Tuesday, January 17, 2012

Building a mountain bike Coconino style.

I thought it would be educational to show some of the major steps in building a common mountain bike Coconino style - feel free to ask questions!

First, I have already drawn my blueprint - I'm not about to go into bike design, and while some people use systems like CAD (computer aided design) I prefer to draw a completely accurate life sized blueprint. Also, this is a repeat of a given design, so I'm not even drawing a new print, but re-using the client's existing one "why mess with perfection" as they said...........
So, here I have chosen some main tubes, BB shell and dropouts & chain stays and need to cut the sections needed from the raw head tube stock and the large curved piece of 4130 top tube which I have previously curved on a 3-roller bender. The seat tube sleeve has also been cut from a 8' length of 4130.

Here I have cut the tubing to usable sections very close to the final cutting length, marked bowing and butting, polished the mill scale from the BB and seat tube sleeve, drilled vent holes in the BB, sleeve and head tube piece which has also been faced.

Now, I have silver brazed the sleeve onto the seat tube, drilled holes for the H2O/ water bottle brazeons, and mitered the ST/BB junction.

Mitering the 90* BB end on the down tube on my vertical mill, the block is to keep the two miters in phase with each other.

.......And, then flipped around for the head tube/top tube miter.

Finished fishmouth straight from the machine. A few file strokes & it's good to go.

Cleaning the head tube with 80 grit emery paper and leaving a rough surface to provide "tooth" for the filler to adhere to on a microscopic level - everything is clean-clean. Every tube is cleaned to this level inside and out.

The tiny dots are where the top tube and down tube will connect.

I have set up the jig to all the measurements extrapolated from my blueprint and installed the head tube and BB shell and am checking the down tube to see how the cutting went - *Ca-Chink!* perfect fit!

All tubes are cleaned inside & out with emery paper & 91% rubbing alcohol - grease & contamination are your enemies for clean brazing.

Now, the seat tube junction has been lightly fluxed inside and out and a fillet laid on the front and the back of the BB shell.

Checking the seat tube for straightness - spot on!

Now, I have drilled the vent holes for the top tube and the down tube, drilled the hole for the seat tube slot, and slotted the seat tube to clamp the seat post.

Next, I have mitered, cleaned, fluxed and tacked the top tube in.

And, then the down tube as well, it has also had the water bottle holes drilled and the compound miter hand filed to fit the seat tube.

At this point the seat tube has been "tinned" all the way around, a small fillet of brass has been laid around it and the compound miter with the down tube.

Now I am figuring out the clearances of the dropouts and the chain stays and slotting them by hand with a hacksaw and a thin warding file.

Here is a detail of the "spearpointed" tips of the chain stays, I have drilled 1/8" vent holes in the bottoms of the stays as well to allow first hot gasses to leave and then during the life of the bike allow water to leave and lube to be introduced.

A big box of the new Coco-Motos!

Stay tuned as we move to the back of the bike, and thanks for reading! - Steve.


Anonymous said...

Nice write up Steve....thanks for posting this. One question, are you new drops stainless.


R.Ajl said...

Great to watch your sequence.

I'm curious, the silver doesn't run out of the sleeve at all when you fillet braze the ST and the SS cluster? I've been brass brazing my sleeves, and they are, for lack of a more graceful term, a bit of a pain in the ass, Silver would make it that much easier on me with my skill set.

Any tips on that front?


steve garro said...

As per silver - braze the brass hot (but not too hot - right at the edge of the heat range) and fast.
Stainless dropouts are something I'm trying to never really use again, I'll do rockers for an upgrade but I see no huge advantage - stainless socket dropouts are super great, however - different joining methods - slip fit vs. slot & fill.......

Sabrosa Cycles said...

good to see that westy went for the coco-motos rather than the slideys. no more creaks for him. RAL?

steve garro said...

Yeah! to be totally fair, the creaking was from his DT hub - I had him hold the phone up to it & diagnosed it over the phone.
I'm over sliders - did you see the window ones are no longer made?
He's going for the same RAL.