To be honest we’ve probably all heard more than we care to of the saga of getting disc brakes into the professional road peloton. But while there were certainly some missteps along the way, the UCI has once again made clear that disc brake bikes will make their way back into the top level of the sport this 2017 season. The most recent concession to the safety concerns of having sharp spinning rotors in the bunch has been to round off the rotors themselves. Last year we saw a couple of different solutions to this (probably overblown) problem, and now we’ve had some feedback from Shimano about what they are doing to address concerns and how it will affect consumers….

After speaking with Shimano’s product development team, it seems that the UCI communique on disc brake rotors has been rather vague, leaving interpretation and the real world solution up to the manufacturers themselves. Officially the UCI is requiring that disc brake rotors to be used in the pro peloton are to have edges that are “not 90°“. There is nothing else specific about required rotor shape and no requirements for either a specific rounded or chamfered profile.

Shimano’s solution has been to chamfer the edge with a very small grind to the edge of the rotor to knock that 90° corner off, quite similar to what we saw from SRAM back in September. The result is a rather subtle difference from the standard Shimano rotor. It is certainly easiest to see if light catches the small <1mm wide chamfered edge of the rotor, as seen above.

When looking down at the top of the rotor the shape is not quite as obvious, but can still be seen if you look carefully. The rotor on the left above is the new RT900 rotor of the new R9100 series Dura-Ace with the “not 90°” edges, while the rotor on the right is a current XTR level RT99 that has not had its edges ground down (and is the one we’ve seen on most pro bikes until now).

Shimano tells us that all of the new RT900 rotors that accompany the new Dura-Ace R9100 series groupsets in both mechanical and Di2 electronic shifting varieties will have their edges chamfered as above to meet the requirements of the UCI. There are no plans currently to do the same thing to any of their other rotors, even though all of the teams that have been testing disc brakes on the road have been using the previous rotor design carried over from the mountain bike. But with that said it seems that it won’t be too difficult for the team mechanics to quickly file of a bit of the 90° edge to satisfy the UCI requirement.


  1. I came across some old hayes rotors the other day, and noticed that the outer edge had a legit chamfer on each corner. I’m amazed that this is even news-worthy. They should all be chamfered, period.

    • Um, don’t quit your day job. No, this will not happen, because the pad does not abrade the rotor at the chamfer.

      Ok, so yes, this is a weird spec from the UCI. It’s about as technical as issuing a memorandum that says “You guy’s, don’t make it sharp, ok?”

      As an engineer, I’d specify a radius for the edge of the disc rather than just “not 90 degrees.” If the disc is 1.8 mm thick, a radius of 0.9 mm centered on the neutral plane of the disc would create a semicircular edge. But to appropriately dull a disc, you’d probably want a 1.5-mm radius that is *not* centered on the disc’s neutral plane…that is, there would still be a flat section in the middle. And that’s what Shimano and Sram’s chamfers approximate, while being easier to machine than radii.

      Consider this: the UCI’s technical committees are neither technical nor committees. Discuss.

  2. Yes the pad does wear the rotor to the edge on most disc brakes. And my favorite internet quote of all time has always been “As an engineer”, well done.

    • Are you sure about that, even on road rotors that aren’t wavy? You know, the only ones we’ve seen that claim to comply with this rule?

      Because that means when you change your pads on these Dura-Ace rotors or SRAM centerline rotors you’ve got unworn ridges of material at the top of your pads. I don’t see that on my pads and rotors. Do you? And if you do, have you observed measureable wear on a disc via your trusty micrometer? Because you’d have to wear a Freeza rotor nearly to the aluminum in order to “sharpen” a disc like this past its chamfer.

      I was indeed sneering at an armchair engineer. That’s because it’s silly to think that Some Guy On The Internet is giving this more thought than did an actual engineer at Shimano or SRAM. It’s not that engineers never make mistakes; it’s that the OP is full of hubris.

