra-legal speeds, packing double in the desert, would logically benefit from a tire that works well for that purpose; a different purpose than the sled ridden solo that runs twisty roads in the rain at a trot. OEM Dunlop tires are by necessity a compromise, but worse, have not progressed notably in years. Meanwhile the discerning rider can find more suitable hoops from Metzeler, Michelin, Avon and even from Dunlop… if you know what you need and where to look for it. We’ll get back to that in detail another time. For the moment just remember, “hysteresis,” “contact patch,” and the “second law of friction.”

But what if I told you that you can improve braking distance as well as feel, almost irrespective of tires? How would you go about it? Most of us tend towards swapping the stock caliper(s) for something else… aftermarket. Not a bad plan, but hardly cost effective. On the other hand, not much thought is given to brake pads and rotors, yet for a lot less money than fancy calipers cost, you can stop better by upgrading either or (preferably) both of those items first! Let’s start with the pads. H-D factory pads are a so-called semi-metallic “sintered” matrix running on stainless steel rotors. Not unlike the OEM tires, they try to cover a lot of ground and do a reasonable job of it. That said, an immediate and notable improvement could be to swap for Carbon-Kevlar (organic) pads. Surprisingly these are a softer compound, easier on the rotor, quiet, dust-free, longer lasting, but more importantly, generate less heat. Let me put that differently: in terms of the proper mastery of friction we spoke of earlier, the way to look at it is in terms of outcome. C/K pad compounds tend to be more heat stable, which translates into consistent braking across a broader range of variables… a good thing.

Fact of friction

The friction performance of pads intended for typical street temperatures is classified under SAE Standard J866, expressed as a two-letter code where the first letter designates the low-temperature (0 to 200 degrees F) friction performance and the second letter the high-temperature (200 to 600 degrees F) performance. The letters generally appear on the backing plate as a prefix or suffix to the part number. If the first letter is lower than the second, the pad works better at high temperatures and needs a warm-up; if the second letter is lower than the first, the pad may fade at high temperatures. The best street pads have good friction at both high and low temperatures. A guide to that level of consistency is when both letters match… meaning the pads perform the same at all temperatures. What the letter code actually represents is expressed in terms of coefficient of friction with a Greek letter that looks kinda like a backwards “y” or a backwards “u” with a long tail… but is pronounced “mu” (moo). (I’m not making this up!) The idea is that a certain numeric level of “mu” can range from 0.0 (no friction at all) to 1.0 (locked-up solid, immobile due to friction). Anybody who ever rode an old Pan, Shovel or Sportster with a drum front brake has firsthand experience with a “mu” of maybe 0.20… if that! Today our stock disc brakes can muster more like a 0.30, perhaps a bit better. Thing is, it ain’t tough to get a solid 0.40 and as high as 0.45 if we do a good job of selecting pads… and the rotor they work with!

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Caption: This chart is a translation of the “mu” ratings for a coefficient of friction and expressed in numerical form (0.0 being the worst, 1.0 being the best) for brake pads. Then the DOT adds their double-letter code, which doesn’t really tell you a damn thing without the info in this chart, but is handier because you can find it on all brake pads sold in America. H-D OEM pads have a “HH” rating and you never want to settle for less than that.

Stainless alloy brake rotors have been a feature of Harley-Davidson OEM brake systems for decades… as much for cosmetic reasons as any others. Alloyed stainless has its advantages, but helping pads with their “mu” isn’t really one of ’em. Cast iron actually works better, but there’s no free lunch… ’cause they are heavy and they rust! Both of these common materials have seen some clever engineering and designs over the years in order to minimize their respective issues, but they can’t be eliminated. Take the latest (’08–’13) H-D stock slotted rotors, for example. Knowing these rotors can “grow” in diameter significantly once they get hot, Harley has cleverly inserted what looks like a button to allow the disc to remain flat when it gets as much as 20-thousandths larger after some hard use. Not a bad compromise for a disc rotor that is built to price. But… Harley also sells superior “floating” rotors, which do the same job as well as maintain full contact with the pads… under pressure. These discs are comprised of two pieces, the “carrier” and the rotor, one bolted to the hub, the other sliding around between the pads. Keeping it all together are “bobbins,” usually retained with a circlip, that allow a controlled bit of movement between the two. This serves a few good purposes, not least leaving a sort of air gap between the two parts that helps cool the disc while allowing the heat growth to remain pretty harmless… and that little bit of wiggle that will let the rotor align more perfectly with the pads for a complete “wipe” of contact between pad and disc. Essentially, more of that proper mastery of friction.

