Aluminium Alloy subframe strenghtening

[lchris21]

Good Morning Peeps,

A mate has asked me to make some mods to the alu-alloy subframe on his offroad bike. He's going to be usuing it on long distance rally raid events ( baby versions of the Dakar) at home and in Europe.
The frame will be carrying an additional +10-15kg. I've worked out where the mods will take place, which is going to include some Tig.
I have a few reservations though on certain aspects.
I don't know what grade alloy it is, therefore don't know what to weld it with??? I'm planning on boxing in some of the extruded sections. Any ideas what it might be made of, what I can use to box it in, what I can weld it with? Any preheating required?? (its only 2-3mm)
I'm concerned about the HAZ?
I'm concerned about the fatigue long term, especially as the bike will have a hard life.

Input greatfully recieved.

ta

 

[chunkolini]

I think I would leave that kind of thing to the experts, long termism and fatigue could get complicated.

 

[hotrodder]

Bike manufacturers typically use one of the 6xxx series (6061 or 6082 are two common suspects) flavours and the odd bit of 7005/7020, the later recovers* better from welding but is more limited with regards to extrusion complexity/formability. KTM are one where 'aluminium 7020' comes up in tech specs sometimes

7005/7020 (effectively the same stuff with a different name) is best welded with a 5xxx series filler, 6xxx series can be welded with several different flavours of filler from both the 4xxx and 5xxx series depending on properties needed/whether it's having PWHT (post weld heat treatment). 5356 is both stronger and, more importantly, less brittle than 4043 'as welded' and so your best choice for this IMO. No preheat needed although taking the chill off is a good idea

Bike frames typically comprise a mix of castings and extrusions/wrought sections which makes PWHT less likely as temps and times are different for each flavour of ally. I'd be very suprised if they aren't designed around the 'as welded' properties

Can you expand on 'boxing in extruded sections'? I'm assuming you'll be using some 6xxx series plate? Fully welding will obviously soften a larger area than stitching while each stop/start in the later potentially makes for some great crack initiation spots... not totally relevant but was asked to look at a job a little while ago which turned out to be a cracked Kawasaki stunt bike. The guy got it welded, both sides of the main rails and the engine mounts. According to him they all reappeared again within a few weeks of abuse and he wondered if i could 'beef it up'. Told him plating it wouldn't be likely to work longterm and the reason it cracked again soooo quickly was probably 'cause whoever welded it had the DT's and was blind- crater defect at the end of every weld, undercut welds with sharp and erratic toes and the cracks followed these perfectly. Told him i could get it to last longer BUT it WOULD crack again as each heat cycle is gonna kinda stack up compounding the 'damage'. He called a few days later to say he'd picked up another frame


* it naturally ages quickly compared to the 6xxx series and with bicycles frames made of 7005 they are typically just naturally or artificially aged after welding while 6xxx series frames need solution annealing and artifical aging (precipitation hardening) to realise their full potential

 

[lchris21]

Thanks for the input Hotrodder. The sub frame is indeed from a KTM. Its main supports are made from extruded alloy in a "U" shape with various brackets and braces Tig'd onto it. I was simply going to complete the box of the U-channel with a strip of compatible material and stich it in??

I think I have some 5 and 4 series filler knocking about in the garage.......

 

[stuvy]

chris, have a browse on the KTM forums and see what the boys on there do.

it will give you a good starting block to go from, then you can plan what your doing with it

 

[lchris21]

cheers for the input. the plan has been formulated, its just the execution/welding side i need clarification of.
the frame is actually KTM, the bike is a Berg......bit of a hybrid longdistance prototype project we're working on.
Berg now builds bikes with plastic subframes

 

[stuvy]

i didnt know that the new bergs were plastic!

i know you want weight to be light but could you make an ally one if it all goes pete tong make a steel one?

due to the vibrations of a single itl crack just after the weld from what iv seen on crossers in the past

 

[Shox Dr]

Further to Hotrodders advise, 'Scott0031' on here, knows a lot about bike frames, swingarms etc, he doesn't come on here much so maybe a PM would be required.

 

[hotrodder]

Scott's not allowed to chill out on here until he's finished machining stuff for me

Is it anything like this one? http://www.oemdirtbikeparts.com/2007-2008-2009-2010-2011-ktm-sub-frame-p/000020-6862.htm

If i was gonna box the U section i'd be inclined to stitch some 6082 flat bar using 5356 however if i picked a relevant example i'm thinking that might just make the ends of the bar where the section's been scalloped the weak link? Replacing the U section with some tube and machined ends may be a better way

 

[lchris21]

Scott's not allowed to chill out on here until he's finished machining stuff for me

Is it anything like this one? http://www.oemdirtbikeparts.com/2007-2008-2009-2010-2011-ktm-sub-frame-p/000020-6862.htm

If i was gonna box the U section i'd be inclined to stitch some 6082 flat bar using 5356 however if i picked a relevant example i'm thinking that might just make the ends of the bar where the section's been scalloped the weak link? Replacing the U section with some tube and machined ends may be a better way

Thats exactly it. My thoughts on the scalloped bits were the same, a potential weakness. I'm going to have to machine some blocks there as adaptors to match the berg frame anyway. I'm looking at beefing that section up as part of it....
Tube....mmm, something else to think about. Cheers