Over the past month I've been plagued with a lot of drive shaft problems.
When I had welded the extension into the shaft I noticed what looked like some inclusions in one of the welds, but left the repair for later. I decided to take care of that a few weeks ago. I ground out the inclusions with an angle grinder using the edge of the disk. I felt like a dentist working on a tooth. Once I got all the bad parts out I welded up the voids and ground off the excess. To stress relieve the weld joint I heated it to a dull red with a torch then covered the shaft with rags to cool slowly.
Here's the setup I used to check the alignment.
The other extension weld looked OK, so I thought I was done with the drive shaft and had a confident feeling about it...
Before my trip to Hollister Hills I tried to do a wheelie over a speed bump to impress onlookers. It ended badly, and needed to bring the bike home in the truck. Here's what I found when I took it apart.
The break was at the other weld that had looked OK. There wasn't much weld penetration, and the crack seems to have started at some inclusions below the surface. As I had mentioned, I decided I didn't like the method of using a pin to align the shaft for welding because I thought the hole for the pin could trap air or machining oil that would contaminate the weld and lead to inclusions. I had used a narrow square butt joint
for the weld, and I thought I would get sufficient penetration with it.
For the repair I made up this fixture. To get perfect alignment of the V-grooves I finished them with a single pass on a mill.
Here's the shaft in the fixture. I ground the ends to make a V butt weld joint
I did the weld, more carefully than usual, removed the excess, stress relieved the weld and aligned the shaft and once again thought I was done with the drive shaft...
After coming back from my shake-down ride at Hollister Hills I noticed a clicking sound from inside the swing arm when I moved the bike around. On taking it apart I found one of the welds on the U-joint caps had broken and the cap was coming out of the yoke. I could also see scrape marks on the top inside of the swing arm where something was hitting.
I don't think the torque through the joint was the cause, but more that the swing arm was going beyond its intended limit while bottoming and putting a large downward force on the yoke when the yoke and swing arm came into contact, and this force is what broke the weld and pushed the cap out. I had noticed before that the yoke clearance was very tight when the shock was fully compressed, and I now think there is deflection in the frame or other parts that is allowing the swing arm to more than expected in relationship to the transmission output flange.
I re-set the cap and put a new tack weld on it. While I had the shaft out I also decided to clean up the two extension welds. As can be seen on the far right of this view the weld is under cut is some places, and also has a few stray grind marks around it. I laid new weld beads over all the bad parts. To finish the shaft this time I did the stress relief, aligned it, then turned and polished the shaft on a lathe to get a smooth finish.
I also decided to make a spacer for the coupler spring to stiffen up the damper. I had a few pieces of 5 mm thick 6061-T4 aluminum scrap I cut out the spacer from.
Here's how the spacer looks in the assembled shaft. This view also shows the polished weld.
To limit the shock travel I made a 3 mm aluminum spacer to put under the rubber bumper of the shock. I didn't take a photo of it though. Because the swing arm is at such an angle I need to jack the back end up to add the swing arm oil.
I fixed up the other shake-down problems and I took the bike down to Hollister Hills
again for another shake-down. I tried to ride it hard to get things to break, but just found a few minor things not worth reporting.