itsmeisthatyou

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

andre nickatina

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.

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.

popluhv

i miss Tom Baker

itsmeisthatyou

not real sure on the type of steel...its an old "KONA" frame

Corcis

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.

word

kona huh

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.

operator

The dent makes the tube stronger!!

Landgolier

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.

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.

braingel

Don't you mean the down tube is in compression and top tube is in tension?

coelcanth

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

lunacycle

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.

Jumbo

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.

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

And then what happend:
https://fixie-king.dk/knak%20(2).jpg

p3ntuprage

https://pardo.net/pardo/bike/pic/fail/FAIL-017.html

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

fsnl
sparky

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?

fatbat

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.

slvoid

Holy sh1t, that is one big mother******** bike.