Crack near front derailleur clamp without overtightening
kilou
Posts: 26
Hi,
my e-bike (bought brand new in march 2013 with now 7400km in total) originally came without any front derailleur (1x10 with a 44 teeth chainring and 11-36 cassette) but the gear ratio was too long for me so I added a second chainring (34 teeth) and bought a front derailleur (Shimano XT direct mount with an "Problem Solver" clamp adpater fitted on the seat tube (http://problemsolversbike.com/products/ ... ur_adapter). I've been riding this setup for 2000km now without any problem. However a few days ago I noticed something that looks like a hairline crack on the seat tube, near the back of the clamp :shock:
My bike frame is made of 6061 aluminum alloy. I do know that it is critical not to overtighten things on that kind of material and I always use a torque wrench. When installing the derailleur clamp, I've been extra careful about that and just tightened the clamp so that it doesn't move. To give you an idea I use a small torque wrench to tighten the seatpost clamp to 5Nm (as recommended for my bike) and I can feel how tight this is. I used the same torque wrench to tighten the derailleur clamp but did not go up to 5Nm as this was not necessary. So I'm absolutely certain the front derailleur clamp was tightened with less than 5Nm, which means that overtightening can be safely rules out as the cause of the crack. I mean is it possible to crack a seat tube with less than 5Nm??? Note that the clamp was the correct size for the seat tube!
I'd like to know if you think it is possible that simply the action of the front derailleur (pressure on the seat tube due to spring tension) may have caused the crack in the seat tube?? Or is it possible that a bike that is not originally fitted with a front derailleur has the seat tube so extremely butted that even a tiny amount of pressure can crack it????
Sounds like nothing can last these days
my e-bike (bought brand new in march 2013 with now 7400km in total) originally came without any front derailleur (1x10 with a 44 teeth chainring and 11-36 cassette) but the gear ratio was too long for me so I added a second chainring (34 teeth) and bought a front derailleur (Shimano XT direct mount with an "Problem Solver" clamp adpater fitted on the seat tube (http://problemsolversbike.com/products/ ... ur_adapter). I've been riding this setup for 2000km now without any problem. However a few days ago I noticed something that looks like a hairline crack on the seat tube, near the back of the clamp :shock:
My bike frame is made of 6061 aluminum alloy. I do know that it is critical not to overtighten things on that kind of material and I always use a torque wrench. When installing the derailleur clamp, I've been extra careful about that and just tightened the clamp so that it doesn't move. To give you an idea I use a small torque wrench to tighten the seatpost clamp to 5Nm (as recommended for my bike) and I can feel how tight this is. I used the same torque wrench to tighten the derailleur clamp but did not go up to 5Nm as this was not necessary. So I'm absolutely certain the front derailleur clamp was tightened with less than 5Nm, which means that overtightening can be safely rules out as the cause of the crack. I mean is it possible to crack a seat tube with less than 5Nm??? Note that the clamp was the correct size for the seat tube!
I'd like to know if you think it is possible that simply the action of the front derailleur (pressure on the seat tube due to spring tension) may have caused the crack in the seat tube?? Or is it possible that a bike that is not originally fitted with a front derailleur has the seat tube so extremely butted that even a tiny amount of pressure can crack it????
Sounds like nothing can last these days
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Comments
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How sure are you it's a crack, the loads from a front mech should never be enough to crack a tube that supports the riders weight (when seated)......Currently riding a Whyte T130C, X0 drivetrain, Magura Trail brakes converted to mixed wheel size (homebuilt wheels) with 140mm Fox 34 Rhythm and RP23 suspension. 12.2Kg.0
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Well I'm not 100% sure, it just look like it's a cracked and I cannot see how a scratch could have made its way at this location. The crack is located just in the opening of the clamp, behind the bolt that tightened the clamp on the seat tube. It is vertical and about 2cm long. Since it goes behind the bolt, I suspect it's a crack as a scratch could not easily go behind the bolt. I've marked the crack end and will monitor its size to see if it's growing. Will also see if I can post pics of it.
