loose carbon bars
 

The bars keep loosening/rotating on my son's road bike. This has happened a few times but Friday with near catastrophic results on a downhill. These are used craigslist bars I installed. When I installed them I did not see any obvious signs of damage. Plus, damaged bars don't fit the symptoms as far as I can tell.

details:
FSA wing carbon bars, 31.8mm clamp area
Easton EA70 2-bolt alloy stem (with a substantial faceplate)
assembled with carbon assembly paste on bar clamp area
I greased the threads of the faceplate screws (bearing grease)
5Nm torque

Symptoms:
The screws seem to be backing out. Yesterday I checked his bike by dialing back the torque wrench and there wasn't even 2Nm of torque on these. I tightened to 5Nm and we went on a ride (with the torque wrench!). In less than an hour I checked again and the bottom screw was slightly loose. By the end of the 2-3 hour ride, they were holding fine.

I was wondering about the grease on the threads. I asked a couple of my LBS mechanics and they both said greasing them was the right thing to do. And wouldn't guess any further without taking the bike into the shop.

I was considering removing the grease and using Loctite.

Something I'm missing here? Any ideas?

edit: one last thing. I've got a 4-bolt ITM stem (with an even larger faceplate) I was planning on swapping in place of the Easton. One of the mechanics blessed this idea. It doesn't address why these screws are backing out, but it seems like a prudent thing to do.

Stem/stem bolts seem like the obvious cause of the problem. Swapping stem should solve the problem.

OK, with the alternate stem, should I grease the bolts with lithium (I've got lubriplate), or bearing grease, or not grease them at all? Does it even matter?

Also, I read here that greased bolts can mess with the torque readings.

If the bolts are stainless or titanium, I'm not sure there would be a reason to grease the bolts.

Why not use a weak threadlocker instead?

You could mark the bolts to verify they are actually turning.

There's a lot of poor engineering related to stems (and other parts), fasteners and torque specs.

I suspect your problem relates to that.

Fastener torque must meet two objectives. The bolt must be tight enough to hold the parts together according to the design needs, ie clamping a bar tight enough not to slip, yet not tight enough to damage the bar.

That part is obvious, but the second, equally important, consideration is that the fastener must be tightened to produce adequate tension to maintain traction so it won't turn and back out. People understand that with respect to spokes and wheels, but tend to forget that it applies to ALL threaded fasteners, except those retained by alternate means, ie. loctite.

So, every screw or bolt has a working torque range, tight enough to hold, but not so tight as to snap.

Reconciling both those specs begins with fastener selection. If the stem clamp bolt is too large, you can't tighten it enough without crushing the bar. If it's too small, you can't clamp tight enough without snapping the bolt.\

So, that is a possible explanation about the cause of the problem, but knowing doens't help.

Here's what may help.

Remove the bolts, and clean the threads of all but a thin film to act as corrosion prevention. Clean the bolt threads and paint a band with a nylon or rubber based material, ie latex paint, or nylon paint. You need something that stays pliable because you're going to jam it as you fit the bolt. There are commercial products for this application, one is "vibra-tite", and if you have a friendly auto mechanic he might have some he can spot you because it's used on set screws all over the place.

Now that you've made a "nylok" bolt, assemble the stem and tighten, but be aware that the nylon adds friction so the torque specs are meaningless, and you'll have to rely on your own judgement.

Threadlock? Does your son have any faith left in this particular combo? would be replacing with new parts if I had what was happening and the parts were 2nd had/unknown history/couldn't verify user error in install.

Have you tried just replacing the screws with new ones? When my wife's seatpost bolt kept slipping, I just replaced the stock bolt with a brand new Grade 12.9 bolt, and haven't had to touch it since. Nuts and bolts don't last forever.

Make certain that you bring the faceplate screws up to torque gradually and evenly and be certain that the gaps on both sides are even. The suggestion of using Vibra-Tite or similar is a good one. Another thing to check is that the bars are actually 31.8mm and the stem is actually for that size bar and is not damaged or distorted.
I prefer a 4-bolt faceplate because it does not have a single point of failure like the 2-bolt kind. Since you have one why not just try it?

Since you seem to think the bolts are loosening in service, then the solution to me seems fairly obvious - use some Loctite on them. Witness paint will tell you whether they're loosening further.

And how are you measuring the torque? If you are measuring 5 n-m with a torque wrench that goes from 5-75, it is probably very inaccurate - no torque wrench is very accurate at the ends of its range, especially the low end. And even if you're using the appropriate range torque wrench, it may simply mis-calibrated. If the wrench has been dropped or stored at a non-relaxed setting for a period of time, it may be way off.

My guess is that you've got the torque a little low and all that grease you're using is making them tend to loosen further in service.

- Mark

OK, removed, cleaned everything, and reinstalled very carefully with the 4-bolt stem, carbon paste and loctite. And I painted each bolt at 12:00 to check for future movement.

I've got this torque wrench, and always store it with the setting dialed down. I'll take it into Performance sometime and see if I can get them to check it against a real shop tool.

https://www.performancebike.com/weba...280&cadevice=t

the carbon bars FLEX and TWIST around in the two bolt stems... the four bolt stem will hold them in two places, and should eliminate the issue...

