Bicycle frames for heavy people: What the frame builder and material expert advises

Large frame manufacturers employ dozens of engineers, designers and material experts, but rarely push the limits of the material or its processing. Falkenjagd and Rennstahl founder Kirschner does this despite having a much smaller team and lower production volumes. The material tinkerer favours steel, titanium and craftsmanship. His frames are considered to last a lifetime.

MYBIKE: Mr Kirschner, let's get straight to the point: What makes bicycle frames for this weight class so particularly robust and rigid? Material, tube shape or the welding techniques?

Andreas Kirschner: Firstly, rigidity and stability, i.e. durability, should not be lumped together. A frame of the same material becomes stiffer in a simplified way when the wall thickness or tube diameter increases. The material, in turn, has a particular influence on tensile strength and therefore especially on durability and resistance to breakage. Steel, for example, can be deformed slightly more than aluminium with the same force, but its molecules reliably return to their original arrangement after "stretching". Aluminium does not do this one hundred percent; every hard stretch makes the material a tiny bit softer.

>> Our test of touring bikes for heavy riders: 6 bikes that can withstand more than 120 kg

Sounds like you have to choose between two evils: Either I have an unreliable frame, a lead-heavy one or a durable one with an imprecise ride feel because it lacks stiffness.

In principle this is correct, but unfortunately worded very negatively. Yes, all frame builders are subject to this "conflict of objectives": the frame should be as light as possible, reliable and durable, it should be manufactured within the set price range and be as stiff as possible, although this does not even apply to all parts. While the head tube and bottom bracket area need to be as torsion-resistant as possible, seat posts or seat stays with a slight flex also have advantages in terms of riding comfort. To achieve these goals, we use very select alloys of steel or titanium on the same model, depending on the position, and employ complex processes from forging to CNC milling and 3D printing as well as drawing the tubes and welding in an argon-protected atmosphere.

I'll just make a quick comment. When looking at the technical details of the test bikes, it is noticeable that some frames have single or multiple butting. -Some of your frames are quadruple butted. What does that actually mean, and is it a sign of quality or not?

Yes, it is! Butting is an efficient way to save frame material and therefore weight without weakening the frame. Every frame tube has to absorb forces along its length, usually more at the ends than in the centre, and the direction of the forces also plays a role. In short, in some parts of the tube less material thickness is sufficient to remain stable throughout the bike's life, in others there simply has to be more wall thickness. We pull our frame tube blanks several times through machines that "flatten" the material from the outside, while mandrels of different thicknesses form the abutment on the inside. This makes the tube longer, but above all it gives it up to four different wall thicknesses along its length. The tolerances here are in the hundredths of a millimetre range.

That sounds like a lot of work. Is this, together with the inert gas welding technology, a reason for the high prices? What needs to be considered when welding and how can you recognise quality?

Titanium and stainless steel must be surrounded by an inert gas during welding. The noble, corrosion-free surfaces would become brittle due to the temperatures of up to 900 degrees and the atmospheric oxygen, and the top properties would be lost. This requires a great deal of technical effort and expertise. The weld seam must also be protected on the inside, which is why we mill openings in the head tube so that the down tube and top tube can be flooded with argon. When buying, the layman should look for small, flat and as even as possible scales on the weld seams. Funnels or bulges would be bad.

Is there any way to evaluate the frame tubes other than on a test ride?

I would first lift the bike to assess whether it is a lightweight frame made of good alloy or whether as much material as possible has simply been used. Branded frames usually show on a sticker which tube sets they were made from. You can google them with the keyword "tensile strength". Values of 400 Newton/mm² for aluminium and 500 for steel are good values, titanium can withstand up to 1,200 Newton. Figuratively speaking, this would correspond to a 120-kilo human being on a titanium wire with a cross-section of one square millimetre.

If the material can withstand so much, what would be hypothetically feasible in terms of maximum system weight for a classic bicycle frame?

That's not so hypothetical. Clever mould construction - chainstays running from square to round, forged parts compacted at neuralgic points, top alloys and a well thought-out welding sequence - would make frames with a continuous load capacity of 215 kilos possible, which would mean a payload of 185 kilos. If budget and riding dynamics didn't matter, a little more would be possible, but even for 120-kilo riders, that would mean luggage weighing over 65 kilos. Who can handle that?

You don't use aluminium. Does the metal even make sense for bicycles?

You can build aluminium frames that are as light as titanium, but the thin-walled tubes are susceptible to destabilising dents, and every bend in the material makes the tube more unstable, which means more movement - a vicious circle at molecular level. If aluminium is to be permanently stable, the wall thickness and tube diameter must be high. But then the weight advantage and good damping are lost.

The weld seam as a calling card

While carbon frames today are laminated together from many fibre mats to form a monocoque, frame builders with metal tubes as the starting material have to weld them in the classic way, i.e. liquefy the metal at the contact points of the frame components using high levels of heat and insert it into the gap. Until they cool down, the ends of the tubes "fuse" with the liquid metal, so to speak, and form a single unit from that moment on.

The difficulty here is to direct the heat at one point long enough to reach the ideal melting temperature, but not too long, otherwise the original frame tube will melt away. Welders must therefore guide the 900 degree hot tip of the device and the wire that supplies the material for the actual joint at the perfect distance and ideal speed - and along a strongly curved line.

By the way, you can recognise the material by the seams. The barely recognisable mini-scales in the picture on the half-left of racing steel can only be found on steel frames, titanium forms somewhat flatter flat seams, those of aluminium are the widest and most bulging, which is why they are occasionally removed afterwards; this is called sanding. As this only involves protruding material, the seam, which according to Falkenjagd boss Dr Kirschner is more stable than the adjoining frame tube when correctly processed, is not weakened. It should look more elegant and protect against corrosion.

Sometimes manufacturers also use grinding to make uneven, less filigree seams look more valuable. On the Koga Worldtraveller with its extremely large tube cross-sections and flowing shapes, the aluminium seams would look like stand-up collars or leaking material; the "smooth" tube transitions are also a calling card for material craftsmanship.