Just bought a Givi E55...
...and it's bloody huge! I was really tempted by the 44L Trekker, but it's square shape meant fitting even one Arai TX3 was difficult enough and I really wanted to stash all of my bike gear (not boots) in a box whilst at a race track.
I also tried the V46, whilst this did a bit better it was a struggle to get much in. So I took a bit of a risk and
bought an E55 on eBay for £175 delivered from France, I paid a bit more for the black version.
The plate was £60 with free delivery.
I might post some photographs later, but in the meantime here's what it can store.
2x full face helmets w/ peaks + odds and sods in between.
-or-
1x full face helmet, HG textile jacket and trousers with winter lining... with some room to spare.
I haven't used the box in anger yet, neither have I riden with SWMBO on the back, but so far so good.
I'm still interested in the Trekkers as panniers, with the added benefit of being able to use either the 33L (exhaust side) or 44L (other side) box as a topbox... if I didn't want to carry round something as large as the E55.
I was a bit concerned about ordering from Europe, so a few words on that. Expect a small delay, lots of these items are cheap as they're sourced when you purchase them, you don't pay import duty tax (whatever it is) on items from the EU/EEC, and Google Translate is your friend
