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.
- 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.
- 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.
- 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.)