      Gotta love the ad hominem; it’s adorable. It’s a good thing you don’t ride a bike, fly on airplanes or ever want radiation to cure your cancer, as engineering is deeply involved in all of these things. (I work on radiation oncology delivery systems). Engineers have plenty of faults, but we do actually think about the problems we’re trying to solve.

    • just because you’re an engineer doesn’t mean what you necessarily know has any bearing on any mechanical topic at hand, and frankly if your opinion is good you shouldn’t have to qualify it with “as an engineer”. like engineers are incapable of making bad decisions.

      • I’m glad you asked whether I have any relevant experience. I’m trained as a mechanical/aerospace engineer and, yes, I’ve worked in the bicycle industry for several different companies, including two bicycle manufacturers. Prior to that, I turned wrenches in shops for fifteen years (on and off; I’ve got about eight years’ full-time experience as a mechanic. Frankly, I’m not a great mechanic. I can *do* everything and do it well, but I’m way slower than good shop mechanics).

        “As an engineer” wasn’t intended to be an appeal to my own authority, though I sort of understand why you read it that way. I meant it to convey “If the UCI had asked me to address this problem, which they didn’t…”

        As to your complaint that I think engineers are infallible, I’d point you to the third paragraph of my post above, where I say explicitly that “it’s not that engineers never make mistakes.”

        • Someone is touchy. I asked a question, you go on a tirade about armchair engineers.

          Apparently Shimano thinks rotors get thinner according to their instruction manual.

          “If the disc brake rotor becomes worn down to a thickness of 1.5 mm or less, or if the aluminum surface appears, immediately stop using the brakes and consult a dealer or an agency”

          But according to our expert engineer here, the engineers at Shimano are WRONG. Rotors don’t wear like that. The engineers that wrote the tech specs for Shimano rotors and instructions should be fired.

          Well that sure sounds like rotors get thinner. Bikerumor wasn’t kind enough to specify anything but less than 1mm, which should be rather obvious, since the rotor is only 1.8mm thick to begin with and you would have one chamfer on each side. Did you even look at the pictures when deciding the radius was 0.9mm using your excellent engineer observational skills? Apparently engineers think that looks semicircular.

          Not exactly easy to judge the actual chamfer based on blurry photos, but it doesn’t look semicircular to me. If the chamfer is less than 0.15mm wide, then it sure does seem like the edge could get sharpened.

          But congratulations on being so smart and finding validation sneering at people you deem to be armchair engineers.

              • Eh…OK, I’ll bite: I’m not addressing his post because he didn’t make any points. But since you asked, I’ll elaborate:

                He (I’m assuming Anonymous is a he, but I may be wrong) isn’t following the distinction between a radius (in this context, a convex fillet) and a chamfer. And yes, a 1.8mm disc symmetrically rounded to a 0.9mm radius does indeed make a semicircle. This is self evident, so I’m not sure what’s up with all the drama. A chamfer needs an angle (provided by the UCI, sort of) and a distance (not provided at all).

                The guy (I’m assuming) gets all worked up about my “excellent engineer observational skills,” when all my numbers check out and don’t depend on observation in the first place. His complaint about whether a 0.9mm radius on a 1.8mm-thick disc is semicircular is incoherent.

                Dude puts odd words in my mouth (“the engineers that wrote the tech specs for Shimano rotors […] should be fired”) when in fact he was the one claiming that the engineers didn’t think this through and that they had foolishly created a self-sharpening rotor. I believe psychologists call this “projection.”

                Of course disc surfaces wear; that goes without saying. But it’s not enough to make these things self-sharpening, especially with a radius applied. And it’s still not clear to me that the pads sweep the outer diameter of the rotor. If they don’t, then these are in no way self-sharpening.

                I can’t help being amused when people write things like, “you ‘engineers,’ you think you’re so smart about mechanical things.” This is about the same as saying, “Those doctors think they’re SO up on human health,” or “Lawyers, amirite? They think they know the law better than other people.”

