Project CB690 (KTM 690 engine in a CB550f frame)

goodoltup

Member
Started making the swingarm pivot mounts on the manual lathe at work. The finish came out a little manky, but that is due mainly to speeds and feeds. These will be welded into the plates and will use the stock KTM swingarm hardware to link up the engine, swingarm, and frame. The last picture shows the threading operation. I also have to fab up a tool to torque the swingarm pivot insert, more on that later.
 

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goodoltup

Member
Slow going on the frame modifications. All those factory welds have to be ground down and the tubes prepped for welding. I've used a combination of an air-hacksaw, angle grinder, and small pnuematic grinder with sanding discs. Not looking forward to having to finish this up.
 

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goodoltup

Member
A drawing of the swingarm pivot that I machined. Inspection of the part showed that I got within 0.05 mm on most dimensions, some bang on, with the exception being the depth of the bore. That was way out. I used a stop on the lathe to gauge depth but something wasn't right, ended up being out by 0.8 mm. For this application it is not important, just need more practice on the lathe.
 

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goodoltup

Member
This tool my colleague made for me on the mill, to torque the swingarm adjuster nuts that are located in the pivot. It has a 17mm hex.
 

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goodoltup

Member
It's looking good. I have assembled the swingarm pivot components and will start welding soon. The plates have to be fettled with the grinder until they fit perfectly, and v-grooved. Also the front motor mount is complete. The spacers are made of mild steel, I may weld them to the mount plates for extra rigidity. Also those bolts will be shortened in the lathe. They were chosen to give the proper amount of unthreaded area.
 

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goodoltup

Member
Have received the frame tube from Pipecraft. I paid £90 for two, and they sent four. They I haven't measured them, maybe two of them are close but not conforming to drawing. I don't really need four, but maybe I can use them for weld trials.
 

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goodoltup

Member
Sneak peak. I ordered new aftermarket sidecovers and seat for purposes of mocking up. The sideplates have been tacked in only at this stage.
 

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goodoltup

Member
Time to choose my rear shocks. I would like to have the shocks in hand for mocking up the mounts that will be welded to the swingarm. Not being sure what length shock to buy I made a calculator in Matlab where I inputed frame dimensions and proposed shock and fork lengths, and get the resulting rake, trail, swingarm angle, shock angle, and seat height. I had targets for all 5, so I fiddled with the front fork length and shock length until everything was satisfied. I will start with a 320mm shock, taking into account the height of the shock mounts themselves. This is with shortening the front forks about 50mm. This can all serve as a basis, and once the bike is rolling I can adjust the front fork length with springs and spacers, and the rear shocks are +-10mm adjustable, too, so I can fine tune it. I could have done this in 2D cad, but this was a fun project. Screenshot 3 shows the program output.
 

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doc_rot

Oh the usual... I bowl, I drive around...
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just caught up with this. Nice work.
 

LMCol

New Member
Awesome build, I've always wanted to drop a big modern single into a lightweight street frame too.

Also, major workshop envy..... I need to learn to clean up after myself.
 

goodoltup

Member
The swingarm pivots have been tacked to the frame plates. The frame plates have been tacked to the frame. The LH frame tube has been tacked into place. Not shown are the bushings I made to line up the frame tube. I made 2 slugs out of mild steel that go inside the tubes, and the tubes are all then plug welded through 6mm holes. All this will allow me to remove the frame from the jig when necessary, it is not longer needed. For now it stays in as I continue with the shock mounts and some other frame tabs, but the alignment between the headstock and swingarm pivot is locked in. When the frame comes out of the jig I'll take it to a welder to finish up and cover over my tacks.
 

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jpmobius

where does this go?
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That's all looking very nice! Those are some beefy mounts. Beefy is good. I think you can safely bore a couple of holes in the swing arm/engine mounts to lighten them up a bit visually. They may take up a lot of real estate visually, but I think you made the right decision from an engineering perspective with the factory arm and pivot location. Nice work on the lower frame tube as well - everyone will think it is factory when you are done! Keep an eye on your trail number - nervous bikes are not fun to ride!
 

jpmobius

where does this go?
DTT BOTM WINNER
goodoltup said:
When the frame comes out of the jig I'll take it to a welder to finish up and cover over my tacks.
Looking at the pics, I'd bet your gas welding skills are up to the task. If the design is good, it doesn't rely exclusively on the welds, and yours is. You'd likely be able to do a good job brazing the new joints. Lots of gusseting/surface area involved in brazing compared to welding, and done well, the connection is super reliable. Worth considering - a lot of the point in doing it at all is doing it yourself.
 

