Brazing a front derailler stub to Dahon frame?
09-22-06, 11:35 AM
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Brazing a front derailler stub to Dahon frame?
I wan to make a steel Dahon frame into a double front chainring. I am thinking of using a braze-on front derailler.
Please look at:
https://www.guybouchard.ca/blogimages...o/dahorear.jpg
Have you installed a front derailler braze-on on bikes like this one?
What's the recommended angle for it?
Any info welcome.
Thanks!
Please look at:
https://www.guybouchard.ca/blogimages...o/dahorear.jpg
Have you installed a front derailler braze-on on bikes like this one?
What's the recommended angle for it?
Any info welcome.
Thanks!
09-26-06, 03:21 PM
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First you need to figure out the height and the only sure way to do that is install the chainwheel. Attach the front derailleur to the braze-on, centering it so you have adjustment up and down. Position the derailleur were it needs to be and mark the frame with a felt tip pen.
Remove the chainwheel and measure the height of the braze-on. Now you have the height figured out all you need is the orientation of the braze-on on the tube. The front derailleur will be more or less parallel with the chainwheel and therefore the BB face. As long as the front derailleur can be set to both toe-in and toe-out you are good.
To be really sure just tack braze it (near the top) just enough to hold the braze-on and put the chainwheel and the front derailleur on to check before you fully braze. At this stage if the angle needs adjustment you can carefully move it on the tack with a screwdriver through the slot. If it is wrong you can break the small tack and start over. Be careful when finally brazing, not to get the whole thing so hot that it falls off. Braze half of it opposite the tack, allow to cool a little, and then braze the other half.
On my production frames I made a simple height gauge for the braze-on with a strip of sheet metal bent to the curvature of the BB and a piece of 3/8 dia. tube cut to the correct length was brazed to this. I would mark a center line on the seat tube by dragging a steel straight edge across the side of the seat and down tube. I would position the braze-on in relation to this center line and clamp it with a small tool maker’s clamp. Then I would tack it, allow to cool, then attach a front derailleur to it to check there was both toe-in and toe-out adjustment. I never had a frame come back because the braze-on was incorrectly positioned.
Remove the chainwheel and measure the height of the braze-on. Now you have the height figured out all you need is the orientation of the braze-on on the tube. The front derailleur will be more or less parallel with the chainwheel and therefore the BB face. As long as the front derailleur can be set to both toe-in and toe-out you are good.
To be really sure just tack braze it (near the top) just enough to hold the braze-on and put the chainwheel and the front derailleur on to check before you fully braze. At this stage if the angle needs adjustment you can carefully move it on the tack with a screwdriver through the slot. If it is wrong you can break the small tack and start over. Be careful when finally brazing, not to get the whole thing so hot that it falls off. Braze half of it opposite the tack, allow to cool a little, and then braze the other half.
On my production frames I made a simple height gauge for the braze-on with a strip of sheet metal bent to the curvature of the BB and a piece of 3/8 dia. tube cut to the correct length was brazed to this. I would mark a center line on the seat tube by dragging a steel straight edge across the side of the seat and down tube. I would position the braze-on in relation to this center line and clamp it with a small tool maker’s clamp. Then I would tack it, allow to cool, then attach a front derailleur to it to check there was both toe-in and toe-out adjustment. I never had a frame come back because the braze-on was incorrectly positioned.
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History, photos and tech articles on "Dave's Bike Blog." 'dave moulton' Registry including a Picture Gallery https://www.davemoultonregistry.com/
History, photos and tech articles on "Dave's Bike Blog." 'dave moulton' Registry including a Picture Gallery https://www.davemoultonregistry.com/
10-05-06, 05:33 PM
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I was just thinking about this. I have a braze on bracket that has a built-in slot. I don't really like that idea since it just seems like something else to fail. I could weld it up and start again, or is this kind of sloted bracket a good system.?
I was also thinking about whether on a touring bike a brazed on mount was a good thing since it would be harder to find a replacement deraileur should a problem occur. Or is the BO version a lot less likely to have problem over all, seems unlikely. Another point would be if the mounting for the non Bo is high enough one could mount it over or under the bracket.
Thanks for the thread, and Dave thanks for the good instructions, you should do a book.
I was also thinking about whether on a touring bike a brazed on mount was a good thing since it would be harder to find a replacement deraileur should a problem occur. Or is the BO version a lot less likely to have problem over all, seems unlikely. Another point would be if the mounting for the non Bo is high enough one could mount it over or under the bracket.
Thanks for the thread, and Dave thanks for the good instructions, you should do a book.
