Truing a Swing Arm

Curious why you wouldn't want to take this off to fix it. If you try to "just" spread it apart with a screw type jack or similar etc... and it deforms further or differently it's gonna have to come off then, or so I would think.

Don't know about every Norton swing arm being weld distorted but do know Cdo's DS leg is the straighter shot back and the TS leg is bent outward more then bent back some to get parallel to the DS. Would be educational to stick it in an oven and see what happens, nothing or relaxes out of align which doesn't seem the issue in this case. Maybe best to just take some off an axle spacer if possible in Atlas. If'n me I'd leave sa on and lay level on arms to watch while cranking them apart and change plan if seeing any significant out of leveling. If the arm are on same level and only pushing straight apart what need to nail down one side for this slight spread desired.

If you can find by measurement the area with the bend then heat this section, then apply force and the unbending will take place in the heated section as it will be the weakest. Its only mild steel so there is no hardening to lose.

Normally when something bends it has a 'molecular memory' so it is best to bend it back cold, heat will destroy this memory and it would likely bend in an arbitrary spot. Therefore spreading the two arms apart cold is more than likely to return it to it's original state.

The logical way to go about this is to assemble the sprocket & spindle and chain and check those with a straight edge or a chain laser.
Anything you bend wrong will have an effect on chain wear-if you get that wrong

You will also need to check the alignment between the swinging arm spindle and rear wheel spindle and correct if it is twisted.

p.s. I would also remove the new wheel bearings that you have fitted and check the width of them against the old ones :!:

Slick,

Is there a reason you think you arm is bent, or could this be a result of compounding factory tolerances ? If it is bent how do you know it is only one leg, both could be in slightly. Both could be bent in the same direction but one more than the other, etc. There are quite a few possibilities. As long as the swing arm operates correctly and there is no structural damage there might be other options. The arm is just a chunk of metal that attached the engine/frame to the rear wheel. You could remove it and get it perfect and find out the bushing bore might be off, or the cradle might be off, the list is end less. You are looking at less than an 1/8" at the axle. I am not familiar with how your axle goes together but I am willing to bet there is at least one spacer in there. Would machining the required amount of material off the spacer be an option. Then just get your rear wheel centered to the frame.
Pete

As Dave said might want to search up cold vs HoT rebending on this tiny scale and mild steel tube, ESpecially if concerned about thermal distortion that may leave one axle flat twisted and out of axle plane. A hot air drier few 100's not usually enough to matter so there goes the paint job for a off the bike repair. Now if bending a straight tube to build a frame then torch it.

All:

Thanks for your replies.

It occurred to me that the problem might be due to a cracked weld where one arm joins the cross tube, and that simply spreading the arm would be inviting a safety issue in the future. So I followed cjandme's advice and pulled the swing arm for a close inspection.

Preliminary results:

No visual defects in the welds.
Dyno Dave wins a point....there is a slight bow in the spindle tube...about 0.006 max
A straight edge (maximum of 8 inches can be applied along the arms before a structural change occurs) laid along the right side arm shows no run-out.
A straight edge laid across the left or drive-side arm shows a bow in the direction to narrow the axle gap. Over the 8 inch run, I measure about 0.055 which proportions out to twice that as the distance from cross tube to axle flats is about 16 inches. It is not all of my missing 0.140 inch, but I'll take 0.110 and be happy (maybe over compensate straightening, get it all, and be ecstatic).

I'll be taking more careful measurements and will try straightening per Concours' recommendations.

FWIW...I never had to force that last spacer in over the life of the bike. This bow has developed from use. Curiously, it is on the drive side, could it be caused by chain pull? My thought is no, but still is curious. I started this thread by telling of two Cdo guys who had the same "I'm missing 1/8 inch" problem. Could there be a new Norton quirk that has just surfaced? Maybe that is fodder for a new thread? Comments?

Slick

texasSlick said:

I started this thread by telling of two Cdo guys who had the same "I'm missing 1/8 inch" problem. Could there be a new Norton quirk that has just surfaced? Maybe that is fodder for a new thread? Comments?

Click to expand...

I had to use some muscle to get that last spacer into the swingarm of a Commando back in the 1970s,
while it was still relatively new. So this 'quirck' is anything but new.

