Just because you can travel light doesn't mean everyone can. I have the Radial 34L and it was the perfect size for my needs. I've had to carry lab equipment, multiple textbooks, change of shoes and clothes, and food for the day. I no longer use the Radial bag (I have an Arkel that converts to a pannier), but it's very good for its purpose.

If I'm going out for a joy ride or if I'm only riding to work, then yes a small bag for the essentials works fine. When I'm riding to work, then to class, and then back to work, and I'm away from home for 14 hours then I need to be able to carry more with me.

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This is the commuting forum, right?

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