On really hot days, keeping the water on your bike cool is an impossible task, and it puts me off drinking it. That’s a dangerous combo. You can use light-coloured bottles, avoiding black, which will help, but there’s another way that might have so far passed you by completely.
Camelbak Podium Chill bottles are a quality item and, as such, have been in every high street bike shop since time began. Still, you probably overlooked them till now, because whilst they look just like any other big bottle on the shelf, they’re almost always way more expensive than every other bottle in the shop.
