Carbon fiber and torque wrenches

dennisn

Recently noticed that more than a few companies are selling torque wrenches for your bike. In reading some of the blurbs about them they describe themselves as very useful on "potentially delicate carbon parts" and "fragile carbon". Ahhh, the fear factor.

Next I read some discriptions of some carbon bars, stems, seatposts, and cranks. It went something like this. "Absolutely bombproof" "super stiff and super durable" and of course the ever present "stronger than steel".

A couple of weeks ago I did the unthinkable. Mounted an "absolutely bombproof" or "'potentially delicate.....", depending on your viewpoint, carbon bar to an aluminum stem WITHOUT a torque wrench. Living dangerously is my middle name. I'm here to report that nothing happened.

Grill

Congratulations, but all parts break, regardless of material or marketing. I have seen all the parts you've listed and more ruined because someone couldn't be bothered to check torque specs and do a proper job, but hey, it's your dime. Do your thing.

pkripper

If its got a torque rating and its carbon I'll adhere to it. No use dragging my face along the road for being ham-fisted

dartmoorscottydog

I'm with dennisn on this. Use common sense and get a feel for how tight is tight! My face hasn't been dragged along the road yet. Doh - that's done it now!!

edten

If you're someone who works on bikes frequently then I'm sure 'getting a feel' is going to come over time. However there are many getting into cycling and heading straight for expensive carbon kit and will have no idea of what 5-10nm feels. The best way to get a feel for the correct torque has to be using a torque wrench or how else are you going to know what 5 to 50nm feels like?

rolf_f

dartmoorscottydog wrote:

I'm with dennisn on this. Use common sense and get a feel for how tight is tight! My face hasn't been dragged along the road yet. Doh - that's done it now!!

The trouble is, unless you calibrate your 'feel' against known torques then it is nothing but guesswork. And, apparently guesswork means that you'll seriously overtorque small bolts and seriously undertorque large ones.

I bet whilst there are loads of folk on here who do have a genuine feel for it, there are just as many who only think they do. Me, I know I'm crap at that sort of thing. A torque wrench isn't that expensive compared to shiny bits of carbon and it is probably quicker working with a torque wrench than without.

edten wrote:

The best way to get a feel for the correct torque has to be using a torque wrench or how else are you going to know what 5 to 50nm feels like?

Whilst I agree with you on use of the torque wrench, it is easy enough to get a feel for what 5-50nm feels like if you have access to bolts that are already correctly tightened to those settings.

uphillstruggle

Fearing the worst i did buy a torque wrench but have been reluctant to use it since although it comes with a certificate if its not callibrated correctly or loses accuracy it would do more harm than good??

on-yer-bike

dennisn wrote:

Recently noticed that more than a few companies are selling torque wrenches for your bike. In reading some of the blurbs about them they describe themselves as very useful on "potentially delicate carbon parts" and "fragile carbon". Ahhh, the fear factor.

Next I read some discriptions of some carbon bars, stems, seatposts, and cranks. It went something like this. "Absolutely bombproof" "super stiff and super durable" and of course the ever present "stronger than steel".

A couple of weeks ago I did the unthinkable. Mounted an "absolutely bombproof" or "'potentially delicate.....", depending on your viewpoint, carbon bar to an aluminum stem WITHOUT a torque wrench. Living dangerously is my middle name. I'm here to report that nothing happened.

Nothing's happened, yet.

edten

uphillstruggle wrote:

Fearing the worst i did buy a torque wrench but have been reluctant to use it since although it comes with a certificate if its not callibrated correctly or loses accuracy it would do more harm than good??

It can't be any less calibrated than your arm! Unless you've been wrenching for some time of course.

uphillstruggle

Id rather trust feel than a cheap inaccurate wrench.

pkripper

I just find it bizarre that people will spend an awful lot of money on carbon kit, and then not spend the £40 or so for a torque wrench. Seems a bit false economy - each to their own i guess.

Chamac

Cracked the seat tube on my trek 5.2 fitting a band on chain keeper under the FD. I was using a torque wrench set to just 4Nm and was shocked at how fragile and easy to damage CF is.
Currently being mended and a FD brazon bracket fitted, when i rebuild on this frame I'll be taking a great deal of care tightening bolts even using a torque wrench.

antfly

Forget the torque wrench, just use carbon paste or some such thing.

estampida

there was a thread about torque wrenches about a year ago and I stuck my oar in........

but the best wrench that cfovered the range that a bike needs with the accuracy and stabilty that ensures lifelong use rrp was £600 or so.......

buy cheap, buy twice....

greasedscotsman

dennisn wrote:

I'm here to report that nothing happened.

