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Pulled the trigger on a Carbon Bike Worried about torque wrench

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Pulled the trigger on a Carbon Bike Worried about torque wrench

Old 09-23-10, 07:11 AM
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Pulled the trigger on a Carbon Bike Worried about torque wrench

I just purchased my first carbon fiber road bike. Up until now I just rode steel and aluminum. I do all my own mechanics and I am concerned with the need for a torque wrench. I've never needed a torque wrench before but I am aware that too much torque on a clamp can ruin a carbon tube. I am most concerned about the stem/steerer tube and the seatpost/clamp.

One thing that confuses me is all the torques are for dry threads? I am a firm believer on greasing all my bolts. so what use is it?

Would the $12 beam-type wrench Walmart sells be sufficient. I have a pretty good collection of socket bits and adapters already.
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Old 09-23-10, 07:22 AM
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Hmmmm there are better options. Go to Sears and get the $24 torque wrench (gauge one) that shows lbs and N-m. 0 to 75lbs range. Or they have a nice clicker one on sale for $59~$69 which has a higher cap. They last. I've purchased cheap torque wrenches in the past and some don't always "click" when they should.

With regards to the Torque specs the seatpost should be about 4- 5 N-m (if I remember right...google for specs). Basically you tighten it until you cannot move it anymore- this is where you stop. Don't start twisting it as you'll make a groove. Just gentle nudge to see if it is still loose. Several folks have told me this.

As for the steer tube I'm not certain. Most have specs published. If it's all carbon then I would be cautious but if it is aluminum then you have more flexibility.

Good luck...
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Old 09-23-10, 07:36 AM
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Originally Posted by BikingGrad80
One thing that confuses me is all the torques are for dry threads? I am a firm believer on greasing all my bolts. so what use is it?
The concern isn't damaging the bolts; its the crushing force on the CF steerer. Thus, I don't think it would make a significant difference with regard to the torque to use whether you greased the threads or not.

Personally, I think that the need for a torque wrench is overstated, if you have some feel for what the right torque is supposed to be. For stems, use Tacx assembly paste, tighten the bolts snug, but not really torqued down, about the tightness you comfortably get with 2 fingers on a small allen wrench.

Most other torque specs, are pretty high (like 40n/m on a lock ring) and you give those what you got. So go easy on the CF parts, and crank down the things that are supposed to be cranked down and you should be fine.

If you do buy a torque wrench it doesn't need to be that precise. Set it a touch under spec on the stem, and you'll account for any margin of error on a cheaper wrench.
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Old 09-23-10, 07:39 AM
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Lubrication can increase the amount of clamping force caused by a given torque by as much as 25%, so if the torque values you are using specifically state for dry assembly, then just reduce the amount of torque you apply by a quarter if you have greased the bolts.

The torque figure is a maximum, not a target. You don't have go to the maximum if you can get reliable grip with less. Better to use assembly paste and get more grip for less torque if you can.
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Old 09-23-10, 08:03 AM
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Originally Posted by scirocco
Lubrication can increase the amount of clamping force caused by a given torque by as much as 25%, so if the torque values you are using specifically state for dry assembly, then just reduce the amount of torque you apply by a quarter if you have greased the bolts.
I think 25% is a low estimate. Per AISC:

In the first edition of this Specification, which was published in 1951, a table of torque-to-pretension relationships for bolts of various diameters was included. It was soon demonstrated in research that a variation in the torque-to-pretension of as high as ±40 percent must be anticipated unless the relationship is established individually for each bolt lot, diameter, and fastener condition. Hence, in the 1954 edition of this Specification, recognition of relationships between torque and pretension in the form of tabulated values or equations was withdrawn.

https://www.boltcouncil.org/files/200...cification.pdf (p.50)
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Old 09-23-10, 08:11 AM
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Ok, I guess I underestimated the effect lubrication would have.

My question is where the assumption that torque specs for stems assume no lubricant comes from.

For example if you look at Thomson's installation instructions, they specifically tell you to lube the stem bolts, and then give a torque spec of 5.5nm.

