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Singlespeed & Fixed Gear "I still feel that variable gears are only for people over forty-five. Isn't it better to triumph by the strength of your muscles than by the artifice of a derailer? We are getting soft...As for me, give me a fixed gear!"-- Henri Desgrange (31 January 1865 - 16 August 1940)

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Old 01-24-07, 08:55 PM   #1
itsmeisthatyou
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dent in downtube...cause for concern?

i purchased an old steel framed road bike for a conversion and there is a small to medium size dent it the downtube....i was told by somebody i should be careful and i was also told its not a huge deal...any advice...is it even worth trying to use it? thanks
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Old 01-24-07, 08:57 PM   #2
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I had a similar thread going where I was informed that while some dents in the top tube aren't really much cause for concern, the downtube is the tube that takes the most stress and pounding. I'd be cautious.
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Old 01-24-07, 09:18 PM   #3
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What kind of steel is it? If it's some thin-walled 853 or something like that, I'd definitely be a bit more wary than say, some old gas-pipe frame.
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Old 01-24-07, 09:21 PM   #4
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Quote:
Originally Posted by Doctor Who
What kind of steel is it? If it's some thin-walled 853 or something like that, I'd definitely be a bit more wary than say, some old gas-pipe frame.
i miss Tom Baker
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Old 01-24-07, 09:22 PM   #5
itsmeisthatyou
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not real sure on the type of steel...its an old "KONA" frame
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Old 01-24-07, 10:24 PM   #6
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Quote:
Originally Posted by itsmeisthatyou
not real sure on the type of steel...its an old "KONA" frame
As with any technical question, more info = better answers. As with most brands, Kona deals/dealt with many types of tubing and many more different kinds when you consider the possible age. A year of the frame and the type of frame it is would be handy. Is it an old Jake or Sutra or what?

You can also find Kona's site at Konaworld.com and a quick google search will pull up a site with old Kona catalogs - I believe it was a German (.de) site that had the old catalogs.
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Old 01-24-07, 10:35 PM   #7
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kona huh
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Old 01-24-07, 10:45 PM   #8
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The top tube is in compression while the down tube is in tension.
Think about it this way, you roll a piece of paper up into a small tube, compress the tube, it withstands right? Now, dent it a little then compress again, it gets crushed.

Do the same and pull on it, dent it and pull on it again.
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Old 01-24-07, 11:28 PM   #9
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The dent makes the tube stronger!!
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Old 01-24-07, 11:56 PM   #10
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I have one big dent and two little ones in my seat tube near the BB shell. No worries, just keep an eye on it and don't ignore it if it starts to creak.
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Old 01-25-07, 10:59 AM   #11
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Well, I think the bike's now less likely to survive a head-on collision with a wall, if that's what you're asking.
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Old 01-25-07, 11:03 AM   #12
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Quote:
Originally Posted by slvoid
The top tube is in compression while the down tube is in tension.
Think about it this way, you roll a piece of paper up into a small tube, compress the tube, it withstands right? Now, dent it a little then compress again, it gets crushed.

Do the same and pull on it, dent it and pull on it again.
Don't you mean the down tube is in compression and top tube is in tension?
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Old 01-25-07, 12:10 PM   #13
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no, he's right
what it means, though, is that the downtube is generally more stressed, which is why it is traditionally a larger diameter than a top tube..

dents are rarely a problem.. unless so large that the tube is no longer straight, or creased so as to concentrate stresses in that area

Quote:
Originally Posted by braingel
Don't you mean the down tube is in compression and top tube is in tension?
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Old 01-25-07, 01:07 PM   #14
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Quote:
Originally Posted by manboy
Well, I think the bike's now less likely to survive a head-on collision with a wall, if that's what you're asking.
Exactly what I was thinking.

Under normal riding conditions, the downtube is subject to tensile forces plus a small amount of bending. If you slam into a curb or a wall, you put the downtube in compression for a brief moment [edit: assuming your ass is still planted on the seat at the moment of impact], because your forward momentum and your white-knuckle deathgrip on the handlebars make the fork steertube act as a lever that pries the toptube forward, while jacking the downtube backwards.

A bending moment or force-couple is introduced into the downtube due to the force exerted by the steer tube at the top and bottom of the headtube.

The dent reduces the effective moment of inertia (resistance to bending) of the shape at that point, and may provide a location for the formation of a plastic hinge (permanent deformation of the metal under bending loads).

The combination of bending and axial compression is the kiss of death for compression members.

So try to avoid walls and curbs.

Last edited by lunacycle; 01-26-07 at 09:16 AM.
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Old 01-25-07, 01:21 PM   #15
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Had a dent i my down tube once. After a couple of monts it started making weird sounds. I could see the crack start at the dent. I then had a blacksmith weld on an extra piece of metal surronding the crack. Rode on that frame for work for half a year after that without any problems.

http://fixie-king.dk/Workbike/Workbike-jumbo.htm

And then what happend:
http://fixie-king.dk/knak%20(2).jpg
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Old 01-25-07, 01:21 PM   #16
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http://pardo.net/pardo/bike/pic/fail/FAIL-017.html

it's thin walled aluminium, but something to keep in mind.

fsnl
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Old 01-25-07, 02:27 PM   #17
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Originally Posted by p3ntuprage
The rider in that account was 175kg? That's 385lbs! The article does go on to say that 140kg riders are more common, but that's still 308lbs. Is that really all that common? Or should I be asking this in Clydesdales?
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Old 01-25-07, 02:37 PM   #18
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Originally Posted by macsaorsa
The rider in that account was 175kg? That's 385lbs! The article does go on to say that 140kg riders are more common, but that's still 308lbs. Is that really all that common? Or should I be asking this in Clydesdales?
Might also have just been damage secondary to an OTB event- not used to disc brakes, puts on motorcycle helmet, heads off fast down a steep hill, and jams on the brakes. Fork compresses, guy flips ass over teakettle, and the tube buckles in the resulting tumble. He's got a monster rotor on the front there.
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Old 01-25-07, 08:53 PM   #19
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Quote:
Originally Posted by p3ntuprage
http://pardo.net/pardo/bike/pic/fail/FAIL-017.html

it's thin walled aluminium, but something to keep in mind.

fsnl
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Holy sh1t, that is one big motherf*cking bike.

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