Often the first question:
Whichever model weight the retracts are designed?
The model weight alone is responsible for the selection of retracts actually not a good criterion.
The decisive question is where and how to land and what's legs, wheels a.s.o. are used. The model requires a suitable spring weight of the legs. The bending stress on the legs depends not only on the weight and length L, inter alia, mainly on the rolling resistance on an uneven  gras runway or on a flat hard runway.
In order to avoid a deformation of the legs by the bending stress as possible, the adapter is made of steel, it has a diameter of 10mm.
A larger diameter of the adapter makes for the retract units offered no sense, since it takes place a weakening of the rotary segment. In other words, a chain is not stable if a thicker chain link is used.
The body of retracts are machined from high grade aluminum CNC. It was possible "rigid" designed. And if the retracts are crashed, we have spare parts ..

With this in mind our Models- were sized retractable landing.
Oh yes, the model weight: Up to 15 kg.

Einziehfahrwerke mit Controller und Fahrwerksbein/Federbein ausgefahren
Einziehfahrwerke ausgefahren