Italian brand Castelli has been providing kit for pro road teams for decades and offers various product tiers from amateur or training gear, right up to top-level World Tour outfits. Yes it’s a piece of roadie kit, but with so much research going into this area of cycling clothing I reckon they beat mountain bike base layers hands down. In my experience the best cycling base layers are actually made for the road.

The Prosecco falls into Castelli’s highest end kit for the pros called Rosso Corsa, where “Every thread and fiber, every curve and contour is obsessed about,” it says. The range uses various fabrics and thicknesses to cover a wide spectrum of base layer usage scenarios at any temperature or workout intensity, and also whether they optimise insulation, wicking sweat or airflow. This Prosecco SS targets cooler conditions of 10-20°, but as it wicks so well, I found it can also handle much warmer temperatures just fine.

Castelli’s top of the range Rosso Corsa line is used by pro roadies… and now switched on mountain bikers too

Design and specifications

The Prosecco is well finished with double or triple layer stitching everywhere and a soft band lining the neck. There’s a fairly wide crew neck opening at the throat to fit under jackets or jerseys without showing and two different types of fibre thread-count/weave in different zones.

The main body (around 75% of the piece) uses a 100% polyester main fabric with Castelli’s special rib knit weave where fine ridges in the material loft fibres slightly off skin. The ribbing helps trap air closer to your skin for optimum temperature regulation, keeping you warmer for less weight. The polyester fibres its made from have a hydropillic treatment to suck moisture along individual strands and towards the outer face of the fabric, this helps it evaporate and dry faster.

The thinner hem material is to avoid your clothing getting too thick where it doubles up under a pair of bib shorts… if you wear them

Two pannels of thinner, almost transparent and gauze like PrimaLoft fabric cover the waist and lower back. It’s made like this because bib shorts overlap the base layer right at this spot, and Castelli has made it so riders don’t get too much material here. This way, the Prosecco aims to balance out rider insulation over the whole torso. The design makes sense for roadies that almost exclusively wear bib shorts, but obviously not all MTBers do.

Performance

Ultra-soft against skin and tight-fitting, but with enough stretch to never feel restrictive or constricting, the Prosecco Tech SS is super comfy and cosy. The fit feels more MTB than most roadie kit, Castelli’s sleeves are long enough not to ride up and the wider neck means you can wear it under a regular riding top without it showing at the collar.

Slonger sleeves, a wider neck and roomy uppershoulder area make the Prosecco ideal for mountain bikers

I found lots of other road base layers a bit too tight around the shoulders, probably because most MTBers tend to have bigger shoulders and lats than road cyclists who sit static for hours pedalling. But the totally unrestrictive fit here is a good start and the Prosecco manages to feel like it has tons of room and freedom of movement without being baggy.

The main material does a great job of insulating and seems to loft just the right amount of air close to skin without being drafty. It’s warm next to skin to keep you toasty when you stop for a rest on a cooler day, but not very windproof, so does let a healthy bit of of breeze through when moving if you wear a very airy riding top over it.

Once you’re working harder, it’s no exaggeration to say that the main Prosecco fabric is sensational at wicking moisture away from skin onto its outer face. In this aspect of shifting moisture from inside, it’s simply the most effective base layer like this I’ve ever used and you can see it in action as the outer face kind of beads and steams a bit like a waterproof jacket while the inside remains dry to touch, even if your skin is really leaking.

The lofted weave is what makes the Prosecco such a quality garment, it wicks wet as fast as you can make it

The inside seems to achieve this by being slightly lofted off skin by fine tufts in fabric weave and also presumably by Castelli’s fabric tech working well and allowing moisture to transfer along strands. The Prosecco also dries so fast that it trounces many other merino tops I use that feel kind of similar and also have excellent wicking properties. Castelli’s top dries so much faster in fact, that, as long as you’re rolling along it pretty much stays dry, no matter how much of a sweaty mess you are.

The one caveat to all this is that the thinner PrimaLoft panels aren’t half as effective at either wicking or drying as fast. This means you often have a sweaty, saturated panel of fabric at the small of the back if you perspire a lot while riding, and I finished multiple rides with the main bulk of the top bone dry and lower edges a bit clammy and saturated.

If Castelli could make the Prosecco SS base layer without the thinner material at the back I’d be giving it full marks

For me, Castelli needs to make a version with its proprietary ‘thicker’ weave throughout as it will actually keep you cooler and more comfortable by not getting clammy and wetted out like the PrimaLoft sections do. I get the brand’s logic of using a finer weave and that it might be cooler for riders that don’t perspire as much, but the thinner weave just doesn’t seem to transfer as much air or be as capable of transferring moisture from inside to out, even though it’s much thinner.