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Up Topic Welding Industry / General Welding Discussion / Aluminum Mountain Bike Frame repair
- - By fakiee Date 08-16-2007 09:23
I have been hunting bike forums to find details on welding/repairing an aluminum bike frame, everyone so far has said it cant be done.  I believe it to be a 7005, or possible alloy composite (M4 - is what the company states but I can't find anything on this).

The bike frame is a Specialized Stumpjumper 2006 and the top tube of the frame has been severally crushed and needs replacing.  I have a friend (builds alloy stair rails, wheels, etc) who says he can replace the top tube but before he begins hacking and slashing the frame I would like to know the finer details of un-welding (not sure of correct term) and re-welding the frame - will it need to be cured/heat treated, does it need treating before the welds, etc?

Any help on this subject will be much appreciated.  Thanks for your time :)
Parent - - By Tommyjoking (****) Date 08-16-2007 10:19
Considering what can be spent on a high quality bike frame I can understand your want of a repair solution.  The damaged areas should simply be cut free and replaced with like material (hacksaw / cutoff wheel).  Any repair should be made via gtaw (tig-heliarc) by an EXPERIENCED welder.  If the frame is truely 7005 then 5180 would be the filler material of choice.  This will allow the frame to undergo heat treatment to restore it to its original condition.    If the frame is actually a t6 alloy more than likely you should use a 4643 filler which again will allow heat treatment.....if you wish to repair without heat treatment use 5356 which will work on either alloy but without heat treatment it will never be in the condition it was when produced.

...... if you have a bit of $$$ tied up in this frame I highly recommend sending it to a repetuable bike frame builder for repair.....he will have access to the materials, heat treat source and welding skills neccessary to restore your frame to proper condition. I don't know if he is still doing it but I might be able to hook you up with somone who builds exotic titanium frames who lives near me if you wish....

Good luck
Parent - - By fakiee Date 08-16-2007 10:54
Thanks for the info, Tommy.

Ive been searching for a decent answer to this problem for a while so your help is much appreciated.  The frame retails at £500 (overpriced for a robot built aluminum frame) so it would be nice to have it repaired.  However, 2nd hand frames of the same standard can be picked up for around £150-£200 and I have already got a new frame to replace the damaged one.  I wanted to see if it is possible to repair an alloy frame and I would still like to go down this path - im not looking for the frame to be brought back to its former glory just repaired so it is safe and ridable (the frame still rides with considerable strength, surprisingly!)

The tubes are hydro formed so the simplest method (i think?) would be to find a replacement tube, remove the old tube and weld the new tube in, trouble is finding an exact replacement tube - almost impossible i fear.  The top tube has been crushed which resulted in kinking and a nice crack/split in the metal so I doubt it can be re-worked?  So the next best solution I think would be to have a new piece of alloy tube welded in.  This would involve taking the old welded cable guide mounts removed and welded onto the new tube and creating similar diameters as the old tube for the new tube. I also assume the frame will need to have the powder coat removed before any work can take place (sand blasted?)

I agree this needs to be done by a professional frame builder, it would be great if you could put me in touch with the Ti frame builder - even if he does say it can't be done that will at least be closure for me :)

Thanks again!
Parent - By Tommyjoking (****) Date 08-17-2007 07:31 Edited 08-17-2007 09:54
Your quite welcome Andy....ppl here get a kick out of being able to be helpfull including myself.  Since you elaborated just a bit more let me see if I can narrow my advice to suit.

I am assuming we are talking about a top tube strut that runs from your steering head to your seat post.  I would imagine the wall thickness of your damaged tube is around .050 inch....if it is round you should be able to find an extruded tube in the proper diameter from a good local aluminum supplier in may be forced to purchase a 20ft length of said tubing. All in all you would probably be better off replacing the tube completely from its welded end to welded opposed to putting weld joints in the middle of its length. 

You are completely correct in that any paint or powdercoating will have to be removed anywhere near the repair areas...I would just have the whole frame BEADblasted to prepare it for a new coat of paint (sandblasting can cause issues with aluminum).  At the point of cutting and welding: What I would do before any work was done is to affix the frame to a table or anything really solid via furniture clamps/c clamps at the points the damaged tube is attached to.  By doing this you will make sure you don't inadvertantly change the geometry of your bike. Then an abrasive disc/cutoff wheel in a die grinder will make a nice quick clean cut to remove the damaged tube..and associated cable can use sanding discs and then scotchbrite discs the clean up the welded area finally going in with a fine file to remove any abrasive residue....then wipe the area with acetone.  Cut your replacement a bit long say 1 inch longer than the distance between the two surfaces and use your sanding discs or aircraft snips to cut (cope) each end to fit as well as possible (follow same cleaning procedure on that part). Fit the part and have your friend tack weld it in...then weld it up as much as can be reached....allow to air cool to almost room temp then flip over reclamp and repeat....any weld area left can be done unclamped after that.  Once again I would do this with no other method than GTAW (tig,heliarc) welding. Use the 5356 filler since we are not worried about heat treat.

If the tube your replacing is anything but round, square or could be very difficult to aquire an exact replacement.  If it is a formed tube in an odd shape it would take a very good metal smith to duplicate from flat stock ...think $$$.  However all is not lost here.  An easy repair method would be to aquire an extruded round tube (or rectangular) that is at least 2/3 as large in dia (or width) of the widest point of your odd shaped tube(elipse).  Then follow all the above but you may be forced to make a v cut on opposite sides of the tube (if your using round tube) along the vertical axis of the bike frame to allow it to be folded in (rubber/compothane hammer) to fit at each end to allow a good weld joint  ...the cuts will probably be no more than 2-3 inches deep and perhaps 1/4 at the top of the v to accomplish this along with carefull shaping via your grinding tools.  the v cut can simply be welded shut along with the rest.  Man I am sorry if thats confusing .

If you cannot aquire 7005 try using a t6061 extrusion with the same filler material 5356.  May not end up exactly like the original but I bet you can bring it back to life and make it usable again.  I am still waiting on contact with the frame builder but since I live in the middle of the USA and I am thinking maybe you are located on a different continent(?) that may be a moot point.    Since  heat treating will not apply here it simplifies things quite a bit..... any good reputable weld shop with GTAW capability in your area should be able to help you out with the work.

Best of luck
Up Topic Welding Industry / General Welding Discussion / Aluminum Mountain Bike Frame repair

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