I would just add to the discussion that if your rucksack fits well, carrying a few extra KG isn't that much of a big deal.
I've just invested a hefty sum in a new rucksack and went for the Deuter SL range. I have the Aircontact Pro SL (Slimline, which is their ladies range) 55+15 for my main sack, and a smaller one around 40l for a "day sack".
Having used an 80l higear rucksack since I was 13 I was somewhat anxious using a newfangled all singing all dancing pack, but I have to say once I got used to how all the bits and bobs work I am astounded at the difference. It actually sits on my hips, for starters, so even a really heavy pack never hurts my shoulders - in fact I don't actually feel the weight on my back as it's all going through my pelvis "as nature intended".
Also lots of other funky features means it saves lots of space. Disadvantage is getting used to having a slimline, smaller rucksack. I have, however, been converted to baselayer systems and hence my clothing is considerably smaller & lightweight nowadays!
I manage to fit in my North Face Tadpole 23, which weighs in at just over 2.3kg, plus a thermarest (500g), sleeping bag (snugpac softie winter, 1.9kg), plus all my cooking gear, waterproofs, food, emergency rations, first aid kit and two poles on the outside without any hassle. Also the large rucksack has a detatchable lid pocket, perfect for popping down the pub as it turns into a mini rucksack complete with straps and everything!
I can wholeheartedly recommend the Deuter rucksacks- both me and the OH swear by them! :)
I borrowed a 25l pack off someone the other week, it felt really heavy and gave me backache! Went to a shop and tried on some and ended up with one that I hardly notice, it's not the weight, it's the fit :)
Absolutely - I completely agree. A friend of mine tried on the sack I have in the store, but just couldn't get it comfortable. She tried on a similar sack made by Vaude and it fit perfectly - sold - so it really is worth trying them on, especially for a big exped/trip where you'll be walking/carrying for long periods of time with a heavy sack.
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