                There’s an old cliche about engineers:

                “Arguing with an engineer is a lot like wrestling in the mud with a pig. After a few hours, you realize the pig likes it.”

                • JasonK, I Personally think your response to the armchair critic/engineer above was well worded, coherent enough for the average person to understand and done in a calm manner. I agree, the only touchy person here is the one who did not want to back his own post up with a name!

                  • I’m glad you found my post informative. I’m really not here to fight with anyone…I initially posted because I, like others, found the UCI spec frustratingly vague and incomplete.


                    • It’s pretty obvious to me that my pads wear evenly and do not build up an edge. This means that the pads would have to make full contact with the disc and there can be no ‘overhang’ which would supposedly cause disc sharpening.

                      Think about it, it would be a really bad design if the pad area wasn’t completely used during braking.

  3. Interesting how these rotors have the little V’s cut out of the brake surface that haven’t been chamfered like the OD. So essentially, Shimano made cutting teeth on their safe rotor lol. I’m sure it was just a cost cutting measure since it’s not as easy to run that section over a grinding wheel, but it might not completely solve the problem.

    A quick question if anyone here knows: Do any pro riders use Mavic Cosmic Carbone wheels with carbon spokes? Or Lightweights/Madfiber? The spokes on those things are damn near razor sharp and could easily tear up a finger/hand/nose/etc while spinning during a crash. Maybe the same with CX-ray spokes? Are bladed spokes banned too? Anyone ever been stabbed by a chainring during a crash?

  4. Not 90 degrees? So 10 degrees is OK then? What a horribly vague requirement. Someone should make a sharpened rotor then sue the UCI for denying compliance.

  5. Really don’t understand the whole scalloped/wavy edge detailing on these. Its design for differentiation only, and in this instance, it’s more likely to lead to injury. Not that I think the risk is remotely high, but given everyone is concerned, why do it/allow it?

    • Scalloped or Wavy edges are for clearing dirt/dust from the rotor surface and for cooling as you use the brakes. It’s a design feature that has been common in Motorcycles for a while.

      • Imo, and based on research I’ve done, slots or holes in the rotor do that way more than the edge profile as the later doesn’t really contact the disc.

        Almost all road motorcycles no longer employ them. It was done for “sporty” looks and that is going away as people realize it amounts to fake air vents or ineffective design only spoilers. It reduces rotor area that can used for heat dissipation or adds useless mass.

        Perhaps what you say is true for heavy mud build-up as it allows debris to be flung out as it enters the pad clearance…and why you still see them on MX/Enduro motorcycles.
        But these are road bikes.

        • Hate to contradict someone on the interwebs, seems like it always ends badly but…………Waves are still a “thing” when it comes to motorcycles, especially in the world of roadracing (check Galfers website) the waves clear the pads and rotors of any debris and do have a cooling effect as air circulates at the pad surface. This isn’t a substitute for holes or slots, but if you’ve done any research you could see that not only are waves still around but that channels are even used on the rotors’ surface, increasing this “cleaning/cooling” effect.

          Not a fake, not for “sporty” looks. As an accomplished road racer with multiple club level wins and some WERA podiums, I can say I loves my Galfer waves!

          • I’m sure Galfer rotors are great, but their “wave”, patented or not, is superfluous for temp control. Numerous other top end brake manufacturers do not require it. MotoGP bikes rarely run it. Top end cars do not, of almost any race organization sans some ultralights looking for weight savings…something Wilwood will admit too, at the cost of ultimate heat control which lightweight on stuff like hillclimbs don’t need. There is also evidence they clean the pad better by forming a planing surface, and this provides more initial bite (note sure that later is actually true)

            But guess what, when someone sees you win, they see your rotors and say “Galfer”! If you didn’t have the waves, they say “are those brembo’s or nissan’s?” Its marketing design for differentiation (unless you really need it to clear mud).

            FWIW – The channels and/or holes are largely there to clear pad gasses that build up between the pad and rotor during hi-temp braking events. They also aid cooling, but that is not really their primary purpose as the added surface area is tiny. Even tinier is the added surface area of OD scalloping or waviness.

