I did all my grocery shopping for the last many months using those same kind of baskets, and I noticed that when I loaded the baskets up with pretty heavy stuff--say, a container of kitty litter, a gallon of milk, and other assorted groceries--the front end of the bike went all wonky and everything definitely felt "wobbly." I found that as people have suggested, getting the heaviest weight as low and as near to center as possible helped a lot.

Putting some weight on the front as a counterbalance helped, too, so I started hanging bags of crushable, fragile stuff--fruits and vegetables, eggs, etc.--from the handlebars, and that also helped. I kept thinking I'd get one of those detachable front baskets, but never got around to it. I think that would have helped, too, having a bit more weight in the front.

However, I just this last weekend retired my double rear baskets and replaced them with a Performance Transit Rack (40-lb. capacity, on sale for $20) and a Jannd grocery pannier that I happened on at a garage sale for a dollar a few weeks ago. The grocery pannier is light, holds way more stuff than one side of the basket did, and is easily removable and carry-able. And I like not hauling the weight and bulk of those baskets around, because I'm starting to do much longer rides (a metric century + 26 transportation miles yesterday) and my old bike is heavy enough already.

I plan to pick up another grocery pannier (they are often on sale at Nashbar and/or Performance for as low as $18) so I'll have two for big grocery runs. The rack also gives me other packing/loading options for the future--an Arkel Bug, a full set of panniers, whatever. Just simpler and more versatile than the baskets.

I like the locking top you devised for your baskets, though. Very clever.