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Strength of welding??

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koolio
Spanner Monkey



Joined: 08 Apr 2011
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PostPosted: 09:35 - 01 Nov 2011    Post subject: Strength of welding?? Reply with quote
Completely new to welding, have a 70s project bike which has some damage to the frame, thank god not major but the support (which is like an arch between the frame at the rear) which holds the rear mudguard and supports the rear passenger if there is one, is missing.

I understand even if it was to be welded back on it would still be a weakness in the frame? Or is a good weld strong enough to support a passenger and everything else?
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stinkwheel
Bovine Proctologist



Joined: 12 Jul 2004
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PostPosted: 10:46 - 01 Nov 2011    Post subject: Reply with quote
A good weld is stronger than the metal. A poor weld can be broken by hand.

You can't weld rust so if it's missing because it's rusted off, you need to replace the whole rusty bit.

If you are still concerned, you can add things like gusset plates.

If the person welding it doesn't know all this, get someone else to do it.
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Fizzer Thou
World Chat Champion



Joined: 06 Aug 2011
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PostPosted: 14:03 - 01 Nov 2011    Post subject: Reply with quote
I have tried welding with ARC but I got a much stronger and neater weld using MIG.
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andys675
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Joined: 08 Feb 2007
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PostPosted: 14:27 - 01 Nov 2011    Post subject: Reply with quote
my rover 214 Embarassed clutch lifting arm split, the mechanic at my old work said it needed the gearbox off to replace or he could weld it, so he welded it and then welded down the sides of the weld, never failed again
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MarJay
But it's British!



Joined: 15 Sep 2003
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PostPosted: 14:31 - 01 Nov 2011    Post subject: Reply with quote
Fizzer Thou wrote:
I have tried welding with ARC but I got a much stronger and neater weld using MIG.


That should be pretty obvious. Arc welding is pretty similar to MIG, except with MIG you have an inert gas to shield the weld from oxidation. Arc welding is good in open spaces where you're joining vast pieces of steel together and any movement of air (IE wind) would blow the inert gas shield away before it had a chance to do its job. For example, shipbuilding.

When welding indoors, with fine work then MIG is undoubtedly better. TIG is actually the best of all IMO but requires a lot of skill. You can TIG weld Aluminium alloy better than you can with MIG. TIG uses a tungsten electrode as well as a welding rod instead of the fed wire used in MIG welding.
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The Artist
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Joined: 06 Jan 2008
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PostPosted: 14:37 - 01 Nov 2011    Post subject: Reply with quote
MarJay wrote:


When welding indoors, with fine work then MIG is undoubtedly better. TIG is actually the best of all IMO but requires a lot of skill. You can TIG weld Aluminium alloy better than you can with MIG. TIG uses a tungsten electrode as well as a welding rod instead of the fed wire used in MIG welding.


I had to have a cast alu propeller fixed when a blade broke off. It was cleaned up afterwards but you couldn't tell it had been broken it was that neat.
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Tuffers
Scooby Slapper



Joined: 29 Oct 2011
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PostPosted: 18:38 - 01 Nov 2011    Post subject: Reply with quote
most good welds can be ground down afterwards to a smooth finish and still be pretty strong if they are deep enough
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kestrel
Nearly there...



Joined: 04 Sep 2006
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PostPosted: 18:50 - 01 Nov 2011    Post subject: Reply with quote
stinkwheel wrote:
A good weld is stronger than the metal. A poor weld can be broken by hand.

You can't weld rust so if it's missing because it's rusted off, you need to replace the whole rusty bit.

If you are still concerned, you can add things like gusset plates.

If the person welding it doesn't know all this, get someone else to do it.


Good advice, however care must be taken when adding gusset plates. Any gusset should be placed on a neutral axis. Wrongly placed gussets cause more harm than good and actually weaken a structure by creating stress raisers at their corners.
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Pete.
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Joined: 22 Aug 2006
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PostPosted: 20:26 - 01 Nov 2011    Post subject: Reply with quote
Aye and they can take out flex in a frame where it's actually desirable.
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kestrel
Nearly there...



Joined: 04 Sep 2006
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PostPosted: 21:09 - 01 Nov 2011    Post subject: Reply with quote
Pete. wrote:
Aye and they can take out flex in a frame where it's actually desirable.


That's correct Pete. Any changes made to a frame, however small, will affect the way it performs it's job. Some changes may result in an overall improvement whilst others could result in catastrophic failure. Before making any changes it's important to have a basic understanding of the possible outcome. There is a book called 'Motorcycle Chassis Design' by Tony Foale, recommended reading for anyone interested in building or modifying bike frames.
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