£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
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road.cc 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.'
Construction is good; these are well-made although the logos started to crack quite quickly, if you're bothered by such things.
The fabric does exactly what is promised. It's not 100% waterproof but it's very water-repellent, especially considering the comfort and breathability
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.
They're comfy and the silicone grippers are really effective.
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
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 worked for loads of bike magazines over 20+ years, and been editor of 220 Triathlon and Cycling Plus. He's been road.cc technical editor for eight years, testing bikes, fettling the latest kit, and trying out the most up-to-the-minute clothing. We send him off around the world to get all the news from launches and shows too. 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 past winner of the Cycling Media Award for Specialist Online Writer.