Why you shouldn’t bend metal
Resist using lockdown time to ‘adjust’ wonky parts unless you’re ready to buy new ones
During these times of garage confinement and increased bike fettling, it’s tempting to tweak that bowed brake lever to straighten it out. But be ready to order yourself a new one if you try. All the metal on your bike has a grain. This is a crystalline structure built on how the atoms are arranged and the lattices growing from them, and it determines the metal’s properties. Iron has a simple cubic lattice. Aluminium is a more complicated but more ductile ‘face centre cubic’, while titanium is ‘hexagonal close packed’ and so hard to make into things.
Grain structure can be tweaked by adding other materials to a molten metal. Aluminium promotes a finer grain in steel, for example. Treatments and processes make a difference too – grain size is influenced by the material’s temperature history, and various heating and cooling processes can tailor the properties. Heating and rapid cooling creates hardness, for example, while annealing (slow cooling) promotes ductility and toughness.
Deforming metal by forging or rolling lengthens the grain and flows it in one direction, increasing strength and other features in the direction of flow – and decreasing it in others. And this is why you shouldn’t rush ‘You’ve messed with the lever’s grain’ to bend a lever straight after a comedy topple. You’ve messed with the grain.
Bending the lever is strain and the material’s internal resistance is stress. Metal has an elastic region, where it ‘gives’ slightly under load before returning to shape. Beyond the elastic limit lies plastic deformation where it stays bent (and actually becomes easier to bend). Levers are usually cast aluminium alloy, where molten metal is poured into a mould and the grain settles how it sees fit (like making a jelly). When your bike falls and a lever hits the ground it suffers plastic deformation. This is a type of cold working, causing massive grain elongation and drastically changing the properties. Cold working also causes strain hardening, reducing ductility and making the metal difficult to work (shape) any further.
So, when you grab at the bent lever and try to straighten it, this extra cold working is too much and the less ductile, strainhardened metal snaps. Metals have a recrystallisation temperature where new, unstrained crystals form from the original distorted grains. This means that heat treatment can be used to restore both your lever’s original shape and its properties. However, the best bet with a wonky lever, for both safety and ease, is to replace it with a new one… so get ordering now.