Pro and cons of "torsion based" torque wrenches

Question

Brands like Canyon are delivering their bikes with torque wrenches that are based on the torsion resistance of the metal that mades up the wrench, like the one pictured below (for the working principle).

To describe it simply: once the bolt is ready to be tightened, applying torque will twist/deform the metal proportionally to the torque. A needle and gauge allow a measure of the deformation, and hence the torque.

Given they only cost a fraction of more professional wrenches, a tool like this would be relevant in the toolbox of those who do their maintenance themselves. I'm wondering how "good" they are:

• does the torsion characteristic of the metal evolve over time or with temperature, of if "abused"?
• do they offer a level of accuracy that is compatible with torque-sensitive applications, such as the tightening of a carbon seatpost?
• what are the attention points when buying one?

What would be the pros and cons compared to a more traditional torque wrench?

Answer 1

Note, except where I mention a specific model, this is a discussion of the flexure type of torque wrench, and not any specific example.

This type of design should be OK for occasional use in easy conditions. It does have a few downsides:

• It's bulky
• You need to watch it constantly. In a busy workshop this isn't good. Consider a set of bolts, like stem bolts: once the first is engaged and you're using the tool, your other hand is already reaching for the next bolt, and so are your eyes. The clicky type won't over-torque doing it up blind
• You really need to be able to get a good look at it to read it accurately. I suspect the Topeak review quoted in the comments was either hard to read or something had got misaligned. The design means you're quite likely to be looking off-axis which will cause an error.

I don't have any carbon bikes, and haven't calibrated my torque wrenches, but I prefer the "torque screwdriver" style. Mine goes up to 8 Nm but I normally leave it on 5 Nm. This doesn't have enough leverage for crank bolts, cassette lockrings etc. I just do mine up tight these days, having got enough of a feel for it. When I did feel the need for measured torque on those I used my automotive-style torque wrench with a 1/2" square drive. That doesn't go light enough for little things

Answer 2

As I understand it, if a torsion torque wrench is accurate on the day it's manufactured (i.e. the scale is in the right place), it will remain that way unless physically damaged.

The click type wrenches need to be recalibrated. Many people do it annually, but in high precision uses, I've heard of them being recalibrated daily.

NOTE: This is what I've read on the internet, and as Abe Lincoln once said, "If it's on the internet, it must be right". However, I've seen it in enough places that I believe are credible that I do believe it to be accurate. I'm sure with some research, actual, tested results could be found.

The torsion beam wrenches do take more work to ensure that you are setting the right torque because there's no indication of when the desired torque is reached. For the inch-pound settings that are common on bikes, this isn't really an issue for those doing their own maintenance (other comments about "production" work, as in a shop are valid).

For working on a car, where you may be pulling for all you're worth to hit 180 ft/lb, that positive audio & physical feedback can be critical because you can't see the scale and turn the wrench at the same time.

Answer 3

Those "beam type" wrenches are the best. Unless you spent few hundred dollars, you cannot buy a better torque wrench, "the clicky ones", I mean. Cheaper ones are rubbish, and if you have a such tool, you also need a decent calibration device. You shouldn't drop them. You shouldn't use them for unscrewing things, or use them as a crowbar, cheater bar, etc. You shouldn't exceed maximum torque values. They are quite fragile.

Beam types are cheap and effective. Problem with all types of torque wrenches is, the holding hand: I've seen guys on Youtube, shooting videos and entitling themselves experts, using torque wrenches like ratchets.Yes; heat effects them, but heat effects anything and everything. Yes; they get "softer" in time, but at that point, you would have probably tightened a few million screws already, and the handle would wear out first :)

Proper carbon fiber is not that fragile, to be destroyed by %5-10 more torque. If it cracks under such torque, let it be, it's too dangerous to ride anyway...

Answer 4

This is one variation of the type of torque wrench that you should be using. The other variation is a beam-type wrench.

Click-type wrenches go out of calibration. How they are calibrated is with a torsion type wrench or a beam-type wrench (well actually not a wrench but a calibration apparatus but its principle of operation is the same).

Also, click-type wrenches can only tighten in one direction usually. A problem on bikes where left-hand threads are sometimes used (pedals, bottom bracket).

My advice, however, would be a beam type wrench and not a torsion type wrench. Beam type wrenches generally are easier to use in spaces where a torsion type wrench can't possibly fit.

The wrenches have sometimes a small parallax error, but making sure you look at the scale perpendicularly is enough to eliminate this error in practice.

The problem with a click-type wrench is that you should calibrate it often, actually so often that it would be a major expense, and the calibration probably costs more than buying a new wrench. Beam-type wrenches are also much cheaper than click-type wrenches.

Remember that the only reason professionals can use click-type wrenches is that those wrenches are calibrated often.

My wrenches are 0-200 Nm automotive beam-type wrench + 0-7 Nm very old Park Tool TW-1 beam-type wrench. However, today you can buy Park Tool TW-1.2 which goes up to 0-14 Nm, and is better than the old 0-7 Nm wrench since it covers more use cases. You still need an automotive beam-type wrench for the jobs that require a lot of torque.