Specialized recently unveiled a disc brake-equipped version of its Venge ViAS aero road bike. The claims of identical weight and nearly identical aerodynamic performance to the rim brake version are impressive, but the reality is we’re still in the early days of aero disc road bike development.
How much more aerodynamic can bikes get, and where are there still gains to be made? Technical Editor James Huang spoke with Specialized aerodynamicist Chris Yu and other notable industry figures to get an understanding for where we are right now — and where we go from here.
Specialized created quite a stir last year when it unveiled its second-generation Venge aero road bike, the Venge ViAS. That bike arrived with an avant garde shape, a wild integrated aero cockpit, and unquestionably bold marketing claims. The feature that seemed to generate the most discussion, however, was the proprietary integrated rim brake design, which used a novel trailing-arm geometry and highly unusual mounting positions: behind the fork crown up front, and halfway up the seat tube out back.
The new Venge ViAS Disc version replaces those odd appendages altogether in favor of wholly standard flat-mount disc brake calipers, augmented by 12mm thru-axles and dedicated disc-specific Roval wheels. Three disc models will be offered initially to complement the existing rim brake bikes — the Venge ViAS Expert Disc Ultegra pictured here, the second-tier Venge ViAS Pro Disc UDi2, and the range-topping S-Works Venge ViAS Disc eTap.
In what is bound to be viewed as a coup for aero disc road bike development, Specialized says all three of them can go toe-to-toe with their rim brake counterparts with no weight gain whatsoever (the parts are heavier, but the framesets are lighter) and essentially no additional aerodynamic drag — in other words, all the performance advantages of disc brakes with none of the usual downsides.
Yet how can that be? Didn’t Specialized even say itself that disc brakes were roughly eight seconds slower than rim brakes over 40km? What happened to #aeroiseverything?
The backstory behind the Venge ViAS Disc
As it turns out, the standard Venge ViAS’s rim brakes come across like afterthoughts because, in some ways, that’s how they came to be. According to Specialized aerodynamicist Chris Yu, it was the disc-equipped Venge ViAS that the company started developing first, not the other way around.
“We put ourselves in the position of someone walking into a store, and they’re presented with a disc aero bike — what are the first questions they ask?” he said. “The first thing everyone wants to know is, ‘Is it heavier?’ So we wanted a platform where we could say, no, weight’s not a disadvantage, or it’s not heavier, period.
“The second question for a race bike is, ‘It’s less aero, right?’ Basically, we wanted a platform where we could say, no, those are not disadvantages. So then you’re left just left with choosing which brake platform you prefer, and the rest of the performance is not compromised. That was the overall vision.
“Late 2012 was when we first started doing shaping on the new Venge, and it was 100% on the disc version initially. We were all-in at that point, and the UCI was showing signs that it was going to adopt this pretty soon.
“A little bit down that project, after we had gotten the main tubes started, there was a pretty clear indication that [the UCI approval] was going to be delayed. So with that uncertainty, and this being a race bike, and needing to supply the teams, we shifted focus to the rim brake version. The disc project was on hold.”
While the UCI’s waffling undoubtedly complicated Specialized R&D plans, Yu says it ultimately worked out in the company’s favor.
“In hindsight, it was actually good. [The Venge ViAS] is a really complicated shape in terms of manufacturing and the mold, and we learned a lot about limitations and better ways of doing the composite design through the rim brake design. And so when we picked up the disc project again, not only were we better equipped in terms of analysis tools for carbon optimization, but we learned more about making the frames.
“That, in combination with knowing exactly what the stiffness characteristics were from a whole bunch of field testing, we were able to redesign the lay-up on the disc frame to knock out weight while holding stiffness the same. That’s how we were able to achieve weight parity after you pick up the additional weight of the components.”
