Guy Yinon

I was looking for a touring MTB frame when I found a Marin Bear Valley Limited frame (4130). It's quite my size and looked great at first glance, with a rigid fork and mid-fork eyelets for a rack. I picked it up and brought it home for further inspection, only to find a dent on the downtube.

I used to be a purist, but not anymore... Is it worth (or okay) to build a touring bike using this frame? My other option is to keep the fork and buy a used alloy frame.
What do you think?

The dent (under the N letter)

FBinNY 

Clean it up and put a Band-aid over the dent. If you didn't already own it, I'd probably suggest passing on it unless it were super cheap. But that bridge has been crossed, so at this point you might as well build it up and enjoy it until you opt to trade up later on.

FWIW the Band-Aid is there to put the dent into proper perspective and hopefully silence those who'll feel the need to comment about the dent. A friend did this years ago and it works amazingly well.

maddog34

build it, ride it, and know that millions of steel bikes have survived indefinitely with similar dents, in higher stress areas....like my '82 trek, dented a seat stay in 1983, still going strong.

Iride01 

If it's already yours and you are going to paint it, then bondo or some epoxy filler will make that go away.

For the most part it's only cosmetic. Although like anything that's damaged. You should always check occasionally to make sure nothing worse is happening. But you'll notice in that new paint if a crack is starting. But still I think it's very unlikely to ever be anything worse than the blemish it currently is. Unless perhaps you dent it again in the same place.

grumpus

Hard to tell from that angle/lighting, what's metal, paint or corrosion. As long as there are no sharp ridges/creases in the metal I'd not worry about it. Maybe remove flaking paint, treat the worse rusty bits, make the orange go away. Then a couple of clear coats if you're going for the rat look.

Iride01 

You might compare the geometry of the frame to bikes that are specifically made and marketed as touring bikes. I'm not sure I found the correct geometry of the frame you have, but the seat tube angle is a little more steep than what you'd want for a more relaxed touring position. And the effective top tube length is way longer than most road bikes of any sort. So you'd probably have to use a short stem and/or drop bars or other type bar with little to no reach or a flat bar.

I think touring bikes are usually 700c, 28" or 29er tires (all 622mm bsd). I'm suspecting this frame is for 26" tires. Though there really isn't anything preventing the use of 26" wheels/tires if the gearing is correct.

Duragrouch

"Bondo in your doors is a sign of courage." - (I think Rufus Parnelli Jones)

If you want it smooth, prep the area, bondo, level off, prime and paint.

If just leaving the dent, I treat small rust spots with Ospho (mild phosphoric acid treatment), let dry for several days, turns red iron oxide to black iron phospate, or other rust remover, wipe with alcohol, paint with touch up paint (brand-x nail polish at the drugstore is only $2-3, very wide variety of colors and easily mixed to match, and 1/10 the price of auto touchup paint these days at the auto parts store).

grumpus

MTB sizing is really based on top tube length and stack height, seat tube length is just about crotch clearance. I have a roadified cross country 26" MTB with six inches more seatpost than my larger framed traditional road bikes - reach and height at the stem are pretty similar.
26" wheels are good for heavy touring because they're rigid, and you can get a 26x1.95" tyre just about anywhere in the world should you need to. Good fat 26" tyres roll nicely and soak up the bumps on a poor surface - they're not going to win any crits, or a Strava commute, but this is touring, or maybe a bit of gravel.

smd4

If you're not a purist, then you're good to go--no need to ask opinions here.

However, I'm still a purist--that frame would be gone. Life's too short to suffer with other people's crap.

grumpus

Wire brush and phosphoric acid is good, the little Dremel wire brushes reach into dents well. But clear coat it - IMHO a touchup job often looks worse than the original damage. A dent like this normally has a slightly raised rim, if you Bondo it I'm not sure you'd be able to blend it effectively.

GamblerGORD53

It's rusty. Toast.
My old handlebar had a rusty spot and snapped apart by the stem one day.

grumpus

Most steel bikes have a bit of rust somewhere, most don't suffer catastrophic failure. Steel handlebar, was that cheap junk or exotic lightweight? A possibly damaged or defective handlebar would ring my alarm bells, failure there is not something you really want to contemplate.

bboy314

Better pictures would help determine the severity of the dent. From here I’d call it no big deal, but it might be worse than it looks in that one picture.

WizardOfBoz

Agree- better pictures would help, but to sum up some things above (and to add one other point)
1) If there are no sharp edges or rips or tears - that is, if its' just dented and not scored - I think that you're ok
2) Steel, especially a high-strength alloy, is often forgiving. That said, sometimes folks use super tube material to minimize the tube wall to almost nothing. If you can find out if the tubes used were not "ultra-lite" or some such, it would add to my confidence.
3) The down tube has some torsion, but IIRC the main stress on the downtube is tension. That's better: dents really, really facilitate buckling under compression. The "stand on the beer can" trick gives good insight into buckling under compression.

Fixes range from ignoring the dent (after stripping and checking carefully for cracks) to aesthetic fixes (bondo) to brazing a piece of 4130 over the thing.

And to answer the question: JB Weld will NOT act as even a temporary repair. Epoxies have tensile strength around 2-3000 psi, 4130 steel properly heat treated has a tensile strength of around 100,000 psi.