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Castelli Nano Flex 2 Review

These are good shorts to have around for early spring base-building when it’s still damp and chilly
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castelli nano flex 2 bike short review
Credit: Competitive Cyclist
Price:  $100 List
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Manufacturer:   Castelli
By Ryan Baham ⋅ Review Editor  ⋅  Jul 23, 2019
  • Padding and Protection - 25% 7.0
  • Comfort and Fit - 20% 7.0
  • Breathability - 15% 7.0
  • Efficiency and Pedal Friendliness - 15% 6.0
  • Durability - 15% 7.0
  • Style - 10% 7.0

Our Verdict

Castelli discontinued this variation of the Nano Flex bib.
Fairly water repellent
Can be too restrictive
Run small
First things first: these run small. Leaner, thinner guys will have a little more success with the standard sizes. That's partially owing to the high nylon content, but Castelli cuts also tend to be a bit smaller and don't consider "corn-fed" to be medium for some reason known only to the Italians. Once you get the sizing right, these are great for spring when you get a bit of chilliness and a bit of extra moisture. The supportive, hydrophobic material helps keep you dry and in-form while you're rebuilding the base, with enough chamois padding to get you through the longer base mile days.

Our Analysis and Test Results

Performance Comparison

We spent weeks researching and testing while comparing them to other top models on the market. Read on to see how the Castelli performs.


Maybe we shouldn't be surprised that in the land of Raphael, da Vinci, Armani, Versace (and let's not keep going), Castelli created shorts that are sexy even in plain black. It'd be nice if they had some flair to them, but their panel design makes it difficult to add colored panels since the entire bottom is made of a single piece of Nano Flex fabric. If you're happy pretending to be K.I.T.T. or Michael Knight, you'll be pretty content with them. If you're after a little more color, you might look to the Gore C5 Bib Shorts+, which have a handful of options, including the Knight Rider look.

castelli nano flex 2 bike short review - even in standard black, castelli puts out really stylish stuff.
Even in standard black, Castelli puts out really stylish stuff.
Credit: Tara Kharrazian


The Castelli walk the line on this one. The shorts are made of a proprietary Nano Flex fabric, which is highly water repellent, helping to keep environmental moisture out and pull sweat off the skin, which is obviously super desirable. The inner lining of the Nano Flex fabric is also brushed, giving it a felt-like quality. But here's where it walks the line. That layer is something of an insulator, so they're pleasantly warm in chilly weather and less pleasantly warm in direct sun.

It's also worth noting that they have a high nylon mix, 86%, which might contribute to moisture and heat retention. Don't get us wrong, they're awesome if you roll through a quick shower and the sun pops back out or if it's just a damp, misty morning, but once they're inundated, they do get that waterlogged feeling.

castelli nano flex 2 bike short review - castelli's nano flex fabric does an excellent job of keeping water...
Castelli's Nano Flex fabric does an excellent job of keeping water out of your shorts and off your skin.
Credit: Tara Kharrazian

If temperature and blow-through are higher up on your requirements list, you might want to look at the Gore C5 Bib Shorts+, our Top Pick for Short Course winner, or the Editors' Choice winning Assos T Equipe Evo.

Padding and Protection

The NanoFlex 2s use the KISS Air2 chamois pad. It has a few features that help it stand out above most of the competition. The first is its small footprint. It has a more targeted, anatomical shape that focuses relief on the high-pressure spots, so you don't have that bunched up cloth problem with your padding - it makes a big difference for those of us with thunder thighs.

The actual pad is high-density foam with the thickest sections of foam in the ischial region, modest thickness along each outer edges of the perineum (thankfully none in the center), and thickness tapers off as move out from urgent, high-pressure zones. It wouldn't be complete without a top sheet that you wish could be your pillowcase. The cover feels something like a cross between fleece and velvet, and it's pretty darn nice.

castelli nano flex 2 bike short review - even when your tail is getting torn up on roads that have turned...
Even when your tail is getting torn up on roads that have turned into cobblestone, the NanoFlex 2's KISS Air2 padding is there to take the pounding for you.
Credit: Tara Kharrazian

All things considered, they're solid for most rides. And, as always, lighter riders will be fine in these for just about any pursuit. Heavier guys may require a little more padding and a size up. If you are looking for a bit more padding, we suggest trying the Louis Garneau CB Carbon 2. If you don't necessarily need more padding, but find yourself fighting your shorts and adjusting a lot, the Assos T Equipe Evo design is nearly unbeatable, and it's worth every rider's attention.

