One of the weak spots of the current 3DR frames is that the aluminum walls of the arms are very thin. The motors mount directly to the arm and in the event of a "landing anomaly" the aluminum wall is easily deformed by forces on the motor.
My solution was to construct "backing plates" for the motors. The plates fit inside the arms and slightly longer screws are used to attach the motors through the plates. The plates are simple 1/16" aluminum sheet drilled to match the 3DR mounting holes. The small indentation you see in between them in the picture below is for clearance of the motor axle, although it is probably not necessary as I don't think the axle protrudes further than the thickness of the arm wall.
This addition should improve the frame robustness somewhat, hopefully without merely identifying the next weakest link.
Comments
Kevin: this is brilliant. I hope 3DR contacts you immediately about implementing this into their frame kits. The weak motors mounts are not a minor issue. It's really a deal breaker.
I use the motor mounts that came with the motors in my 3DR kit.
Sorry guys, but my machining skillz are not good enough to build these for others in either the quantity or quality required.
What Knut said! I already have 2 bent arms so there is a need for this!
I'll buy a set if you sell the plates !
Nice. I'll need to try this some time.
I cut some unthreaded nylon spacers to fit very snugly inside the arms and ran the motor bolts through them. This significantly reduced the amount the square tubes pinched when I had a rough landing crash :)
That said, when I bit the bullet real good the other day, it bent the tip of the arm in a way that was rather difficult to bend back.