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by Matt Wikstrom
September 1, 2015
Photography by Matt Wikstrom
While the pro peloton is only just starting to experiment with disc brakes, consumers have had access to road disc bikes for a few years. Specialized was an early adopter however a pure-bred race bike with disc brakes was missing from their catalogue until recently. In this review, CTech editor Matt Wikstrom compares the company’s first road disc bike, the Roubaix SL4, with the new S-Works Tarmac.
Over the last few years, road disc technology has been slowly emerging but it has one of the few instances where consumers have been able to experiment with the latest technology long before professional riders. Indeed, road disc bikes have been in the mass market for a few years now, albeit in small numbers compared to the range of road bikes with rim brakes.
For many riders, including the professionals, there is still a big question about the value of disc brakes. However, there’s no point in revisiting the arguments because the market has already embraced them. Whether disc brakes completely overhaul road bike design remains to be seen, but one look at MTB bikes suggest that is likely. Moreover, history has repeatedly demonstrated that the conservative attitude of road riders can be overturned: witness the success of lycra shorts, clipless pedals and carbon fibre frames, to name a few.
Specialized created its first road disc bike when it overhauled the Roubaix for 2013. Two disc models were included with the release of the Roubaix SL4 and the number of options has grown steadily such that there are now six models, including a high-end S-Works version.
Specialized signalled its ongoing commitment to road disc brakes when re-designing the Tarmac for 2014. Three disc-equipped models were created as part of the new range, including an S-Works version, a clear show of faith that the company sees a future for disc brakes on race bikes.
Long-time readers of CyclingTips may recall Wade’s initial look at Specialized’s road bike range, followed by separate reviews of the Roubaix SL4 and the new Tarmac. For this review, I take a close look at the disc brake versions of the Roubaix and the new Tarmac and compare them to learn more about their relative strengths and weaknesses.
The Roubaix and Tarmac have both served in Specialized’s road catalogue for over ten years and been refined on multiple occasions. As mentioned above, the current iteration for the Roubaix, the SL4, was introduced for 2013 while the new Tarmac was unveiled last year.
In both instances, Specialized marshalled the new designs in with particular emphasis on size-specific engineering for each frame size. For the Roubaix, this meant variation in some tube diameters along with fork crown bearing sizes that increased with the size of the frame.
A more sophisticated approach was taken for the new Tarmac. Specialized was inspired by their collaboration with McLaren engineers to precisely define which aspects of the frame deserved attention in order to achieve uniform performance across six frame sizes. According to Specialized, they found two aspects were critical: one, the steering response of the bike, and two, stiffness of the rear triangle.
Specialized went on to define optimal values for the stiffness of the rear triangle and the steering response for every frame size. No changes were made to the frame geometry, so this so-called “rider-first engineering” specifically addressed the behaviour of the frameset under load. Whether or not this approach is any different from what other manufacturers (including custom frame builders) have been doing will probably never be determined, but Specialized is adamant that it has improved the performance of the new Tarmac. Indeed, Wade was able to appreciate some improvement compared to the SL4, however they were relatively minor for the frame size that he rides.
The new Tarmac and Roubaix SL4 share a variety of common features. For example, the size of the fork crown bearing increases with frame size for the new Tarmac just like it does for the Roubaix SL4. The bikes also share the same bottom bracket format (Specialized’s OSBB), seatpost diameter (27.2mm), and provide internal routing for the rear brake cable/hose and the gear cables/wires.
From its inception, the Roubaix SL4 Disc has utilised standard quick-release axles for the wheels and post mounts for the disc calipers. The Tarmac Disc follows the same suit, and both bikes have been designed around 140mm disc rotors.
Specialized utilises different grades of carbon fibre for the various models of each bike. The S-Works version of the new Tarmac and Roubaix SL4 are both made from the company’s highest modulus carbon fibre dubbed FACT 11r. FACT 10r is used for Pro and Expert level bikes, while 9r is used for Tarmac Sport, Elite and Comp. A slightly heavier grade, FACT 8r, is used for the Roubaix Sport, Elite and Comp level bikes. According to Specialized, each step in the series offers a weight saving of 150-200g for the frame and provides a “stiffer, more nimble ride”.
There are, of course, some profound differences between the two bikes, the most obvious of which is the incorporation of Zertz dampers into the chainstays and fork legs of the Roubaix SL4. Made from a “viscoelastic” material, the inserts are designed to absorb vibrations travelling through the bike to protect the rider. In contrast, the new Tarmac relies solely on the layup for damping vibrations, and as such, the rider is more susceptible to road buzz and chatter.
