brownie wrote: what about putting a straight edge along the backbone to see if its bent there
Well spotted Ross the left hand tube shows a couple of millimetres clear under the straight edge - right side appears flat. Now I have no idea what to do
Ah I can't take credit for that I was talking about the top tube under the tank The tubes your talking about if one is bent and the other straight you'd think it would put the front down tube out of alignment, to one side (lhs?)
Top tube looks straight to me.............bend will be where the top tube joins the tubes coming up from swinging arm pivot, and if you heated this area with an oxy acetylene torch, I would imagine the down tube could be made to go back into place very easily. Probably just need to tack a cheap welding clamp onto the down tube, and clamp around a bolt going through engine mount, heat in area I suggested, then use the clamp to pull tube back into place.
Before welding tube back together check the headstock is still lined up properly though, as if the jobs not done properly the wheels wont be in line any more. If a frame is made from chrom moly it either going to be brazed or TIG welded, and if it is then you will need to use one of these methods to make repairs.
I think the easiest way is to get a turn buckle and some chain, and attach the chain with a U clamp to the frame and use an other U clamp to anchor the turn buckle, and turn the buckle slowly untill shes home, you can even use 2 or 3 chains and buckles to line it all up nicely.
If it was me doing the repair I'd just MIG weld it. The originals were not brazed or TIG welded so I'd not be to worried about that. The gap was caused by the bike being riden with the brake so just pull it back down and I'm sure it will be close enough to straight or at least as straight as it was before. Another method to get the gap together would be to grab 2 length of 8mm threaded rod and bend hocks in one end of each then put the hocks into the front motor mounting holes and run the other end up to a plate bolted through the streeing stem. This should let you just pull it back into place. I did notice in your photo that the inside of the tube looks to be a bit on the rusty side, check and make sure this isn't why it cracked through in the first place. Happy repairing.
Nobody has stressed the importance of preparing the material either side of the crack prior to squeezing on a bit of mig weld.
If it were me I would clean the surface thoroughly in the area of the crack, and inside the tube and between the engine mount plates.
Chamfer the broken edges to ensure penetration. Pull the parts together however you want. put a fairly substantial tack at the front first then remove the closing tackle and fully weld all round and inside the mouting tabs if you can.
Where are you? I have several large turnbuckles that you could borrow to pull everything back in line.
( I would not trust the line of the existing tubes to indicate the straightness or distortion of the frame. The line of the headstock compared to the swingarm mount along with the wheel spindle positioning and the chain line is what counts. At the 'Factory" these would have been fixed in what Montesa would have laughingly called "The Production Jig" ( obviously in Spanish/Japanese accent ) and the tubes just used to connect these points in space...roughly! )
Then I would cut a small 1/3rd round by about 30 mm long section out of a suitably sized peice of M.S. tube and weld it like a thin patch or strip bandage ( This would be called a butt strap in timber boatbuilding ) down across the horizontal weld at the front of the frame tube, as a precaution against the re-occurence of the crack at the edge of ony weld across the load path and the long axis of the tube, which is viewed with fear and loathing by most sane people. Pipelines are loaded differently so don't count in this case.
It's all a bit of a bodge, anyway.
If you took this to a frame builder who, as a commercial organisation, was expected to warrant the repair and accept/share the insurance implications of a defective repair, the process might involve making up a whole new lower down tube and under-engine cradle and moving the weld joints to the old metal well away from the re-designed front engine mount.
Joe, I thought all that was obvious. With the patches , you should only weld them on along the sides of the patch, so its runing with the tube, never across as it will crack there for sure. I had a crack problem once, until a Catepilar welder fixed it with a tiny patch with the tiny neat welds down the sides of the patch only.
I don't assume anything is obvious as many second hand bikes that have been "restored "will show.
You are quite right about no welding across the load path, although most productiuon space framed motorcycles have some somewhere, and some welders can make you weep with the beauty of their work
Caterpillar agents, I love them, they have some of the best gear and tradesmen in the world. I made friends with the storeman at my local facility and he has been able to supply most of my high tensile bolts at a fraction of the cost from a normal factor's.
You just have to get hold of the Caterpillar parts list/exploded drawings and cross reference.
Hence the need for a friendly storeman and a sympathetic accounts lady!
Keychange,
Go into your local Midas or other exhaust centre, they have welders and mig machines coming out of their ears and may welcome some special job out of the ordinary, if only to break the boredom!
