All Carpenter frames were built from the best available double-butted steel tubesets supplied by Reynolds; cut, mitred, jig-built, pinned then brazed. As Reynolds progressively introduced stronger steel alloys, they were able to offer tubing walls which were thinner and so lighter yet delivered the same strength and stiffness. Mild steel has an ultimate tensile strength (UTS) of around 400 MPa, by the mid-1930s careful alloying and tempering (notably with Reynolds 531) was doubling that.

1898 Double-butting. John Reynolds patented double butted tubing as a means of achieving a steel tube which is thin-walled (so, light) over most of its length yet thicker-walled, (so, strong) at the stressed brazed joints. Prior to this, frame builders had to resort to a strengthening insert at the ends. A billet with a central hole bored in it is forced into becoming a longer tube by angled rollers pushing it over a mandrill. The mandrill is shaped at each end to allow a greater tube wall thickness at this point. The cunning trick is getting the mandrill out of the narrowed end…..

1920s-31 Reynolds AA – High carbon content. Ultimate tensile strength unknown.

Carpenter frames up to approx #2500 were most likely built from Reynolds AA

1931-35 Reynolds HM – High Manganese content. Originally formulated in 1924 for airframe tubing. Cycle tubesets introduced around 1931.

Carpenter frames from approx #2500 to #2900 were most likely to have been built from Reynolds HM.

1935 Reynolds 531 – 1.5% Manganese, 0.35% Carbon, 0.25% Molybdenum. UTS 700 MPa. ‘531’ was quickly adopted by British race-bike builders.

All Carpenter frames from approx #2900 were most likely to have been built from Reynolds 531 tubesets. Both HM and 531 were offered in the 1937 catalogue (presumably printed late 1936) but there is no mention of HM in the 1938 catalogue. The 27.2mm inner diameter of the top of the seat tube is a strong indicator that it’s 531. Weighing the bare frame could help but allowance would have to be made for the frame size.

After WW2 the use of Reynolds 531 was steadily taken up by Continental builders too. When climbing ace Charlie Gaul won the 1958 Tour de France on his light-but-stiff 531 framed bike, this became the tubing of choice in the pro peloton. 26 Tours were won on ‘531’ frames, the last being ‘Big’ Miguel Indurain’s win of 1991.

Reynolds 531 remained the tubing used by Frank Carpenter until the closure of his business in the late ’60s/early ’70s.

Post Carpenter…..

The challenge to steel came from Titanium alloys (strong, only 60% of the density of steel, but extremely expensive), Aluminium – and even a flirtation with Kirk Precision’s Magnesium. These forced a response from both Reynolds (753, 853) and Columbus (SL, SLX). But ultimately it was Carbon-Fibre composites that won the day despite their vulnerability to impact damage, the enormous tooling costs and the need to develop a completely new set of design and fabrication skills. Boardman’s 1992 Lotus track pursuit bike may have cost £500,000 to develop but that was a one-off. The future was clear. That’s unsurprising, the UTS of Carbon composites is double that of the most sophisticated steel alloys, the basic density around a quarter or less, it is more easily shaped into aerodynamically-efficient forms.

mid 1970s Titanium 6Al4V (6% Al, 4% Vanadium) UTS 900-1150, 4.5 g/cm³

Titanium 6Al4V was developed for the cold-war USAF Blackbird bomber. Flema (Germany) produced the first race-bike titanium frame in the 1970s, Teledyne (USA) and Speedwell (UK) also adopted Ti. By the early 1990s this material was seriously challenging steel. Titanium frames are light and have good impact-resistance but it was Carbon composites that pushed Ti out of the mainstream and into a few small speciality areas.

1970s Aluminium 6061-T6. (Mg 1%, Si 0.6, Cu 0.25, Cr 0.02 UTS 310 MPa 2.75g/cm³

Weldable aluminium alloys were first developed in the mid-1930s. Once TiG welding was developed it was used from the mid-1970s for welded bike frames. e.g. 1975 Gary Klein’s welded frame, 1983 Cannondale, 1987 Vitus 979 (lugged), 1989 Giant TCR (ONCE, TdF), 1998 Pantani TdF

1976 Reynolds 753 UTS 1000-1200 MPa 7.5 g/cm³

Reynolds upped the UTS of their tubing by improved tempering processes – but low temperature silver-brazing was needed or else the advantages of the heat treatment would be destroyed. Builders had to submit a frame for destruction-testing before being certificated by Reynolds.

Late ’70s Cyclex Cromo Steel (Columbus SL, SLX). UTS 800-900 MPa. 7.5 g/cm³

SL and SLX use the same steel alloys, SLX tubesets were be made lighter by the use of helicoidal reinforcement in the stressed areas.

1995 Reynolds 853 UTS 1250-1400 MPa 7.5 g/cm³

The last hurrah for steel. It was alloyed with controlled additions of Carbon, Manganese, Chromium, Molybdenum, Silicone and Copper – then heat treated and air hardened.

1992 on Carbon Fibre composites UTS 1200-2400 MPa 1.5 g/cm³

Now frames could be any aero-shape you liked within reason, lighter than ever yet still stiff and strong. The Lotus 108 dispensed with the down tube, the seat stays, one front fork, and one rear stay. Others dispensed with the seat tube & stays, or the top tube. All adopted aero-efficient cross sections. In the end the UCI called a halt to all this innovation. The bikes ridden by pros were looking less and less like the products in the shops….