mystery solved

I have TWO MK3 swingarms that are approx 1/4 wider than stock. So I finally set out to discover why. It appears that the left side plate that the axle goes through is not parallel to the other side. If you were to take 2, 10" pieces of threaded rod, each with two nuts and two fender washers and bolted them through the axle holes the problem would be obvious. Now, is this a factory defect? I would think so because that metal is far too stiff for an owner to bend by accident. Perhaps when welding one side pulled out of position. How many times have you bolted up a rear wheel and seen the axle pull the other side out of position?

seattle##gs said:

It appears that the left side plate that the axle goes through is not parallel to the other side.

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But, is it the LH plate that isn't 90° to the pivot, or the RH, or both?

seattle##gs said:

Now, is this a factory defect?

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Maybe, but I don't see it as being a significant problem. If the arms/plates are splayed 1/4" further apart than normal (approx. 8 1/8") then I suppose they wouldn't be parallel, however, using a threaded rod, nuts and washers, on a spare Mk3 swinging arm, the distance between the plates can be reduced by 1/4" using two fingers and a short spanner.

seattle##gs said:

How many times have you bolted up a rear wheel and seen the axle pull the other side out of position?

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That's assuming the "other side" is correctly positioned to begin with?

Logically, if the 'arms' are of similar construction (tube length, gauge, diameter, etc.) then both should pull in equally as the axle is tightened and not one side pull the other side over, so the question is whether one side is out of alignment, or both?

If you were to take 2, 10" pieces of threaded rod, each with two nuts and two fender washers and bolted them through the axle holes the problem would be obvious. Now, is this a factory defect?

No, it isn't that stiff. I had a problem lining yup the rear axle on my 850 unto lone day I did something like what you suggested. the r/h side plate was twisted out. I crab a sledge hammer and held it against the inside if the plate and belted it with a smaller club hammer and straightened it out. Its been great since.

Dereck

I think Norton frames etc. were made with soft steel and not HT steel. They do bend.

I had a similar problem on my Atlas, only the arms were splayed inward by 0.140". It turned out to be the left (drive side) arm was bowed inwards. As I am the original owner, I can testify:

1) there was never an accident that caused this
2) it was not that way when the bike was new. The proof of this is the right side spacer could be fitted with the fingers when new, but could not when I discovered the problem.

I conclude that, somehow, the arm became bowed in use. It does not seem likely chain pull caused this, but what else could?

In the case of the OP, I suspect the out of parallel left side also resulted from use, and is not a factory defect.

Again, it seems daffy that chain pull could be the culprit, but what else?

Slick

My line-up methods are crude but it was quite clear the problem or at least 90% was on the left side. It just seems impossible that it could be bent by .250 by normal use...this is the 2nd one I've run into. I gave it to a friend who is going to use heat to bring it back into position. The first one meant that I could not line up the disc brake without making a 1/4" spacer to move the disc to the right. A drum brake rear would be easier to solve. But in any case I would not like to crank in the axle and lock in all that tension in the swing arm.

I looked for a Mk3 swingarm diagram/print that had a full set of measurements, but came up dry. I'm sure that such exists, and about 2 years ago a member posted his efforts to insure straightness (excellent post!) with details, but I couldn't find that either.

Is there any concern about shifting the swingarm's arms to the right or left enough such that the rear wheel assembly's center line would not be aligned with the frames center line?

Best.