In order to measure how firmly we are tightening bolts, we use torque. There are two methods of doing this: the instinctive, common-sense version and the scientific version. Both have their advantages. Traditionally, the manufacturer of a part indicated to the mechanic how firmly things should be tightened by fitting an appropriate bolt.
Delicate parts, which just need holding in place, come equipped with small bolts. The spanners that fit these bolts are short so that you don’t have enough leverage to overtighten the bolt. Parts that need to be clamped down firmly come with a big bolt that you can attach a nice hefty spanner to and lean on. This used to work well enough, but as riders we’re demanding lighter equipment all the time, so manufacturers are designing components with less room for error. For example, replacing steel bolts with aluminium ones will save weight, but aluminium bolts are far less forgiving of overtightening; once stressed, they can snap without warning.
Overtightening bolts can also strip the thread that you’re bolting into. This is a common problem with aluminium parts. For example, overtightening the bolts that hold the stem to the handlebars can damage the thread inside the stem, so that the bolt rotates uselessly rather than securing the bars.
The reverse problem, undertightening bolts, has a more obvious consequence; whatever you’re trying to secure will rattle or work loose. Crank bolts often suffer from this – the left-hand one in particular needs to be tighter than people imagine. The first warning is usually a regular creaking noise as you pedal. If you ignore it, the crank bolt works loose, allowing the crank to shift about on the bottom bracket axle. This damages the mating surface between bottom bracket and crank so that even if you retighten the crank bolt, it works loose constantly.
As a consequence, it’s becoming more vital to know exactly how much force you’re putting on any specific bolt. This is especially true for suspension forks, where the bolts that hold the moving parts together are constantly being stressed by the cycling (moving up and down) of the fork. Most components now come with a tightening torque specified for every bolt.
Since torque specifications are a relatively recent obsession, most come quoted in Newton-metres (Nm). The imperial equivalent unit is the inch-pound (in-lb). To convert inch-pounds into Newtonmetres, multiply by 0.113.
However, it’s one thing to find out how tight a bolt is supposed to be and quite another to be able to tighten it to exactly that amount. There is a workshop tool that allows you to do this – a torque wrench. It looks like a ratcheting socket handle and works in a similar way. Standard socket heads fit onto the wrench, which can then be set to the specified torque by turning a knob at the base of the handle. The wrench is then used to tighten the bolt as normal. When you reach the correct level, the handle of the bolt gives slightly, and you hear a distinct click, telling you to stop. These tools are simple and reliable to use and are becoming more and more common in bicycle workshops. A well-equipped workshop will have two torque wrenches. A small one, with a range from about 4 to 20 Nm (35 to 180 inch-pounds), covers delicate applications such as cable clamps and rotor-fixing bolts. A larger one, with a range from 20 to 50 Nm (180 to 450 inch-pounds), covers those that need more force, like crank bolts. The two sizes are necessary because the tools always work best in the middle of their range.
They used to be regarded as too expensive for home workshops but reliable versions are now available from around £60. The smaller size, (4 to 20 Nm), is a particularly good investment. If you don’t own a torque wrench of your own, if you ever get a chance to borrow one use it to tighten a selection of the bolts on your bike to the specified torque setting, to get a feel for how tight they should be. Many mechanics use torque wrenches to set bolts to the correct level regularly to remind themselves what the correct torque feels like.
When working by feel, be aware of the size of the bolts you’re tightening and use this as an indication of the amount of force you should be using. Small bolts take small spanners (or thin screwdrivers) and so should be tightened firmly but not excessively. If you’re overenthusiastic with a delicate bolt, you’ll strip the thread, snap the head off or round off the key faces. Large bolts or those that have to be tightened with chunky tools, like bottom bracket cups, should be wedged home with vigour.
The best place to find torque specifications are the instructions that came with the component, which will have the right torque for your specific make and model. New bikes come with a pack of booklets and leaflets, covering all the parts fitted to your bike. You may have to ask for it when you buy the bike. If you haven’t got the instructions any more, use the Park Tools website to reference general torque specifications. Park Tools’ website is www. parktool.com, and the page address is www.parktool.com/repair/howtos/torque.pdf.
All specified torques assume that the bolt you are using has been greased so that it turns easily in the threads, and that both parts of the thread are clean and in good condition. A dirty, damaged bolt will be harder to tighten than a clean one and so will give a false torque reading.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES