Welding an Alu frame advice (Frankenbike content)

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Welding an Alu frame advice (Frankenbike content)

24 posts & 13 voices | Started 5 months ago by kevj | Latest reply from mick_r

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kevj - Member

I have been planning some major surgery on a frame I have.

I want to modify an existing frame by welding a sleeved seat-tube over the existing seat tube to increase this heigh by around 65mm and then to weld a gusset plate from the top-tube over both the existing seat-tube and the extension to stiffen everything up nicely.

I have been scouring ebay and the classifieds for a suitable donor frame for the extension and gusset with no luck as of yet.

Is there anything I need to look out for? I plan to create a cut-out on the gusset so I don't have to grind back the existing weld.
I also want to get the need seat tube shaft reamed to take out any overspill.

The work is to be done by a local engineering firm.

Anyone had similar work done or have any good advice before I boak perfectly good frame?

Posted 5 months ago #
thepodge - Member

can I ask why you need a longer seat tube?

Posted 5 months ago #
kevj - Member

thepodge

can I ask why you need a longer seat tube?

The frame is slightly too small, which means I have the seat post higher than I would like it to be. I only need an extra 25mm really, but to get the gusset plate to work with a sufficient length of weld, I may as well add in additional length at this stage.

This hasn't been a problem as the rest of the geometry fits me perfectly, but I wish to use a dropper seat post and to do this I would need the frame to slightly higher. I prefer a slacker feel to the bike for going downhill so I am happy with the general geometry.

This is a project bike I am building up which is going to be powder coated so I wanted to get the welding done upfront.

All the drawings are done and I have been quoted a good price for the work. The frame is lovely to ride and I didn't want to swap for another, especially as I wanted to spend some time getting this set up for me.

Posted 5 months ago #
thepodge - Member

fair enough

I'd question if the top tube could take the weld and the extra load.

I'd also look into if you'd need and if the frame can survive heat treating after welding.

Posted 5 months ago #
kevj - Member

thepodge

If the frame can survive heat treating after welding

This was my main concern really, if it were to render the frame brittle.

If I avoid the existing welded area then the new weld will be in areas what have not been previously subjected to heat, so surmise that is should be okay?

Posted 5 months ago #
bristolbiker - Member

If I avoid the existing welded area then the new weld will be in areas what have not been previously subjected to heat, so surmise that is should be okay?

In summary, no If it's a 6000 or 7000 series alloy frame you will need heat treatment on all the welding you do, unless you make the weld beads so big that the average stress in the welds is tiny (then it will look, at best, 'industrial').

Likewise, check the temp of the powercoating system - are you getting up to temperatures/times where you will overage the frame?

Posted 5 months ago #
Singlespeed_Shep - Member

I hope you've covered this but have you looked up making/using a extra-long seatpost?

Posted 5 months ago #
kevj - Member

Bristolbiker,
Would heat treatment then affect all existing welds?

Singlespeed,

I have a seat post that is long enough as it is and the frame is usable, I just wanted to run with a shorter lever arm on the post itself and have the option of using a dropper.

Posted 5 months ago #
grantway - Member

Think for Aluminium you'll need a Tig Welder
here's a link for beginners

Tig Welding For Begginners

Posted 5 months ago #
karl1824 - Member

Be carefull with Aluminium alloy welding, no preheat and you may get HAZ cracks, too much heat and you can get HAZ softening, (ie reduction in strength). A friend of mine had his cracked frame repaired by a local company. Cracked again a relatively short time later, possibly due to one of the above. His repair was on the sea post! You will need an expert in structural aluminium welding, most likely to be sure.

Posted 5 months ago #
cynic-al - Member

Can you please not use the term "Frankenbike" a I feel it is slanderous to my own work.

Posted 5 months ago #
razor1548 - Member

"I also want to get the need seat tube shaft reamed to take out any overspill."

I don't know anything about welding, but I know a swarthy fella who will ream your seat tube for you, for as little as £20.

Edit: He says he makes sure nothing is ever spilled!

Posted 5 months ago #
cruzer - Member

you should be alright without the heat treatment of the welds. Just leave the frame alone for 2 weeks and it should settle down and strengthen. The main reason the majority of frames are heattreated is to speed up this process. just make sure they use backing gas, as this will take the majority of the contaminates out of the weld as it forms.

the main concern i would have is the heat the aluminium produces when its welded is likely to warp the seattube, especially since it will be very thin walled. make sure you get someone who knows what there doing not a man in a shed as it will become alot more costly in the long run.

it might be worth talking to a specific frame builder about this as they will be set up for a job like this. theres plenty of them about now-a-days.

Posted 5 months ago #
cynic-al - Member

I know you'll laugh OP (and others) but this seems to me to be a perfect job for carbon fibre & epoxy wrap!

Posted 5 months ago #
cruzer - Member

I know you'll laugh OP (and others) but this seems to me to be a perfect job for carbon fibre & epoxy wrap!

Joking aside, that wouldnt be such a stupid idea and it would stop the warping problem

Posted 5 months ago #
cynic-al - Member

I wasn't joking!

Posted 5 months ago #
kevj - Member

cruzer - Member

you should be alright without the heat treatment of the welds. Just leave the frame alone for 2 weeks and it should settle down and strengthen. The main reason the majority of frames are heattreated is to speed up this process. just make sure they use backing gas, as this will take the majority of the contaminates out of the weld as it forms.

the main concern i would have is the heat the aluminium produces when its welded is likely to warp the seattube, especially since it will be very thin walled. make sure you get someone who knows what there doing not a man in a shed as it will become alot more costly in the long run.

it might be worth talking to a specific frame builder about this as they will be set up for a job like this. theres plenty of them about now-a-days.

The company are a marine engineering outfit and have a very good reputation. I take it you mean leave it two weeks before I get it powder-coated so as not to lock-in any gasses?

cynic-al - Member

I know you'll laugh OP (and others) but this seems to me to be a perfect job for carbon fibre & epoxy wrap!

That is actually not a bad idea at all and theoretically, I could use thinners to remove it should I ever change my mind.

Hmmm....

Posted 5 months ago #
cruzer - Member

my concern would be that marine engineering and bicycle engineering are 2 to completely different areas of experties. id drop a line to Argos (frame builders) and see what then recommend. you dont want to waste alot of money only to scrap the frame. best to pay that little bit more and get it done by someone who knows exactly what their doing.

And yes, leave it for as long as possible in its bare state. 2 weeks should be enough, but the longer the better.

definately think that the carbon wrap idea might be the best

Posted 5 months ago #
kevj - Member

Yes,I am liking the idea of the carbon wrap as it alleviates all the worries I had about welding.

I have sent an email to Fibre Lyte who are relatively local to me. Just waiting on a reply now.

Posted 5 months ago #
thepodge - Member

I would think you'd be ok with a marine engineer. boats and bikes aren't that similar but the aluminium process will be close enough.

Posted 5 months ago #
smiththemainman - Member

From experience, snapped a trek liquid 30 , top tube inch in front of seat tube and inch above front mech both clean through, machined myself a couple of hollow tube like inserts put frame back together and mate welded round breaks, used the bike next day and many times since, never been a problem, left the weld full, looks pretty much part of frame. Just sat as spare bike/ clothes hanger now.

Posted 5 months ago #
kenneththecurtain - Member

You'll need to make sure that the alloy your frame is made of will age-harden naturally. Many alloys have to be artificially aged (eg, heat treated) to attain their design strength.

Some stuff up there about it being ok with big welds - its the HAZ you want to worry about, big welds won't help (in fact, you may introduce even more heat, making things worse).

Other than the heat treat issue, I would question if you will stiffen the frame in a way which it wasn't designed to handle. You may end up with a monster stress riser in the middle of your top tube, which you don't want.

Posted 5 months ago #
takisawa2 - Member

Just buy a bigger / more suitable frame & save yourself a shed load of hassle.

Says he who joined two frames together to make a long tail.

Posted 5 months ago #
mick_r - Member

Most of my experience is with steel, but most welded frame repairs I've seen (usually around the seat clamp area) have cracked very rapidly.

I think Ceeway sell some aluminium bike tubing that would be a much better source of raw materials than a scrap frame (Columbus?) and there used to be some Columbus tech info about the post weld heat treatment required. You need to make sure it is a similar grade to your existing frame (6000, 7000 series etc) http://www.framebuilding.com

I'd talk to Argos or somewhere specialising in motorbike frame repairs. They might also have suitable raw materials (most readily available aluminium tube stock is the wrong grade or already heat treated so not really suitable).

Or on the cheap I was also thinking that the carbon wrap option and some machined pieces of aluminium tube (Ceeway, ebay, Simmal aluminium etc) would probably be quite reliable and low risk (i.e. you could reclaim the old frame to some extent if it all went wrong).

Posted 5 months ago #

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