I've just started commuting and am currently borrowing a friends Trek hybrid with a rear rack while I decide what rack to use on my bike. I'm using an Ortlieb trunk case that clips on top of the rear rack and while the case is working out great the flex is not. I'm not certain if it's the frame, the rack, or both but the rear of the bike flexes horribly, so much that I cannot stand and pedal or the chain will just start skipping due to the rear end swinging left to right so much.
The fact that it causes the chain to jump between gears (it doesn't happen without the case mounted) makes me think it is just the whole rear end flexing. The pack weighs 20lbs, most of that is my laptop. I'll be switching to a Mercier Serpens next week and have considered multiple options, going to a seat post rack (seems like there would be even more movement in the rear end?), going to a stronger rear rack (would I flex all over with the carbon fiber seat stays?) or buying a fork with front rack mounts and going to a front mounted rack.
Can anyone point me in the right direction? Are any of these decent ideas or should I try something completely different? I thought about panniers but since 15 of my 20lbs is in my laptop I would end up unevenly weighting my bike, plus the single case works out very well when I'm off my bike, I just use it as a briefcase.
I am also unsure of what class of rack to get, I've seen the lighweight racks that are only good for 20lbs, all the way up to racks that are rated at 60lbs. I'm only carrying 20lbs so a lightweight rack would be nice, but will it end up flexing too much?