Specialized may have figured out on its own how to make the disc variant lighter than the rim brake version during the delay but in terms of aerodynamics, the company also got a little lucky. The industry switched in the interim from post mount disc brake calipers — which were essentially just carried over from the mountain bike market — to road-specific flat mount ones, which not only boast more compact shapes, but also cleaner interfaces with the frames and forks.
According to Yu, earlier Venge ViAS Disc development mules with post mount brake hardware had already lessened the aerodynamic drawbacks to rim brake bikes, but it was the switch to flat mount that closed the gap entirely.
“[On the post mount prototype], there was a super small difference in terms of aero — 2-4 seconds slower at low to moderate yaws (below 5 degrees). Once you got out to 15 degrees, it grew to around 8 seconds. What we found in more recent testing on production frames and forks with more finalized shaping and flat mount hardware, that gap consistently shrunk to the point where I’m comfortably calling it essentially the same aerodynamics between the two fully built bikes.
“The difference at those same low-yaw angles, say 0 to 5 degrees, is effectively zero to maybe 2 seconds max over 40km. At yaws of 15 degrees, it was no more than four seconds on one side, and the other side was essentially zero seconds difference. When we test fully built bikes, our margin of error is in the neighborhood of a second and change so we’re really pushing into that margin of certainty, anyway. That’s how close we feel the two bikes are.”
A new rotor and hub design on the way?
Taken at face value, Specialized’s aerodynamic claims on the new Venge ViAS Disc are very impressive. However, given that the rim brake bike uses radically non-standard calipers and the disc version makes do with off-the-shelf hardware, it stands to reason that there is still substantial room for improvement. After all, as good as the latest road hydraulic disc brakes are, they hardly seem like they were designed with low drag in mind.
Yu feels that there are some gains left to be made in terms of more specifically aero-focused disc brake hardware — perhaps even highly integrated, bespoke components — but probably not where you might first expect.
“There still is a little bit left on the table in terms of the entire package but I don’t believe that the caliper is the solution. We’ve done testing on all of our platforms, and some competitor ones, too, and if you isolate the different components, it turns out that the caliper, in almost every case, contributes nearly nothing to that additional delta in the cases where a disc brake is actually slower than a rim brake bike.
“The caliper in almost every situation is actually relatively well protected. For the vast majority of bikes, the exposure of the caliper is smaller than, or no worse, than a typical brake shoe that you would have on even a hidden brake.”
At least according to Specialized’s in-house testing, the main culprit is the rotor.
“It’s mostly the rotor, and the fact that the hubs are usually a little bit bulkier. In that handful of seconds difference among most of the bikes we tested, the difference is greater in a crosswind where the rotor is more exposed. And it’s not just the rotor; it’s also the carrier and all that other material that’s directly exposed.”
Not surprisingly, then, it’s this area where Yu (reluctantly) admitted the most benefits can be potentially gained.
“I would concentrate more on the rotor, and the carrier, and the hub interface — that entire area — before I would concentrate on the caliper. Maybe even how the fork plays with all of those things, too. That entire area near the rotor, the design focus is better used addressing the rotor and hub area rather than the caliper.
“If I ticked those boxes, then I would look at the caliper to see if there’s something I could do there but I wouldn’t spend resources there. I think you can squeeze a lot more from the same effort out of the rotor and hub.”
As good as the Venge ViAS Disc seems to be, the reality is that we’re still in the early days of disc-equipped aero road bikes in general. Yu obviously wouldn’t completely tip Specialized’s hat in terms of ongoing development work, but it does sound like the industry on the whole is on a steep learning curve. So how soon can we expect disc road bikes to actually be more aerodynamic than today’s best rim braked road bikes?
“I think we’re probably one product generation away,” Yu said. “I’m pretty aggressive with what I think there. I think that what most of the big players like us realize is that the racing thing is going to flip relatively quick now, and once that adoption happens, there will be even more resources put in to make those bikes better.
