LBS and Torque Wrench

[slowmotion]

I was taught NOT to use a torque wrench to undo or loosen fittings. What do you think?

That may well be the case, but both of mine have a reverse rotation switch, presumably for some purpose. Both have 3 point hand-written calibration certificates too, but I don't suppose that's going to convince the "finger-in-the wind-experienced-old-timer" lobby.

 

[DCBassman]

It's my understanding (again, probably gleaned from my auto mechanic dad) that reversible torque wrenches would be ok for undoing, but are meant for LH threaded bolts. And, as pointed out, this is all well beyond anything needed for bicycles.

 

[gbb]

I'm in two minds 're torque wrenches.
TBH, a LBS repairing other folks bikes probably should have a TW, if only to protect themselves from criticism, possible damage and to leave customers confident.

On the other side, as a fitter, the only time I've ever felt the need for a TW was on a car. Tòols are my day to day stuff and even on my carbon bike, I've never felt the need to use one. Now that's my choice and my risk, one I'm confident I wouldn't cause any damage, two, it's my stuff, my risk.

It's down to experience IMO, someone not confident or used to using tools may doubt the safety of what they're seeing, someone with years of experience will simply know what feels right (with great certainty....but not infallible)

I'd suggest if it bothered OP, don't go back there.

 

[Drago]

Butnif an LBS does have a torque wrench, do they get it calibrated every 6 months, or even more often? Simply backing off the tension to far can cause many otherwise quite good wrenches to go out of kilter. Simply using a torque wrench isn't enough - the device needs to be very regularly maintained and calibrated to remain effective.

 

[DaveReading]

those torque values are supposed to be upper limits, not targets, I believe?.

On a bike, maybe (though I don't see why). In general, no. You'll have cause to be grateful for that next time you fly, when the engines don't fall off.

 

[winjim]

I would add that I don't think a torque wrench should be used for tightening or loosening as such. It's a measuring device. Use a regular socket set to tighten and the torque wrench to check.

 

[Profpointy]

I would add that I don't think a torque wrench should be used for tightening or loosening as such. It's a measuring device. Use a regular socket set to tighten and the torque wrench to check.

That ain't going to work for tightening as it goes "click" when you reach the mark. If you tighten with a spanner first then if it goes click then all it tells you is it's too tight

 

[tincaman]

I only use a TW when the component I am fitting has more than one bolt which have to have even torque on them such a stems, pedal cranks, handlebars etc. Otherwise its done on feel.

 

[slowmotion]

I can see that the forces required during the process of undoing are not going to be the same as those that were required during the tightening operation, but I still think that the two forces will be very similar at the point at which the screw actually "breaks away" and untightens. I'll be reunited with my torque wrenches in a few days and will try a couple of simple tests.

I just did a quick trial, and it looks like @Sea of vapours is absolutely right. An M8 screw tightened to 17 Nm actually undid at less than 5Nm.

 

[Andy_R]

I do trust the mechanic in this, because I know what 140 Celsius feel like just by touching the milk jug, I know when a food is above 63C by touch, I know when a fridge is below 5C just by opening it.
I have confirmed this knowledge many times at work by checking with calibrated thermometers.
I have also tightened crank arms without torque wrench (got one now), go tit checked by a mechanic, he said I was spot on
What I'm saying is if you do routine jobs at work, after years of doing them you know stuff by feel - not the crank arms in my case, that was just a fluke

...

 

[Drago]

On a bike, maybe (though I don't see why). In general, no. You'll have cause to be grateful for that next time you fly, when the engines don't fall off.

Mate of mine is a maintenance engineer for a budget airline. Hydraulic testing consists of opening the hatch and looking for evidence of a leak. I doubt they've the budget of a torque wrench.

 

[Yellow Saddle]

Hi all,

On Saturday, I had a need to use a bike shop outside of my area while visiting family. The shop has a fantastic reputation in the area. While I was there, I asked him to chuck some carbon grease on my seat post and I was surprised to see him NOT use a torque wrench when tightening it back up, on a carbon frame.

Would you expect this? I am not mechanically minded but I have always been told that you should ALWAYS use the correct torque on any carbon frame fixings but maybe I am naive and falling for industry hype. I guess he was confident of his own judgement but why would a bike shop take any risks.

I welcome your thoughts

Mike

An experienced mechanic can usually do a bolt up within a reasonable margin of error and the correct torque on a single bolt is not that critical. Even a 50% error is not serious. However, once the fixing uses two bolts then things become critical. A good example is a stem. Usually the front cap is held on with four bolts. These should be evenly torqued. The exact setting isn't important (most torque wrenches have a large error in anyway). Consistency is more important than accuracy. The reason for this is that if one bolt is tighter than the others, then that bolt takes dis-proportionally more strain than it's neighbours and it will fail quicker than the others. If there is redundancy (like with a four-bolt stem), then the failure won't be catastrophic, merely inconvenient. But, the picture changes when there are only two bolts.
Bolts made from aluminium need extra care. They don't like cyclical stresses (as in a stem) when they are operating outside their design limits.

 

[DaveReading]

I'm struggling to understand how overtorquing a single bolt is less critical than overtorquing one of a pair.

They are both going to take more strain than intended, and nothing has less redundancy than a single bolt, which has none.

 

[Profpointy]

I'm struggling to understand how overtorquing a single bolt is less critical than overtorquing one of a pair.

They are both going to take more strain than intended, and nothing has less redundancy than a single bolt, which has none.

My understanding is there's actually a fair bit of leeway for any given bolt, but if there's a pair it's better that they are done up similarly even if both wrong (within limits of course).

I've never bothered with torque wrench on a bike, but I've never had a carbon fibre bike which apparently is more critical to over torquing. Even on the car I've only ever bothered for cylinder heads. Rightly or wrongly I reckon I kind of know what's about right whether that's camera screws or car bits. I would bother for cylinder heads, or for things that need doing up bloody tight

 

[Yellow Saddle]

I'm struggling to understand how overtorquing a single bolt is less critical than overtorquing one of a pair.

They are both going to take more strain than intended, and nothing has less redundancy than a single bolt, which has none.

Good point, I should have expounded. Whenever there is a single bolt in a stem, it is usually a 6mm bolt and over torquing it is less critical than over torquing bolts in pairs or double pairs, because the latter are invariably much smaller, with 4mm diameters. The larger the bolt, the less critical its torque becomes (wrt to the bolt, not the device it is clamping) because the margin of error is so much larger.

Even this may not be so obvious, but the area of a 6mm bolt is 28 mm2 whereas the area of a 4mm bolt is 12mm2. That 2mm makes a huge difference. Strength is analogous to area.

On some stems, there are 4 x 4mm aluminium bolts. Those are super critical.