I got a rear rack for my 2nd bike for christmas, and found that it's of the type that has a sprung arm to retain loads, rather than using a bungee as with the one on my commuter. The thing is, it doesn't seem to hold my bag at all well. I commute with a shoulder bag strapped to the rack (usually carrying a ring binder file as well as tools and lock etc) and it never seems to stay put on the rack.

How are these sprung things supposed to work? There must be some way to secure a load with it...

My experience with the old Pletscher spring-loaded rat-traps wasn't very good. Unless the load was just the right size and shape it tended to vibrate over to one side and fall off. I usually combined the rat-trap with a bungee cord to secure things.

I'd just stick to bungee cords.

The rat trap thingy does not stop you from using a bungee cord like you are used to. I grew up believing that a rack without the rat trap was a damaged rack.

You can read about a bike with a rattrap rack here: https://lovelybike.blogspot.com/2011/...-romantic.html
I guess the rack on this bike is a bit bigger than yours.

I am a big fan of the Nite Ize figure 9's. For a rack, the small size is ideal, rather than bungee cords. I've secured bags, a 5-gallon bucket on each side, and even a 32 gallon garbage can filled with plastics to take to the recycle place on one side of the rack using the Figure 9's (not all in the same load, of course). Most of these loads were not on a rat trap style rack, but rather the Jandd Expedition rear rack.

I have a couple of the rat trap racks, but I rarely use the rat trap feature. If you roll a jacket up tightly, the rat trap will hold that fine, but anything hard will likely work its way out.

back in the day the old rat trap did a good job of holding a lucky lager 12 pack from the store to the apartment. it was just the right size for a secure fit. Otherwise and in addtions use bungee cords