What nothing? You didn't go faster? Or feel less road buzz? The ride wasn't transformed? You must feel pretty disappointed or are you just happy they didn't break?

dennisn

pkripper wrote:

I just find it bizarre that people will spend an awful lot of money on carbon kit, and then not spend the £40 or so for a torque wrench. Seems a bit false economy - each to their own i guess.

I'll go along with that. Sort of like people spending mega dollars on a bike and it's components and then putting clinchers on it.

trek_dan

Chamac wrote:

Cracked the seat tube on my trek 5.2 fitting a band on chain keeper under the FD. I was using a torque wrench set to just 4Nm and was shocked at how fragile and easy to damage CF is.
Currently being mended and a FD brazon bracket fitted, when i rebuild on this frame I'll be taking a great deal of care tightening bolts even using a torque wrench.

So when fitting a frame protector you snapped your frame by over tightening the frame protector? Top work.
This is why when building a bike you need to use your common sense not a torque wrench.

dennisn

greasedscotsman wrote:

dennisn wrote:

I'm here to report that nothing happened.

What nothing? You didn't go faster? Or feel less road buzz? The ride wasn't transformed? You must feel pretty disappointed or are you just happy they didn't break?

I do believe that there is a bit less road buzz. Damn well better be for the price you pay for those things and that is one of their big claims. So, yes, I guess.

As for the breakage part I simply don't believe that a reputable company would build something that was "fragile" or "potentially delicate" and then offer it to the cycling public in the hopes that it won't break. If those carbon parts are as fragile as torque wrench manufacturers seem to insinuate then the breakage and lawsuits would be never ending.

pkripper

dennisn wrote:

greasedscotsman wrote:

dennisn wrote:

I'm here to report that nothing happened.

What nothing? You didn't go faster? Or feel less road buzz? The ride wasn't transformed? You must feel pretty disappointed or are you just happy they didn't break?

I do believe that there is a bit less road buzz. Damn well better be for the price you pay for those things and that is one of their big claims. So, yes, I guess.

As for the breakage part I simply don't believe that a reputable company would build something that was "fragile" or "potentially delicate" and then offer it to the cycling public in the hopes that it won't break. If those carbon parts are as fragile as torque wrench manufacturers seem to insinuate then the breakage and lawsuits would be never ending.

Well, I know it's not the torque wrench manufacturers that put torque values on all the components, as they'd be as busy as santa at Christmas otherwise.

There are pretty large safety margins in some kit, but the lighter and more niche / cutting edge the components are, the closer they're likely to sail to the safety margins. When you consider the variability in components that are mixed and matched, manufacturers probably do err on the side of caution, and we probably could get by largely by feel on bolts etc. But conversely even a slight overtightening could potentially induce a stress riser that may not manifest immediately, but could result in premature failure.

Hell, it's your face.

rolf_f

trek_dan wrote:

This is why when building a bike you need to use your common sense not a torque wrench.

The two are not mutually exclusive

drlodge

Rolf F wrote:

trek_dan wrote:

This is why when building a bike you need to use your common sense not a torque wrench.

The two are not mutually exclusive

I think the point being made here is not to use a torque wrench blindly. I'm pretty confident I can torque appropriately by feel - tight enough that the joint will hold, but not so tight something will get damaged.

I don't own a torque wrench, but then I don't own a carbon bike

Chamac

Yes i hold my hand up to my stupidity there. Only ever worked on metal bikes previous to this, big lesson learned. Luckily frame was S/H off ebay so no huge financial loss, and its just a project really for a rainy day bike. Nevertheless i will never again be wedded to blindly folowing Nm guidance.

mike6

I trained and worked as an engineer for 22 years, using allen keys and socket sets every day. Can I guess a torque setting? No I cant, I would hazard not many can. I have "feel" for when a component is tight, but not too tight, but that is subjective. I could still not tell you what setting it was, re torque, without a good wrench.

Light weight alloy and carbon frames are fragile with regard to "crush" forces, but are, in most cases, stronger than steel. Don't over tighten a stem on an all carbon fork, unless you want serious damage or injury.

antfly

You can get torque keys for that sort of thing, they have one setting and cost about a fiver and are probably more reliable than a cheap torque wrench.

greasedscotsman

dennisn wrote:

I do believe that there is a bit less road buzz. Damn well better be for the price you pay for those things and that is one of their big claims. So, yes, I guess.

As for the breakage part I simply don't believe that a reputable company would build something that was "fragile" or "potentially delicate" and then offer it to the cycling public in the hopes that it won't break. If those carbon parts are as fragile as torque wrench manufacturers seem to insinuate then the breakage and lawsuits would be never ending.

I dunno about that, but I did have a pair of carbon bars and a normal alloy stem which I managed to overtighten. Wasn't the bars that broke, it was the stem that split!

mikenetic

"getting a feel for tightness" and being accurate to within 2-3 Nm are totally different things. Seeing as 3Nm is quite possibly the difference between a slipping seat post and going over manufacturer's recommended max torque I'd buy a torque wrench, personally.

