The weight saving for titanium over steel for components like bicycle frames was never as significant as a comparison of their relative densities would seem to suggest, because of its lower Young’s Modulus, larger tubes with thicker walls are needed. These days even that small weight advantage has been to some extent negated by the widespread use of carbon fibre, which in most cases is both lighter and cheaper.
My preferred material for frames, forks and luggage carriers is steel. But for a Brompton folder, getting a lightweight steel frame and forks custom-made would cost more than the same parts in titanium, which are available as factory-produced parts. I don't like aluminium alloy forks and frames, because they don't have the "springiness" of steel and titanium, and aluminium has a different failure mode than steel and titanium.
Aluminium alloy is good for some things, especially rims used with traditional rim brakes, where it works much better than steel. In my experience the sidewalls on aluminium rims generally wear out before there are any metal fatigue issues.
I don't have any carbon fibre parts. This material isn't suitable for my idea of a touring or utility bike, where the weight doesn't matter much for me. I don't want carbon fibre on a folder because I don't trust it to resist the lateral shocks that occur sometimes when carrying the Brompton on public transport. I am sure it is OK for a lightweight road bike that is looked after carefully and replaced fairly often, but I saw lots of broken carbon fibre parts at my local bikeshop before it closed a few years ago. And I once saw a carbon-fibre racing bike literally shatter when the rider touched a moving car.
It’s attraction for me lies in its anti-corrosion properties; titanium frames retain their good looks and require no costly repainting, a property also shared by martensitic stainless steels.
The anti-corrosion properties of titanium don't really matter to me for frames, forks and luggage carriers. I regularly touch up the paint on my steel-framed bicycles and steel luggage carriers, and repaint them completely (using a paintbrush) if I reckon this is needed (not very often). So my bikes don't look so good as when they were new, which I take as a plus as it reduces their appeal to thieves. I still have my 1977 steel-frame lightweight tourer, on which the Reynolds 531 steel frame and forks are the only original parts left.
