zukahn1 

As mechanic I'm always a little uncomfortable with the practice of hard setting bending parts to fit and double hate doing it on frames to just get stuff to ok work. Typically I won't do it most of the time at the coop where I voluteer, with the exeption of DR hangers on a steel bike, with a correct tool. Is this wrong?

SJX426 

What does your gut tell you? I would guess you already know.

Chrome Molly

Great topic.

When cold setting a frame, I have wondered where the frame gets bent (just past the supporting lugs, somewhere along the tubing, etc)? It seems to pressure a frame into bending somewhere along a tube's length must produce a "pattern" to where the stays bend. A "sharp" bend would seem to be the start of an eventual crack. Then again, I've never heard of a failure directly due to cold setting. Slightly perplexing, and I'm also not sure what to think about it.

Road Fan

I understand these uncertainties, but all I can say is, I've had three frames cold-set by careful guys at various LBSs (different cities where I lived), and no negatives. One was 531 or SL as far as I can tell, one definitely SL, and another OX-Plat with very thin walls.

But I can also theorize as usual: If you bend a frame tube that is tapered, the cold-setting (permanent change) will tend to be at the end with lower-diameter, since lower-diameter tubing needs a lot less force to flex or cold-set than does the thick end, which is near the BB. OD is one of the major factors in tube stiffness, more significant than wall thickness. I think it's unlikely to cause damage at the BB or bridges, if the force is applied at the dropouts. Some Columbus seat stays are double-tapered: narrow at the dropout, wider at the brake bridge, narrower at the seat lug. Such a seat stay will also bend more at the dropout end than at the bridge.

It's easy to overthink and to look for problems around the corner, and it's good to do so. While people have reported problems, I haven't had any when the work is done by experienced and careful techs.

Many frame builders have to cold set in order to deliver a new frame that is aligned correctly. It's not a new practice.

dddd

I've cold-set hundreds of frame/fork tubes and have never initiated a crack to my knowledge, nor have I ever had any brazings fail.

There are however about a hundred or more considerations in terms of the actual methods used, which is just like any mechanic's bag of tricks for getting things "fixed" but not "broken".
I have aligned several frames and several forks in the field sans tools, usually using only the wheel's axle as a measurement instrument while forcing one dropout at a time this way or that.
I did manage to slightly buckle a department-store mountain bike's fat fork blade in the process, but it was more than ridable when I was done (and also was as straight as could be detected by visual ionspection and while riding). I even did several rides on it afterward since it was my own bike that I had payed $6 for at a garage sale.

I don't usually even look at the seat stays when aligning a bike's rear end unless the brake doesn't center, so normally all such work is done with the tools (if any) and force aimed at the more-rigid chainstays.
Some chainstays are already highly asymmetric in terms of bending effort in one direction vs the other, usually where crimping is involved, but also where residual stresses (usually from previous incidents of aligning or from the original accident) can make the metal very easy to bend back a small amount in the opposite direction to which it was last bent.
Note that this yield-point asymmetry or "memory" doesn't affect the elastic bending feel of the metal at all until the yield point is reached where some actual bending occurs.

Steel frames often turn up needing some cold-setting work to align some part of the frame or fork, and the cold-setting is simply the only way to go in most cases where extremely hardened metal is not involved.

My only warnings about re-aligning or "spreading" frames/forks are:
First, accurately determine the best course of action as opposed to relying on simple intuition. Think it through as this can get complicated.
Second, don't ever use a file to alter dropouts for alignment reasons unless all effects are considered as part of the first warning. Removing metal is irreversible.
Third, limit how far that a bending attempt can move, so as not to over-bend and risk the tube buckling. Repeated efforts are typically needed to verify where the required force level is, and leverage is your friend.
Fourth, never apply opposing force to dropouts expecting both chainstays or fork blades to yield equally (they never do). Measure and bend only one side at a time when spreading.

A somewhat related warning is to limit twisting force applied to a handlebar stem while holding the front wheel from turning, as I have seen dropouts break from the flexing that occurs down near the axle (a thus-weakened dropout attachment would seem to be an even much worse outcome).

Velognome 

Wrong? No....an unfounded fear...maybe.

You should clamp the brake/fender bridge if your setting the rear. If you examine a set frame you can see the bend/alignment begins at that point on the seat stays. I'd assume the Chain stays bend would begin at the brace or BB lug if a brace is not present.

Agreed, it is a difficult think to stomach if your setting an expensive frame, but I've only heard of one failing and that was at the bridge and it was unclamped to begin with.

To get over your fears, take a old useless frame and bend it up (if you can) I'll bet you give up before the frame does!

On the plus side, it's better to set a frame and ride it then let it hang on a hook unused.

arex

Yes, you're bending the frame, but not very much at all...just opening it up a little. Unless you're really gorilla-ing it, you're not going to harm it.

busdriver1959

The picture shows a fork blade getting its curve. If a fork blade can withstand this without cracking, a couple of millimeters bend in a stay means nothing. There is a noted Japanese builder, whose name I forget, who purposely overbends and then corrects his fork blades. Not sure why. Bottom line, steel is very forgiving.

JohnDThompson 

It does depend to some extent on the tubing. The thinner the tube, the greater the propensity to collapse. And some tubes, e.g. Reynolds 753, simply will not cold-set without a very high risk of failure (753 fork blades are raked at the factory, prior to heat treatment). But these aren't the types of bikes you're likely to run into in a co-op setting.

3alarmer 

...just guessing, but probably the same principle as stress relieving the elbow bend in spokes.
It is true that when you bend steel it sets up some stresses, and there is some cold work hardening if you bend it back and forth too much.

But yes, it seems to be a pretty forgiving material.

...this. And to it I would add that if you go to the trouble of checking the stuff you either buy used or help
people work on at the co-op, you'll soon discover that there are a lot of bikes rolling around that have already
suffered some bending in use that misaligns them. There's no way to fix that except bending them back.

I find very few of the bikes I buy for my own restoration and use here to be free of some minor misalignment,
either in the frame or in the fork, sometimes both. I often do some headscratching just trying to figure out
what was the original spacing for something.

rando_couche

...and how do you think frames are aligned in the first place? With very few exceptions they're not straight immediately post-brazing.

SP
OC, OR

dddd

This process is called stress-relieving, where in this case the yield strength of the fork blade is lower, initially, as the fork blade is first stressed in the direction opposite from which it was just bent.
This is what I referred to earlier as "yield point asymmetry" or "memory" that can greatly reduce a frame's ability to maintain it's original alignment.

After the stress-relieving "reverse-bending" of perhaps a couple of millimeters, the fork blade now has a symmetric, uniformly-high yield strength in both bending directions, helping the frame to better maintain it's original alignment and geometry.