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The SL8 frame weighs just 723 grams - a mere whiff of nothing for a racing bike that has to perform under all circumstances. Downhill in the hands of speeding downhill riders, in the race finale under the kicks of the strongest sprinters, in the spring classics under the hammering blows of the cobblestones. The SL8 easily reaches the minimum weight of 6.8 kilograms prescribed for professional bikes when ready to race; however, it is not only particularly light, but also achieves its advantage because it combines low weight with good aerodynamics (209 watts for 45 km/h in the TOUR wind tunnel measurement). It is not the most aerodynamic bike that TOUR has ever tested, but it is better than the other lightweight all-rounders and not far off the specialists.
The team is the star
Aerodynamic optimisation generally results in more weight due to flatter frame shapes. Keeping this impact as low as possible is key to the SL8's outstanding overall performance. Specialized estimates the impact of the distinctive nose in front of the head tube, one of the few noticeable aero features, at 25 grams. Other parts of the frame must have been cut to make it lighter overall. At the same time, the frame has become more torsionally rigid. How did the engineers do this? Do they use different materials than the competition? Is there a new manufacturing process? The engineer responsible, Sébastien Servet, answers in the negative. He has no other fibres than the competition and manufactures according to known principles.
It is not the special fibres, but the unique development process and his team that are responsible for achieving the ambitious goals, says Servet (see interview). Eight developers worked for three years to shape the frame, fork, handlebar-stem unit and seat post - in short, the chassis of the bike - into the best possible form for the seconds poker in professional sport. In terms of the amount of material to be moulded, there are probably few lightweight objects on earth that have taken more time to develop. Even space travellers can't keep up. Using simulations on supercomputers, the team searched for the best mix of aerodynamics, weight minimisation and tangible mechanical properties: stiff in acceleration, safe and precise, but with the right amount of compliance in the saddle.
The fact that the Specialized Tarmac SL8 looks rather unspectacular is presumably due to the development process: the solution to the classic optimisation problem of weight, aerodynamics and structural rigidity was sought from a purely technical point of view. Machines proceed without prejudice within the set limits. There is nothing baroque, no spectacular design edge, no imaginary aerofoil, except perhaps the extremely flat upper link of the integrated cockpit. The outer shape is reduced to the technical minimum - and the result is something familiar, visually close to the cut of the classic diamond frame. Fascinating! On the other hand, despite its visual similarity, the SL8 is aerodynamically much better than a round tube frame from a craftsman.
However, the formidable, direct and precise riding behaviour is based on inner values. Unlike the tubes of metal racing bikes, the structure of carbon tubes is not homogeneous. Carbon fibres give the creators endless possibilities to determine the properties of the construction from the ground up, at every point. This very wide-ranging freedom of design gives the designers a certain omnipotence and easily deceives us onlookers. After all, it is difficult to judge what a component is capable of by its external appearance alone. We have already measured racing bikes in the laboratory that achieved much lower stiffness values than the SL8 despite their much bulkier appearance. In this respect too, the bike is pure understatement.
A look inside the SL8 reveals more about the construction than the outside view. Take the bottom bracket node, for example: The inner walls of the frame are perfectly smooth and free of creases - which shows that the laminate has been compressed very well. The classic aluminium BSA thread sits on a thin carbon collar. How the touch of aluminium and the bit of carbon were combined is not clear to the observer. However, it is clear that a lot of thought has gone into minimising the weight and keeping the function practical. This detail is representative of many others - nothing about this racing bike is a coincidence. It follows the design principle of omission. Only when nothing can be left out without compromising function is the optimum form achieved.
Specialised Tarmac SL8: An honest product
There is no question that Specialized goes to enormous lengths. Beyond all the marketing, this can be proven by the facts of the TOUR test. Seen in this light, the SL8 is an honest product. Insanely expensive, but also insanely good. Do you have to have it? No. But it's great fun to ride. Knowing how much effort went into its construction makes it even more fun to ride. Because the bike represents what we strive for in sport: to ride higher, faster and further.
What else could come after the SL8? An even lighter bike? As long as the UCI leaves the limit at 6.8 kilograms, the SL9 will in all likelihood become even better, especially aerodynamically, because racers would benefit more from it.
