Specialized S-Works Tarmac SL6 Disc review: Goes faster, stops harder
It’s hardly been a secret that Specialized had a disc version of its new S-Works Tarmac SL6 on the way; prototypes were shown back in July, after all. But it’s only been within the last two months that we’ve been able to ride one, and CyclingTips U.S. technical editor James Huang was among the very first, taking delivery back in late January.
Specialized claims the new S-Works Tarmac SL6 is substantially lighter, slightly more comfortable, far more aerodynamic than the previous generation, and the handling has mellowed just a bit as well. If you’re already a fan of disc brakes, and were eagerly awaiting the disc-equipped version of the SL6 that Specialized introduced a few months ago, wait no more; your ship has not only finally arrived, but it’s shown up with a stock dual-sided power meter, too.
New editions of long-standing bike models are often just mildly updated versions of their predecessors, but that’s not the case with Specialized’s new flagship S-Works Tarmac SL6 Disc road racer.
First and foremost, the new frame gains a sleek aerodynamic shape, with a silhouette somewhat reminiscent of the current-generation Venge. The lowered seatstays yield a similarly offset seat cluster, the wedge-type seatpost binder occupies the same location in the top tube, the slightly sloping top tube takes a straighter path from front to back, and there’s a slight cutout in the seat tube for the rear wheel.
The tube shapes are less dramatically teardrop-like in profile, however, with greater use of truncated airfoils and D-shaped cross-sections that are lighter and more structurally efficient.
Put into numbers, Specialized claims the Tarmac SL6 offers a time advantage of 45 seconds relative to the SL5 over 40km — roughly on par with the previous-generation Venge (which, depending on your level of cynicism, means the new bike is either just that good, or the old Venge was just that average). Perhaps more impressive is the fact that Specialized also claims that this new disc-brake iteration of the SL6 essentially posts the same drag numbers in the wind tunnel as the rim-brake version, despite the protruding calipers and rotors.
Specialized is placing even more emphasis than before on its so-called “Rider-First Engineered” design philosophy, which supposedly ensures consistent performance and ride characteristics across the size range (which, by the way, now spans a very generous 44-61cm spread). As testament to this effort, Specialized says each Tarmac SL6 frame is now built with roughly twice as many individual pieces of carbon fiber than the SL5 – almost 500 in all.
Interestingly, part of the Tarmac’s redesign includes a move back to constant 1 1/8-to-1 1/2in tapered steerer tube diameters across the board, which would seem counter to the argument of consistent ride qualities regardless of size. According to Specialized, however, the larger lower bearing diameter provides straighter fiber paths (and, thus, improved strength) for the new low-profile fork crown. To offset the additional stiffness on smaller frame sizes, the fork blades are now downsized instead to provide more flex on bumps.
Speaking of which, improved rider comfort was also a priority for the Tarmac redesign. According to Specialized, the D-shaped seat tube and seatpost profile that is so good in terms of aerodynamic drag also makes both more likely to flex under impact loads. And since the upper section of the seatpost doesn’t have to withstand any clamping forces, the carbon fiber lay-up is supposedly modified to provide more movement up there as well.
Tire clearance is also more generous if you want an even cushier feel. Whereas the Tarmac SL5 was approved for 28mm-wide tire, the SL6 bumps that up slightly to 30mm, which should be more than ample for all asphalt applications, and even sufficient for light dirt-road use.
Alterations to the bike’s handling characteristics are more subtle, but generally lean toward making the new Tarmac SL6 a little more mellow.
Slight changes in head tube angle and fork rake yield tiny differences in trail (a single millimeter) that should make smaller bikes (44-54cm) a touch more stable, but larger ones (56-61cm) more nimble. In reality, however, the change is so tiny that only the most seasoned Tarmac veterans are likely to notice. Either way, Specialized has dropped the bottom brackets and lengthened chainstays across the board for greater stability at high speeds and a more secure feel through corners.
According to Specialized’s director of integrated technologies, Chris Yu, the change in bottom bracket drop is mostly a wash on account of the new bike’s 26mm-wide tires, which place the bike higher overall than the 23mm-wide ones once favored.
“The the only significant difference across the board is the increase in bottom bracket drop,” Yu said. “This is purely to account for the fact that riders are using much larger tires these days, and the fact that we’re specifying 26c tires. The prior geometries were based on designing around a 23c (or smaller) tire. If we had kept the drop the same, most riders would be effectively higher than before due to the tire size difference.”
Riders on wider tires also tend to run them at lower pressures, though, so the change will perhaps be more readily felt depending on a rider’s particular setup.
Weight-wise, though, the Tarmac SL6 is a giant step forward.
Specialized says a painted 56cm S-Works Tarmac SL6 Disc frame tips the scales at just 800g — 250g lighter than the Tarmac SL5 Disc frame, and the same weight as the current rim-brake Tarmac SL6 (an even-lighter rim-brake Tarmac SL6 is said to come in at just 733g thanks to a more spartan finish).
Built up with a Shimano Dura-Ace Di2 disc groupset, Specialized S-Works Power crankset (more on that in a bit), Roval Rapide CLX 50 carbon clinchers, and the company’s house-brand finishing kit, a complete S-Works Tarmac SL6 Disc supposedly comes in at just 6.59kg (14.53lb). If you’re careful about pedal and bottle cage choice, you could theoretically be race-ready and still come in below the 6.8kg UCI-mandated minimum weight limit.
