Torque setting for seat post?
 

I have a 2011 Giant Defy Advanced 2 carbon fiber road bike with the aero seat post. Has 2 opposing 4mm bolts for the seat post collar. The collar says "max 50 kg cm". Not sure what torque setting to tighten it with after adjusting my saddle height. It's my first carbon bike and I don't want to over tighten it. Thanks for any help.

50kg-cm is roughly 43in-lbs.

http://www.convertunits.com/from/inch+pound/to/kg+cm

So, less than that would be best. Maybe 35-40 in-lbs? :)

The proper torque is enough to keep the post from moving under your weight while riding over whatever terrain you plan to ride on....without exceeding 50 kg/cm.

Use carbon paste and start by tightening to 25 kg/cm (@ 22 lb/inch), then check for movement. If movement is detected then tighten to 30kg/cm and recheck. So on an so forth until the seatpost is held solidly.

-j

What nonsense to use a torque wrench on a seatpost bolt. You need to learn how to tighten the bolt by hand, so you can make an adjustment out on the road. Same goes for just about every M5 bolt on the bike.

And how do you propose that we learn this? By breaking parts until we get it right?

http://www.bikeforums.net/showthread...ecs?highlight=Is there one of these tables for Carbon Fiber Components?

On a carbon bike? I don't think so.

Actually, if you let your hands listen, the bolt tells you.

If you were to graph the torque as you tightened it, you'd get a line whose slope changes at various stages. It starts out flat until all the slack threads are taken up, then begins to steepen on a gentle curve as it actually tightens the clamp. When the clamp bottoms out the curve steepens as you begin to flex the ears and stretch the bolt itself.

Correct tightness or actually maximum tightness is just past the point of distorting the bolt and parts, on the beginning of the steepest slope. In most cases, and almost always with things like clamp bolts it's a very sudden and obvious increase in the torque needed to keep turning it.

Before we became a paint by numbers society, every budding mechanic learned to read bolt load early on, and though maybe a bolt or two were broken, it's an easy thing to learn at lower cost than a torque wrench.

the grease+ a little abrasive, gets used .. 'carbon assembly paste'..

generally , if you are conservative on the wrench,
tight enough is just when the post stops slipping down..

I was not happy with the bolt force, I was using, but still getting slipping,
so I added a band clamp around the seatpost itself , sitting on top of the frame's seat post clamp.

Worked OK before on my Brompton, too, though the seatpost band clamp is another QR in this case..


but, I understand, the OP doesn't have a round seatpost..

I've owned numerous carbon frames, forks and seatposts. Never owned a torque wrench that can register as low as 5Nm and never had a problem with a bolt loosening when I was riding and never broke a bolt or carbon part.

If you're clueless, buy a torque wrench to learn with, but keep in mind that loosening a bolt tightened to 5Nm require moe than 5Nm of torque. If you tighten a bolt by hand and then apply a torque wrench to it, it will require a bit more torque to get the bolt turning again.

My point is to not rely on a torque wrench for tightening M5 bolts on a bike.

You can get those single-value 'torque keys' that are preset at low values like 5NM just for jobs like this. :)

http://bikeforums.net/attachment.php...hmentid=307811

No muss, no fuss, and more importantly, no '....*CRUNCH* <forehead slap>' ;)

Did not know these existed - Great - I have never worked on a Carbon bike but I sympathise having worked with glass parts in the lab that had to be assembled with milled graphite hardware - What a nerve racking, humbling, mess...

I have a Park 1/4"-square drive beam torque wrench that covers the range of 0-7 Nm. it can certainly discern 5 Nm. Cluelessness has nothing to do with it. A lot of studies have shown that even very experienced mechanics badly miss the mark when asked to tighten a bolt to a given torque without a torque wrench.

Some people are inept mechanics. Nothing new there. As I noted, not only have I found no need to own a 1/4" drive torque wrench, I've never had a single problem with tightening the bolts on my carbon bikes. Nothing has ever come loose or broken. I'm sure not going to pack a torque wrench with me on a ride. I do use a large torque wrench for tightening cassette lockrings, BB cups and the large bolt connecting the spindle halves on an ultratorque crank.

The chances of damaging CF parts are exaggerated, IMO. I've done tests, where I took a stem, mounted it on a cutoff piece of carbon steering tube and tried to break the tube by overtightening the bolts. I usde a long handled hex wrench and gave the bolts ridiculously hard pulls, but it had no effect at all on the tube, even with no internal support. The majority of breakage problems are due to a poor fit between the two parts, that causes pinching.

Yes, and I would imagine especially clamping bars with those 4-and esp. 2-bolt faceplates, and not tightening the bolts alternately and gradually - just whaling on one and then the other. ;)

+100,

It's nearly impossible to crush or otherwise damage a carbon bar by over tightening. The bolt or ears are likely to crack off long before that.

The culprit is poor fit, and that's been mostly resolved as tolerances were tightened to accommodate the needs of CF parts. Much of the mythology and paranoia about CF post or bar breakage dates back to the beginning of the CF era. CF posts were used in steel frames without consideration of the difference between how a CF post would clamp vs the alloy post it replaced.

When two non matching (almost) round parts are clamped one must distort to match the other (or both distorting part way). With an aluminum post in a steel frame the thin steel seat tube took the shape of the stiffer post, and there wasn't a problem. But when a carbon post was put in that same frame, it's thinner wall was ovalized by the stronger steel tube, and began to crack or split lengthwise, compromising much of it's strength. Also the ear design of many frames or clamps caused the ears to dig into the back of the post, in a way similar to putting you thumb into an egg when holding it too tight.

So there was a learning curve, but today's, slightly stiffer posts, combined with good roundness and tolerance in frames, and better clamp design have basically resolved the problem. Still it pays, not to be too ham-fisted because it's fairly easy to break a bole or the clamp's ears.

The best solution is to use a traction compound so you can get reliable clamping hold at lower clamping force.

Just how much torque difference between slippage and non-slippage have people noticed who have used the gritty carbon assembly paste on things like seatposts and handlebars? (As opposed to when using no paste)

It depends on the type of traction compound used, and also whether you're comparing to a dry mount or one with grease or typical anti-seize.

Grit based compounds that actually bite into the fitted parts make a day and night difference and can hold with barely any clamping force. Many makers of CF handlebars and stems are incorporating these into the gripping area of either part.

Those that don't bite will offer improvement, reducing clamping force by nearly half or so for the same hold.

I don't say that CF assembly pase is always needed, but if you're having trouble getting decent hold without maxing out clamping torque, a traction compound will solve the problem, and prevent damage from over-tightening.

Ah, that's interesting - so it can practically halve the amount of torque needed? Although I presumed that pretty much all of the carbon assembly pastes had 'bite' to them...

I wonder if it's just a matter of degree - some have more 'bite' (are grittier, presumably) than others - or whether some pastes are just the carbon version of anti-seize, preventing stickage and not really providing any extra grip.

I'm a fan of grit based traction compounds, having used lapping compound for things like keeping handlebars from slipping long before CF parts even existed. However grit has the problem, that parts must be completely loosened before they are moved or the grit will score the surface. Even a fine grit will dull a carbon post inserted too deep and lifted back up, so many of the pastes use tiny plastic or rubber media for traction, which increases friction without marring.

Which is best depends on how much added grip you want, and your or the applications tolerance of minor marring.