So far there’s probably not much news that you didn’t already know. From here on, however, there might just be a surprise or two. I’m going to touch on these, then leave more of the details for next month, since I don’t have the space to ramble on to completion this month. First, brake rotor materials are in the midst of a major rethink, cast iron and stainless alloy being so “last decade.” Among the frontrunners today are what’s usually referred to as “Metal Matrix Composites”… as the name implies, usually a metal alloy of aluminum or magnesium blended with ceramic. The result is stiffer, much lighter, longer lasting, as well as being capable of dealing with friction and heat in ways no “monolithic” metal can. It’s well known that most of the heat from friction in a rotor is contained at the outer edge of the disc. MMC rotors can be “blended” to deal with this fact far more effectively than plain metal rotors. Adding a bit more “thermally inert” ceramic to the “blend” at the outer edges and less in towards the cooler center is a giant leap in proper mastery of both friction and heat dissipation. The other thing is, speaking of benefits and improvements; two of these rotors weigh less than one stocker! But we’re not done, as we’ll see when we get going again next month. For now… let’s just stop.

EBC brake pads

Caption: These EBC pads contain quite a lot of basic information, right on the package. We know they are “HH” rated for friction and that they are “sintered” (i.e., the compound, however proprietary it might be, is “cooked” onto the metal back at extremely high heat). Most organic pads (which ironically have almost nothing organic in ’em these days) have their compounds sort of glued on instead. Since the hype says these pictured pads are for racing, it’s not a giant mental leap to presume they are intended for use with iron rotors and chew ’em up pretty well under hard use. At least comparatively speaking!

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Caption: When Honda burst forth with front disc brakes in 1969, it was a revelation! Shortcomings followed… almost as astonishing and as quickly. Early discs were solid; they could warp, crack and definitely worked very poorly or failed to work at all when wet. Pad compounds were primitive.That soon changed. Discs themselves were slower to evolve, but one of the first steps forward was drilling holes in the rotor as seen here. Too many manufacturers have stuck with this for too long. Holes in the rotor let the pads out-gas and give water a place to go for sure. Thing is, they also wind up looking like this, with uneven pad surfaces, striations, cracks, grooves, warping—you name it—right where the holes are! Braking abilities take a huge dive on rotors this screwed up.

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Caption: I have to apologize for this picture (best I could come up with) of a rarity. This is another interpretation of rotor design, borrowed from what’s common on cars. This is a 1982 CBX front brake grafted onto a custom Vincent and its rotor is vented… just like your auto. Notice by this time calipers were stronger, longer and sported more than one piston and pad per, usually “live,” meaning moving the pads towards the rotor. (Early calipers had one side that moved while the other was stationary: “You pitch—I’ll catch” thinking, which thankfully didn’t last long!)

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Caption:  Sorry to be the one to say it, but over three decades later, we still see big, heavy bikes with caliper designs, pad materials and “holy” rotors on either side of the wheel. This is time-warp stuff compared to what’s possible with modern materials and thinking.

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Caption: CVO/Screamin’ Eagle dressers come with rotors like this. They look great, but functionally they just lose the carrier by virtue of bolting directly to the wheel on special lugs. (New for 2014 are true CVO-style “floaters,” but they only work with certain wheel styles like “Chisel”.) BTW, identifying a true floater for a Harley (especially among all the aftermarket offerings) can be a bit tricky. There are lots of two-piece rotors that are not floaters! Most of the time the giveaway is the attachment hardware. Two-piece discs bolt up tight, whereas floaters tend to use hollow “retainers” and circlips. And all floaters have discernable movement between the two parts of the disc.

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Caption: Better is the setup used on the XR1200: stiff caliper design with four “live” pistons in differentially-sized bores working on both sides of each floating rotor, but still with holes and still heavier than need be. The upside-down forks are intended to reduce unsprung weight and improve suspension compliance… then they go and do this!

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Caption: A Buell front brake is closer to the (Hall)mark in almost every way. A single (but huge and light) rotor, plenty of cooling, minimal disruption of suspension compliance, light 6-piston differential-bore caliper, upside-down fork and ultra-light wheel to reduce unsprung weight… and so it goes. BUT…

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Caption: … DIS is da bomb! Seriously… even a casual glance gives the (correct) impression that this brake works… seriously! There are arguments against the so-called perimeter design, mostly based on outdated information and options. Let’s overcome a couple right now! A moment’s thought tells you that if original 6mm rotors expand with friction, expanding against the pins that hold them could cause trouble with uneven heating of braking surfaces, temp-affected inconsistent shape, pad drag and more gyro effect than is desirable. Well, 10 years ago when this design was fresh and revelatory, maybe—just maybe—it was valid for the abusive track-day guy. But look at the current refinements evident in this picture. Here we have a single, 5mm, ultra-light floating rotor with cooling slots (or grooves) rather than holes and a “partial vent” in the form of a heat-dissipating groove in the outer perimeter. A well-engineered cooling “scoop” directing air onto exactly the places it should go to be effective. Top-of-the-line USD forks with aluminum lower blocks for absolute minimum unsprung weight and excellent suspension compliance. Eight-piston, “diff-bore”, mono-bloc caliper for flex-free high pressure braking, and pinpoint two-finger control/feel at the lever. Examination of EBR’s new 1190 street bike is a master class in excellent “first-principle” brake design! But Erik isn’t the only one with fresh ideas, as we’ll see… soon!

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