They come in very handy for things like Hollowtech 2 cranks where having both a pair of bolts at the same torque is important for fitment.

ricky1980

carbon fibre composite is weaker than aluminium alloy?! god i thought we are suppose to advance on material not digress!

the torque setting for carbon is not because it is fragile. it is purely because carbon composite is brittle, it will fail without much of a warning in terms of material deformation. thus all the manufacturer specify a low torque setting than what the material can actually sustain, as well as factor in material/manufacturing tolerance and life time fatigue cycles.

by feel is fine most of the time when the applied force is around a circular component, i.e. seat post collar, stem collar to the handle bars. but if you have carbon rail on saddles and/or carbon seat posts, then the bolts connecting to the saddle should be tighten by torque wrench as your body weight will shift on the saddle which can increase the stress in those components.

Runtothehills

ricky1980 wrote:

carbon fibre composite is weaker than aluminium alloy?! god i thought we are suppose to advance on material not digress!

the torque setting for carbon is not because it is fragile. it is purely because carbon composite is brittle, it will fail without much of a warning in terms of material deformation. thus all the manufacturer specify a low torque setting than what the material can actually sustain, as well as factor in material/manufacturing tolerance and life time fatigue cycles.

Not quite - carbon fibre is very strong and stiff along the fibre direction but is very weak perpendicular to it: when it's crushed across the fibres it tend to then fail very easily along the fibres as well, especially as when it's crushed the matrix fails which means the load distribution between fibres is shot.

As you say it's also brittle which means it's very susceptible to stress concentrations. If something made from aluminium/steel is overtightened or unevenly tightened so one side is stressed more than the other the material will yield plastically and sort itself out- CFRP won't, it'll just fail.

vorsprung

antfly wrote:

You can get torque keys for that sort of thing, they have one setting and cost about a fiver and are probably more reliable than a cheap torque wrench.

Like this http://audaxing.wordpress.com/2010/11/1 ... orque-key/

I find carbon fibre to be a counter intuitive material. It is stiff in the bottom bracket for better power transmission. It is flexible to give a better ride on the road. It can do this magic because it's strength is directional.

This has two consequences:
• Adjusting by feel, especially if you are used to steel bikes, isn't going to work. They feel different
• Some parts, like the steerer tube, are strong in one direction but not as strong in the perpendicular plane to this. So it will hold you weight and flex when you hit a pot hole 10,000 times. But overtighten the stem one time to many and there may be trouble.

I am not an engineer but...isn't this just obvious? It's just the way materials work

The other thing is the assessment of risk. There is a good chance Dennis can have his hand tightened stem for several years, ride on it, take it off, put it on another bike, run into a tree etc etc and have absolutely no problems whatsoever.

But using a torque wrench to ensure the pressure on the parts is in the correct range removes a tiny bit of risk. Sometimes part failures can be catastrophic and cause injury. I am only talking about a *tiny* risk reduction on a *highly unlikely* threat of injury. But torque wrenches are not that expensive, so why not?

dennisn

In the end I think I will alaways think about the video, that made the rounds on this forum a few years back, in which a guy took a pretty big hammer to BOTH a metal frame and a carbon frame. The metal frame easily suffered the most damage in the least amount of blows. I realize that this doesn't PROVE much of anything but it sure made an impression on me. So much for fragile carbon fiber.
Gets me thinking that people want their cycling parts to be precieved as fragile and delicate, hence hi-tech, and therefore they are riding and working on rocket science type machines instead of simply bicycles. Feeds their egos when they tell the unknowning that "this is carbon fiber - very hi-tech"(aren't you impressed with me?).

pkripper

dennisn wrote:

In the end I think I will alaways think about the video, that made the rounds on this forum a few years back, in which a guy took a pretty big hammer to BOTH a metal frame and a carbon frame. The metal frame easily suffered the most damage in the least amount of blows. I realize that this doesn't PROVE much of anything but it sure made an impression on me. So much for fragile carbon fiber.
Gets me thinking that people want their cycling parts to be precieved as fragile and delicate, hence hi-tech, and therefore they are riding and working on rocket science type machines instead of simply bicycles. Feeds their egos when they tell the unknowning that "this is carbon fiber - very hi-tech"(aren't you impressed with me?).

you're right, it doesn't prove much of anything at all - maybe that the elastic / direct impact absorbtion characteristics of the two materials are different. Oh, and the way they exhibit damage (which you also should really define). However, if I then had to ride either bike, I'd almost certainly choose the metal frame, as I'd have a reasonably accurate guage (to the untrained, albeit with some experience with material engineering eye) and a fairly reasonable understanding of the potential failure mode. CF, I wouldn't have the former for sure.