Specialized Tarmac SL8 Review

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Eight is great. Or: What happens when you combine Aethos weight, Venge aerodynamics, and buttery smoothness.

Building a great bike is hard. Making a great bike better is even more challenging. Yet every three to four years, brands that make race bikes for their professional road racers take on this challenge.

When the brand is Specialized, we’re talking about A LOT of professional racers. The company provides its Tarmac race frame to Bora-Hansgrohe, TotalEnergies, Soudal Quick Step, Team SD-Worx, AG-Insurance-Soudal Quick Step, L39ION of Los Angeles, Team Medellín EPM, Trinity Racing, and others.

Before we get to the new-new, let’s take a moment to recognize that the Tarmacs SL5, SL6, and SL7 were—and still are—well-mannered and great riding race bikes. Specialized’s goal with the new Tarmac was to improve upon well-regarded and ultra-successful bikes.

That’s a far tougher ask than improving a bike with glaring deficiencies. But raising an already-high bar was what Specialized set out to do with the new Tarmac SL8. It’s a frame that is more aerodynamic than the SL7 and lighter—thanks to some lessons learned from the fantastically lightweight and smooth Aethos and the devastatingly fast Venge.

The result is a bike that answers the question, “What do you get if you Aethos-ed a Venge?”

On paper, the results look stunning: 685 gram claimed frame weight (115g lighter than an SL7), over 16 seconds faster than the SL7 across 40 kilometers (using a 45km/h average speed), with (compared to the SL7) notable improvements to the frame’s stiffness to weight ratio, and a smoother ride in the saddle.

Price: $6,500 to $14,000 ($14,000 as tested)Drivetrain Options: SRAM Rival AXS, SRAM Force AXS, SRAM Red AXS, Shimano Ultegra Di2, Shimano Dura Ace Di2. All builds have power meter.Bike Weight: 14.8lb (54cm S-Works Di2)Claimed Frame Weight: 685 grams (S-Works 12r), 780 grams (10r)Frame Details: Electronic drivetrain only, disc brake only, BSA threaded BB, 12x142mm rear and 12x100mm front axle spacing, hidden brake hose routing, proprietary aero seatpost (20mm or zero offset options)Sizes Offered: 44, 49, 52, 54, 56, 58, and 61 centimeters

Specialized touts the SL8 as “The most aero road bike we’ve ever made.” It’s more aerodynamic (it claims) than the now-discontinued Venge model.

If you’re unfamiliar, the Venge was Specialized’s aero road bike. And it had the traditional hallmarks of competitors in that category: A deep truncated airfoil downtube and deep and skinny seatstays. The Venge was a bike you’d lump in with bikes like Trek’s Madone, the Cervelo S5, and Canyon Aeroad—Bikes that scream, “I’m aerodynamic as hell!”

The SL8 doesn’t look anything like those aero-road bikes. The SL8’s downtube is essentially round, and the small seat stays are only slightly deeper than their width. How can a bike that eschews what for years was told (and sold) as the hallmarks of an aerodynamic frame be more aerodynamic than bikes with massive airfoil downtubes and the like?

I’d guess a small part of it comes down to cycling brands’ ever-evolving understanding of the very tricky science of real-world aerodynamics.

But based on the presentation and white paper provided to me ahead of the SL8’s launch, there’s more at play. It comes down to who and what Specialized optimized the Tarmac for, which is encapsulated by this sentence from the SL8s sell sheets, “We’re putting aero where it matters - the leading edge - not just where it looks good.”

For background, understanding how rider speed affects the wind direction they encounter is essential. The faster a cyclist rides, the straighter the wind angle they face on the road. That means pros averaging 40 to 50 km/h deal with more direct headwinds than amateur riders traveling at 24 km/h. Those slower speeds mean we’re more likely to encounter apparent wind angles of 15 to 20 degrees.

The average apparent wind angle a rider encounters matters when deciding on aerodynamic tube shapes. If you’re designing for higher average speeds and the straighter wind angles they create, you can concentrate on leading edges and making a bike’s frontal profile as tidy as possible.

Designing for lower average speeds (and/or greater apparent wind angles) means you must find ways to clean up the air from the side as much as the front. And that is when you need those large airfoil shapes on the downtube and seat stays.

However, assume the straighter wind angles of pro speeds and things like downtube shape matter less. When subjected to low-angle winds, the downtube is hidden behind the headtube, fork, and front wheel and takes little direct wind. If you take managing airflow out of the equation when designing a downtube, you can use the most efficient shape for weight and stiffness: Round—like the SL8’s.

Specialized bases the SL8’s aero claims on a rider speed of 45km/h (28mph). That’s freaking fast. Specialized’s white paper includes a graphic that indicates its benchmarks are with apparent wind angles of zero to about seven degrees.

