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Steel Frame Repairs


Procrates
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So a friend had a little boo-boo and bent his steel framed peace.

Does anyone know a framebuilder / engineering firm that will be able to save the frame and let peace continue.

 

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From what I can see in the photo I don't think it is that serious.  From the picture it looks like a ding only, and it doesn't look like the seat stay is bent.

 

If the seat stay is bent, get a new frame.  When steel tubing bends, it goes into the plastic region(beyond the Young's Modulus) which makes that part of the frame harder, which in turn creates a stress concentration at that point in the seat stay.  By bending the seat stay back to straight will only increase the stress concentration at that point in the frame.

 

The following recommendation is only applicable if the damage to the seatstay is a ding only.

 

If your buddy have insurance on the bike I would recommend claiming a new frame. 

 

Otherwise I would suggest you get some body putty and fill up the cavity and get some primer to spray on followed by the gloss black of the frame.

 

I have worked with steel and other metals since I was a little boy.  For my final year project at Varsity I designed and built a roll cage for an off road go-kart, that took part in the SASOL mini baja challenge.  The one guy on our team bumped the frame against a boullard on a test ride which dinged the frame. 

 

I included the ding on the 3D model of the frame and had a rerun on my FEM Analysis.  The results were for all practical purposes identical. 

 

This frame of the GT Peace is constructed out of AISI 4130 which is also known as chrome-moly, which is a steel alloy which has about 1.5times the tensile strenght of mild steel at the same weight.

 

Frames like these(I also own a GT peace) are generaly over engineerd, so I don't think that this ding(small) in the frame would be a problem at all.

 

But as I said, if you have insurance, this is the perfect excuse for a new frame.

 

 
Stephan2010-05-22 09:09:57
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Stephan

 

Well said, you are spot on. Also from personal experience a simple ding on metal or alu not serious. If ridden you will not have a catastrophic failure causing an accident. However the rider needs to inspect that part after every ride. What happens in my experiene is that a hairline crack may start to develop.

 

Once this happens you can eithe repair it or replace the frame depending on the value of the bike but the bike can not be ridden safely from here again.

 

By the look of it being a steel frame its probably cheaper to replace than to repair.

 

 

 
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You've heard a whole bunch of scary things and I think most of it is over reacting. That steel will NOT work harden from a ding like that and it poses Zero risk to the life of the frame or the rider's safety.

 

The photo doesn't tell the whole story and we don't know if the seatstay is now bent and the rear triangle out of alignment. The way to check would be to remove as many components as possible - crank, wheels etc and wrap string around the frame, say from the left rear dropout, over the head tube, to the right rear dropout. Now check for diffenent gaps on the left and right.

 

It may affect things such as drop-out alignment, something that will have a short-term effect on hub bearings.

 

Unfortunately even a framebuilder won't replace that seatstay. Iti is TIG welded and custom-drawn for that bike. To find a piece of cromoly tube in that shape is difficult and replacing a TIG-welded piece isn't as easy as replacing a lugged part.

 

If the drop-outs are stil perfectly aligned, any repair can safely be a cosmetic one only. Some body-putty and a paint job.

 

If the drop-outs are crooked, you can get them back into alignment, but that still won't fix the ding. Treat it cosmetically again.

 

There is certainly no reason to dump the frame. It is either good, or can be repaired (aligned).  No need for FEM, CAD or FUD.

 

I am 100% certain that a crack will not develop. That area of the frame is not in the heat-affected zone (from the weld) and can safely be bent backwards and forwards a few times.

 

Framebuilders will tell you they regularly "cold set" (framebuilder speak for bend) rear triangles to accommodate 120mm and 130mm hubs as and when the frame's purpose changes.

 

 

 

 
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Just imagine if this was a carbon frame.Shocked

 

No worries dude, don't be scared of spastic plastic it won't hurt you Big%20smile. I ride an FLX team that was repaired at exactly the same spot and after 18 months it's still all good and though none of you know me I am a 'big' guy Shocked
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@ Johan Bornman,

Only saw your reply today.  This is just discussion, and no personal attack.

Work hardening did occur in that area.

When you view the stress strain curve of a material, there are mainly two parts:

Elastic region ? If external load is taken away the material will return to its original form

Plastic region ? If the external load is taken away in this stage the material will not return to its original form and permanent deformation will occur.

In this instance the seat stay went into the plastic region and permanent deformation occurred.  Whenever a material is strained into the plastic region work hardening does take place.  Many text books on the subject also refer to the plastic region as the work hardening region.

Chro-moly(hard) has a smaller plastic region than mild steel (mealable) which means it work hardens quicker than mild steel, higher grades of chro-moly (higher than AISI 4130) which is harder, also requires heat treatment before, and sometimes also after welding.

There will be a stress concentration in that seat stay due to 2 reasons:

The sudden change in cross-sectional geometry(because of the ding)

The sudden change in material properties due to work hardening (although small).  This can be rectified by heat treatment

This stress concentration is small, but the seatstay will weaken quite a bit if it is bent, and then bent back to the correct postion.

When a frame is ?cold set? as you say, it bends evenly in four equal deformations.   If you bend this frame back, it will bend just beneath the ding, which will weaken that section further.

Since this is a cheap Taiwan chro-moly frame I don?t think it is over reacting when I recommend the guy to replace it if it is bent.

I am not a phantom who all of a sudden thinks he knows everything about everything, but I studied my ass off to be where I am today, and to do the work I do every day, and this problem was right down my alley.

From my time on the hub, and the number of posts I have(ie posts per day), you will be able to see I am not trying to be a smart ass.

PS:  Why were you on the hub during the Bulls vs Crusaders game?Wink

Stephan2010-05-28 09:13:31
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