The Roubaix uses a simple, traditional seatpost clamp that every rider will be familiar with, while the new Tarmac has an internal clamp. The design is very tidy, but as Wade noted in his initial review, once the post is removed, the clamp will fall out.
The differences between the Tarmac and the Roubaix extend to the geometry of the each bike, starting with the angle and length of the head tube, as shown in the tables below:
With a taller head tube and slower steering, the geometry of the Roubaix SL4 is not as aggressive as the Tarmac. A close look at a detailed geometry chart for the Roubaix SL4 reveals that the bike has more fork rake, longer chainstays, and a longer wheelbase than the Tarmac at every frame size, with smaller differences in steering rake and bottom bracket height.
For this review, Specialized Australia provided an S-Works Tarmac Di2 Disc and a Roubaix SL4 Pro Disc Race Di2 from the 2015 catalogue. As mentioned above, the S-Works Tarmac is made from Specialized’s highest grade carbon fibre, FACT 11r, compared to 10r for the Roubaix SL4 Pro, while there are further differences in the parts specified for each bike.
The Tarmac was equipped with Shimano’s Dura Ace Di2 derailleurs and R785 hydraulic levers and disc calipers along with a Dura Ace chain and cassette (11-28). The remainder of the build is furnished by high-end parts from Specialized: S-Works carbon cranks (52/36 chainrings), Roval Rapide CLX40 SCS Disc wheels with 24mm S-Works Turbo tyres, S-Works SL alloy stem and carbon handlebars, FACT carbon seatpost and a Toupé Pro saddle with carbon rails. Interestingly, the cranks and wheels utilise ceramic bearings from Ceramic Speed.
The Roubaix SL4 Pro Disc Race Di2 is equipped with the same levers, brake calipers and wheels as the S-Works Tarmac (albeit without Ceramic Speed bearings) however the rest of the build features mid-level parts starting with Shimano’s Ultegra Di2 derailleurs, chain and cassette (11-28). Specialized supplies its carbon Pro FACT cranks (50/34 chainrings), Pro SL alloy stem and Expert alloy bars, carbon CG-R seatpost, 25mm Turbo Pro tyres, and a Toupé Expert Gel saddle with hollow titanium rails to complete the build.
There is just one colour choice for each of the bikes. Specialized opted for a stealthy aesthetic for the S-Works Tarmac with a matte black finish and black parts. The Roubaix SL4 Pro Disc goes in the opposite direction with a full gloss white frameset and black parts. With the logos largely camouflaged, the bikes will appeal to buyers that prefer an understated finish, where the eye is free to roam and discover the finer details.
The Roubaix SL4 Pro Disc Race Di2 retails for $7,999 while the S-Works Tarmac Di2 Disc retails for $11,999. The size 54 S-Works Tarmac sent for review weighed 7.00kg (sans pedals and cages) compared to 7.80kg for the Roubaix SL4 Pro (also size 54). For more information on either bike, including a look at the new colours and specifications for 2016 models, visit Specialized.
At face value, the Tarmac and Roubaix appear quite distinct, and the marketing language that surrounds each supports this notion. But is this something that is immediately evident out on the road? The answer is largely yes, but it depends upon the terrain and the effort of the rider.
For example, while I was riding a smooth stretch of road at a steady pace, I found the two bikes provided the same kind of sensations and ride quality. Neither telegraphed much road buzz, and both offered the same kind of efficiency. At this point, the only difference between the Tarmac and Roubaix was the number of spacers under the stem.
Venturing onto chip-seal roads didn’t offer much contrast either, though I was impressed with how smooth the Tarmac continued to be. The bike almost glided over the rougher surface, though back-to-back comparison with the Roubaix revealed that there was a little more edge to the sensations arriving at the handlebars and saddle.
That all changed once I hit truly terrible roads, the kind that have cracks running through them. Under those circumstances, the Tarmac was completely overwhelmed and I had to contend with a lot of vibrations as well as the sound the valve stems vibrating in the wheels and the levers rattling on the handlebars. In contrast, the Roubaix was relatively unperturbed—my hands and arms were still shaking to some degree—but the magnitude of the vibrations was diminished.
Swapping the wheels (to test the influence of the different tyres) and seatposts between the bikes had a mild effect on the ride quality of each. Starting with the posts, Specialized claims that the CG-R seatpost that is supplied with the Roubaix SL4 offers 18mm of vertical compliance but it doesn’t bounce around like other suspension seatposts.
