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Both climbing and descending is shit with rear loading, cornering at speed with weight on the back the bike wants to stand up/go straight on. With balanced/front loading it’s much easier to corner, the bikes wants to lean in.
It's very hard to get a rear loading to be as stable as possible, even if it's well packed.
Only a handful of saddle bag have addressed this issues, like the Porcelain Rocket Mr Fusion which have it's own saddle support system that nearly completely eliminated the side swing.
Riding on a flat, smooth, straight road ok, I’ll give you that.
Both climbing and descending is shit with rear loading, cornering at speed with weight on the back the bike wants to stand up/go straight on. With balanced/front loading it’s much easier to corner, the bikes wants to lean in.