Reaming swing arm bushes

Thanks for the photos. It's interesting to hear Pat's view on the backbone as I've always noted different degree humps in various Norton frames and have been told countless times that is normal.

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Never post after a few drinks unless you are sure you have seen the second page of comments..

Good night.

Russ

Kudos for going to the effort to get it all done properly. More expensive and maybe stressful, but you'll have the satisfaction of knowing it's right when you're finished.

Ken

RennieK said:

Thanks for the photos. It's interesting to hear Pat's view on the backbone as I've always noted different degree humps in various Norton frames and have been told countless times that is normal.

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No doubt it is normal, however that does not mean it is right. It happens during the manufacturing process as a result of welding. Some are curved more than others. It just depends on the individual doing the welding.

It does reduce the rake and trail and makes the bikes handle quicker and they may be more prone to shake their head if it is reduced too much.

It is also common to find them with the steering head angled a bit to the right. Right off the showroom floor. That is also a result of welding.

Usually if they have been crashed you will find signs of buckling . Jim

comnoz said:

RennieK said:

Thanks for the photos. It's interesting to hear Pat's view on the backbone as I've always noted different degree humps in various Norton frames and have been told countless times that is normal.

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Usually if they have been crashed you will find signs of buckling . Jim

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Good to know Jim. The last almost rolling basket case I bought had the worst one I'd seen. I insisted it was bent but the owner insisted he'd gotten several opinions on it and it was "normal" to have the hump. There was no other signs of trauma so I bought it anyway. I'll have to do some comparisons with a straight edge.

The guy that checked my frame said that the hump is probably the result of the order the frame is welded up and that the Widow Maker-fix tube is probably put in last, after it cooled both the backbone and the smaller tube would have some tension in them that would make the backbone warp.
He also noticed that the headstock tube didn't end up round where the races go in after welding. He put in a set of tapered roller bearings and saw that the thinner races took on the warped shape. Around 5-7 thou, big as far as bearings go. He didn't think it would be as big a problem with the heavier ball bearing races.

Guido said:

Well, I got my frame and swing arm back from the Frame Man today. That's the actual name of the business, in Sacramento.
I posted earlier about the "hump" in the back bone and a few people replied that it was normal and that they all have it.
Pat at the frame shop said it wasn't normal and that it should be straight.
SNIP

.

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All the replys that said it is normal are correct...
In over 30 years I have NEVER seen a (non widowmaker) commando without the bow in the backbone tube. My 75 MKIII with 5 miles on it has a bow!!!!

The bow though not desireable, IS "normal".

It is a result of shrinkage from all the concentrated welding on the bottom of the tube. Any competent welder knows about that.
To declare it is not normal brings in to question Pat's technical credibility or business ethics.
If you want the bike straight ....fine...have him blueprint the frame. But if he told me it is not normal I would walk out the door.

It may be influenced by the order they are welded. Anytime you do heavy welding one one side of a tube the shrinkage of the weld will bow the tube in that direction. By the time you weld two downtubes and the stiffening plate plus the long weld for the headstock reinforcing tube the backbone is going to be bowed just from shrinkage. It is hard to prevent without preheating the whole area of the weld and then welding in small segments with low heat.

I have generally preferred a frame with a bit of a bow. It makes the steering lighter and quicker.

I have never seen any good come of tapered steering head bearings on a Norton. Jim

I wonder if the tendency for the backbone to bow is what precipitated the change in the 850 head geometry.

Yeah, look for the high spots and scape to taste.

comnoz said:

It may be influenced by the order they are welded. Anytime you do heavy welding one one side of a tube the shrinkage of the weld will bow the tube in that direction. By the time you weld two downtubes and the stiffening plate plus the long weld for the headstock reinforcing tube the backbone is going to be bowed just from shrinkage. It is hard to prevent without preheating the whole area of the weld and then welding in small segments with low heat.

I have generally preferred a frame with a bit of a bow. It makes the steering lighter and quicker.

