Pirelli P-Zero SL-R test: The Italian manufacturer calls its new concept "PAAS," which is designed to aerodynamically merge the rim and tire into a single unit. This is complemented by an optimized carcass construction, which significantly reduces rolling resistance without making the tire too fragile.
Anyone currently involved with road bike technology knows: it's all about aerodynamics. The absolute fixation on weight is over – modern racing bikes are still light, but that's no longer the most important thing. In an era where even heavy classic bikes are ridden at around 45 km/h, it's clear that professional cyclists are trying to extract every last watt. And, as usual, this trend continues through the lower ranks and down to amateur cycling.
Numerous components on racing bikes have now been aerodynamically optimized, and tires are no exception. This seems only logical, since the front tire is exposed to the wind. Various manufacturers have therefore focused on developing tire profiles that promise an aerodynamic advantage. However, Pirelli recently introduced the Pirelli P-Zero SL-R, a racing tire that takes a completely different approach.
What's so special about the Pirelli P-Zero SL-R, which the manufacturer claims has 20% better aerodynamics than its previous top model, saving up to 10 watts in strong winds? Pirelli has actually taken a step back in terms of the tread pattern, designing the SL-R as a slick with a completely smooth contact patch. Its unique feature lies in the special shape of the sidewall, specifically the transition between the tire and the rim. This transition isn't a tight fit; instead, the tire is constricted by the rim, creating a U-shaped gap between the two components. Furthermore, the tire is usually wider than the rim.
Pirelli P-Zero SL-R: Faster with aerodynamic sidewalls
Both of these configurations are aerodynamically unfavorable, as they prevent the airflow from flowing smoothly around the tire and rim. This is crucial, because if airflow separation occurs, the advantage of a deep rim profile (keyword: "sail effect") can be largely negated. Parallel tire sidewalls appear to be optimal, with the tire being slightly narrower overall than the rim. Pirelli achieved the former with the P-Zero SL-R by adding rubber to the sides. The widest point of the tire is thus located just above the rim edge, whereas it is considerably higher on a conventional tire.
The manufacturer calls this design, currently unique, the "Pirelli Advanced Aerodynamic System" (PAAS), and it certainly fulfills its purpose of creating a smoother transition between tire and rim. This is clearly visible on the Zipp 303 SW, whose 33 mm wide rim features 4 mm wide, flattened rim flanges. With a conventional Pirelli tire, these surfaces are clearly visible; the sidewall of the P-Zero SL-R, on the other hand, almost completely covers them.
This is partly due to the fact that the nominally 30 mm wide tire expands to a full 32,5 mm thanks to the 25 mm internal width of the Zipp rim. The manufacturer is, of course, aware of this, and so the SL-R box includes width measurements for rims with internal widths of 21, 23, and 25 mm. On the 25c Zipp rim, the Pirelli tire takes on a shape that also has a positive effect on rolling resistance.
The unusual shape of the new racing tire is also noticeable on a conventionally shaped carbon rim – here a Hunt model with a 22 mm internal width and a 30 mm external width. The P-Zero SL-R should therefore have the potential to provide slightly improved aerodynamics, regardless of the wheelset used. The rims should be as wide as possible to harmonize optimally with the tire, which is available in 28 and 30 mm widths. According to Pirelli, a 32 mm wide SL-R is also in development.
Easy assembly even when new.
Mounting is straightforward on both rims. It's noticeable that the PAAS tire pops into position in the rim bed with an extremely sharp pop when inflated – on both hookless and hooked rims. A tire lever is needed for initial mounting; a compressor isn't required. The pre-stretched tire can easily be pushed over the rim edge with your thumbs, but it won't fully inflate with a floor pump.
Pirelli has also refined the tire's construction, presenting a new carcass design with three layers of fabric on the sidewall and two under the tread. One layer is wrapped around the bead core from the inside out and folded upwards. In contrast, the previous top model, the Pirelli P Zero RS, had three carcass layers under the tread and a two-layer sidewall, plus an airtight layer on the inside.
What's the point? Reducing the number of fabric layers under the tread reduces rolling resistance, and it works. According to Pirelli, the SL-R rolls 10% more easily than the RS; the measurements from the Bicyclerollingresistance.com website are even more conclusive: at high tire pressure, they measured 8,4 watts versus 10,5 watts, representing a 20% reduction in rolling resistance. The three-ply sidewall, in turn, provides stability at low pressure, enabling dynamic cornering. The "SmartEvo 2" rubber compound, which Pirelli has continued from older models, also contributes to this with its high level of grip.
Noticeably reduced rolling resistance and very high cornering grip
What aspects of the new tire construction are noticeable in practice? When it comes to rolling resistance, the situation is relatively clear: In contrast to Pirelli's solid standard tire, the P Zero Race TLR , which absorbs 5 watts more (i.e., 10 watts for both tires), the Pirelli P-Zero SL-R definitely feels faster. This "aha moment" is, of course, temporary, as you get used to the improved performance; however, when switching back to the old tire, cycling initially feels somewhat more sluggish.
The aerodynamics are a more complex issue. Wind tunnel measurements are usually based on 45 km/h, and outside of competitive racing, hardly anyone maintains that speed for extended periods. Unless, of course, you're going downhill – and those who let the bike roll can certainly achieve new personal bests without changing anything other than the tires. This applies even to twisty roads, as the Pirellis offer secure grip even at extreme lean angles and provide clear feedback with a distinct noise when approaching the limit of their traction. And last but not least, you get the feeling that the Pirelli reduces negative wind effects on the front wheel – it feels noticeably more stable at high speeds on descents.
Low weight considering its actual width
At 290 grams, the new Pirelli is light for a 30-series tire, especially one that actually measures up to 32,5 mm. The downside is that material is naturally missing somewhere, namely where it's most needed for puncture protection and durability. However, the lack of a puncture protection layer didn't have any negative impact during the approximately 600 kilometers of testing so far. Even the occasional gravel section and poor asphalt with potholes and small stones didn't cause any damage; even on the rear tire, the fine center seam was still just barely visible after this mileage.
Enough rubber for many kilometers
According to Pirelli, the thickness of the rubber layer in the center of the tread is 1,2 to 1,3 mm, which is quite substantial – wear indicators on racing tires are typically 1 mm deep, and it can take thousands of kilometers for them to disappear. The SL-R can therefore certainly be expected to have a long lifespan. However, Pirelli has omitted these wear indicators on this model. How many kilometers the tire will last remains to be seen; what is clear is that it is designed more as a competition tire or should be reserved for special occasions. This is due, in no small part, to the high price of €104,90; however, Pirelli links to online retailers on its product page who sell the tire starting at €67.
The end result is a very fast, supple tire whose unique sidewall suggests a certain improvement in aerodynamics. The racing model doesn't feel fragile, making the Pirelli P-Zero SL-R perfectly suitable for everyday use. Long training sessions on poor roads can always be done on a more affordable tire.