bent swing arm

I wasn't happy with the way my Roadster rear wheel looked from the back. It seemed to tilt over to the left, so I put the bike on a flat surface and made sure that the top rear frame loop was level using a spirit level, I then removed the wheel and put the spirit level across the swing arm in a couple of different places as well as on a long dummy axle through the wheel slots and found that the left arm was quite a bit lower than the right. I removed the arm and substituted one that I took off another bike and this arm was perfectly level. I then took the bent arm, which I assumed had suffered some accident damage, to a friend with an engineering shop and asked if he thought it could be straightened, he put it on his flat plate where it indeed rocked diagonally between it's four contact points. He pointed out that there had been a weld repair at the front chain guard mount and he surmised that this additional weld applied to the front had caused the fork to bend under heat-stress. I mention this as a warning because many of the Commandos I see need some repair at this bracket and in future if I need to do this I will be extra careful to check the alignment before putting the swing arm on the bike.

Great, something more to check. I did get my chainguard front mount re-welded two seasons ago. Apprieciate the advise, though.

The later swingarms were reinforced at the front "corners". Has anybody welded reinforcements on an earlier swingarm to achieve the same effect? If so, was fairly straightforward, and did it warp or twist the swingarm as described above? And has anybody had any experience in straightening bent swingarms. I am guessing that if you can immobilize the tube, that it would not be that hard to move the swingarm "arms" around fairly easily, hot or cold.

Stephen Hill
Victoria, BC

dave M said:

He pointed out that there had been a weld repair at the front chain guard mount and he surmised that this additional weld applied to the front had caused the fork to bend under heat-stress.

Click to expand...

I find that hard to believe, unless the welder was a hack. The swingarm tubes are thick enough to hold their shape for the few seconds it takes to weld a small piece to.

Maybe it was gas welded, I very much doubt that damage would have been cause by a Mig weld on such a small repair,
Dave.

maylar said:

dave M said:

He pointed out that there had been a weld repair at the front chain guard mount and he surmised that this additional weld applied to the front had caused the fork to bend under heat-stress.

Click to expand...

I find that hard to believe, unless the welder was a hack. The swingarm tubes are thick enough to hold their shape for the few seconds it takes to weld a small piece to.

Click to expand...

sounds plausible to me, have heard of the same thing happening to auto differential tubes, welding on just one side (e.g. adding a bracket, etc) can distort them and they are plenty strong

I vote welding distortion but who knows I also tweaked a swing are by a drag strip pavement hi side that also took out my knee and by a Gravel ditch hi side save that took out the RH shock rubber/mount. The drag strip event prevented correction by Norton Professional chassis corrector to ever pass a spindle again, while the shock event hasn't affected anything I can detect. Luck of the draw only an attempt can answer if do-able.

Of particular note on your report is that is took close visual and measuring to detect and issue existed, same thing applies to most the basic Commando structure, engine flats machining to frame bent faults or anal rear tire alignment, if it just assembles w/o much trauma - your can't not detect it in riding or handling quality thank goodness. Not to say that's not something to aspire too or detectable at very extreme limits of handing loads, but who ever rides that way in public or even race tracks.

Cradle can get tweaked too. I'd say best best is try to over force it back by the long axle lever while mounted in cradle. Heated mild bends generally not a strong stable as cold bends.

This is another reason I'm so pleased that JBW is magnetic guidable and makes a great cold weld.

hobot

Hobot, Some discrepancies and inaccuracies are less important than others, for example the slight bow in the frame top tube that many have ascribed to a previous crash, when this is actually also due to heat distortion during the manufacturing process. As long as the bow is in a linear direction then there is no compromise in terms of geometry. In my case the swing arm itself was only noticeably out by about 4 mm when put on a flat plate, however when this discrepancy was magnified over the large diameter of the wheel it was significantly tilted to one side. As a postscript, I have subsequently examined another swingarm off a friend's project. He has also had the front chain guard mount rewelded on and his swingarm is indentically bent. The weld itself is quite a nice neat job and not done by someone with a new arc welder for Christmas going at it like a man possessed.

This is all repairable, but I mentioned this observation in the first instance because I myself couldn't believe that such a small weld could distort such a beefy tube and it is a repair that many of us I'm sure will need to do on a restoration project at some time, so it is useful information and easily checked by simply placing the swing arm on a reasonably flat surface and seeing if it rocks about fore and aft. I would also join Ludwig in warning that you need to keep the swing arm pivot pin in place during straightening or you will likely find that the front bores no longer match up.

I wonder if it's 30+ years of having it pulled on by the chain that distorts it a bit? I bought a couple of them while restoring my 72 and both had the same rock while laid on a flat surface. Neither had been repaired to my knowledge. Just a thought.

Might be a possibility. Makes you wonder.

Yep, 2nd similar small weld case tends to vote for heat distortion over chain tug.
I know it visual irks me to see un-centered or angled chassis, but had enough experience now I don't detect much from them unless interferes with assembly.
This is good news to me as means I have no fears to bash and brute force lever stuff back to assemble ease, not a thought to getting it exactly right, just good enough.
Obviously spindle and axle passage must be pretty aligned so wish you luck on finding or fixing tweaked swing arm. Most handling issues in Commando's is slack in more than one place that can pile up on each other in steady lean states, whowhoo.

After I paid pro to fix Combat sing arm to pass spindle, it still didn't at home so would just buy another until got one a straight one. But can be a joy process to do something the hard way and win, so have at it and get back with good news.

Another point for accessnorton!

Mine's out of the bike for the spindle tube repair and someone said "put the gussets in it like the 850s have. . . . "

So, I'm gonna put the spindle tube in it and clamp it down at all four corners to a welding table before I add any gussets to mine. Or . . . . maybe I won't bother.

I've seen good welder fabercators box in the 750 swing arm but boy howdy better really nail down the axle and spindle as you say putting the kinds of thermal stress on chewing gum like swing arm. Repair chain mount and spindle clamp bolt plate too.

Ladies and Gentlemen, A final report on this issue; I have just gotten my swing-arm back from my friendly welder. He drilled a small hole in the left swing-arm leg to let hot gas escape and then used a dodgy old arc-welder on the tube on the opposite face of the original repair. It now measure square on a flat-plate and I have put the arm back on the bike for checking and it is perfectly square with the frame, using a spirit level as a reference. I will paint it this weekend and hopefully be back on the road by next week.

Appreciate the reality check of how wax like thin steel tube can be to localized heat and a good welder/body worker can apply that in reverse. hobot