KTM 640 Subframe Repair & Strengthening

I just had a crack repaired & some bracing added to the subframe on my 640 & thought I'd post up some pics.

The crack was in the rear end of the tube that the tool box mounts to. (see pic) The right side subframe rail was bent down a little, probably from a big endo I had a while back, & I think this has put stress on that tube & then it has cracked. I recently did lots of corrugated roads in outback Australia with a load on the rear rack & that was likely the final straw. Most of my gear is carried in my panniers that are mounted well forward, apart from one stretch of the Oz trip where I had 10L of gas on the back there is usually at most 7kg on the rear rack.

There was a thread over in the Oz forum about broken 640 subfames but I thought I should post this up here for the masses. There is also a pic of Loaded's similar repair efforts in this thread (scroll down) The std failure mode seems to be for the tool box tube to crack like mine, then the left side rail gives way just behind where the gusset on that side ends. So keep an eye on the end of that lil' bit of tube, especially if you carry a load on the back.





I cleaned it all up & made the gussets then got a friend who is a pro TIG man to weld it up. I just used 2mm steel. Cro Moly would have been the proper thing to use but I couldn't find any closer than the USA & several weeks delivery.

Edit: A little more bracing had to be added after a few more years of hardcore touring, see below.

Cheers
Clint

That is some serious bracing! I have already lost my toolbox and the tubing it was bolted to, broke right off. Have had some welding done to the left side of the subframe as well. May have to look strengthening both sides at some stage.

Thanks for the post Clint.

Trev.

I'd look at adding some bracing sooner rather than later if I was you, without the toolbox holder/brace there's a good chance she'll just crack again. I firmly believe that it is a waste of time just welding up anything that has broken under normal use - a redesign or some extra metal is required to prevent the same problem happening again.

Cheers
Clint

Ya know, since I saw Loadedagain's subframe bracing years ago, and countless posts about bent/broken subframes I wonder if the few pounds saved by the "racing" subframe are really worth it... on a supertanker.

Thanks for the nice how-to post Clint, and thanks for bringing your link in too crazybrit.

Do you guys know if the subframe on the LC4 is made of the same steel as the main frame - 25CrMo4?

Cheers

Nice work Clint


Good to know there is an expert nearby when I get my Adv

We really need a KTM factory mole. I bet the German fora have them.

I actually called HQ here in Austria and managed to get through to a factory engineer a while ago. Subframe is indeed made of the same CrMo steel.

Depending on how serious you take your welding you could use a different rod and maybe pre/post heat treat the welds. Different opinion depending on who you ask, although race car guys and roll cage fabricators are probably a good source. Carroll Smith recommended to pre-heat the metal and to normalize the welds afterward in "Engineer to Win" (I think....).

Thanks. Metals seem much happier with Annealling - look at some welded structure fractures and it often occurs at the edge of the weld. I gather the welding of (eg) titanium can involve a large degree of pre and post (welding) heating where access to ovens is reqd. I guess Akra would be familiar with this material.

Sorted

the subframe is fine and all, but i hardly see it with the 70 series in the background

If a glimpse of a half disassembled 70 turns you on that much, you'll certainly enjoy the full story http://www.offroadexpress.co.nz/Forums/viewtopic.php?f=18&t=2123

Cheers
Clint

For Cromo you could braze it.
Cool it slow>?
Am I wrong?

Well anyway I have.
bill

Yep, nothing wrong with a good braze. If an oxy acetylene plant is available Cro-mo can apparently be welded quite well by a skilled hand on a torch too. From my dim memory of engineering class Cro-mo does need to be post heat treated for totally optimum weld strength, but one reason for its popularity is that just letting the weld cool slowly gets you near as dammit.

Cheers
Clint

This is a project that has been on my mind for a while now, just another reason to buy a welder!

So that repair lasted about 35K km (& just the one or 2 jumps with full luggage )



Found this yesterday. So off with the subframe, out with the TIG. Added an extra gusset.



Still a bit of a learner on the TIG so the welds are slightly ugly. Will see how it goes. Just used ER70S2 rod. Word on the street is that if the weld area stays good & red for 5-10sec after welding post heat treatment should not be required.

Cheers
Clint