The Shimano Lark Mini (DD-400) is (along with the SunTour Love) one part of a 1970s Japanese fashion for making tiny derailleurs for children’s bikes. It’s just like a real derailleur - but impossibly small.
I suspect that the Lark Mini, with its 34mm parallelogram plates would struggle to cover the width of a 5 speed freewheel - and was probably most at home when used with the 3 and 4 speed freewheels used on Japanese children’s bikes at the time.
The Lark Mini does have a couple of interesting technical points. Firstly the it has only one sprung pivot, which is not Shimano’s usual style. Secondly the cable clamp is mounted on the outside parallelogram plate (not the front knuckle). This is a design feature used by nearly all Shimano designs today, and I believe that it first saw the light of day on the Lark Mini.
Finally there is the question of style. While SunTour was camping out at the junction of Haight and Ashbury with the ‘Love’, Shimano went all Carnaby Street, using the word ‘Mini’ (think Austin, skirt and Moke) - and writing it in a swinging, groovy, right-on font.
Turn on, tune in and bolt it into your drop-out.
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