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6/4 is tubing, 3/2.5 is for plates and solid pieces.
Other way round I believe. 6/4 isn't normally available as tube, so you end up having to form 6/4 tube by bending plate into a tube and welding the seam, rather like ERW steel.
Brommers-
This^. I believe.
My first Ti frame was 6Al/4V.
Only one of a handful sold in the UK.
When it developed a crack around the BB, it was replaced under warranty, but with a 3Al/2.5V frame, on the basis that Reynolds deemed 6Al/4V tubing too expensive to produce commercially, and would only do so as a special order.ETA - It was a f'ing stiff frame, which still had springiness.
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Correct! My mistake. I’m a certified aircraft engineer, saw more titanium than you’d believe but it still used to make me go “oooh, lovely” each time.
JurekB
jj72
Titanium is generally only made in two commercially-available grades, 6al/4va and 3/2.5 (aluminium and vanadium being the alloying elements).6/4 is tubing, 3/2.5 is for plates and solid pieces. Neither is exactly cheap so generally cost savings are in production. It’s a bugger to work with (it’s extremely abrasive and eats cutting tools, for example) so components tend to be pricey, but assuming the manufacturers can get the tubing rolled out to the right diameter there isn’t much to do to make it into a Brompton seatbpost.
Incidentally, most early ti bicycles used commercially-available aviation hydraulic and fuel line tubing, which was only produced in a limited range of diameters, as it was far too expensive to get custom-drawn tubing, and so early ti frames were very skinny.