Getting to know the SL6 Disc
Seven weeks isn’t enough time to really get to know a bike (and two years is maybe too long), but the seven weeks I’ve had has been much more than we usually get by the time the curtain lifts on a new model, and certainly more thorough than the usual first-ride review.
Here’s the short version: if you love the feel of race bikes, and have made the move to disc brakes, the new S-Works Tarmac SL6 Disc is pretty fantastic.
Specialized has notably made no claims of increased frame stiffness on the SL6 relative to the SL5, although given how things have been progressing recently, that isn’t surprising. Along with a number of other brands, Specialized has been saying for several years now that the industry has essentially reached the pinnacle of chassis rigidity; bikes could get stiffer, but doing so wouldn’t confer any performance advantages without generating substantial compromises in other areas of performance.
Long story short, the SL6 is just as wickedly responsive as the SL5, both in terms of power delivery and handling input. There’s little sense that any energy is wasted, and the front end is impressively reactive to steering inputs. Perhaps more importantly, the overall feel is very balanced and cohesive, with excellent predictability in how the bike reacts to mid-corner imperfections — neither the front nor rear triangle shows any signs of being significantly stiffer or softer than the other.
Handling changes are subtle, but existent nonetheless. Ridden back-to-back with a rim-brake SL5 (with the rim-brake version of the same wheelset and same tires), the new SL6 does feel slightly less twitchy. Front-end response isn’t appreciably different for my 52cm test size, but the lower center of gravity feels more planted through corners, at least to me.
Specialized claims more comfort in its bench testing, but there’s not much of a tangible difference on the road. Overall ride quality is still quite firm, even with the stock 26mm-wide Turbo Cottons inflated to a modest 70-75psi. Riders who live in areas with anything short of perfect roads might find the ride to still be a little too firm. In fact, I’d argue that that continued ride stiffness even squanders some of the SL6’s newfound stability. By the numbers, the SL6 should be an absolute beast through corners, and in most typical road-going situations, it is.
However, the choppy ride can make it challenging to carve a turn on coarser surfaces, where slightly softer chassis tend to stay more planted. Later in my testing period, I swapped the stock wheel-and-tire setup to Enve’s AR 4.5 Disc wheelset and 28c Schwalbe Pro One tubeless tires (which puffed up to nearly 31mm on those ultra-wide rim beds). The end result was still fast, and still light, but with better traction and ride quality.
If I were building a Tarmac SL6 Disc from the ground up myself, this is the way I’d go.
I didn’t tear this bike down for an actual frame weight, but the grand total is bang-on with Specialized’s claims — actual weight for my 52cm test sample is 6.58kg without pedals, or an insignificant 10g heavier than official figures. Equipped with a set of Shimano Dura-Ace pedals, a Wahoo ELEMNT Bolt computer and aluminum Bar Fly mount, and a pair of Specialized Rib Cage II bottle cages, the total weight is a still-impressive 7.15kg (15.76lb), with room to move downward.
True weight weenies will argue that the bike could be lighter still with rim brakes, and they would absolutely be correct. Were I still living in the American Midwest (or any flatter region in general) and predominantly riding in dry conditions, that’s perhaps the way I would go, with the rim-brake bike also carrying the additional benefits of being simpler and easier to service.
That said, the performance of Shimano’s latest disc brakes is just so good that I would have a hard time opting for a pair of rubber blocks squished against a rim. Lever feel on Shimano’s latest Dura-Ace hydraulic controls is firm and positive, there’s superb power, and outstanding all-weather control. The setup is a bit noisier, yes (especially when wet), but at least for this rider living in mountainous Colorado, there’s only one choice.
About that power meter
The new S-Works Tarmac SL6 is the first bike from a major brand to include its own house-brand power meter as standard equipment; no upgrades required. Specialized claims best-in-class “real world” accuracy (+/-1.5%) with its new dual-sided S-Works Power crankset, thanks in no small part to rigorous testing on the 4iiii-based system by Dr. Rodger Kram at the University of Colorado-Boulder. But is it as accurate as Specialized says? As is always the case with power meters, that’s hard to say. While the numbers tracked pretty well with other power meters fitted during testing, that’s hardly a confirmation of trueness. In effect, it’d be like saying two pieces of fruit look and taste the same, but without a known reference, you still don’t know if they’re apples or oranges.
Nevertheless, it’s an impressive piece of hardware if only for the salient features. The large-diameter carbon fiber arms are incredibly stiff (verified by independent testing, not me), but yet remarkably light at just 440g (claimed) without chainrings or bottom bracket. Dual ANT+ and Bluetooth wireless protocols make for wide compatibility with virtually any computer head, over-the-air updates help keep the system up to date, and advanced metrics will provide additional information such as left/right balance, torque effectiveness, and pedaling smoothness on compatible head units.
Even the covers for the CR2032 coin-type batteries are held in place with real metal screws so as to create a tighter seal for improved weather resistance.
But does the S-Works Power really provide power data within +/-1.5% of the true figure? Stay tuned on this subject; we’re just getting started.