I can't imagine they would give you a torque spec based on no lubrication after they told you to grease the bolts.

https://bikethomson.com/blog/wp-conte...s/stem_rev.pdf
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Old 09-23-10, 08:12 AM
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Or you can just lube the threads like normal and forget the torque wrench. Tighten multiple-bolt interfaces by alternating between bolts to evenly distribute the tension, then tighten just enough that the bolt does it's job. If someone is setting torque specs on dry, 5 or 6mm threads, then it's stupid and you should ignore that torque spec. You can control lubrication, but if you have to assemble dry, then it's a crap shoot what kind of thread friction you are getting. It becomes a lawyer torque spec, as opposed to an engineering torque spec, at that point.

Besides, in general, specifying torque values on small bolts is kinda stupid in the first place: lots of thread area compared to cross-sectional area. It's even more stupid to specify maximum torque values. It's one thing to specify a minimum torque; it's open ended - the torque spec can overshoot the mark by some sort of engineering factor of safety and ensure the joint doesn't fail (picture lug bolts on a car wheel; you ain't stripping those threads, but there is a min torque you need to hit to make sure the wheel doesn't fall off). But specifying a maximum torque value means that you are caught between that maximum and whatever minimum value which causes the joint to fail; essentially you have a bounded torque range, even if it's not stated explicitly as such.

Because torque is basically indexing friction to a tension value, on small bolts, the torque is small and very small variations in thread friction cause big variations in bolt tension for the same torque. If the spec is open ended, then you simply imagine a high friction value and set the minimum torque to that value. If it's a bounded spec (like all these carbon bike bit specs are), then what criteria do you use to justify your specification? A high friction value assumption leaves the part crushed. A low friction value assumption lets the joint fail.

So... best advice is to ignore the specific torque number, heavily grease your threads, and work the bolt tension by hand. Tighten the joint slowly, evenly, and methodically. Check often to see if the joint is stable (say, the stem doesn't rotate around the steering tube). When it's stable, give the bolt(s) just a little more torque and then yank or twist on it hard (imagine, you are going to be riding this thing, so don't treat it with kid gloves) to see if the joint comes out of position. If the joint stays put, then you are done. If it moves, put it back in position and tighten the bolts a little more and retest.

And yes, I own and wrench on a couple carbon bikes, including one with a carbon steering tube. I don't own a torque wrench. Haven't broken anything and haven't died from my bike coming apart.
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Old 09-23-10, 09:30 AM
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My concern is I tend to use my L shaped allen wrench and 2-3 fingers and a tumb and get things like the stem and seat post as tight as I can until I start to grit my teeth. I tend to favor over-tightening things. That is fine on steel and aluminum but could wreck my steer tube or seat tube.

I also don't want to have my seat post slide down during the ride or lose control of steering because the stem isn't tight enough.

I might just be a bit over-intimidated as it is my first carbon bike and I don't want anything to happen to it.
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Old 09-23-10, 09:54 AM
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Definitely over-intimidated. If you can stand in front of your bike with your front wheel between your knees and torque on the handlebars and have it not move, then the stem is tight enough. If you can really twist on the saddle and have it not rotate around the seatpost, then it's tight enough (it'll twist before it'll slide down).

Definitely don't just torque down on the bolts. Be slow and methodical. On the other hand, these ain't eggshells either. If you have questions, take your bike down to your most established bikeshop in town (mom-n-pop is better than chain for this one; small shop is better than big shop). Go during a really slow time and ask the oldest dude in the mechanics part of the shop to show you how to correctly tighten a set of bolts without a torque wrench.
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Old 09-23-10, 09:58 AM
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and use carbon fiber assemply paste like TACX. It reduces the torque needed to hold things in place.
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Old 09-23-10, 10:00 AM
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Originally Posted by merlinextraligh
and use carbon fiber assemply paste like TACX. It reduces the torque needed to hold things in place.
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Old 09-23-10, 10:40 AM
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Old 09-23-10, 12:42 PM
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Go buy two good torque wrenches from sears, an inch-pound one for small fasteners, and a foot-pound one for large. They run about $70 per, and are the right tool for the job. Also remember to dial it back to the stop or minimum torque spec in between uses, if you leave it loaded (spun up to a torque) it will detension the spring and result in lower torque values than specified. For everybody that is saying to do it by "feel" unless you are building 100 bikes a day, you will never be able to accurately and repeatably acheive torque specs.