With those parameters assumed, where Specialized applied the aero touches (and where they didn’t) make sense.

The front end sees the bulk of the aero tuning. The head tube extends well in front of the fork steerer to create a pointy nosecone (Specialized calls it—and I’m not making this up—the “Speed Sniffer”). The fork is more aero-looking than the SL7s: deeper legs, a narrow leading edge, a truncated trailing edge, and a sizable crown. Interestingly, the SL8’s fork and headtube strongly resemble the last-generation Venge’s.

I questioned Miles Hubbard (the SL8’s product manager) if the claim that the new Tarmac is Specialized’s most aerodynamic road bike ever only applies to pro riders. Or does the claim also hold for amateur riders traveling slower and encountering more wind from the side?

“I understand your perspective, and of course acknowledge the common knowledge that traditional aero road bikes with deep tube sections may perform well in wind tunnels at high yaw angles,” Hubbard said.

He added additional context for riders in the real world, “Since air is turbulent and ‘dirty’ after passing the front of the bike and rider’s legs, we’ve found that conventional deep airfoil tube sections at the rear of the bike do not move the needle in terms of aero performance, even in higher yaw conditions. Moreover, a bike with extremely deep section tubes everywhere can have adverse effects in gusty winds for beginner riders who may be riding at slower speeds.”

“In terms of rider benefit, and the full package for a fast bike, the design ‘budget’ in these areas is better spent optimizing weight, comfort, and ride quality,” Hubbard said.

Putting what Hubbard said (and did not say) through the bicycle-marketing-tech-speak interpretation software I’ve built in my brain after more than two decades of reviewing bicycles leads me to a few assumptions.

If we saw a yaw sweep of the SL8’s CdA, it would look amazing at lower wind angles but more average at higher angles. And I would expect bikes like the Madone, S5, Aeroad, etc. to have less drag at higher yaw angles.

But I also can see where Specialized is coming from with the SL8’s design. Even though there is mass acceptance of the benefits of aerodynamics at the highest levels of racing, riders love light and snappy bikes. And it is still hard to make an aero road bike as light, stiff, and smooth as a bike with fewer aero features.

With the SL8, it seems Specialized aims to get their cake and eat it too by creating a more aerodynamic bike for their fastest pros and lighter and better riding for everyone else.

The S-Works model employs a Roval Rapide one-piece carbon cockpit. Eliminating the two-piece design’s “hardware cluster” saves 50 grams of weight. It also reduces drag by four watts. Being a one-piece system, Specialized offers fewer stem length and bar width options than with a two-piece setup. But Specialized has a healthy 15 combinations of stem length and bar width. I loved the 38cm x 115mm option—the longest stem length available with the narrowest bar—I sampled on my test bike.

The SL8’s seatpost is another component that received attention. Specialized chose a skinny truncated airfoil shape for the seat post and seat tube. The SL8’s seat tube is the same width as the Venge’s seat post, “The SL8 has the narrowest, most aero seat post we’ve ever made ensuring fast-moving, dirty air around the legs can more easily flow backward,” Specialized claims. But this area didn’t get just aero optimization. Specialized says these parts are the reason the SL8’s seated compliance is six percent greater than the SL7’s

The SL8’s seatpost is so skinny that a Shimano Di2 battery will not fit inside the post. Instead, Specialized made a part that clips into the end of the seatpost and dangles the battery into the seat tube below the post. Although I had no issues with my 54cm test frame and saddle height, I suspect there might be clearance issues for shorter riders on smaller frames with lower saddle heights.

Focussing on leading-edge aerodynamics freed the Specialized product team to chase weight savings in areas where “aerodynamics matters less or not at all.” And on this front, they adopted some of the lessons learned during the development of the Aethos.

Many of those lessons aren’t visible but relate to the fine details of where and how the plies of carbon get used in the frame’s construction. Specialized stresses the SL8 wasn’t just a “cut and paste” of the Aethos’s development. From the SL8’s White Paper: “We knew which tube cross sections and shapes would have the best structural efficiency, but with dropped seat stays and higher stiffness targets, the layup strategy from Aethos had to evolve.”

Even so, there are shades of the Aethos in the SL8. With a set of digital calipers, I measured the width and height of my 54cm SL8 test bike’s down tube and the down tube of a 54cm Aethos in several locations—the numbers were within a few millimeters, or less, of each other.

The rear triangle is also minimalist, with very small seatstays and quite-small chainstays. Like its downtube, the SL8’s chainstays look much like the Aethos’. So small are the rear triangle’s tubes that some riders may question its stiffness. However, Specialized claims the SL8’s stiffness “surpasses” the SL7 (but did not provide many details on this claim). With a bit more stiffness and a lighter frame, Specialized gets to crow that the SL8’s stiffness-to-weight ratio improves by 33 percent compared to the SL7.