Single cylinder and plunger dommie axle spacers don't always readily go in either,
so I'd say that Nortons liked to have their axle spacers/spacing closely matching the distance.
So that no inwards clamping stresses were input into the forks ends at the back there.... ?

texasSlick said:

All:
A straight edge laid across the left or drive-side arm shows a bow in the direction to narrow the axle gap. Over the 8 inch run, I measure about 0.055 which proportions out to twice that as the distance from cross tube to axle flats is about 16 inches.
Slick

Click to expand...

Gee... the welding of the chain guard mounts is on the inside...same direction as warp.,..welding 101 strikes again. On mine I find the warp all at the front, mostly focused at the front chain guard mount.
The rear one seems to be to close to the casting to affect the whole arm.

IMO
Heat the left arm at the front chain guard mount and jack out .080" at axle mount and live with the residual.

or disregard the whole issue and wedge the SA open to jam in all the parts. Drive it until the wheels fall off. :mrgreen:

dynodave said:

Drive it until the wheels fall off. :mrgreen:

Click to expand...

The wheels are going to fall off over 1/10th of an inch ??
Pull the other one, it plays jingle bells.....

To close this out:

Bumped the swingarm with a 30 ton hydraulic press. That took out the bow and opened up the opening where the wheel mounts by 0.150 inch....over did it a little, I was looking for 0.140. Attempts to spread the opening with threaded rod and nuts failed...the arm simply sprang back....even with heat applied.

I suggest anyone who has to force the last spacer in place, examine the swingarm for a bow.

Thanks for all the help and recommendations.

Slick

Congrats. So need to know ,was it the weaker arm that moved the tad ?

I had to take Trixie Combat sa to dozer shop with 30 t press to get relief and nothing I had could develop the tons it took to work. Any memory of the PSI to correct yours?

hobot said:

I had to take Trixie Combat sa to dozer shop with 30 t press to get relief and nothing I had could develop the tons it took to work. Any memory of the PSI to correct yours?

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Did not have gauge to measure pressure applied.....only used eyeballs to see slight bend between supports on low side, and ram on high side of bow. There was hardly any effort to pump the ram. I am sure a 10 ton press would have sufficed. The swing arm was set up in the press so only the bent arm had pressure applied...the rest just floated.

@ all who said bent side was weaker and would yield to spreading first:

you all get a point....when I used all thread to spread the arms, I spread a little, and looked a little...both arms spread some, but the bent one went straight, and the straight one stayed straight. However, I spread the arms first 1/4 inch over, then 1/2,& 3/4 over size, each time the arms sprang back to where I started. Next, I repeated using heat where I thought the arm was stressed....got about 0.040 that way, but no more. Another point for DynoDave....I got the 0.040 when I applied heat to the chain guard weld point, right where he said welding would cause a bow. However, the apex of the bow, where the press was applied, was about 2 inches from the chain guard weld point toward the axle.

I am still at a loss to explain how the bow developed....

FWIW....the paint on the swing arm was extremely tough. A professional auto painter pronounced it powder coat, but retracted that when I told him the date of mfg was 1962. The paint was still glossy, unlike that on my frame which has gone dull. Two different paint shops during mfg? Perhaps Reynold painted frames? Had I not applied heat, there would be no need to strip the paint and re-paint. Also, the Clayflex bushings look to be in good shape for 50 years of service....I believe I will leave them alone.

Slick

texasSlick said:

FWIW....the paint on the swing arm was extremely tough. A professional auto painter pronounced it powder coat, but retracted that when I told him the date of mfg was 1962. The paint was still glossy, unlike that on my frame which has gone dull. Two different paint shops during mfg? Perhaps Reynold painted frames? Had I not applied heat, there would be no need to strip the paint and re-paint.

Click to expand...

All Norton cycle parts were STOVE ENAMELLED for very many decades,
and this would certainly have been the case for anything coming out of Bracebridge St
pre the move to Plumstead Rd and the AMC era.
And probably after the move, but yet to be confirmed.
Parts were dipped or hosed with liquid enamel, hung to drip off and then dried/baked in an enamelling oven.

Maybe 2 different batches of mixed up stove enamelling brew ?
Some does seem to survive better than others...

Have you tried to polish the paint on your frame ?
It will often respond well to a cut-n-polish type paste.
Test gently on a hidden area first, as they say.