Wednesday I began working on Steven's Retro 29er.
I began by laying out the fixture to match the sizing and geometry we had discussed. I have a methodical process I follow to insure I get consistent results.
The bottom bracket is fixed and the set up revolves around this known point.
The first adjustment made is to accommodate for the bottom bracket drop, a measurement that sets the difference between the center of the bottom bracket and the center of the axle line, effectively setting the distance between the ground and the center of the axle and the ground and the center of the bottom bracket. This measurement is affected by the wheel size planned and the expected tire range to be used.
Once I know the relationship between the axle line and the bottom bracket, I can then confidently set the chain stay length.
Calculating up the wheel base allows me to then use the straight edge that crosses the bottom of the fixture to find the center of the front axle point, which I mark with a sharpee. Some factors that influence this measurement include the head tube angle, the fork length, and the fork offset.
Once the front axle point is a known quantity, I can then set the point that indicates the bottom of the headtube, taking into account the stack height of the headset and necessary distance to allow for fork crown clearance.
The seat tube angle is then set, a tube placed for reference, and the tape pulled out for the final set up.
With all the known coordinates made to the fixture, the last items for me to set are the headtube angle and the final top tube length.
Now a double check of all my info vs. the reality of the physical measurements and we're ready to rock!
I selected the tubing I wanted for the build, then quickly decided against some of my choices, favoring some smaller diameter stock to maintain the classic lines we want to stay true to. A quick trip to visit with Joe B, my US importer of Dedda and Columbus tubing, and I was on the way home with tubes and canolis :) Yummy!
I rolled out each tube to find the arc centerlines and butts, then got to work prepping the headtube. The 600mm headtube stock is cut to length on the cold saw then chucked in the lathe to square the ends and bring to final length.
A quick check with the calipers...spot on.
The head tube then goes back into the lathe to be relieved on it's inner wall to prep for the headset. Most folks don't realize that headtube stock does not come ready to use and if not properly prepped, can cause increased stress when a headset is pressed inside.
Here we are taking down the id of headtube to match a precision fit for King headsets...
And a view into the final product...
With the base prep done, the tube set is ready to be mitered up into the frame members that will create Steven's new ride.