[MosquitoMoto]

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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 )

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?


K

[cj737]

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.

[MosquitoMoto]

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.



K

[MinnesotaDave]

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.

[MosquitoMoto]

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.



K

[MinnesotaDave]

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

[MosquitoMoto]

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!



K

[Arno]

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.

[Coldman]

What about a doubler on each side?


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[Poland308]

What about a doubler on each side?


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That’s what I was thinking!

[MosquitoMoto]

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.

Thanks for your thoughts Arno.

Interesting that the main frame on this bike is a CroMo tube trellis but the subframe is aluminium and the subframe often fails.

Have checked the other side and it's fine. At this stage the owner has asked me simply to repair the crack, without adding bracing. I'd actually like to see it braced.



K

[Graveyard]

I’ve welded a lot of subframes for a lot of stunt riders, including my own subframe a few times. That’s a pretty minor crack really. I’ve had to repair complete breaks before. I believe the rod I’ve used is 4043. Once they crack you are right there are no guarantees. I typically just clean the area real well, bevel the crack all the way trough and weld it. That thick I usually set my welder at 150 amps and throttle with the pedal. If you want to go a little extra you could weld it and then plate it on top. That’s the best especially if it’s an area where it’s not visible. Good luck.

[ex framie]

This is interesting.
To me that joint suffers a compression type stress.
Yet the crack exhibutes as a tension, flex crack.
As to welding the sides only on manufacture, 2 things, 1 is cost, 2 is not putting a heat effected zone in the V of the joint. Obviously that didnt work.

There is a few photos on the web regarding welding of these, distortion seems to be an issue so bear that in mind.
I would prep and weld it without added bracing and just have your mate keep an eye on it.
Consider bracing if it cracks again or more likely get your mate to buy a new one and keep this one for practice.

Goodluck with it.

[motox]

i don't know the deal with KTM but I've cracked and or broken at least 10 or more aluminum sub frames (mostly Kawasaki and Honda.) i did cut a slight V in the crack and welded it with 4043. i have also cut and lowered several subframes for
shorter riders and have never had an issue with them.
don't over think it.
craig

[kiwi2wheels]

Around 2002 , KTM were using 7020 tube for their moto-x and enduro sub frames, also the Dakar V twins, and welded with 5087 rod. Modified sub frames were never post weld heat treated and never cracked.

As KTM have expanded ( ownership, etc ) it's possible they have changed to a (cheaper) 5000 series tube; either material you'll be fine using 5356.

One point, fit the sub frame with the lower bolts and check the alignment of the two upper points ; any loading from misalignment will cause cracking. Weld as much as possible with all the sub frame bolts fitted.

[MosquitoMoto]

So much good info - thanks people, I really appreciate it.

Love this forum!



K

[bruce991]

My KTM dirtbike subframe broke right at the front where the hole is for the tapered bolts both sides. I welded with 4043 and there was more time sent with stinger cleaning the material than normal, than I added filler after much time watching impurities surface. It has held for two seasons now. Some very impure material IMO.

[ex framie]

Kym,
So have you had a go at this yet?
How did it go?