Broken Cota frame

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keychange
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Broken Cota frame

Postby keychange » Sun Jun 17, 2012 5:32 pm

A beautiful morning and I had thoughts of calling Greg Harding to see if his usual Sunday ride was open - my TLR is still in pieces but the Montesa has been fine - so I wheeled it out of the shed and kicked her over - slightly reluctant but she has hardly been touched in two months. The engine kicked and away she went, so I thought I just better check everything before loading on the trailer and my optimism was crushed to see the front spar of the frame was about an inch disconnected from the base. A fairly clean snap of the joint where the pipe joins the base but what I am not sure of - is the frame welded in a stressed structure or has my frame bent, because I struggle to bring the two parts together in order to weld it. The frame doesn't look bent so it appears the stress is deliberate but wow there is a lot of pressure required to bring it together

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FM350
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Re: Broken Cota frame

Postby FM350 » Sun Jun 17, 2012 7:59 pm

Spanish TS frames are generally not made out of very good tube, and welding often isnt that great either. The last of the Bultacos, which had lower frame tubes removed commonly used to snap the top tubes. Anyone riding an old Spanish bike should keep an eye on the frame, as its easier to repair a crack than an actual break.

Repairing that break is pretty simple, but you will need to clean all the paint off the repair area, and temporarily weld something onto the down tube, which you can use to clamp onto and pull the tube back into position. If you dont want to weld something on the down tube, it might be worth trying 2 ratchet straps, going round lower fork yoke at the top, and front engine mount bolt at bottom

The actual welding needs to be done by someone who knows what they are doing, but as it would only take about 5 minutes to do, if you have it all clamped up properly, shouldnt cost much if you need to pay someone.



David Lahey
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Re: Broken Cota frame

Postby David Lahey » Sun Jun 17, 2012 8:45 pm

Here is a possible scenario
The frame may have developed internal stresses from being moved out of shape over time by use (riding rocky sections, MX jumps or from hitting the lower frame rails on solid objects), and there may also have been some internal stresses left over from when it was manufactured. Those internal stress were relieved when the failure occurred, by the bottom end of the tube moving forwards, causing the positional mismatch at the break.
If the bike had been ridden with that break, I suspect that the frame backbone would have bent like a banana. Good that you saw it in time


relax, nothing is under control

Jon V8
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Re: Broken Cota frame

Postby Jon V8 » Sun Jun 17, 2012 10:27 pm

If you are not worried about originality it may be worth adding some extra bracing to that area,or you could use a bracing sleeve inside the original tube.You can move this into place using a small hole or two in the existing tube and using a scriber or similar to lever it into place,if that makes sense,weld over the hole after which also holds the sleeve in place.I have recently used this method on the front spaceframe on an E - Type Jag which meant I could do an effective repair without completely stripping the front end.(Cant believe how thin the original tube was...)



pop
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Re: Broken Cota frame

Postby pop » Mon Jun 18, 2012 9:34 am

Jon V8 wrote:If you are not worried about originality it may be worth adding some extra bracing to that area,or you could use a bracing sleeve inside the original tube.You can move this into place using a small hole or two in the existing tube and using a scriber or similar to lever it into place,if that makes sense,weld over the hole after which also holds the sleeve in place.I have recently used this method on the front spaceframe on an E - Type Jag which meant I could do an effective repair without completely stripping the front end.(Cant believe how thin the original tube was...)

I was about to say the same, apart fixing the Jag part. I once used a bicycle gooseneck bolt with the angled nut to pull the tubes in to place with 2 well placed holes and bit of steel bar as a big washer, but you can use some threaded rod to anchor it at the back frame tubes.



keychange
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Re: Broken Cota frame

Postby keychange » Mon Jun 18, 2012 2:57 pm

FM350 wrote: it might be worth trying 2 ratchet straps


I tried that and then a wire with turn buckle but no where near enough pull. So I rigged up a fence strainer which is a lever that climbs a chain and using this I can just get it together but I can't hold it and weld it at the same time :roll:

That's why I was wondering if the were assembled in a pre-stressed way or should I be looking a lot closer for a bent frame.



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Re: Broken Cota frame

Postby FM350 » Mon Jun 18, 2012 3:36 pm

No Spanish frames made out of cheap ERW tube were not pre-stressed in any way whatever. Your top tube might be bent a little, and this will mean the repair is going to be more difficult but still possible. If you get a couple of these: http://www.demon-tweeks.co.uk/Product.d ... &%20Plates
and then weld bolts on the side of the down tube to fit the eyes over, and make a fitting to bolt onto the front engine mount which you can drill to accept the threaded rod, then after applying heat to top tube (if its bent to any extent), pulling the tubes back together is just a matter of tightening the nuts on the eye bolts.



keychange
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Re: Broken Cota frame

Postby keychange » Mon Jun 18, 2012 4:21 pm

FM350 wrote: Your top tube might be bent a little.

There is a hairline crack where the down tube joins the steering head on one side only. But running a straight edge on the top and the down tube I can see no discernible bend. Anyway I guess I'll have a go with the bolt - unless anyone else has better ideas.

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Re: Broken Cota frame

Postby brownie » Mon Jun 18, 2012 4:35 pm

G'day Andrew,

I think your model Cota frame is chromoly so maybe should be TIG welded, what about putting a straight edge along the backbone to see if its bent there



keychange
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Re: Broken Cota frame

Postby keychange » Mon Jun 18, 2012 4:51 pm

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




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