When I went back to my hometown of Hayfork, Ca in '88 to get deep into the woods, me and my best friend had our Blackburns loaded to the gills.
Well, about three weeks later, my buddy broke his collar bone one day on a ride. Since I didn't want to leave him up at my old place alone and w/o company, he used to sit his 150 lbs of fat on my Blackburn. This happened many times until he finally got good enough to ride one handed.
That thing never, ever broke. I was amazed. Still steel tubulars are better, but the Blackburn will do in a pinch.
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chepe gp40-2hh
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iik
Blackburn Alloy MTN 1 is 580 grams according to their website. I have a similar one ('Bor Yueh' or something) which is 520 g. They should be strong enough for 6 kg - that's about the weight that I have on the rear. I don't know what 'base weight' includes, but 6 kg is pretty light - i'd be interested to see your packing list (mine is here).
Pro tip