Homebred 29 - Final checks before material purchase.

This should be the last post about design, everything after this should be about production, scary!

The jig is ready, the flux, brass, dropouts, BB, chainstays, bottle boss mounts and cable guides have all arrived from various parts of the world (Ceeway and Paragon).

To make absolute sure that everything is going to fit with no issues I have gone, what has been called, overboard in terms of checking clearances (and I admit they are very extreme). Unfortunately, although brazing is classed as semi-permanent in most frame builders books, once built up there is very little that can be done to rectify any geometry and clearance issues, unlike my carbon colleague Oliver (link to projects) who is able to saw large amounts of frame off and re-mould it. This isn't an option for me.

Front Wheel

I used a 2.6" tyre to check for clearance at the bottom of its travel (the ATC + the stack height of the bottom bearing) and then to the whole length of its 100mm travel to make sure that nothing would clash. This is also taking into account the fork offset used on the forks I intend to buy (44mm offset)

Fig. 1

Crown

This clearance between the crown at full rotation shows the fork crown to clear nicely with 2mm to spare. This is highly dependant on an arbitrary number I chose for the fillet radius on the crown so the down tube may be moved to provide a bit extra clearance in case of misalignment.

Fig. 2

Rear wheel on the chain stays

 Using a 2.55" tyre gives some clearance more typically seen on American designed frames. I will be running a 2.25" tyre instead giving enough clearance for the English weather.

Fig. 3

Rear Disc

A 180mm disc is by far the largest I will ever be using on the rear wheel so I have used that for clearances, it clears on both the chain stay and the seat stay.

Fig. 4

Fig. 5

Chainrings

Using measurements found for a variety of Shimano cranks the extremes of the dimensions were chosen. Along with the chosen dimensions of chain line from a mixture of double and triple cranks the measurements from a hope 34t chainring was taken and then scaled up to produce a 46t chainring in the out most position and a 36t chainring on the middle ring position, neither showed to be a problem.

The distances between the chainrings were measured from a set of FSA Afterburner triple cranks that I own.

Fig. 6

Fig. 7

The thickness of the chainrings is greatly exaggerated at the overall diameter so the clearance in reality will be far greater.

Cranks

Using the narrowest Q factor on a Shimano crank, that I could find actual measurements for, I produced the crank circulation for a 175mm crank with a a large amount of material after where the pedal would thread in to be sure. Again showing now issues.

Fig. 8

Heel and Toe

Using my EU48 Shimano shoes to produce very large blocks in the assembly to check the clearances a minor issue of heel rub on the chainstays was found. Considering the blocks used have sharp edges this will not be an issue but is still shown below. The toe clearance will be greater than shown for the reason previously stated.

Fig. 9

Fig. 10

Hopefully this will be enough to make sure there are no clashes and that I don't need to make any modifications to allow a usable frame.

In terms of calculations for the survival of the frame none have been done up to this point. I have decided up to this point that blissful ignorance is better due to the fact that I am buying the thickest gauge tubes I can from Reynolds. Fortunately to back up my expectations of my frames survival due to the heavy gauge, large diameter air hardened steel tubing, Stanton have released a new bike called the Sherpa (as seen on single track) which is using the same DZB (double zone butted) tubing that I am in the same steel alloy for the down tube. The down tube is the area I am most concerned about thanks to the angle it is sat at to the head tube and the forces I intend to put through the head tube, hence why my Surly Karate Monkey has extra bracing under the down tube (and why Reynolds have produced their DZB tubes). 

The chainstays I am using are from Columbus and come pre-bent, saving me a lot of issues with foundry sand, lead alloys or ice trying to bend the stays into some sort of usable shape. They are similar to the ones used in a Chickens frame designed for riding a cyclocross bike down sets of steps, meaning I have no great worries on these either.

As well as the clearances being checked some small geometry changes have been made so that I do not have to run my saddle at the extremes of what it was designed for. In order to do this the effective top tube has now been lengthened from 599mm to 630mm.