Interview with Sébastien Servet, Senior Engineering Manager Road, Specialized Freiburg
You need the best experts.
TOUR: When did the development of the SL8 begin?
Sébastien Servet: At the end of 2019, we were excited about the SL7, which had just been completed and was not yet on sale, but had already been ridden by some pros. We met the pros Yves Lampaert and Remco Evenepoel in Morgan Hill, at the Specialized headquarters. We asked them: Are you happy with the SL7, what suits you, what would you like to see?
TOUR: What did the two of them want?
Sébastien Servet: Lower weight was the number one priority, which surprised us. Aerodynamics was also a big issue. That was the starting point for optimisation. The goals were 200 grams less than the SL7 and four watts less drag.
TOUR: Who was the driving force behind the further development?
Sébastien Servet: I was the lead engineer for the project. Peter Denk (Development engineer in Freiburg; editor's note) has a big influence. Nine years ago, Specialized gave him the opportunity to set up a team to realise planned projects, for example with the best aero and FE experts (FE stands for finite element method, simplifies strength and deformation calculations; editor's note). This has made our team very international. We couldn't have developed the SL8 without the absolute best experts in various fields. We have 17 people from nine nations in Freiburg for the development of road bikes. Some of the team work in Morgan Hill, while others are based in Asia and oversee production.
TOUR: Is the SL8 frame produced in Asia using the classic tube blow moulding method?
Sébastien Servet: Yes, at the end we bake the frames as usual. Heat is applied. Negative moulds are used. But the details, the effort we put into defining the tools and moulds, that is unique in my seventeen years of experience in the bicycle industry.
TOUR: Can the laminate be calculated on the computer, or is the prototype indispensable?
Sébastien Servet: Computer design using the finite element method can achieve a lot - if you have experience. However, predictions using software are only good if the information that goes in is correct. We used prototypes to test whether the forecast and reality match. We have to test the safety of the product in real life; we can't pass the responsibility onto the FE expert.
TOUR: Towards the end of the development process, the frame was made heavier to increase stiffness?
Sébastien Servet: That's right. That was a conscious decision. We first build as light as possible so that the structure is as efficient as possible, and then we refine it with special layers and fibres. The small iterations at the end are very important to achieve the specific stiffness targets and to make the frame more solid and reliable.
TOUR: Are there special fibres in the frame?
Sébastien Servet: Specialized is big and powerful, but not big enough to develop new fibres. We don't get special fibres, but specific blends, for example with customised layer thickness. Of course, we can also decide which fibre we use where. The special thing about Specialized is the precision with which the layup can be developed and the complexity that is achieved, both in the definition of the ply shape and in the number of fibre types.
TOUR: If you only used the most expensive fibres, would it be possible to build an even better frame?
Sébastien Servet: The most expensive fibre anywhere would not be a solution. It's the mix of design and the right fibre in the right place that makes the difference.
Technical data for the Specialised Tarmac SL8
Manufacturer information
- Price: 14,000 euros >> available here
- Frame sizes**: 44, 49, 52, 54, 56 (tested size), 58, 61
Measured values
- Weight frame/fork/headset bearing*: 723/383/58 grams
- Seat/top/head tube: 495/565/159 millimetres
- Stack/Reach/STR***: 574/389 millimetres/1.48
- Wheelbase/caster: 990/60 millimetres
Equipment
- Drivetrain/gear system: Shimano Dura-Ace (2x12; 52/36, 11-30 t.)
- Brakes: Shimano Dura-Ace (160/140 mm)
- Wheels/tyres (weights)****: Roval Rapide CLX II/S-Works Turbo Rapidair 2BR 26 mm (f./h.: 1,136/1,438 g)
*Weighed weights.
**Manufacturer's test size bold.
***Stack/reach projected vertical/horizontal measurement from the centre of the bottom bracket to the top of the head tube; STR (stack to reach) 1.36 means a very stretched, 1.60 an upright riding position.
****Wheel weights including tyres, cassette, quick release/threaded axles and brake discs if applicable.
