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Motorcycle subframe crack repair

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Re: Motorcycle subframe crack repair

Postby MosquitoMoto » Sun Jan 07, 2018 3:20 am

Arno wrote:
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.


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.



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Re: Motorcycle subframe crack repair

Postby Graveyard » Sun Jan 07, 2018 4:05 pm

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.
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Re: Motorcycle subframe crack repair

Postby ex framie » Sun Jan 07, 2018 6:09 pm

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.
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Re: Motorcycle subframe crack repair

Postby motox » Sun Jan 07, 2018 6:46 pm

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.
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Re: Motorcycle subframe crack repair

Postby kiwi2wheels » Sun Jan 07, 2018 8:40 pm

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.
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Re: Motorcycle subframe crack repair

Postby MosquitoMoto » Mon Jan 08, 2018 7:16 am

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

Love this forum!



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Re: Motorcycle subframe crack repair

Postby bruce991 » Tue Jan 09, 2018 11:50 pm

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.
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Re: Motorcycle subframe crack repair

Postby ex framie » Sun Jan 14, 2018 4:44 pm

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