Frame Straightness

[niagra]

Hi all,

Just been having some fun with the angle grinder and a knotted wheel - a great purchase!

Anyway, I have noticed that one of the arms on this part of the frame is slightly bowed. It may not be that clear in the photo below, but the lower arm (as you look at the photo) is about 5mm out of true.

Is this how it should be or is it actually bent? If it is bent, is it ok to bend it back (using appropriate brute force and scaffold bar, or should I let an expert deal with it)?



Thanks,
Dario

[aobp11]

This old thread might help:
"Sub-frame on 1955 Springtwin straight?"
Albert

[Lance]

Hello Dario,

Interesting. I was meaning to ask the same question as I have 2 1949 G80S rear frames that are exactly as yours. Both are the drive side just like yours and are bowed just so slightly. Both frames are dead straight otherwise too.

I read through the thread that Albert mentioned although that is mostly around a 55 model twin with the difference being that the right side is bent/bowed on theirs.

If anyone has additional confirmation on a left-hand drive side bowing of the rear subframe on early swingarm frames it would be good to know. Glad I haven't tried to take the bow out yet.

I took pictures with a yard stick years ago and could probably dig them up.

Perhaps this should be double posted to the singles or general area too? Will wait to see how many responses.

Thanks,
Lance

[niagra]

Thanks for the mention of the other thread Albert, good to know that other frames are bent but still not a definitive answer.

Thanks Lance for your response, again it's good to know that mine is not unusual.

Perhaps the tolerances at the factory were not perfect and frames had to be adjusted after manufacture to get it to fit each individual bike?

Dario

[Don Madden]

I am sure the factory made the frames accurately, there is too much coindidence in all these to be bowed to be accidental. Has anyone checked one assembled for chain clearance? Cheers, Don.

[Lance]

My two '49 G80S frames are dismantled at the moment so I loosely fitted one back together to check clearances. Somewhat difficult to tell, although, it does seem that if the left side tube was perfectly straight that it would have a greater chance of fouling on the swingarm.

There seems to be a slightly larger clearance on the right side. I didn't go as far to add the chainguard but from pictures I took before the dismantling, it seems it would still have clearance if the tube were straight.

Below is a photo I took a long while ago when dismantling and first noticed it. Both my frames are identical this way but every other aspect of the frame geometry is straight and level.



Again, as Don says, too many like this it seems to be anything other than factory tweaking. The tube may be thin but I can't imagine it bowing like that just from normal use or weight.

Regards,
Lance

[wilko]

I disagree, no factory would do a dodge like that!

[Lance]

Well, I'm all ears to how then? So curious. My other one is identically the same way as is the initial one in the posting as well as the ones on the other thread mentioned above. Are these that weak that the weight of a rider could really do that? I was convinced that at first but I'm not seeing it as very probable. Stranger things have come to light, though.

If not factory, then what, weight? If not factory, nor weight, then what, did someone purposefully do that for some reason during it's life?

Again, maybe tomorrow I'll repost to a larger forum group to potentially reach more singles swingarm riders who may have seen this or know about it.

There's a few of us that would like to know if we should try to straighten or not before we proceed and this could also potentially help others in the future.

Edited by - Lance on 29 Oct 2011 03:01:49 AM