Only 5Nm on a stem/handlebar clamp?

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gregoryoftours
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Re: Only 5Nm on a stem/handlebar clamp?

Postby gregoryoftours » 14 Nov 2019, 11:44pm

By the way bars that have a rough textured section where the stem clamps like these are designed to be installed without the need for carbon assembly paste.

Manc33
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Re: Only 5Nm on a stem/handlebar clamp?

Postby Manc33 » 15 Nov 2019, 12:04am

Tightening 4 bolts to 5Nm each is going to be the same as tightening one bolt to 20Nm. The amount the stem is being clamped is going to be same in each case.

I didn't use the F word this time. :P

So the bars should say something like "Combined Nm: 20".
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Brucey
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Re: Only 5Nm on a stem/handlebar clamp?

Postby Brucey » 15 Nov 2019, 12:15am

Manc33 wrote:Tightening 4 bolts to 5Nm each is going to be the same as tightening one bolt to 20Nm....


sort of; the fastener tension generated depends on the design of that bolt though.

Maybe this thread should be retitled

'is it a surprise that some cheap stuff you can buy on e-bay isn't clearly marked and doesn't come with clear instructions?'

cheers
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Redvee
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Re: Only 5Nm on a stem/handlebar clamp?

Postby Redvee » 15 Nov 2019, 1:21am

A lot of people overtighten faceplate bolts without realising it. Like Mick I do mine up so the gap is equal, to the eye, top and bottom, then I got a torque wrench and tightened the bolts to the recommended Nm and was surprised how much more I could turn the bolts with an allen key after using a torque wrench.

Brucey
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Re: Only 5Nm on a stem/handlebar clamp?

Postby Brucey » 15 Nov 2019, 1:42am

richardfm wrote:
CX-3 wrote:An engineer told me before he died...
(Hands up if you know the rest of that pub outing song!!!)
...that an allen key would tighten to the correct torque for the size of bolt being tightened if it was done up hand tight, firmly but without using undue force. Something to do with the diameter of the head and the length of the key. Obviously he was talking about "proper" workshop keys, not the cut down version carried by cyclists. No need to lean on the key or use an extender bar.

An Engineer Told Me is one of the songs we used to sing at sea when I was in the merch many years ago. That was also where I learnt that if you use the correct spanner or allen key then hand tight firm is right. I've only ever used torque wrench for cylinder head bolts and wheel nuts/bolts on cars and caravans


Tests have shown that skilled mechanics can be about as good as a torque wrench at setting bolt tension, using standard tools. But torque wrenches don't get hangovers, or distracted, or have off-days. And sadly, skilled mechanics are vastly outnumbered by the other sort. The very simple fact is that folk vary considerably in strength, so it immediately becomes a nonsense that 'the tool is automatically the right length to do the job'.

Added to this is the fact that the same size bolt heads are used on bolts with different threads on them and the same size threaded bolts have different heads on them too. So for example on a bicycle you may find M5 threaded nuts/bolts with

8mm hexagon
9mm hexagon
5mm hex socket
T25 torx
Other torx size
4mm hex socket
3mm hex socket
2.5mm hex socket

heads, amongst others, depending on whether they are standard bolts, various kinds of allen bolts or grub screws. The torque required in each of these applications is not always related in any way to the spanner/key size in use.

So I think that many folk who avoid breakages and loosening of fasteners may not so much be very consistent about how tightly they do bolts up as lucky or using up more of the tolerance built into the design than they might imagine possible.

cheers
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NickJP
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Re: Only 5Nm on a stem/handlebar clamp?

Postby NickJP » 15 Nov 2019, 2:00am

Mick F wrote:I'm nowhere near having them so tight that I break anything.
I reckon the threads would strip way before anything broke.

That might have been valid in the days of quill stems and sleeved aluminium handlebars (eg Cinelli Campione del Mondo), but modern lightweight parts are not nearly so forgiving. Here are a couple of photos I took of the top and underside of the central clamp section of an ITM C-F handlebar that I managed to persuade a friend he should no longer use. The carbon fibre has been indented by the faceplate bolts on the threadless stem being over-tightened - failure wouldn't be far behind:

Image

Image

pwa
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Re: Only 5Nm on a stem/handlebar clamp?

Postby pwa » 15 Nov 2019, 8:59am

Manc33 wrote:Tightening 4 bolts to 5Nm each is going to be the same as tightening one bolt to 20Nm. The amount the stem is being clamped is going to be same in each case.

I didn't use the F word this time. :P

So the bars should say something like "Combined Nm: 20".

