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Friday, 4 July 2014

Homebred 29er - Progress!

So far the design has changed slightly in a few key places since the last blog post on the matter. The top tube has grown by 40mm so that I can have the option to not have my seat slammed all the way back on a layback post and then put myself in a better position on the bike. Along with that I have bought some new Columbus pre-bent 29er Zona chain stays as I was no longer confident of the shear condition that the braze would be under in the previous stay design so bought some new stays for ~£40.

Other than these design changes the jig has been set and all the measurements are correct, as can be seen in the pictures below. Everything is set central and my biggest fear the head tube not being vertical has now been sorted.

Fig 1. Head Angle.

 Fig 2. Seat Angle

 Fig 3. ~447mm Chain Stay Length

Fig 4. 55mm BB Drop.

Fig 4. Bottom of the HT
Fig 5. Centre of the HT
Fig 6. Drop Outs central and square
Fig 7. BB Central

Now that the jig is complete there is really only the tubes left to mitre. Having found a program online to produce profiles for the mitres I was able to input all the information and this then I could stick the profiles onto the tubes and file away until I got up to the line. I then finished them off by getting the tube that mates to them, wrapping it in sand paper and filing the last bit as such, resulting in very little, if any, clearance. Thus far the seat tube and chain stays have been completed, half the top tube and down tube have been done.
Fig 8. HT/DT Junction
Unfortunately the chain stays can not be done in the same way as the other tubes as they are oval at one end, thankfully  I do have the CAD for them that will allow me to get a rough mitre that can be finished off with sand paper wrapped around a 1.5" block. This can all be checked (albeit roughly) as I offer the cranks up to the jig to make sure nothing is going to clash once assembled, which would be very unfortunate. As the chain stays were nearing completion it was found to be very difficult as the tip was only 13mm and the block to be mitred to is 1.5" so although the profile is perfect, the angle that it sits at could be better (Fig. 12). This shouldn't be a problem with the amount of brass that can be put in there.

Fig. 9 CS/drop out Junction
Fig 10. Chain stay/BB Junction
Fig 11. Chain stay in place with blue tac
The next blog post will either be about the completed frame or how something has gone disastrously wrong.

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