https://www.craftsman.com/shc/s/p_101...&blockType=L12

https://www.craftsman.com/shc/s/p_101...1&blockType=L1

And no you can't use just one, the large wrench won't go down low enough for small fasteners, and the small one won't go high enough for large ones. If you are only going to buy one initially, get the inch-pound version first, you will use it more often. Also don't be fooled, I love Park Tools, but they charge $250 for the same inch-pound wrench that craftsman sells, the only difference is that it is blue.
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Old 09-23-10, 01:26 PM
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Originally Posted by noise boy
For everybody that is saying to do it by "feel" unless you are building 100 bikes a day, you will never be able to accurately and repeatably acheive torque specs.
But the point is you don't have to "achieve torque specs." In most cases, you just have to not exceed the maximum.

I wouldn't tell you I can feel exactly what 5.5nm is. But I can tell you that I can feel what will hold, and is not so much torque to risk breaking parts.

With the Cf parts, you need to get the part to hold while not exceeding the maximum. It's not a big trick to get your stem not to twist on the steerer and still not exceed 5.5nm, particularly if you use TACX.

For other specs, such as 40nm for lock rings, you need to crank it down, and putting elbow grease until the lockring starts stuttering is the right torque.

Building a bike, even a CF bike is not rocket science.

All that said, I own a torque wrench and used it when I built my last bike. But I usually don't bother when I'm adjusting a seat, or changing stems or handlebars, and I haven't crushed any CF parts or had trouble with things slipping.
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Old 09-23-10, 03:10 PM
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Originally Posted by noise boy
...For everybody that is saying to do it by "feel" unless you are building 100 bikes a day, you will never be able to accurately and repeatably acheive torque specs.

...
Not the point. As merlinextraligh said, the point is to build a bike that won't fall apart without breaking anything. The torque spec is a compromise... and for small fasteners (anything less than a quarter inch bolt the torque spec is BS), it is an especially bad compromise. The only reason why it's there is so the company can put out a number for numbskull mechanics to fixate on.

Case in point? Why are the torque specs for a stem written and specified by the company which makes the stems? In the olden days before carbon fiber and ultra lightweight aluminum, this was because the torque spec was a minimum spec. Anything less and the interface wouldn't be guaranteed hold.

But if the torque spec on the stem bolts were meant to prevent the steering tube and handlebar from crushing, shouldn't those maximum torque specs be written on the handlebar and steering tube? The stem doesn't care how much it is tightened, as long as you don't start stripping threads. The bar and steering tube certainly do.

The only answer is that those torque specs written on the stem are actually minimum torque specs written to keep the joint from failing or slipping. Has nothing to do with carbon fiber crushing or whatnot. This would be why Thomson has a spec is written for ungreased threads even when it explicitly tells the user to grease the threads; it is a minimum spec, not a max. The Thomson stem doesn't care if you crush the handlebar or steering tube. Thomson doesn't make handlebars or forks. They care that their bolts are tightened enough to prevent the joint from slipping and causing loss of control.
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Old 09-23-10, 03:16 PM
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BTW, remember the Trek fork steering tube failure thread(s) a couple months ago? This would be why Trek says to only use Bontrager stems on their forks. Because the Bontrager torque spec can be tailored to the Trek steering tube limitations, or at least tested against them (Trek owns Bontrager, if someone didn't know that already); the fork and stem can be designed as a system. But, say, for a Ritchey stem, the torque spec written on the stem only takes into account the stem's needs to keep the joint from slipping; Ritchey doesn't care about such and such limitations on the steering tube of any particular bike.
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Old 09-23-10, 03:18 PM
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You all are entitled to your opinion, and that is fine, but the reality is that if there is a torque spec on a part, it is generally there for a reason. Every single item on my Cannondale has a torque spec engraved on it, and all of them are clearly labeled max torque. I feel that "until I feel it is tight enough" is at best a guess, and would bet real money that if you did it your way then put a torque wrench on the bolt, you will be higher than spec more often than not. The human arm is not accurate enough to hit 40 inch pounds reliably. I come from an automotive background, and torque specs exist for a reason.
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Old 09-23-10, 03:26 PM
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Originally Posted by noise boy
You all are entitled to your opinion, and that is fine, but the reality is that if there is a torque spec on a part, it is generally there for a reason. Every single item on my Cannondale has a torque spec engraved on it, and all of them are clearly labeled max torque. I feel that "until I feel it is tight enough" is at best a guess, and would bet real money that if you did it your way then put a torque wrench on the bolt, you will be higher than spec more often than not. The human arm is not accurate enough to hit 40 inch pounds reliably. I come from an automotive background, and torque specs exist for a reason.
The automobile industry does not deal in bolts so small and the reason for the torque specs in the auto world are vastly different than the reason for torque specs in the cycling world.