The load of the front mech is horizontal while the weight of the rider puts a vertical load o the seat tube. Could that explain it? Also note that since the bike did not come with a front derailleur originally the seat tube could have been extremely thin at that location due to tube butting... The thing is that I'm 99% sure the clamp was not overtightened (the 1% remaining are just here because we're never 100% sure of anything). But I know at least that the clamp was tightened with less than 5Nm.
Is there a simple test to know if it's a crack or just a scratch without ultrasounds or x-rays???0 -
Post a picture?I don't do smileys.
There is no secret ingredient - Kung Fu Panda
London Calling on Facebook
Parktools0 -
Here are some pics (sorry for the quality it's quite difficult to spot on). The front derailleur clamp has been removed for clarity. The arrow shows the upper end of the crack. The 2 other marks on the right shows another "scratch/crack" but this one is less clear. The most obvious "crack" is the one below the arrow. What do you think?
http://postimg.org/image/4cmsb6vuh/
http://postimg.org/image/b2s2j5ry3/
http://postimg.org/image/3xg3vwzax/
http://postimg.org/image/3yun32qih/
http://postimg.org/image/ee9jkao65/
The last pic is probably the best. You can see that the line is not straight, there is a sharp deviation in the middle. Looks more like a crack in the alloy than a scratch to me...0 -
Looks more like a scratch than a crack to me....mark the ends of it and see if it spreads at all......Currently riding a Whyte T130C, X0 drivetrain, Magura Trail brakes converted to mixed wheel size (homebuilt wheels) with 140mm Fox 34 Rhythm and RP23 suspension. 12.2Kg.0
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If it were just a scratch, shouldn't I see bare aluminium (light gray) underneath it? After all a scratch means that the paint got removed so the aluminium should be visible. This is definitely not something I see here: the scratch/crack is very dark and I can't see any bare metal in it.
I'll maybe try to pour some paper towel down inside the seat tube and spray some WD40 on the crack from the outside. WD40 is a good penetrating oil so if the paper towel gets wet, the tube will be cracked for sure...0 -
The WD40 trick didn't work, or at least the paper towel didn't get wet at all. That doesn't mean the tube is not cracked though...
Here is another pic with the front mech clamp attached. See how the crack/scratch is located at the edge of the clamp and also goes slightly behind it. A scratch couldn't really grow behind the clamp, no?
Note that we're talking about the scratch/crack on the lower left. The one on the upper right is probably really a scratch and it looks quite different than the one on the lower left.0 -
Maybe it got electrocuted.I don't do smileys.
There is no secret ingredient - Kung Fu Panda
London Calling on Facebook
Parktools0 -
Still looks like a scratch....Currently riding a Whyte T130C, X0 drivetrain, Magura Trail brakes converted to mixed wheel size (homebuilt wheels) with 140mm Fox 34 Rhythm and RP23 suspension. 12.2Kg.0
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Hope you're right! I'll keep an eye on it and see if it's getting bigger. Thanks for your help guys!0
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It looks like a scratch. If the clamp were tight enough to damage the frame, it would have left a few more marks on the paint.Specialized Roubaix Elite 2015
XM-057 rigid 29er0 -
You could try this:
Remove clamp. Get long thin screwdriver. Jam blade against bb so you don't scratch paint. Press handle against ear so it works like a stethoscope. Tap tube with side of wooden dowl or similar. Compare noise of tapping known good area with that of tapping on suspected crack. If there is a crack it will probably sound different.
Not saying it would work but worth a try. I've used technique before to detect cracks in car drum brakes.Sometimes. Maybe. Possibly.
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DIY NDT: run a spirit-based marker pen along the mark and then try and wipe it off with a cloth and solvent. If its a scratch, it will simply wipe-off. If it's a crack, the marker pen dye will seep into the crack and be far harder to remove.Make mine an Italian, with Campagnolo on the side..0
