The joints in the tubing comprising the main frame are not welded right round, leaving open areas for water to get in. You can see this at the headstock and where the rear subframe attaches to the main tubes.
Now, what goes in must have some means to come out again or there will be trouble stored up. This particularly applies to the side frame which has the side stand attached to it.
If you you look at the front left tube of the frame where it terminates to join the removable side frame, you will see that it is plugged. But that the plug has a hole in it.
Well and good, you would think. The water getting into that tube can run out.
BUT. The removable side frame is also plugged with the same plug with a hole, so the water running out of the main frame front tube now runs into the side frame!
It gets worse. Water getting into the rear tubes can also run round the frame and run down into the rear of the removable frame, courtesy of the same type plugs!
On the right hand side of the frame, it simply runs into the bottom of the frame. Here's where the trouble starts.
So where do you think that the water is supposed to run away out of the frame from?
Believe it or not, the small holes in the bottom of the engine mounts (as shown) are the drain holes!
No, I didn't believe it either at first, but I have done some experiments which prove it. The main frame has drain holes which dump the water into the engine mounts on either side. The engine mount drain holes are barely an 1/8" in diameter and I will guarantee that when you go and check yours they will be choked solid.
The side frame in the pictures is as solid as you are likely to find. But look at the rust that I knocked out of it! It can rust such that the it seems the only thing holding it together is the paint! Then when the bike is leaning on the side stand the thinned tube will twist, and in the worst cases split, causing the extra lean angle as mentioned.
If yours is showing signs of bubbling or worse, remove it and check!
If it should fail, your bike will suffer damage in the fall-down plus you get to ride it home with no frame integrity!
Once I had the worst of the rust out, one end was sealed up, as were the sides of the engine mount, and a large quantity of Wayoyl anti-rust fluid was injected into it from the other end.
Proving that the drainage system was as I have described, the fluid ran out of the small engine mount drain hole. The side frame was vigorously shaken to disperse the anti-rust fluid around it after the other end had been sealed.
(This particular fluid becomes wax-like when exposed to air and has 'creep' properties, so it gradually covers all the surface.)
OK, so now your side frame is sorted or replaced and you've cleaned out the drain holes, how are you going to keep them clear, short of getting down on your hands and knees every so often and poking them clean?
Check out the pics and make a couple of 'Jiggle Pins' (yeah, they really are called that!) from split pins, as shown. They don't need to be amazingly beautiful works of art; just make sure they are not able to fall out but are loose enough to 'jiggle'. You might want to drill out the holes a bit larger.
Now, while you are inserting the pins, check out the engine mounts too. Since they fill with dirt and rust particles they can and do rust out as well. The bottom picture is of a '76 right hand engine mount.
Next to check is the bottom right hand frame tube around the engine mount. If this is rusted through at any point the engine will have to come out to repair it. An awkward and difficult place to do a repair.
Right, now we have to check out the centre stand problem.
The large tube that the stand pivot bracket is mounted to can corrode at each end where it joins the main frame tubes. If your centre stand is stiff through lack of maintenance this will impart a twisting motion to an already weakened area, then when 600 pounds of motorbike hits the stand stops, the tubing splits! The stand tube on my own bike was so split I could have almost pulled it off by hand!
You may also find that the large sheet metal bracket which the stand pivot goes through is also rotten.
As this is an exercise in checking the frame only, I am not going to try an explaination of how to fix this here. It requires considerable fabrication ability and proficiency with a welder to make a safe and solid repair.
It can be done without stripping the bike to get at it, but with difficulty. Most times it would be better to have it apart to check all the rest of the frame.
The centre stand tube rots out and then splits each side at the points indicated.