            • Here we go………

              Moto GP bikes use carbon rotors which need heat to work properly

              Cars uses a sandwich rotor design that are vented in the middle, which is the best way to vent/cool

              Channels and/or holes don’t add surface area

              Scallops or waves don’t add surface area

              But this is the internet where everybody with a keyboard thinks they know everything.

              • Not all MotoGP bikes use carbon rotors. Nissans are still run as steel and some teams use them…they are not scalloped. Many superbikes run steel rotors…most not scalloped.

                If scallops or waves don’t add surface area, how to they enhance cooling? Can you explain physics of that.

                Channels and holes allow pads gasses to escape. Hell scallops probably help remove out gassing…but is that needed at the expense of heat load area on a properly designed and sized rotor?

                Galfers makes a quality rotor. There is no evidence that its due to their scalloped edge. It does aid in differential marketing.

                • You guys both make valid arguments but I hope you know you’re talking about completely different materials for rotors and that is the primary reason for the feature differences..

                  If the rotor is steel, or iron, it’s probably going to have holes, slots, or ‘scallops’ (cutouts on the edge). I’m pretty certain the holes are mainly to reduce rotor weight and to help with brake pad out-gassing. Slots really only help with brake pad bite by removing the ‘glazed’ layer during initial braking. Scallops work a lot like slots, but, they also reduce weight and are better at clearing debris in muddy conditions.

                  Now, you’re not going to find ANY of those features on a carbon rotor. Probably because the cost of adding those features is prohibitive, and they may not be necessary. Holes aren’t necessary because the carbon is so light. Slots aren’t necessary because the carbon only works at high temperatures anyway (very different thermodynamic conditions). And scallops wouldn’t be necessary either.

                  Possibly the brake pads on the carbon rotors have slots, or are individual pads to aid with cooling (one per piston).

  6. I was on the sidelines at the Amstel Gold race in Holland a few years back when Frank Schleck and Matty Lloyd crashed right in front of me. Matty’s bike hit the curb and bounced up at me. I reacted by turning my back to it and it hit me from behind. The chain ring cut my butt cheek.
    if you watch the youtube video you can see me (in Skil Shimano kit) get the medics attention because Matty was on the sidewalk and they couldn’t see him and at the 40 sec mark you can see me ask my friend if i’m bleeding because i could feel the cut but couldn’t see it..

  7. Will they issue a technical bulletin on rounding off the outer teeth of chainrings, or does that violate the 3.1:1 rule?

    In all seriousness though, now that all groupset suppliers have some form of disc brake (licensed or no), the UCI would be better off sitting down with the World Tour-level pro teams and deciding on a few races that will be disc only.

    If we’ve survived exposed chainrings for 70 years and bladed spokes for 40 (?), we’ll do fine with chamfered rotors. Sharp things are one issue, but the deceleration difference at 90kph in the wet might actually get someone killed.

    • So will they sit down and test any and all brake combinations (i.e. “x” pad with Al rim, “y” pad with carbon rim, “z” pad with Al/C rim with “b” style finish….).
      And then they need to test the riders because what if rider A is a brake threshold master, while rider “B” a ham fist (yes they exist in the peloton, regardless of what people want to think, it’s why some riders crash or fall way back on descents compared to others).

      The riders need to know their bike. If they are coming into a corner and the guy in front of them isn’t braking when they feel they need to, its up to them to brake early. Alternatively, if I trust my brake/tire/skill combo more, I may dive into a corner hotter than others. Its called racing to your equipment and skill limit and large differentials exist even if everyone was on the exact same equipment. Nobody has really proven that discs cause huge deceleration differences to the point that causes massive crashes.

      • “Nobody has really proven that discs cause huge deceleration differences to the point that causes massive crashes.”

