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Ian ... you have been given a great deal of feedback upon your question.... but really have you not answered the question yourself in the above snippets.
Let me summarise what I am sure you already recognise as being blindingly obvious:-
Let's say you get advice (from any source .. expert or otherwise); ... you go ahead and make these things; ... the owner/user (despite your warnings/advice and against common sense) takes the vehicle on the road with the nuts in place and they fail; ...someone gets injured (not necessarily lethally or seriously); ... lets assume a kiddie crossing the road gets knocked down by the now errant wheel and rim.
Forget the Regulations and the Police and every other area of Law and Enforcement that would no doubt be 'interested' in the event and asking all sorts of difficult questions of the person who manufactured these wheelnuts that failed ... lets just think of the parents, grandparents, brothers, sisters, aunts, uncles who are looking for someone to blame... Who do you think they will raise civil actions against even if the police take no action?
The person who made the item that failed.
So there you are in court and you have already publicly admitted to this group, before you even attempted to make the items, that :-
A) You dont know what the relative strengths of the materials are
..snip..
and
B) You have no idea how to calculate relative sheer loads, tensile strengths, etc etc
So what defense do you plead?
As written ... I believe you have already answered your own question. You don't know therefore would be wise to decline the 'offer'. Remember
- Ignorance and Stupidity are no defense in Law. Insanity might get you off if you wanted to try that approach.
Regards
IanB