Truing a Swing Arm

texasSlick · Nov 28, 2014

I have a problem with my Atlas swing arm, but this situation applies to Cdos as well, or for any swing arm for that matter, and is thus of a general nature.

Somehow the inside width of my swing arm, where the axle mounts, has been reduced by 0.14 inch. The swing arm is mounted on the bike.

First of all, it must be determined, which of the arms is displaced. Any thoughts on making this determination?

Next, how would one return the arms to proper width? Simply jacking the swing arm apart (for example, by placing a screw jack between the arms) will not ensure the proper result, as there is no control over which arm will yield to the pressure.

If I pulled the arm off the bike, I could probably answer my own questions. I am trying to avoid removing the arm.

Slick ....(not so slick on this one!)

Deets55 · Nov 28, 2014

Slick,
Did you measure everything that fits between the swingarm flats. Perhaps something is oversize there. Don't discount you speedo drive. My was messed up and caused my rear axle to sit funny.
If everything is up to specs there and to avoid taking everything apart, carefully spreading the arms with a jack should work. The only thing that might change, IMO is the rear tire center line , it might shift a bit. Whether for better or worst will depend on where it sits now. That can be fixed by re-truing your wheels. Only a suggestion from a real novice
Pete

Torontonian · Nov 28, 2014

Above advice all good , a more involved method would be to find another known good one on the same model and ask the owner if you can take some measurements please.

texasSlick · Nov 28, 2014

All:

Measurements taken and results concluded are given in thread under 'Other Nortons', titled Atlas Rear Wheel...etc.

Dyno Dave and Madass Don provided measurements, taken with my own, that leads to conclusion that swing arm has been "squashed" somehow.

Slick

madass140 · Nov 28, 2014

maybe a length of threaded rod with 2 nuts (9/16" ideal) and washers inboard to spread?
the side that spreads is the one that spreads easier than the other side, which would of been the side that closed in , in the first place, I'd try that or car or hydraulic jack.

concours · Nov 28, 2014

I'd start with a decent flat table (Acorn platen, but something less will do) and a 2' framing square, stand it on it's front tube with the legs skyward, check to see if it's obvious which side isn't straight, the axle mounting plates should be parallel with each other and perpendicular to the front tube. JMHO and a lifetime of fabricating. :mrgreen:

To straighten, restrain the pivot tube AND the known straight side. Make a temporary lever from a piece of 2" pipe 5' long, pad with a wood block at the end touching the pivot tube, connect to the axle mount flange using a 5/8" bolt and a short length of chain. Be careful, go slow, it wil bend easier than you may have thought.

Rohan · Nov 28, 2014

concours said:
check to see if it's obvious which side isn't straight, the axle mounting plate should be parallel.

If its only 1/10" of an inch out, good luck with spotting that. !

My experience with Commandos is that the swingarm always seem to have sprung (inwards) slightly,
and need a heave to get that final spacer in.
Speading it is almost certain to flex it at a diferent point. ?

DogT · Nov 28, 2014

I may be dense, but is it the swing arm or the cradle that's out of dimension? Just a thought.

texasSlick · Nov 28, 2014

@Madass...... that is exactly how I would apply pressure to spread the arms, but what makes you certain, the deformed arm will be the one to bend?

@Concours.....that is exactly how I would proceed if I pulled the arm off the bike, but I am trying to avoid that.

@Rohan....I am sure Concours method WILL pick up 0.14 inch. I agree with you...why would the bad side bend?

@DogT.....No cage...it is an Atlas....so stated at start of thread. Two Cdo guys whom I have been in contact with may have similar problem, so a solution here is of benefit to Cdos as well.

I appreciate your input, guys!

Slick

concours · Nov 28, 2014

"why would the bad side bend?"

Because we've restrained it both at the pivot tube AND the other leg. Experience with weldments, not conjecture. :mrgreen:

texasSlick · Nov 28, 2014

concours said:
"why would the bad side bend?"

Because we've restrained it both at the pivot tube AND the other leg. Experience with weldments, not conjecture. :mrgreen:

I do not have an issue with your method, Concours....it is the concept of others, of spreading the arms with a screw or jack, indiscriminately, and expecting the bad side to bend, that bothers me.

Keep the ideas coming, please. Particularly how I might deduce the bad side on the bike.

