Nick Müller wrote:

The other funny experience was, that what they sell as wheigh tlifting
disks are in reality the cheapest flywheels you can get.

NOOOOOOOOOO!

for gods sake do not just grab something semi-round and think it will
work as a flywheel, the day it bursts those who live through it will
regret your lack of engineering rigor.

I cannot stress this strongly enough.

There are scrapyards full of flywheels, made out of an appropriate
material.

You MUST KNOW THE YIELD STRESS of your flywheel material, that word is
KNOW, as in FOR A FACT, as in you have absolute knowledge of the
precise composition of the material it is made from.

Hand on heart, I would MUCH rather take my (minimal) chances standing
in front of a handgun bullet in preference to standing in front of a
bursting flywheel.