I'm an experienced audaxer (completed first PBP in 2015) planning on building my first bike this year and want it to be an Audax mile-eater. I've been riding my first road bike since I bought it in 2009 (Bianchi Via Nirone C2C) but now I want to build something more audax friendly to my own specifications. This will also serve to teach me bike mechanics, something which I always feel hopelessely inadequate at.
I have already made some decisions on components, including fixed mudguards, disc brakes, pannier dropouts, dynamo hub etc. This thread is to gather advice on the benefits and drawbacks of suggested components, starting with frame type.
What type of frame would you recommend for Audax and why? Steel, Titanium, or Aluminium? I don't really want to know what you ride unless it is backed up by some reasoning and/or comparison to lesser previous experiences.