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Bicycle Mechanics Broken bottom bracket? Tacoed wheel? If you're having problems with your bicycle, or just need help fixing a flat, drop in here for the latest on bicycle mechanics & bicycle maintenance.

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Old 04-14-12, 11:45 AM   #1
saddlesores
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Cracks in chainstays...why?





aluminium diamondback mtb, used for light touring. lots of rough
roads in se asia, usually about 20-25 pounds on the rear rack, in
addition to my 185.

after a cleaning following a particularly rough ride, noticed a deep scratch,
about 3" long running lengthwise, midway down one chainstay. closer inspection
found it to be a crack, with another crack on the opposite chainstay.

no dents, no accidents, nothing i can think of to cause this. all chainstay cracks
i've read about have been at the weld near the bottom bracket, unless caused
by auto accident.

should an aluminum frame wear out after three years and maybe 20k miles?
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Old 04-14-12, 12:54 PM   #2
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Are the stays dimpled there for crank clearance? If so, the dimpling (done by the manufacturer) could have caused the cracks. If the frame is under warranty have it checked for replacement.
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Old 04-14-12, 01:39 PM   #3
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Aluminum frames CAN wear out in 20K miles, or less; time is less relevant than mileage/use.

I put about 15K on my Dakar XLT, and all of those were either communting or LIGHT urban. The XLT is an all-mountain frame, one I thought would handle my ruggedness (Clyde, 230# +). It did, too -- for that relatively limited span of time. (It's not like the money I paid for it was flushed, I saved enough on not buying gas/doing oil changes & tuneups/carrying car insurance to pay for it twice over!)
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Old 04-14-12, 02:25 PM   #4
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Those cracks are in an odd area and don't follow the line of the stress put on the stays. You would expect stress cracks to run perpendicular to the stays, not along them That and the fact both stays cracked and at the same place makes me think there had to be a manufacturing error of some type.
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Old 04-14-12, 06:57 PM   #5
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Aluminium fails catastrophically, and has no elastic tolerance - or some such phrase. Basically, every bump has an impact and the cumulative effect will be failure like the cracks you've got. Steel, as long as it's kept within its (usually quite wide) range of flex, doesn't deteriorate an iota.

You don't mention whether you're still riding that frame. Personally, I wouldn't swing my leg over a bike in that state.
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Old 04-14-12, 08:03 PM   #6
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my guess, the dimples caused a high stress point, which lead to premature failure of the tubing. Either that or the frame has been over hardened during the heat treating process.

I see, in the last picture, you have hose clamps on the chainstays, but those will do nothing to stop a vertical split there.
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Old 04-14-12, 08:42 PM   #7
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Oi! Those cracks look pretty bad if they are cracks. If they are, you should be shopping for a new frame immediately and not ride this one anymore, unless you can get the manufacturer to replace it under warranty as already suggested.

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Old 04-15-12, 12:13 AM   #8
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Do you ever ride off any drops?
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Old 04-15-12, 12:45 AM   #9
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Riding with loaded bags will have put far different strains on the stays than the sort of thing which would occur in normal riding. If they are indeed cracks in the metal which you can flex open and closed then STOP RIDING THIS BIKE RIGHT NOW! But another possibility is that the paint is separating from the metal below.

If you're not sure if it's simply the paint or if the cracks are indeed in the metal then chip away the paint. It's not doing any good anyway. If the paint flakes or chips off easily then you likely have your answer. But inspect the exposed metal anyway to be sure.

If you find that the stays are indeed cracked and flexing the stays causes the cracks to spread and close then this frame is done. Your hose clamp bandaids are not going to do diddly towards avoiding any final disaster. Instead it's time to call this one done and consign it to a scrap metal bin and go shopping for a new frame or bike.
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Old 04-15-12, 12:51 AM   #10
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Looks like crash damage to me.

- Mark
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Old 04-15-12, 02:36 AM   #11
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*chainstays are not dimpled for crank clearance. however, the tubing on the
front third or so is ovalized - narrower in the horizontal plane.

*no warranty, not that it would necessarily be honored in china. i got my money's
worth out of it.

*lotsa rough roads in laos and cambodia and southwest china. the last trip before i
found the damage was about 1000 miles thru guizhou and guangxi provinces, with
hundreds of miles of steep, potholed dirt and mud. with a nice 32-km screaming
descent down a 'road' not maintained since mao was in diapers.

*no crash damage, no drops, i always loaded/unloaded on buses in thailand.
no apparent damage by airline baggage handlers.

*you can see some scrapes near the cracks - where i used a dental pick to pry off
the loose paint. pressing on the pedals could cause the cracks to open slightly.
the hose clamps? maybe would hold things together on flat riding around town?

*this frame is now living a second life as a truing stand.
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Old 04-15-12, 07:30 PM   #12
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Looks like there is a dent in the stay, caused by some sort of impact. Maybe a hit by the crank arm? Metal fatigue?
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Old 05-14-12, 10:01 PM   #13
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no dent. that's the lighting on the ovalized tubing.

finally got in touch with diamondback. the guys in the warranty department (now comes under raleigh)
tell me they have no relation at all to the diamondback who sell bikes in china. and golly, they don't
even have a contact for that other company.

now riding this one....
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