Carbon fiber BENDS and TWISTS... this is why we all love the ride provided by the CF frames.... same for the old steel tubed frames... aluminum is very stiff, and it rides rough. Once aluminum YIELDS to a load, it fails/cracks, or will, eventually...

Not knowing the sequence you are using to tighten the clamp bolts, I would caution against tightening either the upper or lower fasteners, then the other. Snug them evenly maintaining as equal distance between the faceplate and the stem body before torquing to the final value.
If you would look at the side profile of the stem with the cap seated and no bars in place, you will see the opening/machining is not concentric.

Not correct. Al has about 1/3 the stiffness of steel. But since it also has a limited fatigue life and will propagate a crack quickly the structure (bike part) needs to be designed stiffer then other materials require, to avoid the liability of failure. This is why Al framed bikes have a stiff ride, they're designed to have this. The exceptions are the bonded AL frames of yesteryear. They used traditional tube diameters, relatively thick walls and without any welds tended to be less brittle. But they also didn't have the lower weight of modern welded AL frames. Andy.

Many carbon bars have rough surfaces where stuff is supposed to bolt onto them (stems/brakes).

One the bars start to rotate, I presume that roughness is gone, and it could cause more rotation.

Be careful to always check for cracks or signs of damage to the CF.

It may not be a bad idea to start hunting for a set of replacement bars.

ummmm, i was giving an answer that was easily understandable, not getting into the characteristics of materials before manufacture. Aluminum is actually stiffer, per pound of material, and yes, the designers take that into account. thicker tubes... YIELD STRENGTH is what you are terming as BENDABLE. Aluminum has less MEMORY, and is not very springy, compared to steels and CF.

properly annealing can, and does, remove most of the brittleness around welds.

Hydro-forming, forging, etc., the aluminum creates an improved grain structure that lends strength to the shape.... and carefully engineered shapes also create more strength per structural member. Eliminating sharp junctions increases durability, too.... as does larger tube diameters in ANY material.... once a tube wall is thinned tho, they dent/deform more easily, which then creates a STRESS RISER in the form....

compression or tension loading in design must also be considered... materials yield more quickly under tension.... the downtube/head tube, or the chain stay/BB junction typically fails first in a jumping accident, eh?

and then, there is the selection of the correct alloying elements, and adjusting their tiny, tiny percentages...

I use a tiny bit of antiseize on my stem instead of grease.

Al and stiffness
 

[QUOTE=maddog34;19532036]ummmm, i was giving an answer that was easily understandable, not getting into the characteristics of materials before manufacture. Aluminum is actually stiffer, per pound of material, and yes, the designers take that into account. thicker tubes... YIELD STRENGTH is what you are terming as BENDABLE. Aluminum has less MEMORY, and is not very springy, compared to steels and CF.

properly annealing can, and does, remove most of the brittleness around welds.

Hydro-forming, forging, etc., the aluminum creates an improved grain structure that lends strength to the shape.... and carefully engineered shapes also create more strength per structural member. Eliminating sharp junctions increases durability, too.... as does larger tube diameters in ANY material.... once a tube wall is thinned tho, they dent/deform more easily, which then creates a STRESS RISER in the form....

compression or tension loading in design must also be considered... materials yield more quickly under tension.... the downtube/head tube, or the chain stay/BB junction typically fails first in a jumping accident, eh?




and then, there is the selection of the correct alloying elements, and adjusting their tiny, tiny percentages...[/QUOTE


]We are in the same area WRT understanding properties. I just dislike the "Al Is stiff" misnomer. To me this is marketing. I now know you don't think so. Here's hoping that our readers also know better. Andy.

I take this one step farther and find fault with ALL of the general characterizations of properties of the various materials.

Yes, of course materials have individual properties, such as density, ductility, fatigue resistance, and so on.

But mechanical or riding properties of structures made from these materials are just as dependent on the physical configuration, ie. tube diameter & wall thickness, along with construction method, temper (where applicable), and other human controlled aspects.

So, in the end, it's not as much about the material itself, but the designers thinking in choosing it and the overall design objectives.

For example, it's not that aluminum frame are inherently stiff, but that the material was chosen, because it was possible to use larger diameter tubes, and benefit from their higher moment of inertia, without a weight penalty.

I think this is 90% of the solution to your problem. Those 2-bolt stems just aren't as secure as the 4-bolt ones. And yes to evan326's suggestion of anti-seize instead of grease, although loctite will obviously work even better.

Turns out, because of a light mounted on the bars, I got the left /right location a little off-center so we had to do it all over again - this time paying a lot of attention to balancing the four bolts during reassembly. The big plus is that my son - at this point - was very focused on the issue so I let him do it. It should give him confidence and also give him the knowledge to check the bar on his own. We painted the bolts so he can see any movement, and he's taking my torque wrench so he can test it over time (at a lower setting) if he wants.

We also checked the bars for damage (which I never suspected anyway), used the carbon paste and loctite.

They are backing out because of the grease. Use some thread lock or Teflon.