Frame materials
(previously published in 'de Wereldfietser', 1-1999)

(c) m.s.gerritsen 1999

A standard bicycle-frame consists of 11 tubes, fried together in an intelligent way so that several functions can be achieved. You should be able to attach all parts in their proper position (fairly important for handlebars, saddle and crankset), the construction should have sufficient stiffness and strenght,  and the contraption should handle safely even when fully loaded up. And it will be nice if it doesn't look to bad, and isn't too heavy. The frameset is a deciding factor in the overall quality of the bicycle as a whole. You can hang a fortune in parts of a poor frame, but it will never be a pleasant bike. If it fits, you'll have more fun with a better frame and simpler parts. Quality in a frame depends on:

  • longivety, achieved by a design which takes account of the material properties, has carefully designed joints and a properly executed construction,
  • the proper mix of strength and resilience, again a function of material properties and the choosen dimensions. A frame should be stiff enough so that it will handle faithfully in fast descents, or on a steep climb. But too much stiffness isn't a good thing either because tyre adhesion will suffer during cornering and bumps will be transmitted even more.
  • good geometry, for handling and roadholding
  • a good finish, with all braze-ons in their proper place and a paintjob which will last longer than the first holiday.
Touring bicycles are invariably made from steel or aluminium alloy, although on tradeshows you might occasionally encounter a titanium or carbon frameset.
Steel is the classic choice, but from the MTB-scene you see aluminium more and more gaining a foothold..

Specific weight: the weight of a material by volume. A liter of steel will weigh 7.9 kgs, aluminium 2.7kgs, and Titanium 4.5 kgs. Within practical limits the alloying af the basematerial has no influence on the specific weight.
Tensile strenght: the tension (e.g force per crosssection) where the material breaks. Most metals will exhibit flow before breaking. At a tension below the tensile strenght the material wiill flow, i.e. deform permanently. Below this elastic stress limit the material will spring back, if you go higher you have a bent frame! The elastic stress limit is thus more important than the final breaking strenght. With alloying, heat treatment and coldworking the strenght of the alloys can be changed.
Stiffness: If you apply a load to a specimen it will deform. Keep the stress below the elastic limit and the deformation is lineair with the stress applied. The E-modulus is the coefficient which decriibes how much a given material will deform. For steel the modulus is three times as high as for aluminium, eg a given crosssection in aluminium will be three times more elastic than the same shape in steel.
Alloying elements have little influence on the stiffness, so a more expensive steel tube will be just as stiff as a cheap one. On the other hand it is quite possible that the more expensive tube can be stretched 4 or 5 times further before it takes a permanent set, because the permissible elastic stress limit is so much higher.

Steel is a group of alloys which are based on the element Iron (Fe) By the insertion in the cristal lattice of several exotic elements with funny names ( Chromium, Molybdenum, Vanadium) and by coldworking the elastic limit can be raised. The weight and the stiffness of a given tube remains the same, but the difference in strenght between a cheap tube and a high-class bicycle tube can be a factor of five. If you keep the strenght the same, an expensive frame with the best tubes could weigh a factor of 5 less. Unfortunately although this frame wouldn't fail, it would also be 5 times as flexible, as the E-modul remained unchanged!
In practice the better frames will weigh about half, and be a bit more flexible than their cheaper brethren and quite a bit stronger. This reduction in stiffness is acceptable, because nobody worries about stiffness when designing a cheap frame, sufficient strenght is the priority. A gaspipe frame is thus too stiff and the excessive stiffness is all in the wrong place too. 
Apart from more strenght there is also another reason to choose a more expensive tubeset, and that has to do with the dynamic material properties. That all steels are not the same becomes obvious when you pick up a tube, as they sound a lot different. I will not bother with a discourse on energy dissipition, micro strain or the area of the hysteresis curve, but you can remember that a higher strenght material will result in a better riding and more responsive frame.

There are quite a few manufacturers which offer (or have offered) a great many tubesets in varying alloys. The best known are Columbus and Reynolds, but there are quite a few more with Falck, Dedacciai, Oria, Mannesmann, Poppe & Pothoff (Stainless), Tange, Ishiwata, Vitus and True Temper etc. Usually you will find a sticker explaining what you have spent your money on, usually with a fancy name and sometimes the specification. Most manufacturers start with a tubeset from 25cromo4 with a 0.8 mm straight wall, followed by several series in butted 25cromo4, some higher end versions with modyfied cromo (up 20 %)  and some high-end version of such strenght that the wallthickness can be reduced to 0.4 mm.
If the sticker says 25cromo4, it will decribe a steel tube (the 95% Iron content is so obvious it doesn't need mentioning) 2.5 % carbon (again carbon is always used, so we will leave out the C) 4/4=1% chromium and some molybdenum. But the alloying elements sound wonderfull (I love Oakley with Plutonite and (where did they get this) Unobtanium) so copywriters will make the most of it and then some.Titanium is used in stainless steels to improve the weldability, but ad-writers will let you believe that you're getting a Ti frame. I'm waiting for the day 'they' discover it is also an important ingredient in white paint!

