Fixed Super Light Super Safe

Determining the weight target will be difficult. You set yours based on the current legal competition limit based on a sport that happily has all the parts pre-weighed and actively produces light weight parts.
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That sounds like an excuse.

How much does Dammit weigh and when was the last time someone from the uci weighed his bike before a race? - Point is there is a method to why he chose that set of criteria, but he still just made it up.

Just choose 5kg with no rider weight limit and a budget of £2.5k and get on with it.

As for parts, the only one which gives you an "issue" is the frame*... which isn't really an issue as you can still use a road frame conversion, a Seta, or maybe even a stripped carbon road frame with new dropouts bonded on.

• maybe you could argue a rear hub too, but even there you have ww choices.