I have never seen any good come of tapered steering head bearings on a Norton. Jim

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All very well;, but the head stock , back bone and support tube are welded up as a sub assembly and then refitted to jig for the rest to be attached to, so the bow is irrelavant,it is also possible that they punched the holes for the seat rail tubes to poke through which would also distort the tube

Well, this is all very interesting indeed.
So, am I to assume that the frame being welded up, with a straight back bone to start and then distorting to a bow in the end and then not corrected when all was finished is another one of Nortons cost cutting plans? Similar to when the iso tubes were welded to the brackets causing it to distort and then the ends not being trued square in relation to the bore.
Maybe, just maybe, in the original drawings and specifications the rake in relation to the back bone and the level line of the frame changed after all the welding and being it was not corrected it gave the bike it's snappy steering, less rake.
Do you really think while the frame was originally designed they took into account the warpage from the welding and figure that when it cooled that that was the final dimensions which would mean the there are one set of dimensions for the jig and then another set of dimensions when all the welding was done.
Yes, the bow may be "normal", but is it "right", just like the iso tubes.
According to the drawing I have, the back bone is straight.

and does the drawing handle any better?, no they wouldn't have adjusted the drawings to suit, but the jigs would have been

I know I have a frame jig here that has had a lot of Commando frames on it and they very quite a bit in steering tube angle.
I have not seen anything that would make me believe they changed the steering tube angle consistently on a Mk3 frame but I have
probably only had 4 or 5 MK3 frames on the jig.
I have seen where the frames with a pronounced hump in the backbone tended to have a steeper head angle than straight backbones.
I am measuring them by comparing the steering tube angle and height to the rear isolastic mounting bolt. Jim

My '73 has less of a hump.
Now this one is humpless. Curious to see if I can feel a difference.
I just have to trust in what people tell me. I don't want to start a controversy here.
Anyway, i'm painting it up and makin' it all perdy now.

Guido said:

My '73 has less of a hump.
Now this one is humpless. Curious to see if I can feel a difference.
I just have to trust in what people tell me. I don't want to start a controversy here.
Anyway, i'm painting it up and makin' it all perdy now.

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I don't want to infer that what has been done to your frame was wrong.

I suspect that if your frame man has the specs he would do the cosmetic straitening of the backbone tube and then tweak the headstock and mounting points around until they are right. With a torch and a frame jig it is easy to tweak a Commando frame. If the headstock ends up perpendicular to the rear axle you will be better off than the majority of Commandos. Where the tubes go between the rear axle and the headstock only matter as far as chain alignment and isolastic alignment are concerned.

Getting the spring rates and ride height correct is the most important consideration when you want it to handle good. Jim

Uhg, I've found or created enough frame distortions in two Combat's to come to conclusion only thing that matters is it all bolts up fairly easy with or w/o offset shims in head steady and the rear axle adjusted so chain sprockets are pretty much in line. Frame or tires in same line or even aimed in same direction hardly matter, undetectable in handling, hands on or off, as fork self centers on road following and frame flexes to take it up w/o notice. Until I fitted Peel's crash cage I'd not noticed/detected that front down tubes were both shifted to the LH of center, as viewed thru front tire/forks center. When I had Peel frame stripped for coating I'd found two breast buds on these tubes, to realize she'd been crashed on normal crash bars that crusted both tubes where bars clamped on and welding was done to repair. I'm leaving it alone at she assembles easy and is expected to out handle even the new robotic bikes coming online. As Jim C. says what matters is suspension spring rates/dampening and front-rear height, but failed to mention the tires and frame itself as springs too when the going gets harsh.

I'm in contact with Ken Augustine of The World's Straightest Commando fame, so not w/o his deeper insights and handling mods not mentioned in the Phantom Oiler article. Ken says to shorten up the wheel base by re-making the cradle for one thing else to try.

If Peels triple 3D triangulated crash cage makes chassis too stiff I've provisions to rubber mount some attachments to let her twist up enough not to notice the ringing rebounds that limit ordinary cycles. I think the Al alloy and length of tubes will be just enough spring not to interfere with the factory slop I've come to bet my life on.