Instructables
The pictures aren't the best because I never intended on making an Instructable but here goes.

I have wanted a 29er mountain bike for a while and stumbled across an old Trek 720 hybrid that I was going to just fix up to make into a commuter bike but was inspired. I had many parts around my shop and what you have will dictate where you can take this. For those of you unfamiliar with a 29er, it is a mountain bicycle with 700c wheels (like that of a hybrid) using a larger tire width making the overall tire size around 29" instead of the typical 26" found on most mountain bikes.

I am a welder with a number of years experience in making goofy creations and sculptures, this is the first really high end bicycle. Producing strong welds is crucial in this project so if your not experienced get someone who is. I used a MIG welder for this and am self taught I can't afford a TIG welder yet and have never taken formal classes but have become fairly proficient over the years.
 
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Step 2: Geometry and Head tube

This bike being an older design used a 1" threaded head tube. This design is heavier and out dated so this was the biggest obstacle I faced. I had 2 forks to chose from and both were 11/8 threadless. I researched and found a great company called Nova Bicycle Supply and was able to order just the head tube and some other braze on parts from them that you will see I used later.

I measured the geometry and used the geometry from a bicycle made by Salsa as a template for how I would line everything up and weld it together.
B4SEC4MP6 months ago

Nice work! I've wanted to do this for a long time. How did you ensure that the head tube was vertical in the plane of the front triangle (i.e. parallel with the seat tube)? Did you have to face the top/bottom and ream the inside of the head tube for any warping due to welding?

axlejor3 years ago
Awesome job, its funny I have wanted to put disc brakes on my chro mo vintage commuter for a long time but have been scared to weld on a caliper mount. I have access to everything I need at work, you may have inspired me to take the leap!

...But I wont blame you if I screw it up.

mntbkrguy (author)  axlejor3 years ago
Take the leap, I just attached the caliper on the frame mount installed the wheel with the caliper squeezed onto the disk where I needed it, then tacked the mount in place. It's not too difficult and with all the adjustments you can make on the brakes themselves it doesn't need to be perfect.
BtheBike3 years ago
Woe , that is quite an undertaking . think i've seen a 29er cheaply made with just adapters for disks and a fork before , but yours is all out . Beautiful re-working and reuse .
mntbkrguy (author) 3 years ago
Thank you, I have made many more things and seriously need to make a few more build threads. I have since added a table top belt sander to my collection of machines which allows me to make the notches in the tubing a lot faster and easier than before.
I have the same frameset, and funnily enough, I was considering doing the same thing with it! I had one question though: did you do anything to the rear stays, or did you just leave the rear triangle alone? It seems like maybe a bigger tire on the rear would be more feasible for hard trail riding that a two-niner is suited for. Maybe I can build one of these guys; I was looking at a Fargo, but this mod looks a lot cheaper!
jon.karak3 years ago
Like the setup, but typically I see disc-brakes mounted on the upper seatstay. Why did you mount the caliper is on the chainstay? Did it preserve the ability to mount a rack and fenders? Was it difficult to find a compatible caliper given the unusual placement? Did you feel the need to reinforce the stays to resist the stress of the brake on the frame?
Its commonly found on the touring bikes. It allows the user to still mount a rack without any special adapters.

its like the salsa vaya. Theoretically the steel's strong enough to stand the force of the disc brake. It will stress it a lot and possibly lead to early frame failure but thats very unlikely.
What about the the caliper? Did you have trouble finding one that would fit? It looks like you've got an an Avid BB5. Do you like it?
mntbkrguy (author)  jon.karak3 years ago
Sorry I never responded in the past I need to keep better track of things on here.

