Bromptons have a non-inconsequential amount of aluminum now: the cranks, rims, bars, brake calipers, hubshells, Pentaclip, folding pedal. I think Brompton is alone* in recommending the owner/rider swap out some of these aluminum parts for new on a scheduled basis: "As with all bicycles, aluminium parts such as crank arms and handlebars are subject to material fatigue. Aluminium is a great choice for these components for a number of reasons: it’s lightweight, stiff, and cost-effective. However, it is also less resilient than steel, and for this reason, we recommend aluminium parts, particularly safety-critical components, are replaced within 5000 miles - sooner for vigorous riders, and especially after an accident or significant impact. To give an indication of time; assuming you commute 10 miles per day in total, and use your bike 3 times a week for 48 weeks of the year; your aluminium parts will need replacing approximately every 3.5 years."

I never see any used Brompton ads that mention the aluminum parts have been replaced or discounted prices because the parts are (over)due for replacement. Shrug. Anyway, a Brompton aluminum frame with a 3.5 year life is a non-starter in the marketplace.

1) In present discussion, welding is a non-issue. For reasons unknown to me, Brompton has stuck with the old high labor content torch brazing method of joining the frame tubes.
B) Folding bike designers usually consider an additional frame design parameter: denting from being knocked about. For this reason, you don't see many folding bikes fabricated with relatively thin wall tubing.
iii) Exactly what the Brompton frame is fabricated from today is unknown outside the company. They used to advertise the frame was "hi ten steel" without specifying any particular alloy. WB-A recently wrote that the Brompton used many different steel alloys for the various frame parts but did not elaborate as to what any of them were.



*Hmm. Do I remember Birdy recommends replacing the fork assembly on a scheduled basis?