13l should be fine so long as you can secure it well enough on the bars, see my bodge from a few pages back to allow extra bungee to hold it in place:
On bike:
Not best pic but 2 alpkit compression straps plus 1 bungee with that bodge and it was v.stable didn't affect handling. Contents were bivvy, sleeping bag & neoair xlite mat.
13l should be fine so long as you can secure it well enough on the bars, see my bodge from a few pages back to allow extra bungee to hold it in place:
On bike:
Not best pic but 2 alpkit compression straps plus 1 bungee with that bodge and it was v.stable didn't affect handling. Contents were bivvy, sleeping bag & neoair xlite mat.