Slick

concours · Nov 28, 2014

texasSlick said:
concours said:

"why would the bad side bend?"

Because we've restrained it both at the pivot tube AND the other leg. Experience with weldments, not conjecture. :mrgreen:

Click to expand...

I do not have an issue with your method, Concours....it is the concept of others, of spreading the arms with a screw or jack, indiscriminately, and expecting the bad side to bend, that bothers me.

Keep the ideas coming, please. Particularly how I might deduce the bad side on the bike.

Slick

Sorry I lost sight of the "in situ" requirement. Bending one side would be no problem, but determining WHICH side isn't clear yet...

Stephen Hill · Nov 28, 2014

The swing arm has closed up by 0.14 inch, from your email.
And you don't want to remove the swing arm to straighten it.
Even if you could figure out which side moved, I doubt you will be able to move it back with the swing arm in place.

I would just spread it using a threaded rod or jack.
Worse case scenario, the wrong side moves. So you are 0.14 off centerline. Almost insignificant.
More likely, both sides will move, in which case you are 0.07 off centerline. Insignificant.
Even more likely, as mentioned earlier by madass, the sprung side moves the most (Yes Virginia, steel has a memory). Sub-insignificant.

Stephen Hill

Torontonian · Nov 28, 2014

Which side would bend first ? I'm no welder or frame straightener , but I do know from 2 past experiences (straightening steel ) is that the chances of the bent piece bending back into place are greater ( than the other never bent side ). Correct me if wrong.

Hobot · Nov 28, 2014

.14" ain't that much to cause anything but tight fit fight and too little to matter which arm or both bend some to allow easy assembly -so correct me if dumb- why not just put allthread through and crank open in cycles til it springs back about right? If was spindle area it'd have to be nailed down to control what gives before it spring back.

texasSlick · Nov 28, 2014

hobot said:
If was spindle area it'd have to be nailed down to control what gives before it spring back.

Precisely why I am reluctant to simply jack it open.....no control over what gives!

But I am siding with Madass, Torontonian, and S Hill, that the bent side is the weaker, and will preferentially yield to the spreading pressure. There is some comfort in knowing there are some who agree before I do it.

I am still open to ideas, guys. Thanks.

Slick

mschmitz57 · Nov 28, 2014

What if the 2 arms are straight and it's the pivot tube that is bent slightly in the center?
I've seen an Atlas swing arm bent like this by an over-enthusiastic P.O. pressing bushings in, probably with a hydraulic press and deforming the pivot tube. Put a straightedge on the pivot tube to be sure.
Probably not the case but just a thought.

Hobot · Nov 28, 2014

what other ideas are there but lack of mentioning slice/dice/weld back or buy new. Hope ya get good satisfaction on the robust jigging and heave ho's with some entertaining educational visuals of it.

dynodave · Nov 29, 2014

mschmitz57 said:
What if the 2 arms are straight and it's the pivot tube that is bent slightly in the center?
I've seen an Atlas swing arm bent like this by an over-enthusiastic P.O. pressing bushings in, probably with a hydraulic press and deforming the pivot tube. Put a straightedge on the pivot tube to be sure.
Probably not the case but just a thought.

You are correct the cross tube is bent.....they are ALL bent. I have 3 out of the frame and 1 still has the original factory elastic bushes. A 12" scale laid on the 8-1/8" tube shows the bend which is caused by the original fabrication weld shrinkage of the 2 swing arm tubes. This is well understood by the end of your first welding class...if awake during the class.
I'm not a particularly good welder, but for 8 years my friend at work and I would try and work out the best fabrication welding sequence to reduce or eliminate the warpage in complex assemblies.

For the same reason EVERY commando back bone is humped...every one I ever put a 12" scale on.

At a tech seminar put on by Spyder (Lotus aftermarket specialty chassis builders) they told us of the learning curve to weld up these complex ladder frame chassis and still end up with a "in spec" frame. I have one for my 67 Lotus Elan.

Rohan · Nov 29, 2014

Which would suggest that all that needs to be done is to jig the 2 axle slots of the swingarn at a suitable width,
and then heat the pivot tube until the stress goes out of the assembly and the 2 arms will stay at the appropriate width.

While not melting the rubberlastic bushes...

??

Truing a Swing Arm

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