Castelli Nanoflex knee warmers

£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.

Verdict

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