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 Post subject: Bowed top tube
PostPosted: Sun Dec 08, 2019 8:33 pm 
Dirt Disciple

Joined: Fri Dec 08, 2017 10:05 am
Posts: 19
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!


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 Post subject: Re: Bowed top tube
PostPosted: Mon Dec 09, 2019 10:50 am 
Retro Guru
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Joined: Mon Mar 02, 2009 4:22 pm
Posts: 1773
Which way is it bowed? Has it been in a front-end crash if it's bowed upwards? Forks definitely straight?


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 Post subject: Re: Bowed top tube
PostPosted: Mon Dec 09, 2019 1:13 pm 
Dirt Disciple

Joined: Fri Dec 08, 2017 10:05 am
Posts: 19
It is bowing slightly downwards. The straight edge was rocking when placed on the underside of the tube. Forks are straight.


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 Post subject: Re: Bowed top tube
PostPosted: Mon Dec 09, 2019 1:29 pm 
Old School Grand Master
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Joined: Fri Oct 14, 2011 8:41 pm
Posts: 9926
Location: Cumbria
Is it a parallel angled frame?


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 Post subject: Re: Bowed top tube
PostPosted: Mon Dec 09, 2019 4:14 pm 
Retro Guru

Joined: Sun Aug 22, 2010 7:07 pm
Posts: 1720
Location: Cotswolds
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


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 Post subject: Re: Bowed top tube
PostPosted: Mon Dec 09, 2019 4:24 pm 
Dirt Disciple

Joined: Fri Dec 08, 2017 10:05 am
Posts: 19
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.


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 Post subject: Re: Bowed top tube
PostPosted: Mon Dec 09, 2019 4:25 pm 
Dirt Disciple

Joined: Fri Dec 08, 2017 10:05 am
Posts: 19
I have tried to upload a couple of photos but apparently the file size is too big....


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 Post subject: Re: Bowed top tube
PostPosted: Mon Dec 09, 2019 10:35 pm 
Gold Trader / MacRetro rider
Gold Trader / MacRetro rider
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Joined: Mon Feb 11, 2008 12:03 pm
Posts: 7176
Location: held captive by baby haggis in a cave in Scotland
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


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 Post subject: Re: Bowed top tube
PostPosted: Mon Dec 09, 2019 10:41 pm 
Old School Grand Master
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Joined: Fri Oct 14, 2011 8:41 pm
Posts: 9926
Location: Cumbria
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?


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 Post subject: Re: Bowed top tube
PostPosted: Tue Dec 10, 2019 10:33 am 
Dirt Disciple

Joined: Fri Dec 08, 2017 10:05 am
Posts: 19
[url][URL=https://s44.photobucket.com/user/jamiephillips1972/media/20191209_162931_zpstrhi8dqq.jpg.html]Image[/url][/url]

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


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