john
yes... And what I am saying is that if you _ just_ use a torque wrench,
you are "hillbillying up" !
Especially with carbon. Torque is great with steel, and its the easiest we
have for achieving preload, especially if tested and friction is consistent
and repeatable.. .
Once again in the situation of stem, the clamp force is way more
complicated than just the preload of the bolt which an inaccurate result of
a torque value. stiffness of the stem, the tube and the fits between the
two need to be accounted for. So obviously the fits/ roundness have to be
very good.
So what are we trying to achieve?: A joint that doesn't fail. Where, the
critiera of , a joint "fails" is discusion unto itself. I personally go to
where it holds, where nothing slips, And then a tad more. I like to
sandwich my carbon. (meaning a good inner plug in the steerer tube... ) .
but I am tad conservative I don't think I will use carbon seatpost for
example or handlebars...
On Tue, Mar 6, 2012 at 8:01 PM, Steve Brown wrote:
> Time to get serious, which I do not do often. Fastening systems with
> composites are always a difficult subject. There is always a lot of serious
> engineering considerations when joining composites with methods designed
> for metals. Always follow the manufacturers suggested recommendations.
> When you compress a tubular section made with metal there is a wider
> margin of error than with composites. Too much pressure, the composite
> does not compress or deform, it cracks. With the risk of not being
> politically correct, carbon is not the best of materials to "hillbilly up".
> As the representative for a company that sells composites and talks with
> some of the leading engineers in a wide range of industries from aerospace
> to recreation, the best thing I can say is to follow the manufacturers
> recommendations.
>
>
> Steve Brown
>
> On Mar 6, 2012, at 6:31 PM, Dan H wrote:
>
> I agree clamping force is the ultimate goal but since that's difficult to
> measure, torque will get you in the range.
> I started this. I wish I could stop it now.
> Seems I usually regret posting anything here but I don't learn.
>
>
>
> On Mar 6, 2012, at 3:58 PM, john wrote:
>
> So Let's see 90%-95% of the torque put into in a bolt or nut goes into
> friction. So what happen if your friction is off a little ? Answer: your
> preload/ bolt force is off a whole bunch. In a stem or etc clamping a
> tube, you also have to worry about fits and the stiffness of the stem and
> stiffness of the tube. Which are perhaps the biggest unknown in this
> situation.
>
> So, Like I said a torque wrench will get you to an order of magnitude, in
> the ball park.
>
> >>>Which is important! <<<
>
> But otherwise you are still pretty much guessing. :)
>
> I don't have many carbon parts (well only fork) and it seem to me what
> really is being conveyed with these low torque numbers is to clamp
> it enough that it holds and then maybe a touch more for some preload that
> won't get totally lost given some load....
>
> Many fasteners and threaded things on a bicycle are not used in any sense
> of engineering, and certainly are not an engineered joint. I guess
> everyone likes 5mm bolts or something, the smarter thing to do would be to
> use an appropriate fastener size (preload appropriate), but i digress, 5mm
> bolts are probably a commodity....
>
> What is the end goal? Achieve that.
>
> But yes its Ok, I dont' listen to my Medical Doctor either, I will only
> listen to someone with maybe first aid training, if that : ) And for a law
> advice, would I ever listen to a lawyer, Hell no !! : )
>
> It's all fun. keep the bald side up.
> And do NOT throw away your torque wrench.
>
> I have one too somewhere...
>
> John, PE, MSME
>
>
>
>
>
>
>
>
> On Tue, Mar 6, 2012 at 10:14 AM, Steve Brown <
> sbrown@stevebrowncompany.com> wrote:
>
>> Hate to take the chamois cream out of anyones shorts, but unless you use
>> a torque wrench every time you are still guessing.
>>
>>
>> Steve Brown
>>
>>
>> On Mar 6, 2012, at 10:07 AM, Ben Fischler wrote:
>>
>> On Mar 5, 2012, at 11:09 PM, Rick Johnson wrote:
>>
>> The simple answer is that unless you do the job every day you need a
>> torque wrench for anything more than old school steel parts.
>>
>>
>> What Rick said. When I was building frames and wrenching often I would
>> have trusted myself w/o a torque wrench. Now that I wrench a computer a
>> hell of a lot more then my bike (sadly!) I like knowing when to say when,
>> and I'm consistently surprised at how little torque is called for on some
>> parts. I haven't broken anything, but I would have come close w/o
>> checking.
>>
>> -Ben
>>
>> AnimationMentor.com
>> [ ben.fischler@gmail.com ]
>>
>>
>>
>>
>>
>>
>>
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