I've been riding into London now for nearly 5 years, every working day, with distances ranging from 4 miles to 54 miles... I've tried various methods of luggage transportation, from a big 35L Karrimore daypack, to a small 15L backpack I was given on the 2012 Etape du Tour. I've tried taking in new clothes every day and i've tried taking in everything on Monday (this is currently my preferred method).
I now use an Alpkit Gourdon 20, and have been using this for the past two years.
Well, where to start? It's a fantastically light, 100% waterproof 20 Litre rucksack, available in a range of colours (I opted for the fluro yellow for extra visibility) The minimalist and simple design no only looks good, but is fantastically functional, with two side pockets for bottles (also very useful for carrying shoes, wine bottles, beer) and an elastic drawstring on the back for extra bulky items, such as a coat, more shoes maybe? The Gourdon is fastened by a roll top buckle system, which allows for a large opening at the top of the rucksack. This opening means you really can stuff clothing in, well as long as you don't need your shirts ironed, and utilise the space to the maximum. I have also on occasions ridden with the top open and items poking out. The buckle fastening system also serves a secondary purpose of carrying bike wheels. Ok so I don't do this everyday, but on the 5/6 occasions I have and commuted home they have been very secure (proof). The adjustable chest and waist straps provide ample support when on the bike or running/walking, whether the Gourdon is fully loaded or half empty
The 20 litres are surprisingly roomy, and not only has the Gourdon been used for 5 shirts, Laptop, work items, i've also used it for weekends away, hiking, trail running, mountain biking (there is a pouch for a water bladder), shopping trips to Aldi (very useful) and general storage of stuff that can't get wet when camping (important)
Even after two years of solid use, almost everyday, the Gourdon is still 100% waterproof, and apart from mud, oil and dirt is still in great condition, and literally no signs of wear to the material. I can't really see me needed a new rucksack for a while unless I damage this, and even then Alpkit provide free repair patches when you buy the Gourdon.
Ok, so what is the price for this lightweight, multi-purpose, waterproof rucksack? £22.50. Yeah, that's right. Ow I bet shipping is loads? Nope, that's free too. Alpkit are a great British company, offering a range of bags, climbing gear, bike-packing gear, lighting and clothing. They also have some pretty epic blog posts, enjoy!
Overall - A fantastic piece of kit that not only every commuter should own, but anybody who needs a rucksack really. You can't go wrong.
Alpkit link: https://www.alpkit.com/products/gourdon-20
*I bought this bag myself and have not been sponsored to write this post, I just think it's a bloody brilliant bit of kit