goodoltup

Member
Thank you jpmobius,
It was actually tig welding you see there. I'm not sure of the penetration I got on those, and I was concerned about the heat buildup on the frame, as evidenced by the blueing. My technique needs work, and I move kind of slowly and the heat builds up.
The plates are 10mm, which I am hoping is enough. But they are quite long, and I think they will need some gusseting in the perpendicular axis to keep them from flexing. I think I will mock up some triangular plates to tie them into the frame in that direction.
I read your forum entry on the RD, I like the result very much. Did you use YSS shocks? I have some purchased and ready to mock up, just wondering if you like them?
 

jpmobius

where does this go?
DTT BOTM WINNER
Well, just make sure you let the work air-cool. Definitely getting a lot of heat in the weld area, but that shouldn't be an issue as long as you don't make things brittle by quenching. 10 mm is very robust. I think everything is ok as is and adding additional gussets probably won't be of a lot of benefit. You could potentially add a bit of stiffness by doing so, but I doubt if you would detect any benefit when you ride it. Accurate fitment of the spacers in the front and good fitting fasteners all around will likely will prove more helpful. The rear mounts with the swing arm pivot incorporated into the back of the engine case is about as good as it gets. The weak "link" in fabrications like this is usually in the connection to the tubular portion. You have to have fairly frail brackets or mounts for them to give up before the connection to the frame. Look at any factory design and I think you will note that mounting anything including the engine, is accomplished with comparatively thin stamped sheet metal components. Heavily loaded mountings are often made in two thin pieces and weld to the two "sides" of any tube instead of the center. Single element brackets also attach to the side in most cases as well. This is because (depending on the direction of the load) doing so spreads the load out in a larger, friendlier area on the tube and reduces the likelihood of fatigue cracking in the tube. This is usually much more catastrophic than cracking the bracket, and consequently you see cracked brackets more often than cracked tubes due to this design preference. In your case, the rear mounts are very strong, and will feed all the loading into the original frame tubes. Unfortunately, the result is that there will be large forces at the ends of the plates where they meet the top of the tubes. This is the place where it is much better to have a bracket design that attaches to the side of the tube rather than the center, because the tube can "move" a lot more in this area when loaded in this direction which is easier on the material. Or in other words, the load is in effect spread over a larger area even if the weld is exactly the same size. So down the road you would be wise to keep a close eye on the tubes for cracking at the ends of those big mounting plates. This of course would be the likely location of trouble even if you did design the mounts for welding to the sides of the tubes as well, just less likely. Not trying to raise alarm here, just advising you keep an eye on these areas as the miles rack up. The good news is that steel is nearly universally used for good reason. All steels have a fatigue (aka endurance) limit. That means that if you don't load or cycle your load beyond a certain threshold, your design will last indefinitely. Exceed those values and your design can be relied on to fail in (hopefully) a predictable way. Materials with no endurance limit (like aluminum) will always eventually fail, no matter how kindly they are treated, so you have to be very careful to decide what an acceptable lifespan will be. Hopefully in your case, your design will be on the "good" side of the curve and you will live happily ever after - just keep an eye on things! This would be a good (I think I mentioned this previously) place to braze the joint. This is good because the fillets made with brazing will be much larger than welding and spread the load over a much larger area. Additionally, (with sufficient skill) the steel will be heated to a much lower temperature which will not disturb its original mechanical properties. You can braze with your tig torch, but as you are still getting acquainted with using it to weld, I suggest that you weld both sides of your plates, but do NOT weld the very ends. This will spare the tube from getting so much of a stress riser in the center, and move those loads into the end of two welds, one on each side.

Also, as a guess, maybe you are not using enough power when welding. This may be counter intuitive, but the logic is this: The quantity of heat you place into the material is a combination of energy and time. Too little energy takes too long to build up enough heat to start welding, so a much larger area is heated up very strongly. Turn up the amperage, and the weld area melts very quickly, allowing you to weld and keep moving the heat impacted area forward and allowing the just welded area to start cooling - net effect is less heat injected into the parts. Aside from the gas shield, that's the great value of tig welding - speed and less heat.

The shocks on the black RD are from Race Tech and built to order. I don't have any personal experience with YSS. Very highly recommend Race Tech if you want to upgrade down the road. They don't sell you what they have, they determine what you need and build it for you. Results are exactly what you would expect.
 

goodoltup

Member
I made a temporary upper shock mount for mock-up purposes, here it is held on with a G clamp. Once the lower mounts are made the top mounts can be finished and welded in place.
 

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