What's so special about bike-specific torque wrenches?

Question

Prelude

I had figured that since I'm riding an alloy bike with a carbon fork, I will only need a torque wrench if/when I will want to change the stem—and even then, that's assuming the steerer is of the same material as the fork.

But then I lost the setting of an entirely ordinary bolt—the 22-Nm-rated saddle rails bolt—and along with it lost the fore-aft setting for the saddle, which I had carefully found last year by adjusting by 5mm, then 2mm, then 1mm over several weeks (I am wondering how a pro bike fitter could find this ideal setting in an hour or two, but anyhow). This year something rattled—only while riding, never on the stand—and it took me again several weeks to figure that the rails bolt was loose, meanwhile losing the setting.

Now onto the question

Torque wrenches are not a tool specific for bikes. I carry a tiny tool on the bike, and will leave the torque wrench at home, hence the weight and size make no difference. Quite the opposite even: Since I will use this tool only at home, I'd rather get something adult-sized, not one I'd have to tweak with the tips of my fingers.

There appear to be three distinct types of torque wrenches.

1. A wrench with a dial. You adjust the dial to a given value. When you use the wrench you hear just one (faint for 2-5 Nm, audible for 5-20 Nm) click. If you continue cranking past this point, you will exceed the dialed torque.
2. A wrench with a dial. You adjust the dial. When the value is reached, you continue to hear "click, click, click, ..." and you can continue to crank as much as you want, with no increase in the torque applied.
3. The wrench does not have a value to preset. You just use it. When the bolt locks, you start to pay attention to the dial and increase your cranking until the desired torque reading is visible. These in turn come in two varieties: (a) In a "beam-type" torque wrench, the entire dial is visible outside the tool. (b) More modern variations bury the beam and dial inside the tool.

It's easy to guess that the third kind just has a calibrated spring. I'm not entirely sure how the first two work (that detail would perhaps take us too far out of this question).

But assuming I'd prefer a wrench of Type 1 or Type 2, why would I look specifically for one made/marketed for bikes.

Non-bike-specific torque wrenches are rated at 20-200 in-lb (2.xx-22.xx Nm), 40-200 in-lb (4.xx-22.xx) Nm, as well as higher torques that are pointless for a bike.

This illustrates the first issue. General-purpose torque wrenches are rated in imperial (inch.pound), whereas bike-specific wrenches are (thankfully) rated in metric (Newton.meter) values.

Besides having to translate from metric to imperial, what's so special about bike-specific torque wrenches?

(Needless to say, a general-purpose wrench comes with no bits; one will need to assemble those oneself.)

Answer 1

Micrometer torque wrenches all have a minimum value below which they aren't accurate. In a perfect world this would be the lowest value displayed on the wrench, but in practice some have low-end values where things are dicey. (This is just my experience from using various of them on bikes and various bike-specific ones.)

A good small torque wrench for a bike is one where 3Nm and up are well into the accurate working zone. On a lot of more generalist wrenches, 3Nm or 5Nm are flirting with the bottom of the scale. There are a few bike specific wrenches out there that do this well in a way that's kind of hard to find outside bike tools. There are also a few purportedly bike-specific ones that are mediocre at it.

Many general or automotive type torque wrenches are sized such that the more cranky bolty type values (40Nm etc) are well situated in the accurate zone. There is nothing special at all about the bike-specific ones of this type, i.e. the Park TW-2.

Answer 2

The "click-style" torque wrenches need to be calibrated, so I shy away from them for bike use.

The simplest deflection "beam-style" torque wrenches, like Nathan's mention of the Park TW-2, just make use of the inherent resistance to bending of the metal "beams" they are made of (no muss, no fuss, no springs). It's easy to spot if one is out of calibration, as the metal deflecting rod's pointer no longer points to zero on the attached scale. Recalibration simply involves carefully bending that rod so the pointer again points to zero at rest.

The latter are cheap, too, especially old used ones. I find they tend to come with either 1/4" or 3/8" square drive and have both metric and imperial markings; to cover the full range of torques I see, I keep one of each size around.

Here is a 1/4" drive wrench (example only) with a 0-9 Nm range:

Here is a 3/8" drive wrench (example only) with 0-90 Nm range:

Answer 3

In addition to what @Nathan has said, essentially they are nothing special, bike specific wrenches often come with a set of hex and torx fittings that suit most uses of a torque wrench on a bike. So while the wrench is not bike specific, the and the fittings are general purpose, it is rare outside the bike world to need just the sizes supplied together. Personally I find having 'bike sized' hex and torx sockets in the same case as the wrench to be far more convenient than having them in a sperate case with a dozen others I never use.

Many home bike mechanics have no other need for a torque wrench, let along one with 3-20Nm range, and automotive shops typically do not carry them in these smaller sizes needed for bikes. Without 'bike specific wrenches' fewer home mechanics would own a torque wrench, leading to broken bikes, denied warranty claims and unhappy customers.

Cost wise without the bicycle industry, the volumes oft these size wrenches is low- it is possible to buy very good value, acceptable home use quality wrenches only because of the need for them in cycling pushing volumes up. Without the 'bicycle' wrenches we would all be stuck between the cost of the likes of Park tools (or other premium workshop quality brands) or not using one at all.