The Tarmac SL8 arrives in levels: The 12r with the highest grade carbon composites, and the 10r, a less expensive and heavier carbon blend. Both frame versions are disc brake only, electronic shifting only, and employ the BSA threaded bottom bracket standard. Some frame details include a hidden seat post binder, a removable front-derailleur tab, and two bottle cage positions on the downtube.

S-Works models receive the 12r carbon frame. Specialized uses this variant as the basis for its 685-gram (56cm) claimed weights and pairs it with a 358-gram fork.

Pro and Expert models feature the 10r carbon frame and matching fork. This frame is about 100 grams heavier—780 grams for 56cm—than the 12r, and it gets a 371-gram fork.

Both frames borrow the lightweight thru axles from the Aethos and the Aethos’ non-standard front disc caliper mount (only compatible with 160mm rotors).

Although the SL8 has clean and aerodynamically optimized routing, Specialized uses a friendlier system than many brands. Crucially, they route the disc hoses (and drivetrain wires if present) under the stem instead of through it. And when coupled with a round 1-1/8-inch steerer tube, riders can use a two-piece system (Pro and Expert models ship with a two-piece bar and stem) with the SL8.

Most bars should be compatible. And as far as stems, Specialized recommends riders use the “Tarmac SL7 stem” which has a little bolt-on cover to neatly wrangle the hoses and direct them into the headtube. Most traditional stems should work with the SL8, but they won’t neatly integrate with the SL8’s headset transition spacers (I smell an opportunity for 3-D printed parts makers). And you might need to zip-tie the brake hoses to keep them neatly in place under the stem.

The Tarmac SL8 arrives in three trims and as a frameset. All are available at Specialized dealers and www.specialized.com. Riders can choose from seven frame sizes: 44, 49, 52, 54, 56, 58, and 61 centimeters.

At the top of the line is the S-Works Tarmac SL8 ($14,000). Riders can pick Shimano Dura-Ace Di2 or SRAM Red AXS parts for the S-Works model. Both builds have a power meter (4iiii Precision Pro on the Dura-Ace build and Quarq Dzero on the Red model) and come with Roval Rapide CLX II wheels, Roval Rapide Cockpit integrated carbon bar/stem, and a carbon-railed Specialized Power saddle.

Next is the Tarmac SL8 Pro ($8,500). This model uses the 100-gram heavier 10r carbon frame. Again, riders can choose a Shimano (Ultegra Di2) or SRAM (Force AXS) drivetrain. Both bikes have power meters, but there is a notable distinction: The Shimano build has a 4iiii power meter that measures left-side power only, while the Force has a Quraq power meter in the spider that measures left- and right-side power individually. Pro models have a two-piece bar and stem instead of the S-Works one-piece cockpit. The two-piece isn’t as slippery to the wind(about a four-watt aero penalty), but the two-piece is more adjustable. Plus, it makes swapping stems and bars easier.

The “base” trim is the Tarmac SL8 Expert ($6,500). This model has the 10r carbon frame and comes in one drivetrain option: SRAM Rival AXS, with a power meter (left side measurement only). Like the Pro, this model features a two-piece cockpit, but instead of the aero-top handlebar, this model uses a good ol’ round-topped (less aerodynamic) handlebar. And the Roval C38 rims are a bit less aerodynamic than the Rapide rims on the Pro and S-Works models.

Specialized also sells both the S-Works 12r frameset ($5,500) and the 10r frameset ($3,500) used by the Pro and Expert models. All models and frames—except the Pro with SRAM Force, which only comes in black— are offered in multiple colors, and the S-Works frameset comes in a raw carbon “Ready to Paint” version if you want to work with a custom painter.

I received my 54cm S-Works Dura Ace test bike roughly a week before the SL8s public launch. And thus far, I’ve put over 200 miles on the bike.

For a good chunk of my first ride on the SL8, I was not impressed with its ride quality. I felt the pavement’s every crack, pebble, and minute depression. While it did feel very light, extremely responsive, and enormously agile, I was confused about statements in the press kit like, “…more compliant, floating over rough road and diving into corners with a supple, balanced feeling unique in the high-strung world of race bikes.”

What bike were they riding, I wondered.

However, it was about the bike(s) I wasn’t riding.

For more than a year, I have not been riding race bikes. I spent most of the past 12 months riding road, all-road, and gravel bikes with 35 to 50mm wide tires. So, when I jumped on a race-optimized bike with 26mm tires—a centimeter (or more) narrower than what I usually ride—yeah, I noticed it.

For context, I took the wheel/tire package off the SL8 and stuck them in my wife’s S-Works Tarmac SL6 (we both ride a 54cm). The SL6 is a pretty smooth riding race bike, yet in this apples-to-apples comparison, the SL8 is superior.