“I think they’re close enough today with the bikes where you can literally just take off the rim brake calipers and bolt on the disc brake calipers. The difference there isn’t that big to begin with. I’m relatively certain that with the generation of design and development that’s out there, not just us, but probably a couple of players out there, will have a bike faster than what we see today.”
Other manufacturers weigh in on the future of disc aero road bikes
Specialized isn’t the only industry player with an interest in aero road bikes, nor does the California company hold an unusually bullish attitude toward disc brakes. The fact that Specialized has managed to get the Venge ViAS Disc on nearly equal footing with the standard bike will invariably raise a few industry eyebrows, though, and that it was launched without UCI approval perhaps serves as a good indicator of what we may expect from other companies in the near future.
Felt was a very early proponent of aero road bikes, launching the original AR almost a decade ago at the 2008 Tour de France. The current AR range doesn’t include any disc variants, but Felt has demonstrated a strong support of the technology on other platforms in recent years, including on the latest FR.
“I believe that it will be possible to develop a disc-specific frame that is more aerodynamic than what’s currently available,” said Felt road brand manager Hubert Otlik. “I would not be at all surprised to see proprietary integrated components in the future.
“Companies have created custom components in the past for their bikes – Cervelo’s collaboration with Magura on the P1 brake comes to mind. I could see a bike brand partnering with a component manufacturer to design a proprietary caliper. I imagine it will be small, elegant, and beautiful to look at. However, the UCI rules and our inability to sheath or fair the rotor presents an issue as well.”
“With the advancement of rapid manufacturing technologies like 3D printing, materials such as carbon and ultralight metals, it’s not unthinkable that in the next 5-10 years we will have hydro levers, calipers, and discs that are equal to or lighter weight than what’s currently available,” Otlik continued. “We’re always exploring new technologies and ideas.”
Giant, meanwhile, recently debuted a number of disc-equipped TCR road models to complement the current range, but the aero-focused Propel line remains rim-only — for now.
“The number one issue is overcoming the misconception that a disc brake equipped aero road bike will be aerodynamically inferior than a rim brake equipped model,” said Giant’s global product marketing manager, Andrew Juskaitis. “[Overcoming that] will require engineering and design that isn’t currently available on the open market. This means that if manufacturers want to build an aerodynamically superior product (over a rim-brake equipped bike), they will need to consider bespoke equipment.”
“Current disc brake equipped aero road bikes appear to have existing technology simply mated to current frame designs, which we don’t feel is the best way to launch this new trend. If Giant enters this segment, we will be sure to design a product from the wheels up, meaning we start with a custom disc brake system, then engineer an aerodynamically superior frameset/wheelset/cockpit around the braking system.
“Giant won’t enter the arena unless we can manufacture a disc-brake equipped product that is aerodynamically superior to our current (rim brake-equipped) Propel Advanced SL — and that is equal to (or less) in overall weight. Let’s just say that we’ve booked more wind-tunnel time this development year than all other previous years combined.”
Trek’s statement on the topic is a bit more confusing. On the one hand, the company is clearly standing firm behind the performance benefits of the current Madone, and seemingly has no interest in releasing a disc version.
“The Madone currently has a great solution for brake integration,” said Trek road brand manager Michael Mayer. “It has incredible integrated braking performance that is clean and fast. As much as [Specialized] say there is not a penalty, there is a compromise for using disc brakes on an integrated aero bike. We are not going to compromise the performance vision of the Madone to switch to disc brakes today. When there is a great solution to disc brakes and aero integration that adds a true benefit to our design goals, we will be there.”
That all said, Trek nevertheless hinted at future development with a stunning design concept that was on display at its recent TrekWorld dealer event — and yep, it was fitted with disc brakes. Trek officially says the bike is its vision of what a commuting bike will look like in ten years, and in keeping with the theme, it features a belt-driven internally geared drivetrain and an electric-assist motor. Nevertheless, it’s impossible not to notice the strong resemblance to the current Madone.
Make of that what you will.