Bowed top tube

[jamie72]

The top tube on a 1985 Mercian I have recently acquired seems to be slightly bowed. To the tune of about 1mm when I hold a straight edge to the underside of the tube. I wonder if it has always been this way right from the time of it's manufacture? Or the result of some form of wear and tear. Checked all my other bikes and they are all straight apart from my 1977 Gazelle which has about the same amount of rocking of the straight edge.
Will probably just live with it to be honest unless it is pointing to something sinister, although it is all I can think about at the minute!

 

[Jonny69]

Which way is it bowed? Has it been in a front-end crash if it's bowed upwards? Forks definitely straight?

 

[jamie72]

It is bowing slightly downwards. The straight edge was rocking when placed on the underside of the tube. Forks are straight.

 

[Midlife]

Is it a parallel angled frame?

 

[keithglos]

One possibility is that it could have been brazed in a rigid jig, and the cooling created a stress. This is why the old frame builders did not use such jigs, but corrected as they went.

Keith

 

[jamie72]

73' head angle 75' seat angle. That is according to the Mercian catalogue spec for a 21.5" King of Mercia racing frameset (which this is) I know that it has stock geometry as it was built for a bike shop, Howes of Cambridge to be sold on by them as a complete bike.

 

[jamie72]

I have tried to upload a couple of photos but apparently the file size is too big....

 

[Jamiedyer]

Mercians, especially KOM are usually built to a pretty high standard and not jigged. That's not to say it hasn't been like that since it was built but I would have thought unusual for a Mercian.
For the photos, most people will use an image host like Flickr, Photobucket, google images etc. Direct hosting on the site is very limited hence the small size.
Be interested to see it and also to hear how you get on.

Jamie

 

[Midlife]

I just wonder is some judicious springing of the top tube downwards was required to fit the top tube in the head lug with such a small frame and difference in angles?

 

[jamie72]

[URL=https://s44.photobucket.com/us...ips1972/20191209_162931_zpstrhi8dqq.jpg[/img][/url]

Apologies for the poor picture but there is clear daylight here with the straight edge held on top tube.