I have found that a wheel comes off in flight
not because the collar gets loose, but because
I didn't notice it. Putting notches in the axle didn't
really help. It just took a few more flights of being
ignored before it fell off. Fuel tubing and O-rings
also fall into this category.

My solution was to solder a washer onto the
axle instead of using a collar. I put a piece
of cereal box between the washer and the
wheel for proper clearance. With a regular
soldering iron and regular solder it doesn't get
hot enough to melt the wheel.

It does take about 5 minutes to do each wheel,
compared to the one minute using collars, but
I find it is well worth the time. Since I started
doing this about 28 years ago, I have never
lost a wheel.

Jenny