You’ve probably seen the little numbers on your stem and various other places on your bike where hex bolts are. They usually say something like ‘5 Nm’? If you don’t know what they mean or what their significance is, then you’d better keep reading.
Torque wrenches have been around for years in mechanical engineering – motor mechanics and aeronautical engineers for example – and they are used to ensure that the bolts the engineer is tightening are tightened to exactly the correct amount. A good example of where they are used in in the multiple bolts that hold the cylinder head of an engine together. As you might imagine, it is important that the bolts are equally tightened to ensure that the two flat surfaces are held together with equal force. Fine, you may ask, but what has this got to do with the humble bicycle?
In years gone by, bicycles were more often than not simple mechanical objects made mostly of steel. The mechanic would tighten everything up, and that would be that. The steel was strong, and he knew not to overtighten it. But the modern bicycle is engineered quite differently from the steel relics of the past. Today they are made of exotic carbon and aluminium and titanium; altogether lighter and more delicate materials which need more careful looking-after than steel. Carbon Fibre, in particular, especially when formed into lightweight components, can be quite brittle when stressed in the wrong way. The relevant torque settings must be applied or the material can crack and the carbon lose its structural integrity.
As a result, the torque wrench is now the staple tool of the serious bicycle mechanic, and the different torque settings are liberally printed on the relevant parts so that they are done up to the correct pressures. Many new bikes bought directly are assembled by owners who may not have the know-how of the professional mechanic, so the manufacturers try to help out as much as they can by printing the settings on the parts.
So how does the torque wrench work?
As you might expect, there is more than one type. The cheapest and simplest is the beam-type wrench which uses a scale and a needle to let you know when you’ve reached the correct tightness. After this simple tool come the pre-set meters which either click or spin round freely when the tightness setting is reached. It all sounds simple enough. Sadly it’s not. Below are some important things to remember when you’re using a torque wrench,
- Make sure that both the threads of the two parts you are connecting are clean of dirt and/or rust. Muck in the thread will make the screw harder to do up in the thread and could inadvertently trigger the stop point in the torque meter before you’ve reached the required tightness.
- Carbon Fibre and aluminium have very little tolerance to overtightening so don’t assume that ‘one more turn’ will be better. Engineering is a science, so when they say 5 Nm, they mean 5 Nm!
- If you’re tightening two bolts to hold, say, the seat post or the stem, ensure that the two bolts have an equal amount of torque applied. Keep tightening them up a little in turn to spread the load evenly between them.
- The dangers of over or under tightening can be severe; incorrectly tightened stems and seat posts can break off when under pressure – such as when you’re sprinting, and bolts can sheer in the thread which is a very expensive failure.
Finally, a little note about ‘Nm’. It stands for a Newton metre, and one Newton Metre is equal to the torque resulting from a force of one newton applied perpendicularly to the end of a moment arm that is one metre long. Got that?
Joking aside, those little numbers printed on your frame and elsewhere are very important and should not be ignored. You do so at your peril. If you haven’t got one yet, then it is time to get a torque meter.