Is steel is the ideal material from which to make a high impact bike frame?

Tensile Strength.

When considering how strong a material is, we have to think about where we are measuring this strength.

All the metals considered for the construction of bike frames are ductile. This means they stretch before they break. Steel is very strong and takes a lot of force before it yields, a good feature of a high impact bike frame. It also yields a lot, so it gives warning of impending failure. Aluminium, although it yields, is not as strong as steel and does not yield to such a degree, and so we find it outshone by its ferrous partner. Cfrp, although it is immensely strong, hardly yields to a degree that the yield point is nearly the same as the failure point. Therefore no warning of breakage would be given to the rider and a sudden failure could result in injury. Titanium has fantastic elongation figures and yield strength, but is not commonly used for frame construction. The reasons for this will be discusses in the ‘titanium’ section of this presentation.

Density

Steel, although it has many other admirable qualities, is a fairly dense metal. Weighing 0.283 pounds per cubic inch, it's almost twice as dense as titanium (0.160 lb/in2) and nearly three times the density of aluminium (0.098 lb/in2). This seemingly heavy weight however, when weighed up against its other properties, such as its good strength and stiffness can be all but forgotten.

CFRP

CFRP is a very strong material with which to fabricate a high impact bike frame. Areas in which it is let down, however, are as follows. CFRP has a very low elongation at breaking point, which means that it gives virtually no advanced warning of failure. It is also easily terminally weakened. A steel frame would shrug off a stone impact with little damage. On a CFRP frame however, the same impact would break fibres in the tube structure causing a weak point in the frame that spreads. CFRP is also almost prohibitively expensive to anyone other than sponsored riders.

Titanium

As titanium has been compared to steel in other sections of this presentation we have seen how titanium often out performs steel in such areas as density and percentage elongation before failure. Titanium is also inert in any atmospheric condition on this planet, a property that steel cannot nearly match. So why aren’t all frames made from titanium? The simple answer is cost. Titanium is expensive to extract and is extremely difficult to manipulate and form, leading to further increases in cost for production of a frame. Although it may seems like a ‘wonder-metal, titanium is simply too expensive for the high impact type market where frames regularly get broken.

                                       Stress             

     Young’s Modulus =                     =                                                                           Strain      

Aluminium

Aluminium outperforms steel in many areas such as density and corrosion resistance, and with the right manufacturing procedures can be made stiffer than steel (with the use of over-sized tubing for example). Aluminium is a very good, relatively inexpensive material to make a frame from, but steel still surpasses it in key areas where high/repeated impact situations are to be considered. Steel is naturally more elastic than aluminium, and can deform with applied loads and return to its original form with no detrimental effects to strength etc. Every applied load, however, fatigues aluminium. This is not a property idea in high impact situations. Aluminium is also more brittle than steel and deforms less than steel before it fails. In a situation with repeated impacts, advanced warning of failure is a very good idea.

Summary

From the analysis of contending materials in this presentation, I can conclude that steel is the ideal material from which to make a high impact bike frame. It has a good modulus, good elongation percentage and high strength and its inherent ‘springiness’ gives a comfortable ride. It is also relatively cheap. A good, high impact design steel frame will cost around £250, whereas a good alloy frame will cost twice as much or more. Price, in a section of the sport in which frames are regularly damaged, is an important factor.