Materials for handcycles
People often ask me what is the best material for bikes? Steel, Aluminium, Carbon, Ti? After a quick search I realized that there is a ton of misinformation and partial information out there, and that the record needs to be set straight.
In the bike industry aluminum comes in a couple of different alloys: 6061-T6 and 7005-T6. 6061 is alloyed with both magnesium and silicone to make a moderate strength alloy, and 7005 is alloyed primarily with zinc to form a high strength alloy. T6 refers to the heat treatment and temper.
Welding aluminum alloys
The most important thing when considering aluminum is the ease with which they can be welded, and the strength of the weld.
When aluminum is welded, the weld zone becomes annealed. The key difference between the alloys is that 7005 recovers some of its strength after welding in a process known as age hardening (or low temperature precipitation hardening), while 6061 requires an expensive high temperature heat treating process to regain strength and temper.
- 7005 has approximately 60% of its original strength after welding and age hardening.
- 6061 has approximately 30% of its original strength after welding.
Due to the cost and inconvenience of heat treating, 7005 aluminum is gaining popularity with smaller builders and for lower end bikes. 6061-T6 heat treatment requires that the metal be solution heat treated and then artificially aged. Typically this involves dipping the welded frame into hot brine at about 1000 degrees farenheight for 2-3 hours (solution heat treating), and then quenching and keeping the frame at a high temperature for an additional 2-3 hours a process known as artificial aging.
The result is a joint that is roughly 90% of the strength of the base material.
For this reason it is vital that all bicycles and handcycles that are made from 6061 aluminum need to be heat treated. If they are not heat treated they are dangerous.
With 7005 the frames should be made from material that is 30% thicker than is necessary (which makes the bike 30% heavier). So comparing the materials:
|Material||Density (kg/m3)||Yield Strength MPa||
|Weld compensation factor||Strength to weight ratio|
In the absence of heat treating this makes 4130-N cromoly is the strongest and lightest material of the three considered here for bikes and handcycles. There are of course many other factors to examine when considering materials, but I will save that for another time.