When you read 5Nm on a part it is telling you to do all the bolts at 5Nm max. That is all you need to know. With a face plate that has 4 bolts, naturally it is essential to have them evenly done up (to the eye) before applying the final turns with a torque wrench or equivalent. The fact that we have to be so careful with some modern parts is down to manufacturers striving to give us components that are made as if for a Formula 1 car, with no excess (safety margin) material. You wouldn't catch Formula 1 mechanics doing bolts up around carbon without a torque wrench.

Darkman
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Re: Only 5Nm on a stem/handlebar clamp?

Postby Darkman » 15 Nov 2019, 10:35am

Did mine up to 5Nm and while it doesn't seem a lot, they've never moved. The most important thing is to tighten them up evenly - hand-tight first, then half a turn each in a 'cross' pattern with a torque wrench (assuming there are 4, one of our bikes only has 2). That way it ensures the pressure is evenly distributed evenly across the front plate.

I have three torque wrenches and use them often, on bikes and on the car. Between them they cover everything from 5Nm to 210Nm. When there are recommended torque settings, guessing at it is potentially very bad.

pwa
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Re: Only 5Nm on a stem/handlebar clamp?

Postby pwa » 15 Nov 2019, 10:56am

Slightly off topic, if I have to loosen a stem (steerer) clamp to turn bars for transit, and the clamp has been set by torque wrench which I'm not carrying, I count the turns done to loosen and use the same number of turns to tighten later on.

Samuel D
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Re: Only 5Nm on a stem/handlebar clamp?

Postby Samuel D » 15 Nov 2019, 11:01am

Darkman wrote:(assuming there are 4 [screws], one of our bikes only has 2).

Which in itself renders the concept of a torque value printed on the handlebar a bit daft, doesn’t it.

slowster
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Re: Only 5Nm on a stem/handlebar clamp?

Postby slowster » 15 Nov 2019, 11:13am

Darkman wrote:Did mine up to 5Nm and while it doesn't seem a lot, they've never moved.

Adjustable torque wrenches have handles that are typically substantially longer than even a long handled allen key and a large diameter handle to make them comfortable to hold while applying pressure. That extra length/leverage and comfortable grip makes it much harder to gauge by feel how much torque you are applying before the head flops or clicks. Although I usually use a torque wrench where values are specified, I prefer the feel of resistance I get when I tighten a fastener using an allen key, i.e. 'user feedback'.

Even though I know my own estimation of what is adequate torque will simply not be consistant or reliable like a torque wrench, I still like to try to maintain a degree of mechanical sympathy for what is an adequately tightened bolt, not least because I won't have access to a torque wrench if I need to make adjustments or fix a mechanical while on the road.

Darkman
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Re: Only 5Nm on a stem/handlebar clamp?

Postby Darkman » 15 Nov 2019, 11:38am

Samuel D wrote:
Darkman wrote:(assuming there are 4 [screws], one of our bikes only has 2).

Which in itself renders the concept of a torque value printed on the handlebar a bit daft, doesn’t it.


No. Torquing up four bolts to 5Nm each does not magically give a total force of 20Nm, and consequently, two bolts at 5Nm does not give 10Nm. You end up with a force of 5Nm spread equally across the plate and it doesn't matter whether there are two bolts, or twenty - the force applied to each isn't cumulative.

Samuel D
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Re: Only 5Nm on a stem/handlebar clamp?

Postby Samuel D » 15 Nov 2019, 11:58am

Darkman wrote:You end up with a force of 5Nm spread equally across the plate […]

You’re using torque units to describe force, like Manc33 did earlier.

If the clamp is the same size, there’s about twice the clamping force if it has four screws rather than two at the same torque. Of course there are a few gotchas here, and two-screw clamps are sometimes a bit smaller (but rarely half the area). The plate has low bending stiffness compared to the screw tensions, so pressure is not evenly distributed across it, etc.

The whole affair is unsatisfactorily vague before you even consider the myriad ways that torque on a screw does not correspond to a predictable axial tension.

Cyril Haearn
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Re: Only 5Nm on a stem/handlebar clamp?

Postby Cyril Haearn » 15 Nov 2019, 12:08pm

Torque wrenches may be faulty, can one easily get them calibrated?
Feeling by hand is quite accurate I think
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Samuel D
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Re: Only 5Nm on a stem/handlebar clamp?

Postby Samuel D » 15 Nov 2019, 12:59pm

Cyril Haearn wrote:Torque wrenches may be faulty, can one easily get them calibrated?

Sure, not that careful calibration is needed to beat gut feeling.

Cyril Haearn wrote:Feeling by hand is quite accurate I think

It’s not remotely accurate. Even the best mechanics are all over the place. That works for a Massey Ferguson, but lightweight bicycles are getting more and more finicky. You don’t have a factor of three to play with any more.

I’d put money on many of the equipment failures in the pro peloton being caused by bad torque from mechanics too rushed, cocky, or ignorant to use a torque wrench.