Framebuilders - Need advice on bending aluminum frame
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03-08-09, 08:53 PM
I'm looking for some advice from those with more experience working with frames than I have. I have an aluminum, 29er mountain bike. I crashed it last year and bent the rear triangle out of alignment. Today I was attempting Sheldon Brown's method for bending the rear triangle back into alignment. Here's a link to his method, which involves using a 2x4 as a lever:
Now my frame is aluminum, not steel, but I needed only to move both dropouts right by about 1/2 cm, and I was willing to take the risk. Unfortunately, I misjudged the amount of force required, and my first bend resulted in a about a 2.5cm move to the right for the right side of the rear triangle. Then, of course, I had to bend that side about 2cm back to get it where it really needed to be.
While everything "looks" good now from the outside, I'm a bit worried about the excessive bending.
Should I toss the frame? I'm thinking that I will, and just chalk it up to "education".
03-09-09, 09:16 AM
Well, after sleeping on it, I've decided to toss the frame into the garbage. I'd never feel secure on it now to ride it.
It is no great loss. I had the frame in the trash pile once already, but pulled it back on the advice of my LBS. But the mis-alignment really gnawed at me. That's why I took a shot at re-aligning things yesterday evening. And I did get things realigned, but the bending is just too much, I think, so I'll just toss the frame and move on to a new one.
The frame was for my around-town, errand-running bike. Am thinking this time to go with either a Karate Monkey (flat bar, good color) or a Fargo (drop bar, not-so-good color). Either would probably work. What I'm after is a rugged frame that'll easily accept rack & fenders along with big, honking tires like, say, the Schwalb Big Apples in their 2.3" incarnation.
03-09-09, 11:47 AM
My son is not light on bikes and we have a good relationship with the LBS. On one bike, he broke off a canti stud. Not sure how he did it, but Trek replaced the frame... or, rather, gave us the equivalent price of a new frame off another bike. After tallying up all the wear items and time I'd have needed to dump into the frame swap, a new bike got the nod.
Then the wife backed over the bike after the son left the bike leaning against the tail gate a couple weeks later. We don't have the kind of money to keep buying bikes, and oddly enough the shop wouldn't cover such damage under waranty. Go figure... It was bent at the chainstays just behind the bottom bracket. I rigged up some stop blocks and used a car jack to bend it back into some kind of alignment.
This was an aluminum frame. I know the bending that went on severely compromised the frame--there was enough give when I was doing the bending to know that the frame would not be long for the world.
But it lasted him a few more years of abusive riding before he got a 29r to replace his old bike... which he wrecked in any number of other ways, but the frame stayed whole.
I am not at all suggesting that fixing an aluminum frame by bending it back into place after a catastrophic incident is a smart thing to do, but I was pleasantly surprised by how long a bike fixed like this lasted. I was fully expecting it to break, but gave up looking for cracks about the first year after the fix.
Worked for us; might kill you. YMMV...
As you were bending the heck out of it, or during the initial crash, did it seem that the bending occured all in one small area, like where the welds attach the the seat tube, or the bridge, then you have a possible problem in a concentrated area. If you were just bending the tube, it might be as well to look at the tubes themselves, do they have fancy S curves in them? It's not as though all bending is fatal.
The other thing, is that your main loads riding the bike are in the vertical axis. The bending may have weakened the tube in a horizontal axis (and assuming from the above check you have ruled in or out damage to the joints) but for the most part that is pretty lightly loaded.
Also it is the rear end. I'm not saying nothing bad happens if it falls off, but is it likely to collapse all at once, and even if it did... The fork is a different mater, if it collapses it's potentially fatal complication. Let's say you had a aluminum fork, and you crash it so your wheel base shortens an inch or 2, then you bend it back forward, well if that goes it could be interesting. What are we talking about here? Is it a comuter or are you racing it downhill?
I'm not trying to talk you out of trashing it, that has to be your safe option, but you can also analyse the specific tweaks and try to rule various failures modes either in or out. If there aren't any dents crimps, kinks or heavy workings of the welds, it starts to sound more like a general bend, one that spread the strain over a large area. So you would be concerned about being in a place where a certain force applied to a tube across a wide area was sufficient to cause the whole tube to blow. That would be quite a trick.
03-10-09, 07:48 AM
Peterpan, thanks. Good things to think about in your post.
I do, in fact, believe that the force was concentrated. It's hard to explain without being able to show you the frame and point, but the chainstays are dimpled (not sure that's the word I want) by design in a certain spot. They are thinner there, and the paint flaking away makes me pretty certain that those thinner, flattened areas took the brunt of the bending.
Other than having paint flake away from the bending, I see no physical damage. However, I cannot see inside of the tubes.
I do agree that a failure of the rear chainstay is unlikely to be a huge problem, especially not for an "around town" bike. But I've decided I'd just rather not worry about it at all. So I'm turning this event into my excuse to buy that Fargo frame I've been wanting.
And the Fargo is steel, so perhaps my newly-minted knowledge of frame bending will come in handy someday. :D:D:D
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