I mounted the caliper on the chainstay as said in the other persons comment so that it wouldn't interfere with the bike rack. Any caliper will fit you just need to have the distance from the center of the disc to the caliper correct in order to be sure the pad makes full contact. getting the angle took some playing around but once I had the right spot I clenched the brake and tacked the brake mount in place that way it was lined up as close as I could. The only difference in the caliper I used is the model. The "BB5 Road Mechanical Disc Brake" has a shorter pull allowing you to use brake levers for drop bars. They work great I have always been a fan of Avid brakes. http://www.sram.com/avid/products/bb5-road-mechanical-disc-brake
bobhickman5 years ago
Which head tube from Nova did you use? Was it this one: OS CRMO 37.0 X 200

I'm contemplating a similar project as I'd really like to update an old bike of mine, but am limited by the 1" threaded head tube.
mntbkrguy (author)  bobhickman5 years ago
Thats the one
lealamb5 years ago
I loved this frame build! I have a question about the head tube. Was it just a hollow tube that you cut to length, and is 1 1/8th diam.? How did the head set work? Was it pressed in?
mntbkrguy (author)  lealamb5 years ago
I ordered the head tube from a bicycle frame supplier called Nova Cycle Supply then cut it to the length I needed. The headset I am using is a Chris King 1 1/8 standard which is pressed in. You can order integrated head tubes from Nova and other suppliers but they are more complicated to install.
SirRideaLot5 years ago
As to my previous question concerning level. OK got it, I see you used axle center as level. Why didn't I catch that? Must be mesmerized by 29ers...LOL Again great job! Thanks!
mntbkrguy (author)  SirRideaLot5 years ago
I am sorry I didn't get to your question, you are correct based off of the illustration from the Salsa diagram I made the chain stay level and measured the head tube angles from there.
SirRideaLot5 years ago
How did you determine the bicycle frame level? I assume the table and mounts are level, but what part of the bicycle did you use as the key part to determine level? Thanks and great instructable!
mattias law5 years ago
was th trek frame originally intended for 29" wheels? have you weighed it yet it would be interesting to see, damn i wish health and safety hadn't taken away all the fun in my school or else i would go and pick up a second hand frame tomorrow and start chopping and welding. have you ever considered buying tubing and just making a whole bike, then you could get everything just right for you size and preferences
mntbkrguy (author)  mattias law5 years ago
The original bike already had 700c wheels so the wheel size was the same but the rear can't accept a tire wider than 2 inches wide. The Fork came from a 29er so it has no difficulties with size. I have looked at tube sets and many of the web sights have good sales but before jumping into that large of a project I figured this would be a good test of my abilities. I haven't gotten a specific weight but I would say it falls somewhere in the middle of where any steel 29er would be.
peteragain5 years ago
Interesting project, Could you post some pictures from the side of the final result? and how about mudguards? You do not live in Dublin I suppose..
ledzep5675 years ago
now you just gotta make it a single speed :D
Honus5 years ago
Cool build! Another place that's really great for framebuilding supplies is Ceeway.
http://www.ceeway.com
mntbkrguy (author)  Honus5 years ago
Thanks I will check them out
goodgnus5 years ago
Very cool, would like to see closeups of your welds. Are you using 100% co2 or 75/25 or straight argon? What gauge wire?
mntbkrguy (author)  goodgnus5 years ago
I use 100% co2 only due to cost, I haven't sprung for a large enough tank to get anything else. I am pretty sure I have been using .035 but I would have to confirm that. as for the welds smoothed out the welds around the head tube to better match the machine welds on the rest of the bike but I can get a shot of the brake mount.
Uumm, I have actually found that 75/25 is fairly cheap. To get a K sized shop bottle you might need to spring upward of about 500ish, but a refill is only cost you about 75. (This is all in Phoenix, AZ)
mntbkrguy (author)  mntbkrguy5 years ago
put a new pic up of the brake mount but that doesn't even show the weld. I used .030 wire not .035 which makes sense because I am welding much smaller things now.
i personaly think you ruined a trek! but thats just my idea. also it must be uncomfprtable with no shocks :/
A lot of people prefer to ride fully rigid bikes these days; I'm one of them. They give you much better feedback of the trail, they climb like goats, and with large volume tires, they descend better than you'd think.

mtnbkrguy: Sweet bike!
mntbkrguy (author)  beauwalker235 years ago
I would totally agree with you if it weren't a mid 90's comfort hybrid. as for the ride its actually pretty good that's why it has the 2.25 tire on the front. It is a very common set up these days.
Nice build up! (Much more actual building that most build ups!) Makes me even more excited for the cyclocross bike I'm buying soon (Only one more month. One more long, agonizing month) Enjoy the fruits of your labor!