Bicycle Mechanics - Are these scratches on a carbon bar dangerous?
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05-07-06, 04:14 AM
I have purchased a new MonkeyLite XC carbon bar about a week ago. I installed it on my bike, on a 2-bolt Raceface Prodigy stem. Since a torque wrench is not available in my area, I used an improvised lever/weight system to apply a torque of 10-12 N.m, following the stems' manufacturer instructions. Upon testing the screws, they were indeed not overtightened at all. After a 40Km ride on pavement, I inspected the bar and found two scratces in the stem clamp area. They are both shown in the photos attached. The big scratch has now peeled away, and it appears to be detachment of a very thin surface coat, revealing a jet black opaque surface below, with no carbon pattern. The small scratch seems not deep at all, but can be felt with a nail passing over it. My guess is that is was caused by a small scratch in the stem clamp (shown in a photo of the stem. Even though it seems sharp, the stem scratch is almost impossible to feel by passing a finger over it). The stem features no sharp edges. The large scratch, appeared on what would be one of the 4 corners of the stem.
Do any of these two scratches pose a risk? Are they enough to initiate a stress riser? If not, now deep should a scratch be to be compromise the bar strength?
I notice that the area which corresponds to the stems' perimeter, has lost the glossy appearance and became something that looks ready to become another 'peeling big scratch'. Is this failure of the outer coating normal?
In one of the photos, a bubble-like structure of a diameter around 0.5mm is shown. This is on the bottom of the handlebar, about 15cm from the edge. Is that just a surface problem or does it compromise the bar structurally?
I know I'm asking a lot of questions, some of them making me look like an anal retentive freak, but it's my first carbon bar, and I don't like worries holding me back from enjoying the ride.
I sent the same email to Easton, and they ignore it.
I'd appreciate your input!
the glossy part is just the top coat to make the bar look more attractive, so a scatch on that wont hurt the bar. however the bubble part of the one scratch has me concerned, it may or may not be an issue and photos don't really tell me much. your best bet is to take it into the LBS where you bought it and have them examine it. the one thing you dont want is a bar breaking while riding, and carbon stuff breaks without warning, and it can cause a nasty crash should it break.
05-07-06, 05:00 AM
Hey freako, thanks! Are you talking about the bubble-like structure on the big scratch? That's actually clear coating peeiling away, it has peeled and the area below is jet black. How deep should a scratch be to get me concerned? Like, 0.5 mm?
05-07-06, 11:46 AM
05-07-06, 05:17 PM
I'd like to hear from those who have experience with breakage of carbon bars. I've only broken one (Scott LFX ca. 1995), and that was a crash where the bike hit the dirt and snapped the bar. The fiber matrix was actually for the most part intact, and although the bar was spongy because of the break, I was still able to ride back to the car via fire road. Scott USA replaced the bar free of charge and I still have it on my hardtail. I trust that bar.
05-08-06, 03:17 PM
Come on guys, all this talk about carbon bars snapping catastrophically, and no actual examples of what kind of scratch can compromise a bar?
05-08-06, 03:30 PM
if sydney were here....
he'd tell you to quit whining.
05-08-06, 09:41 PM
Yeah, I kinda expected that :P
05-08-06, 10:27 PM
it's not so much the scratch that has me concerned, as much as the fact that you torqued them down to twice what easton recommends.
most carbon bars have a torque spec of 4-5Nm (hell, there's an aluminum ITM road bar that'll deform at 2Nm, and that doesn't even hold it still). i have my monkey lites torqued to 4.5Nm and they've never slipped on me. the 10-12Nm spec. for the stem is just that, the spec for how tight the stem bolts can go before the threads strip out. the spec for the monkey light is roughly half that. you never know how the structure is underneath after being under that pressure. i wouldn't ride them.
easton recommends that you sand the stem with 320/400 grit sandpaper to remove any burrs that can scratch or gouge the bar, creating a stress riser.
i'm not really sure what you mean when you say that a torque wrench isn't available in your area, even a low-end hardware store should have one of the cheesey (but better than nothing) deflection torque wrenches. abiding by the recommended torque specs. is important on any part, but especially so with carbon. i once mis-read the spec on a carbon stem and tightened the steerer clamp to 8Nm. it started to crackle at about 6.5, and at 8, the area around the bolt started to sheer.
05-08-06, 11:41 PM
Well, after rethinking what I did, I must not be even close to 10Nm. I never let the lever system go its full course, and I just tightened the bolts by hand with a screwdriver and no leverage. So that should put the torque value at approximately 5Nm. the bars were actually tightened by hand, and stopped the moment anything more than very slight flexion of the wrist was required, so that the burs just wouldnŽt slip. According to Easton, that would be aroun 5Nm. And if 10Nm is the maximum of my stem, then this torque is much higher than the one I applied, since I remember the torque I used on my steel bars on the same stem!
I guess IŽll just order a torque wrench online and see how much torque I put, and if I see I indeed overdid it, I wonŽt ride it. ShouldnŽt a snakebite pattern appear if the carbon is cracked?
05-11-06, 09:55 PM
Well just to update, both scratches are gone now, with the clear coating gone, leaving a jet black surface below. Easton still doesn't reply my emails. Anyone tried emailing them in the past?
05-11-06, 11:09 PM
This is why I don't ride carbon anymore, I just got fed up with being afraid of every single scratch and crack. Sure, a CrMo fork may be 1 pound heavier, but no more worries about damage, scratches, etc. and the vibration characteristics are way better anyway.
OK, here's my carbon riser bar experience:
1. I can't remember the brand, it's Taiwanese.
2. I fit bar ends and despite taking care, promptly crushed the bar with the bar end clamp. The failure was a crack along the length of the bar where the clamp caused the collapse.
3. I hacksawed the cracked/crushed bit off and made some aluminium sleeves to fit inside the bars to prevent collapse.
4. I have ridden them like that on my SS; now since it is a SS it suffers very severe torque on the bars when I climb the steep bits in my commute. I pull so hard on the bars my hands literally got blisters before callouses formed.
5. The bars also had a tendency to rotate in the stem clamp creating many scratches; I just kept on torquing until it no longer rotated under my grip.
With all that abuse, the bars are holding their own.
As for scratches, I would be concerned if they were deep enough to break the carbon fibres; the strength of the bar is from the carbon fibre, not the resin. If the resin fails completely, the bars would become spongy but should not suddenly fail catastrophically. So you can stop worrying.
05-12-06, 04:07 PM
I guess I'll be checking mine periodically to see if anything funny begins to happen. Is it obvious when a scratch has reached the carbon cloth? How deep should it be, approximately?
Meanwhile, I'll use clear nail polish to cover the two scratched patches, is that OK?
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