The specific weight of aluminium is one third of steel. Unfortunately the same goes for the E-modul. If you copy a steel design you have to increase the wallthickness by a factor of three to achieve the same stiffness, so you will win very little. Often you will find you have compromised longivety, because the permissible stress in aluminium alloy welds is more than three times lower than the permissible stress in a steel joint. Weightsavings can only be attained if you increase the tubediameter instead of the wallthickness. (this is called oversizing)  An oversize tube will be stiffer in bending (because more of the material employed is at a greater distance from the neutral- or centerline). The same effect can be observed with a rolled up newspaper, this is a more effective weapon than when folded flat. Because the tube is stiffer, it is strained less and longivety is improved. Compared to a steel frame a welded aluminium job will be heavier or stiffer to be able to achieve the same life expectancy. For this reason the current crop of aluminium frames all have fat but thinwalled tubes.. However there is a limit to how much the tubes can grow, because the maximum diameter to wallthickness ratio is limited. Overdo it and you get a beercan, which is very light, but also very easily dented. Ofcourse in a bicycleframe that would not permissible.
As a rule of thumb, an oversize aluminium frame will be stiffer than a steel counterpart in order to survive. But also the transmission of roadshock and the frequency response will be different. Do not buy an aluminium frame without riding it first, if all you have ridden are steel frames. It could be quite different, although fat tyres or a carbon fork might mask this entirely.
A theoretical aspect of aluminium is the fatique limit, which it doesn't have as opposed to steel. Fatique is the forming of cracks under cyclic stress, at levels well below the elastic limit. In cycling circles this is well known as the broken carrier in Nowhere. For steel there is a well defined stresslimit: stay below and the construction will last forever, but aluminium will always break if you keep loading it long enough. However neither steel or aluminium bikeframes are designed for an infinite life, the designer aims for long enough life with the envisaged use. To build that undestructible steelframe you would have to add more weight, which no-one wants to lug around, and is difficult to sell. For both materials the question should be what standards did the designer apply, and as you usually don't know it is not practicable to condemm alumnium on this aspect. However if you do break a frame, a steel job is more easily fixed than an alumnium one.

Butted tubing
Butted frametubes feature a variable wallthickness, taking into account the expected loads. The middle of the tube is usually the thinnest part, and the ends are about 0.2mm thicker. The reason for this is that the stiffnes of a frameset is not only determined by the stiffness of the tubes, but also by the deformation in the joints. With thicker tube-ends, the stiffness is increased at very low cost in terms of weight. Longivity is increased as well, and the dressing of the joint is less critical. Butted tubes are made one by one, so they are more expensive than the plain gauge stuff which arrives at the factorydoor in 7m lenghts. A frame with butted tubes will be a bit lighter and a bit more responsive than a plain gauge version.

To lug or not to lug
Steel frames are usually brazed (with or w/o lugs) or welded. Aluminium frames are either welded (usually necessitating post-heat treatment) or glued with lugs. 
Lugs are the fittings which can be used to join the tubes in a frame. The lugs are made in large series and are very convenient for series production or the small builder. The tubes are fitted in the sockets, making the assembly is self-jigging, and necessitating very few fixtures. Downside of the proces is that the tubediameter and the angles are determined by the lugs available. It is also possible to join tubes without lugs. In that case the tube-ends have to be mitered precisely, and the assembly is held in a jig. Steelframes can thus be brazed, or welded as we see with aluminium frames. By doing away with the lugs the designer gains absolute freedom with regard to tubediameters and angles. With steel lugged construction, you are usually limited to a 1" or 1.125" horizontal toptube, and a drop dependent on chainstaylenght, seat-angle and the available bracketshell. This is the reason you can often find small frames with a higher bracket than is used in the versions for longer legs, or the ghastly double toptube, in cases where a single large diameter tube would be more more efficient and less obtrusive.


Due to the lugs used, the tubediameters were fixed for a long time. If you wanted a stiffer frame the only way was to increase the wallthickness. This increased weight and stiffness in equal amounts. If you use an oversize tube, where the wallthickness is kept constant, but where the extra material is used to increase the diameter, things change. Look at the following table: a single 35 mm tube is twice as stiff as two 28.6 mm tubes, but weighs 30% less.
diam. x wallthickness 
22.2 x .8
25.4 x .8
28.6 x .8
28.6 x .9
28.6 x 1
31.7 x .8
34.9 x .8

Step through frames.
I'm not in favour of the step through design. You give away stiffness for free, which you have to try to recover with fat and heavy tubes. On the other hand, if the hips aren't what they used to be, or if you want to use the bike outside the holidays you might not agree. The worst thing to do then will be to buy a Dutch roadster, the opoefiets (grandma-bike) This frame is made with small diameter tubing, curved for extra lenght, and the tubes are quite close together, making a very flexible frame. What you want is a frame where the wheels will still track on the same line, even if you are honking up a hill and pulling on the handlebars. Torsional rigidity is achieved by using a stout toptube instead of two thin laterals, by raising the step-through height as far as possible and by using large diameter and thick tubing. Any attempt at design and elegance will usually result in a poorer frame.

Frames to measure
Do not buy a custom frame if you can be happy with an off-the-peg frame. This saves money and a long wait. However a custom frame will be sensible if you have special requests with regards to the construction, or if your bodydimensions do not conform to the averages used by the frame-factories. That will be the case if you are very large or small, or a woman, but there are also quite a lot of people with short legs-long bodies and vice versa. For instance if you have unusual short legs you are better served with a 'dachshund' frameset than by a small frame with a very long stem, where you will put an excessive amount of weight on the front wheel.

Whether a steel or aluminium alloy frame survives your holiday depends on the design, not on the material. However steel is more easily repaired. A frame from high-quality tubing will ride nicer than a cheap version with more heavy metal. An aluminium alloy frame will probably be stiffer than a comparable steel frame. Make a long testride and buy what you like. A decent fitting frame is worth more than lots of shiny parts.

Ranking of available -steel- tubesets
525 531 
753 853 631
Thron SL 
Nivacrom Thermacrom
  Uno series 
Zero series