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Motorcycle subframe crack repair

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Motorcycle subframe crack repair

Postby MosquitoMoto » Fri Jan 05, 2018 7:49 pm

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I've been handed the subframe of a late model KTM990 SMT by a good mate with a request that I repair a crack. The significant crack was revealed when said mate was undressing the bike for a thorough cleaning after a trip to the Phillip Island round of the 2017 MotoGP.

It seems failures on SMT subframes are all too common - there's a ton of repair/bracing jobs to be seen online.

Now...I normally never fabricate or repair motorcycle parts for anyone but myself; I don't want to put anyone at risk. Wouldn't do it for paying clients either - compared to many, Australia isn't a very litigious society but again, why risk legal hassle. But this guy is a very good friend and he understands that the repair comes with no guarantees and no comebacks. (He's priced the new replacement part :o )

Now...the part is probably 6061 (I know...cue the guesswork already!). I would normally clean, create a jig and weld with 5356. I am aware that none of this is ideal, that annealing will be a concern and that ideally, this part really needs professional welding and re-tempering.

Would love to hear any thoughts and advice you people might have. I seem to remember that Dunkster is the man for this kind of thing, but there are lots of other smart people on here. Cheers!

EDIT: Is it just me who finds it strange that the factory welds are only along the sides of the joint?


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Re: Motorcycle subframe crack repair

Postby cj737 » Fri Jan 05, 2018 9:27 pm

I personally doubt that material is 6000 series, but it may be. Your idea of gouging the crack and welding it up will be fine in my opinion and is very similar to what I have done many, many times for a Dirt Tracker pal of mine. His KTM is also only welded on the sides. Must be a stress/flex calculation? Its not like the brace will pull away as the stress load is in the other direction, right?

I did not anneal the brace or frame after my work, and 3 years on they're still in tact.
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Re: Motorcycle subframe crack repair

Postby MosquitoMoto » Fri Jan 05, 2018 9:44 pm

cj737 wrote:I personally doubt that material is 6000 series, but it may be. Your idea of gouging the crack and welding it up will be fine in my opinion and is very similar to what I have done many, many times for a Dirt Tracker pal of mine. His KTM is also only welded on the sides. Must be a stress/flex calculation? Its not like the brace will pull away as the stress load is in the other direction, right?

I did not anneal the brace or frame after my work, and 3 years on they're still in tact.


Thanks CJ. I've also done plenty of my own stuff that tends to hold up well, but making a repair for a friend makes you ponder a bit longer.

Interesting that the KTM you have experience with is also welded 'sides only'. Seems KTM have some work to do on this particular subframe design for the 990 SMT as there are lots of failures posted up online. I suspect that the next generation of this bike will have a brand new design of subframe.

Thanks for your input. Will post repair pics when all done.



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Re: Motorcycle subframe crack repair

Postby MinnesotaDave » Fri Jan 05, 2018 10:41 pm

Any chance you can call the company for material type and/or filler metal recommendation?

I've done a few steel ones (for family), have not seen an aluminum one up close yet.
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Re: Motorcycle subframe crack repair

Postby MosquitoMoto » Fri Jan 05, 2018 10:45 pm

MinnesotaDave wrote:Any chance you can call the company for material type and/or filler metal recommendation?

I've done a few steel ones (for family), have not seen an aluminum one up close yet.


Hey Dave.

Although there's a chance I might soon uncover details on what specific alloy has been used to make the OEM subframe, KTM are far keener on selling a new replacement subframe than they are on sharing information regarding a repair. Business is business, I guess.



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Re: Motorcycle subframe crack repair

Postby MinnesotaDave » Fri Jan 05, 2018 11:41 pm

Some junk yards have an electric gun that zaps the metal and reads out its composition.
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Re: Motorcycle subframe crack repair

Postby MosquitoMoto » Sat Jan 06, 2018 2:00 am

MinnesotaDave wrote:Some junk yards have an electric gun that zaps the metal and reads out its composition.



Never knew that - I'll have to ask around. I suspect I'll eventually discover the alloy type via a KTM forum or a technical manual somewhere.

Mind you...I'll probably hit it with 5356 regardless!



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Re: Motorcycle subframe crack repair

Postby Arno » Sat Jan 06, 2018 7:58 am

MosquitoMoto wrote:Seems KTM have some work to do on this particular subframe design for the 990 SMT as there are lots of failures posted up online. I suspect that the next generation of this bike will have a brand new design of subframe.


Definitely... Seems like the stresses on this section are actually bending/flexing the connection to the rest of the bike and it's breaking at the point where the system transfers to the more rigid/triangulated section.

Pretty much a design issue and likely it'll crack again here in the future as this remains the point where the stresses peak between the flexing 'leg' and the more solid triangulated section. As this seems common, have others added some sort of bracing or trangulation on this section to reduce the flex as part of the repair? (if there's any space..)

BTW.. Did you inspect the other side/leg? May be good to get some crack detection fluid and developer out on that leg too. May already have some small cracks starting out..

Bye, Arno.
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Re: Motorcycle subframe crack repair

Postby Coldman » Sat Jan 06, 2018 8:11 am

What about a doubler on each side?


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Re: Motorcycle subframe crack repair

Postby Poland308 » Sat Jan 06, 2018 10:40 am

Coldman wrote:What about a doubler on each side?


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