Steel has been literally the "backbone" of cycle frames for the last 100 years. For much of that time it has been unchallenged but now, with newer materials coming along, it is worth taking a good look at the material that most professional cyclists and nearly all cycletourists still choose to ride.
Steel is an alloy of iron with small quantities of other chemicals, principally carbon, but manganese, chrome, molybdenum are common. How these alloyed materials are used has a huge effect on the steel, and how the resultant tubes are treated after drawing has an equally important effect. To say a bike has a steel frame tells you very little about it's capabilities.
There's a lot of techno-waffle written about frame materials, but basically they have to fulfil two criteria. Firstly it's got to be stiff enough (resists bending forces) so that your effort isn't absorbed in bending the stuff back and forth and the thing keeps the wheels in line, and secondly it's got to be strong enough so it doesn't fail either now or in the future. These two are intimately related as I'll explain.
The first myth to dispense with is that top quality steels are stiffer than cheap tube. Sadly it's not the case - the "Youngs modulus" (the measure of stiffness) is the same for all steel. I've heard talk that top quality steels have less internal friction than cheap steel, but as I've never seen a tube manufacturer even mention this as a selling point I'm inclined to add it to the "myth list!". Lastly steel frames don't go "soft", the Youngs modulus of steel doesn't reduce with age or use (unless it's beginning to fail through fatigue), it's just a story put about by knackered pro's at the end of a season.
What all this means is that a frame made of mild steel will feel exactly like one made from a top tubing so long as the tubes gauges (wall thickness) and diameters are the same. In order to make a stiffer frame the wall thickness must go up, or the diameter of the tube must increase. By ovalising a tube the stiffness in one direction can be increased whilst it decreases by a larger amount at 90 degrees. This is used in tubing like Columbus Max to stiffen the bottom bracket area. Increasing the tube thickness obviously increases stiffness but adds weight, as does increasing tube diameter.
Compared to other metals steel is very stiff, far more so than Aluminium or Titanium, so tubes tend to have a skinny appearance. Aluminium needs bigger tubes, as its poor fatigue performance means that it cannot be allowed to flex back and forth like a steel frame as fatigue cracks will appear. This is why Alu frames have to be stiffer and more uncomfortable than their steel counterparts. This actually brings up an interesting point - how stiff does a frame need to be? This varies hugely with personal taste, strength and weight of a rider and the use to which the bike is going to be put. Most pro road cyclists like a frame with some flex to give a shock absorbing ride for long competitions. On short time trials or crits a stiffer frame might be better, though the amount of energy absorbed by a flexing frame is not great. More of a problem in a whippy frame are things like chain rub, instability when cornering hard or "honking" and in extreme cases "ghost changes" where the flexing frame causes gear cables to be stretched to the point where the bike changes gear on its own. With close spaced 9 speed cassettes this is a growing problem and one of the reasons that steel frames are increasingly becoming "oversized" to increase stiffness despite the poorer ride.
So why buy quality steel rather than gas pipe if there's no advantage in stiffness? Well its all down to the most important quality - strength... Lets look at the strongest steel cycle tube in the world, Reynolds 853. This tubing is made of a complex Chrome-Moly alloy. The tube is cold drawn, i.e. forced though a die at quite staggering pressure, then though rollers to give the final tube size and wall thickness. So here we have a basic tube about twice as strong as mild steel, and with twice the yield strength (the point at which it will bend perminantly rather than spring back). Then it goes through heat treatment - repeated heating and cooling in oil baths to increase the UTS (ultimate tensile strength) to three times that of mild steel. Now the clever bit. When most steel is joined using heat the resultant heating and cooling looses some of the UTS. With 853 (and 653) the heating and cooling of the joining process is taken into account and acts as a final stage of the heat tratment, lifting the UTS still further so that after joining an 853 frame is not only nearly four times as strong as mild steel, but over twice that of typical high quality Chr-mo, incidentally giving the same strength/weight ratio as titanium...
Now we can go back to the statement that a mild steel frame will be identical to ride to a top quality frame. The catch is that the mild steel frame would, if drawn to the wall thickness of an 853 frame, simply bend or fail entirely! If we say that we require a given frame to be capable of withstanding the power input of a pro sprinter for a number of years we end up with a strength specification. To meet that specification a Chro-Mo frame would need to have twice the wall thickness of an 853 frame, a mild steel frame four times! This translates into weight. A typical strong 853 frame can come out at 3 pound, a Chro-Mo 5+ and the mild steel option 8+.
Lastly when you buy a top quality tube it is butted. That is the wall thickness is greater at the ends where the greatest stress is and thinner in the middle. The lightest 853 frame tubing is 0.7 mm thick at the butts and only 0.4 mm at the centre. There is a downside to all this of course, and that is that the lighter tubes will be much less stiff, and it is the job of a frame designer to use different guages of tube to produce a frame which is light yet stiff enough for a given rider. This is why the 853 frame isn't a quarter the weight of the mild steel one, it would be too whippy. At the other end of the scale the mild steel frame will feel heavy, dead and unyielding.
So a class steel frame should be light, certainly competitive with other materials, and have a distinctive springy, shock absorbing quality that makes it feel alive. It's these qualities that make steel as popular as ever. Add its excellent fatigue characteristics, ease of repair and relative cost and you can see why steel has a long future ahead.©Geoff Husband