Comfort and Fit

The Castelli has a few things going for them in the comfort department. The Nano Flex fabric is fairly water repellent, and the KISS Air2 chamois is well-ventilated, so they do a good job of keeping you dry. The fabric is also brushed on the inside, so it has a great fleece or felt-like quality to it. It's also nice and warm, so it's a great choice for cool weather. The leg grippers are also a good design, using shark skin-style grippers to limit sliding, but they do tend to slide up after as the sweat gets worked up.

On the fit front, they have a close, tight fit and they're about one size small by American standards. Somewhat exasperating that sizing is that the Nano Flex fabric is 86% nylon, so it has pretty limited stretch, despite the name. That situation understandably improves with proper sizing (medium Americans would do better in a large).

castelli nano flex 2 bike short review - the nanoflex 2 leg grippers feel like very fine shark skin. it's an...
The NanoFlex 2 leg grippers feel like very fine shark skin. It's an effective, comfortable design.
Credit: Ryan Baham

While we liked the NanoFlex 2s, we felt there were a few other models that offered more comfort. For another tight-fitting training model, take a look at the Top Pick for Short Course winner, the Gore bib shorts. For premium cycling shorts that won't disappoint, take a look at the Assos T Equipe Evo bib shorts.

castelli nano flex 2 bike short review - when you get the sizing right, the nanoflex 2s are pretty well...
When you get the sizing right, the NanoFlex 2s are pretty well suited to those early spring base miles.
Credit: Tara Kharrazian

Efficiency and Pedal Friendliness

While "Flex" is in their name, the Castelli NanoFlex 2s have limited flexibility. That's probably because of their 86% mix of nylon and tight cut. As with other high-nylon, athletic models like the Gore C5s, there's a degree of desirable compression there, but it can interfere with the ease of pedaling. That's a good deal farther down the spectrum of resistance with these shorts. The good side of that is that there's no concern about excess fabric bunching up around either the moving parts or the sitting parts, so that improves comfort and reduces the potential for fabric to get caught on the nose of the saddle.

Shorts that did better here tended to be both tight and flexible. The excellent SUGOi Evolution Pro Bib Shorts meet that definition, and they're a great place to look for efficient, pedal-friendly shorts that won't get in the way. The ultimate shorts in this category are the Editors' Choice Assos T Equipe Evo, which even has a special chamois that moves and flexes with the body, removing the problem of chunky, meddling chamois padding.


They use tough nylon as an unusually high basis of their Nano Flex fabric, so you can be sure that the fabric will be a lot more resistant to fraying, tears, and other degradation. They also minimize seams to improve comfort, which has the added benefit of reducing weak points where stitching could come undone or loosen. The only real limitation we found was that the nylon can actually be too inflexible, leading them to lose their ability to retract over time - especially the leg grippers. The standard mesh shoulder strap and upper design also tends to develop tears and fray sooner than is ideal.

If you're looking for something that might last a bit longer, try the slightly more flexible Gore C5s. If you like the extreme tightness and compression of the thicker fabric, take a look at the Pearl Izumi Elite In-R-Cool Bib Shorts.

castelli nano flex 2 bike short review - the castellis' mesh upper design is pretty common in bibs, but it...
The Castellis' mesh upper design is pretty common in bibs, but it can also fray and develop tears quickly, especially if your bibs are extra tight.
Credit: Ryan Baham

Best Applications

The Castelli is well-rounded and just fine for any type of riding. However, we preferred them both on shorter rides and chilly misty mornings when their insulating inner layer helped to keep us just a bit warmer, and the water repellent Nano Flex fabric helped to keep us just a little dryer.


Ninety-nine dollars is the right starting point for these bib shorts - find a good discount, and you've got yourself a good set of spring training or spin shorts.


These were great to have around on damp, cool mornings when we didn't want to get our premium shorts muddy and nasty - or deal with the constantly soaked feeling. Their affordability makes them an easy, accessible option for most riders. We suggest trying a size up if you're American-sized. They're also really tight with their high nylon mix, so riders wanting a little more support or compression will appreciate that aspect. For guys looking for something with a little more freedom of movement, you might try looking at the Gore C5 Bib Shorts+. If you're after a bit more comfort, take a look at the Assos T Equipe Evo.

castelli nano flex 2 bike short review - the castelli nanoflex 2s can be seen here powering josh bonnici...
The Castelli NanoFlex 2s can be seen here powering Josh Bonnici through a tough segment.
Credit: Tara Kharrazian

Ryan Baham

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