As for the bet, it depends on the part. If it's a carbon part, you might find it looser than spec. If it's an aluminum handlebar, it might be tighter than spec. On purpose.

BTW, I enjoy your snootiness about who is entitled to their opinion. I generally assume up front that I am entitled to my opinions, as long as they are well supported opinions. You are certainly entitled to entrust your knowledge to the unknown higher power of whoever-pulls-those-torque-numbers-out-of-their-asses-that-keep-the-lawyers-happy. If that makes you feel better at night, then good on you.
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Old 09-23-10, 03:33 PM
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Also, if the torque spec is engraved, then it's on an aluminum (or some metal) part, which means the torque spec is simply to keep you from stripping the threads as you reef on it. Different issue than the one about crushing carbon bits.
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Old 09-23-10, 03:40 PM
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Nothing snooty about it, opinions are just that, opinions, but the OP was asking if a $10 Beam type torque wrench would do it, which says to me "I am new to torquing bolts" and in that reality he is really better off with a torque wrench. He doesn't seem to have the experience that would be required (via stripping a few bolts) to be able to tell how tight small torque values can be. Notice I never said you were wrong, I watched the Ducati factory mechanic build a race engine with a regular ratchet, when you do something often enough you can train your muscles as to how much is enough.

On another note, the shop I bought my bike from used one of those ritchey (or one just like it) plastic torque wrenches all over the bike during the fitting process, when I got it back to the house, every single fastener they touched was overtightened by a substantial margin (some as much as 100%). I am not entirely sure I would trust one of those on small fasteners.
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Old 09-23-10, 04:14 PM
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Wow, this thread has become a great example of interweb knowledge...

To the OP, just buy a torque wrench and torque your bolts to spec, that mode of operation has never failed me, however plenty of my expert friends have broken carbon parts with their "calibrated" arms.

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Old 09-23-10, 04:24 PM
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Get the torque wrench from Harbor Freight that measures in inch-lbs. Its rediculously cheap and so far has proven to be very accurate.

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Old 09-23-10, 04:37 PM
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I bought a couple of torque wrenches when I bought a carbon frame. My opinion? Torque Wrenches are not necessary. Mine has remained unused. Just tighten stuff to the bearest minimum to hold and you'll be okay.

Also, I use Finish Line Fiber Grip on all things carbon.

YMMV.
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Old 09-23-10, 05:02 PM
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Originally Posted by JaceK
Wow, this thread has become a great example of interweb knowledge...

To the OP, just buy a torque wrench and torque your bolts to spec, that mode of operation has never failed me, however plenty of my expert friends have broken carbon parts with there "calibrated" arms.
The mention of "calibrated arms" is a good reminder that one of the best ways to avoid crushing stems and seatposts by overtightening is to keep the lever arm as small as possible. You can achieve all the torque you will ever need on these sort of items (around 5Nm) with thumb and forefinger around the short arm of a hex key, but will be unlikely to overtighten them without a really hard effort that you will be in control of. Compare that with merrily swinging on the end of a foot-long wrench, where the difference in perceived effort between 5 Nm and, say, 15Nm is much smaller and harder to control.

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Old 09-23-10, 11:13 PM
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You do not need a torque wrench, period. Like others have mentioned, using bike specific tools are designed with the leverage in mind to achieve the torque you need.
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