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Castelli Nanoflex knee warmers



Water-repellent warmers for autumn and spring use - and they actually work

At every product is thoroughly tested for as long as it takes to get a proper insight into how well it works. Our reviewers are experienced cyclists that we trust to be objective. While we strive to ensure that opinions expressed are backed up by facts, reviews are by their nature an informed opinion, not a definitive verdict. We don't intentionally try to break anything (except locks) but we do try to look for weak points in any design. The overall score is not just an average of the other scores: it reflects both a product's function and value – with value determined by how a product compares with items of similar spec, quality, and price.

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£37 for a pair of knee warmers? A bit steep, isn't it? Ah yes, but prepare to be surprised... Castelli's Nanoflex are water repellent so rain beads up and rolls off.

Yeah, yeah, yeah, you've heard it all before. That's what we thought too. But Castelli's NanoFlex fabric actually works.

NanoFlex is actually Castelli's Thermoflex fabric with a coating added - more on that in a bit. It's a medium-weight material, about halfway between a standard Lycra and a fleecy Roubaix, so you get a good level of warmth here. I've been using these on some summer mornings and evenings but I'm a bit reptilian; most people will find the level of insulation about right for spring and autumn use.

The NanoFlex warmers are as soft, stretchy and breathable as any others out there. Nothing unusual on any of those scores. Their real trick is that when water gets on them, rather than soaking in, it mostly just flows off.

Castelli don't claim that these are completely waterproof. Some water does get through, especially if it's hosing down, but most doesn't. I was genuinely taken aback - yep, taken aback - when I put these under the tap to check the claims. It doesn't look like it should work, but it does.

Of course, the sink test is okay as a party trick but it only counts if the fabric can repeat the performance out on the road... and it can. Road spray doesn't get in at all and neither does drizzle. Heavier rain can get through, especially in the areas that scrunch up behind your knees, but far, far less than normal. It's genuinely useful wet-weather protection, especially on this rainy old island where the next shower is never far away.

How do Castelli do it? Nanotechnology. Everything is nanotechnology these days. What does 'nanotechnology' mean? Pretty much anything a manufacturer wants it to mean, apparently, but in this case Castelli say it's a five-phase treatment that they give to their Thermoflex fabric which involves adding 'millions of tiny silicone nanofilaments'. Yup, nanofilments - little bits of silicone, if you prefer. It's all Top Secret but the important point is that it works, and many washes down the line it's still working.

The elasticated grippers, by the way, have a silicone band on the inside to hold them in place on your legs, and another on the outside to hold them to your shorts... another neat little touch.

The only not-so-cool bit is that the heat-transfer logos have started to look cracked pretty early on, although that doesn't affect the performance. The price? Well, no getting away from it they are expensive, (you can find them for under £30 online) but they do work, and if you put a lot of miles in rain or shine they do make a positive difference to your comfort on the bike which I'd is worth it.


Water-repellent warmers for autumn and spring use - and they actually work test report

Make and model: Castelli Nanoflex knee warmers

Size tested: Medium

Tell us what the product is for, and who it's aimed at. What do the manufacturers say about it? How does that compare to your own feelings about it?

It's for Lycra-clad performance types. This is what Castelli say:

"HOW DO THEY DO THAT? We always strive for total performance and total innovation, but sometimes we surprise ourselves. This NanoFlex fabric behaves just like our Thermoflex fabric in dry conditions, but then provides unbelievable water repellency when it starts raining. We don't claim that it's fully waterproof, but it's the most effective thing we've ever made for keeping you comfortable in all conditions. It's just barely short of a miracle.

Water-resistant NanoFlex fabric

Same breathability, stretch and warmth as our standard Thermoflex war mers

Great in every possible condition"

That's fair. It's not totally waterproof but it does make a big difference.

Tell us some more about the technical aspects of the product?

This is what Castellis say about NanoFlex: 'Exclusively engineered by Castelli, we take our thermal fleecy stretch Thermoflex fabric and coat it with millions of tiny Silicone nanofilaments to create the most water-repellent fabric finish ever. Drops of water stay as spherical balls on top of the fabric or bounce off the fabric without leaving a trace. The Silicone nanofilaments also trap a layer of air between them for excellent insulation and prevents water droplets from soaking through the coating to the fabric underneath.'

Rate the product for quality of construction:

Construction is good; these are well-made although the logos started to crack quite quickly, if you're bothered by such things.

Rate the product for performance:

The fabric does exactly what is promised. It's not 100% waterproof but it's very water-repellent, especially considering the comfort and breathability

Rate the product for durability:

The performance has proved durable so far. As I said, the logos started to crack and look a little old quite early on... but that's not a biggie.

Rate the product for comfort, if applicable:

They're comfy and the silicone grippers are really effective.

Rate the product for value:

You can get cheaper knee warmers but considering the water-resistance you get here, I reckon this is a pretty good price.

Tell us how the product performed overall when used for its designed purpose

They do exactly what they promise.

Tell us what you particularly liked about the product

The water repellency and silicone bands both inside and outside the grippers.

Tell us what you particularly disliked about the product

There's nothing to dislike, really.

Did you enjoy using the product? Yes

Would you consider buying the product? Yes

Would you recommend the product to a friend? Yes

Overall rating: 8/10

About the tester

Age: 40  Height: 190cm  Weight: 74kg

I usually ride:   My best bike is:

I've been riding for: Over 20 years  I ride: Most days  I would class myself as: Expert

I regularly do the following types of riding: time trialling, commuting, sportives, general fitness riding,


Mat has been in cycling media since 1996, on titles including BikeRadar, Total Bike, Total Mountain Bike, What Mountain Bike and Mountain Biking UK, and he has been editor of 220 Triathlon and Cycling Plus. Mat has been technical editor for over a decade, testing bikes, fettling the latest kit, and trying out the most up-to-the-minute clothing. He has won his category in Ironman UK 70.3 and finished on the podium in both marathons he has run. Mat is a Cambridge graduate who did a post-grad in magazine journalism, and he is a winner of the Cycling Media Award for Specialist Online Writer. Now over 50, he's riding road and gravel bikes most days for fun and fitness rather than training for competitions.

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elstado | 12 years ago

Just bought a pair of these for 23 quid for cool/wet winter commutes in the morning. Should be good.  16

mandy | 12 years ago

37 quid i only paid 30 but there well worth it

AdamML | 12 years ago

I bought a pair of these and the matching arm warmers prior to a jaunt up to the west coast of Scotland. I have to say I really do think they work and are well worth the premium.

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