        While I wouldn’t call it “proof”, the GCN guys did an interesting disc vs. rim brake test and found that while the discs outperformed the rim brakes in all conditions, the biggest difference in stopping distance occurred in the wettest, most challenging conditions.

        I bet we’ll see some guys on rim brakes ride into guys with discs this year, if for no other reason than wet carbon wheels/brakes take some distance to begin working after the brake is applied where discs work immediately in the same conditions.

        • Dinger – if what you say is true, the disc guys are either braking too early, or the pad guys braking way too late and will overcook the corner regardless.

          If anything, I expect a disc equipped rider to hit the back of rim brake riders as the later brakes earlier due to their inferior modulation and control. The disc rider will want to dive farther before braking

          • That is so ridiculous. Have you ever used disc brakes?

            I’m sorry, but there’s a reason they exist and a reason people use them, because they make one of the least controllable aspects of bicycling, completely controllable and confidence inspiring.

            I honestly can’t understand why rim brakes were around for so long. They SUCK! They are a hugely compromised design, they add a TON of inertia to the rim, they can CAUSE rim failure during extreme braking events. Honestly, they are old tech, and should not be featured on a professional bicycle in 2017. I honestly think discs should be mandatory.

            The technology is there, why do people hesitate to use it? It increases SAFETY!

  8. Has anyone done actual testing to see if chamfered edges significantly reduce the cutting potential of a thin rotor? Yeah, the initial sniff test says “safer”, but I wonder how big the difference is when you’re hitting it at 30+ mph, or when it’s spinning, or when it’s searingly hot during a long descent. You can do serious damage with a butter knife under the right conditions.

    I wonder if calling for “not 90” is just a way for the UCI to appear like it’s doing something when in reality the benefits are negligible. I welcome an engineer’s input.

    • Timbo, these are good questions for anyone to ask, engineer or not.

      A perfectly sharp 90-degree edge could be a hazard. Luckily, a manufacturer would have to try have to make their disc that way on purpose. The manufacturing processes often used to make discs (laser-cutting, stamping, etc.) leave imperfectly sharp edges. Chamfering those edges helps reduce sharpness, and a radius instead of a chamfer would help even more.

      I agree that the discs-as-spinning-blades idea makes intuitive sense, but lots of things that make intuitive sense turn out not to be true. And your point about butter knives is well taken, especially when you consider that a chamfered disc is considerably less sharp than even a butter knife.

      I think you’re right about “not 90” being some hand-waving at a non-problem. But rounded/chamfered edges are duller than the stock edges on most discs, and duller is not a bad thing in this case. My frustration is that the UCI dictates things like saddle position to the millimeter, but can’t be bothered to issue a real engineering spec for disc edges. The UCI simply isn’t all that technically sophisticated. Cycling has been this way since Henri Desgrange.

      To my chagrin, pro cycling is not nearly as technical as Formula 1. There are not gaggles of engineers assigned to each team to tweak a panoply of variables. The closest we get was the team around Lance and the team that works with Sky. But those things are the exception, so we have relics like tied-and-soldered wheels and mechanics who overtension wheels “to make them stiffer.”

  9. These new DA are supposed to have an even greater cooling capacity than XTR Freeza rotors. I wonder how they would work on mountain bikes…

  10. Guys, if you can find it, Watch the New Zealand Pioneer MTB Stage Race. A rider cut his arm badly on a spinning disc rotor to the point of needing stitches.Its not just the sharp 90 degree edge thats the problem but the rough surface finish of the edge and the wheel speed. On an MTB, most weekend warrior aren’t going fast enough for the disc rotor to present a problem.

  11. I don’t really think it really matters if the rotors do wear and take the “rounded” edges off. The pros who are concerned about the issue of a rotor being a dangerous, hot knife blade just waiting to cause severe cuts and burns, won’t have to worry about the wear since I’m sure the rotors will be changed by the team mechanics before any wear would happen.
    This whole issue is so freaking ridiculous and most any rider other than a pro, shouldn’t worry. Myself, my next bike will have an electric drivetrain and disc brakes.

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