If you like working on your bike, by now you should be used to turning and tightening bolts with a hex key or Allen wrench. Have you ever wondered that perhaps you’ve turned a bolt too tightly though?
WHAT IS TORQUE?
Most bolts on a bicycle actually have a torque specification. If you don’t remember your basic physics, torque is a measure of rotational force applied to an object along an arm of a given length. Torque is usually expressed in newton-meters (Nm) in the metric system, or pounds-feet (lb-ft) in Imperial measurement. In mechanics’ case, this arm is usually a wrench of some sort.
Normally you can turn a bolt in by hand until its head becomes flush with (and snugs up against) the surface you’re screwing it into. Once finger-tight, further tightening happens with a wrench, where you apply more and more torque to the bolt. As this happens, the tension also builds up along the length of the bolt you’re tightening, due to the interaction of the threads of the bolt and the receiving object. Exceeding the torque specification for that bolt can mean breaking it clean in half, as the material gives out under the tension. These broken bolts are quite hard to extract.
For the most part, metal parts are quite tolerant of a little excess torque. Torque specifications become really important, however, when you’re dealing with carbon fiber parts. Carbon fiber is a wonderful material, but one thing it’s not strong against is crushing force. If, for example, you have a carbon fiber seatpost and exceed the torque spec on its seatpost clamp, you will almost certainly damage the seatpost. Proper torque is also very important for the parts of a folding bike’s hinge and latch mechanisms, even though they’re made of metal.
TORQUE WRENCH TYPES
The dangers of applying excess torque are why a torque wrench is a worthwhile investment for serious mechanics. There are two general types.
A beam-type torque wrench is a socket wrench with a graduated scale, which you use to read the deflection of a freestanding secondary beam. That beam’s deflection off the main wrench arm is the applied torque. Beam-type torque wrenches are simpler and cheaper, and don’t need any maintenance, but require you to keep an eye on the scale. They’re best used for high-torque applications, such as crank arms and lock rings for cassettes or Centerlock brake rotors. These parts see torque loads in the 35-55 Nm range.
A micrometer-type torque wrench is different. Like its namesake, the end of the arm has a rotating collar used for measurement. On a micrometer caliper, this widens or narrows the caliper arms; on a micrometer-type torque wrench, it dials in the desired torque value. Once this value is reached, a clutch mechanism will release and give you both the characteristic “click” sound and slight deflection of the arm. Micrometer-type torque wrenches are more suited for the low-torque applications of most other bolts on a bicycle, but they do need periodic recalibration – and proper storage to reduce the need for recalibration.
A sub-type of micrometer-type torque wrenches is the torque key. These are simpler devices which retain the clicking clutch mechanism, but ditch the adjustability. They are, in essence, made to stop at just one preset torque value, usually the most common 4 or 5 Nm – although Park Tool’s newly released ATD-1 adjustable torque driver has five fixed torque values you can select.
We’ll be looking at Bike Hand’s YC-617-2S micrometer-type torque wrench today. This exact torque wrench can actually be seen sold and rebranded under various names.
- Micrometer-type construction
- 1/4″ square drive socket
- Reversible ratchet head mechanism
- Included bits: 3mm, 4mm, 5mm, 5mm long, 6mm, 8mm, 10mm, T20 Torx, T25 Torx, T30 Torx
- Torque rating: 2-24 Nm
IMPRESSIONS AND USE
Bike Hand’s torque wrench comes in a blow-molded black plastic box, made to house the wrench itself and all its bits. A metal double-hinge clasp keeps it shut.
The selection of bits should guarantee that it will work with almost any bolt on your bike. On my Vitesse, 95% of its bolts can be worked on with a 5mm hex key. The larger size hex key bits are for use on kickstand mounting bolts (8mm) and freehub bodies (10mm). The 5mm long hex key bit is particularly useful for tightening the clamp band bolts on my TCX SLR 2‘s STI levers, which are slightly obscured by their brake hoods. The T25 Torx bit should be handy for working on brake rotors, as well.
I like that all the bits themselves have a knurled collar around them. This enables you to turn loose bolts in by hand until finger-tight, before bringing in the torque wrench to tighten to correct torque. You press in the button on the head of the torque wrench to fit the bits onto the 1/4″ square drive socket, where they will stay on very securely.
Turning the collar sets the torque, displayed by a moving red column along a vertical graduated scale. This lets you use Bike Hand’s torque wrench on low torque applications such as stem and seatpost bolts, as well as higher torque uses, such as the 15-18 Nm needed to tighten the TCX SLR 2’s saddle clamp.
I find that for torque loads of less than 4 Nm, extra vigilance is needed as the clutch mechanism clicks very softly and the deflection isn’t very noticeable. At 5 Nm and up, the clutch mechanism works much better and more noticeably.
For more instruction on how to properly use a torque wrench, the boys of GCN have a video on it below.
I’ve had this torque wrench for quite a while now, and it’s a reliable tool to have, especially while working on the TCX SLR 2’s seatpost and stem bolts where setting correct bolt torque is critical. At PhP1,800 to PhP2,000, it’s not exactly cheap, but it’s one important tool to have in your arsenal if you’re serious about wrenching on your own bikes.
This article was originally published on the now-defunct United Folding Bikers blog on July 22, 2015. It has since been slightly updated.