A torque wrench is a tool that allows you to tighten a bolt to a precise amount, so that it's as tight as it needs to be and — importantly — no tighter. Most torque wrenches let you know with a distinct click when you've reached a set tightness.
Why is getting the tightness of your bolts important? Too loose and you run the risk of a bolt coming undone, too tight and there’s the danger of causing serious damage to your bike and, as a result, to yourself. Over-tighten a seat clamp, for example, and you could ruin a carbon-fibre frame.
You’re too smart to let that happen? It’s easily done. The mechanics at your local bike shop will tell you about people who’ve cost themselves a lot of money by getting it wrong. Torque wrenches aren’t exactly cheap but buying one could save you a lot of cash in the long run.
You'll also need a torque wrench to install some power meters so they provide accurate measurements, though this is less common than a few years ago.
The amount that you should tighten a bolt varies between components, so always check manufacturers’ recommendations.
This Shimano Ultegra crank, for instance, comes with the instruction: “Each of the bolts should be evenly and equally tightened to 12-14N·m by torque wrench”. The N·m stands for Newton metre.
If you're wondering what a Newton metre is, it comes from the definition of torque. A torque is a rotational force. Force is measured in Newtons, as you'll recall from GCSE physics. Torque is the force multiplied by the distance between the point where it's applied and the centre of the bolt. You get a torque of 4 Nm by applying 4N to the end of a spanner a metre long, or — if you don't happen to have a set of stupendously large spanners — a 40N force on a 10cm spanner.
The right torque for a particular bolt depends on what it's made from, what the parts it fits into are made from and — if it's part of a clamp — what the thing it's clamping is made from, among other things.
Torque wrenches have become a must-have in the last few years because there's so much carbon fibre and very light aluminium in modern bikes. Clamps around carbon components can easily do damage if over-tightened, so a torque wrench is essential if you're handling such gear.
A torque wrench is also useful for big jobs, when you may not realise just how tight something needs to be. Square taper cranks, for example, typically need around 40 N·m, which is surprisingly hard to achieve without a long spanner.
Torque wrench types
Different torque wrenches work in different ways, but one common type allows you to set your required torque by turning a knob at the end of the handle. This one (above) from BBB costs £52.48. You fit the appropriate head, then turn the wrench until a distinct ‘clunk’ tells you that you’ve reached the correct torque.
To maintain accuracy, manufacturers of adjustable click-type torque wrenches usually recommend you send the tool back to the factory to be calibrated after a certain amount of use: check the manual for your tool's particular requirement.
If you can't live without an LCD display, then there are torque wrenches that'll feed your desire for digits. You can either read the torque from the display as you tighten the bolt, or set a target torque and it'll buzz and flash a light when you reach it.
One other option is to use something like this Preset Torque Driver from Park Tool (£39.99). This one allows you to tighten 3, 4, 5mm and T25 bolts accurately to 6N·m, clicking when you’ve reached the required torque. Drivers set to other torques are available.
You might also run across a beam type torque wrench like the Park Tool TW-1, above. This indicates torque with a pointer that simply indicates how much the tool's main arm has deflected as you turn the bolt. Beam wrenches are incredibly simple, very tough and don't have to be sent back to the factory to be recalibrated. If the pointer isn't on zero when the wrench is at rest, you just bend it until it is.
However, you can't set the torque in advance and get a satisfying click when you reach it, so beam-type wrenches have all but disappeared. Park no longer makes the TW-1 or its big brother, the TW-2.
5 of the best torque wrenches
Effetto Mariposa was one of the first brands to offer a high-quality, high-precision torque wrench specifically for bike use. This Pro version has a two-way ratcheting head for speedy tightening, a handy addition to the original's fixed head. It's not cheap, but it has a very useful 2-16 N·m range, it's very accurate and it oozes class.
Not as sophisticated as the Giustaforza, but much, much cheaper, this is a decent generic torque wrench at a very reasonable price. There are several very similar tools available: the X-Tools Pro Torque Wrench and Pro Bike Gear torque wrench are almost identical. Buy whichever you can find cheapest.
The Birzman digital wrench we mentioned earlier is no longer available, but this Topeak torque wrench with a digital display is very similar. It has a ratcheting head, a range of 1-20 N·m and can be set in 0.01N·m increments. To be honest, that's a tiny bit silly. It's hard to imagine needing more than 0.5N·m precision, but it's amusing for geek points.
If you need more oomph, the £159.99 D-Torq DX has a range of 4-80N·m.
Most bike-fettling jobs that really call for a torque wrench require fairly low torque values, like the 4-6 N·m range of the Park Tool ATD-1.2. It's quite expensive for a limited-function tool, but does what it does so well that it's very highly regarded.
Ritchey popularised the idea of a single-setting torque wrench with its first Torqkeys, but they were also supplied with just one hex size, which was fine if it was the one you needed, but a bit limiting if not. The latest version bundles a selection of useful bits with either a 4N·m or 5N·m body. Moulding a driver for a Shimano Hollowtech crank cap into the handle is a nice touch.
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Mat has worked for loads of bike magazines over 20+ years, and been editor of 220 Triathlon and Cycling Plus. He's been road.cc technical editor for eight years, testing bikes, fettling the latest kit, and trying out the most up-to-the-minute clothing. We send him off around the world to get all the news from launches and shows too. He has won his category in Ironman UK 70.3 and finished on the podium in both marathons he has run. Mat is a Cambridge graduate who did a post-grad in magazine journalism, and he is a past winner of the Cycling Media Award for Specialist Online Writer.