I then reread my notes and reviews of race bikes I’ve previously tested to refresh my memory. And at that moment, the SL8’s smooth ride began to take shape in my mind: For this category, the SL8 does have a supple and well-damped ride, even on the 26mm wide rubber.

Size up to wider tires, and the SL8 only gets smoother and more comfortable. I confirmed this by trying the SL8 with a pair of 28mm Specialized Turbo RapidAir tires (the same model as the stock tires), and, big surprise: A wider tire at lower pressures made the SL8 even smoother. If you bump up to a 30 or 32-mm tire, the SL8 will only get floatier.

The ride quality is extra impressive when you factor in the SL8’s weight and responsiveness. With the Specialized computer mount (but without cages or pedals), my size 54cm Shimano Dura Ace-equipped model alighted my scale at 6.7 kilograms (14.8 pounds). And that includes a dual-sided power meter, 60mm deep rims, and converted to tubeless with a generous helping of sealant.

Specialized paired a low bike weight with impressive frame stiffness. The SL8 is sharp and explosive in ways that would exhaust a thesaurus. But is that a surprise anymore?

It’s 2023—we’re going on 37 years since the carbon-tubed Look KG 86 first poked its composite nose into the Tour de France peloton. And it’s 24 years since that Texan was the first to “win” (and then un-win) the Tour riding a carbon bike for the whole event. Sure, brands are still learning how to maximize the potential of carbon fiber composites, but they also have literal decades of experience with the material.

So, it shouldn’t be surprising that Specialized—a large and experienced company that also prioritizes racing and pours tons of resources into it—drops a bike that is as light, stiff, and smooth as the SL8.

Even so, my ancient and jaded ass still finds it impressive how light, stiff, and smooth the SL8 is on the road. The bike brings grins to my face whether I’m suffering on a climb, bombing a descent, trying to hold onto a fast group, or flicking it through corners.

The SL8 carries on with the same geometry as the SL7 and many earlier generations of the Tarmac. It seems safe to call it “proven” after years of success on the sales floor and the highest levels of racing. However, I should point out that modern race bike geometry is very homogenous. There is little in the SL8’s numbers that vary from the geometry of other brands’ race bikes.

Numbers are just one part of a bike’s handling—Frame stiffness and compliance play a massive role in how a bicycle moves on the road. And it is here where the Tarmac’s magic lives. I’ve sampled many other bikes with geometry similar to the SL8’s, but this bike is special.

The SL8 is both reactive and stable to levels that are difficult to wrap my head around. It initiates a turn smoothly and holds a precise line through turns smooth and rough, and it feels explosive when called upon but serene in less intense moments.

I know I spent a ton of time dissecting the aerodynamics of this bike, but even if its aero is ultimately more beneficial to pros than us normies, the way the SL8 reacts, climbs, handles, and floats down the road makes it a masterpiece of a bike.

But there’s no denying it is a race bike and all that comes with it. If you want a light and responsive bike with a comfortable ride, the Specialized Aethos is a smoother bike (and 100 grams lighter) than the SL8. The Aethos is also far easier to live with as it has traditional brake hose routing, takes a standard bar and stem, and uses a round 27.2mm post. And there are growing numbers of high-performance road/all-road bikes that fit 34, 36, and even 40mm tires that are more comfortable and far more versatile than the Tramac SL8.

However, those all-road bikes aren’t racing bikes, and most don’t have the aerodynamic advantages of the SL8. Sure, the aero benefits of the SL8 are most suited to the fastest riders, but they’re still beneficial to the rest of us because they save watts and provide more speed. In 2023, aerodynamics matter so much that if you’re racing (or even trying to get Strava PRs), a bike can’t be considered a “serious race bike” without some aero optimization.

Indeed, there are good reasons for most of us who don’t race seriously or for a living to choose a more practical and versatile bike than the Tarmac SL8. But racing bikes are special because nothing moves and feels like a race bike. And on nearly perfect road surfaces, there may be no faster and more enjoyable bike than the Tarmac SL8.

A gear editor for his entire career, Matt’s journey to becoming a leading cycling tech journalist started in 1995, and he’s been at it ever since; likely riding more cycling equipment than anyone on the planet along the way. Previous to his time with Bicycling, Matt worked in bike shops as a service manager, mechanic, and sales person. Based in Durango, Colorado, he enjoys riding and testing any and all kinds of bikes, so you’re just as likely to see him on a road bike dressed in Lycra at a Tuesday night worlds ride as you are to find him dressed in a full face helmet and pads riding a bike park on an enduro bike. He doesn’t race often, but he’s game for anything; having entered road races, criteriums, trials competitions, dual slalom, downhill races, enduros, stage races, short track, time trials, and gran fondos. Next up on his to-do list: a multi day bikepacking trip, and an e-bike race.

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