Indeed, I found that the CG-R post was quite firm, and while it did a little to soak up bumps, cracks and ruts, it wasn’t enough to transform the Tarmac into a cobble-gobbling rig. Likewise, the Roubaix continued to offer much of the same ride when fitted with the post from the Tarmac. As for the tyres, the influence on each bike was nearly negligible, which is not surprising given the relative minor differences between them (eg. the Tarmac had 24mm tyres compared to 25mm for the Roubaix).
Specialized promises that the Tarmac is a race bike and once I started to ride the bike aggressively, it was ready to shine. Every time I got out of the saddle, the Tarmac was obviously stiffer and more responsive than the Roubaix, and I revelled in the sensation when sprinting or attacking a climb.
The Roubaix, by contrast, was a capable performer, but it lacked the same kind of edge in performance that the Tarmac had suffered on rough roads. Thus, I was able to sprint and attack climbs with the Roubaix, but the bike wasn’t as energetic or agile as the Tarmac.
The Tarmac had quicker steering than the Roubaix, so it was much easier to negotiate sharp corners at high speeds. Furthermore, I was able to hold a tighter line on the Tarmac than the Roubaix. Riders that enjoy criterium racing will therefore be served better by the Tarmac than the Roubaix.
Both bikes were stable and well composed under most circumstances. The Roubaix had an edge though, because it was more surefooted than the Tarmac, which I presume was due to the longer chainstays and wheelbase. I can’t say that I felt any extra confidence on the Roubaix but it was an easygoing bike compared to the Tarmac.
On long rides, I found myself really preferring the qualities of the Roubaix. Yes, it was less taxing to ride than the Tarmac—I never suffered the same kind of fatigue in my back or shoulders—but there was a measure of luxury in the easygoing nature of the bike. Another way to describe the ride quality of the Roubaix might be “contemplative” compared to the aggressive agility of the Tarmac.
I’ve reviewed Shimano’s R785 hydraulic disc brakes previously and they continued to perform admirably on this occasion. More so, in fact, since the bikes had already done some miles, which meant the pads/discs had been run in and therefore working at their best.
My time on these bikes proved once again that disc brakes are the perfect complement for carbon clinchers. After all, there’s no need to worry about harmful heat build-up in the rims when braking hard on technical descents. And braking remains smooth, light and incredibly effective in all conditions.
There was no discernible difference in the performance of the Di2 transmission on each bike. Part of this reason was that they shared the same integrated brake/shifter levers, but with all the work being done by motors rather than the rider, there is no difference in the feedback from each system. Of course, the Dura Ace Di2 transmission weighs less than Ultegra Di2 so buyers need only decide where they want to make a saving: on the bike or in their wallets.
The same rationale could also be applied to the rest of the build for each bike. With 800g separating the two bikes, there is the prospect that buyers will only enjoy marginal gains despite paying thousands more for the higher-end build such as I’ve found for Scott’s Addict and Trek’s Émonda.
If cycling is the new golf, then the Tarmac and Roubaix represent two very different clubs that could go into your bag. Of course it’s not necessary to have more than one bike to enjoy the sport, but the analogy serves to illustrate the distinction between the two bikes. The Tarmac is sharply focussed on race performance, especially the S-Works version, which is as light and stiff as Specialized can manage. In contrast, the Roubaix is more adventuresome with much more focus on the comfort of the rider.
I don’t expect the quarrel over disc brakes is about to subside, but it’s interesting to note that Specialized appears equally committed to rim and disc brakes. Thus, the S-Works Tarmac is currently available with rim or disc brakes, where the builds are essentially identical except for the braking systems. The same applies for the Roubaix SL4 Pro, at least for 2015, but a look at the 2016 catalogue shows that rim brakes have been relegated to entry- and mid-level models.
It was easy for me to forget about the disc brakes while I was riding the Roubaix and the Tarmac. Even at this early stage of development, road discs have been integrated seamlessly into current bike design. Of course, one look down at the front hub was enough to refresh my awareness, but it was always short-lived. In short, the new braking system is not difficult to live with, just a little different.
With the recent news that the professionals have started trialing the use of disc brakes in competition, it is interesting to note that not all bike sponsors have disc-equipped race bikes. However, the UCI is not expecting an official introduction for disc brakes until 2017, perhaps in recognition of the work that is ahead for some manufacturers.
For a manufacturer like Specialized that was early to enter the road disc market, they may be well placed to pioneer new disc brake designs in the coming years. Since current brakes are essentially re-purposed designs taken from MTB, it will be interesting to see how they will be refined to better suit road use and/or racing.