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Discussion Starter #1
I've been reading this forum for the better part of a year now, and as a senior in high school, I'm building a rigid 29er for my CADD project. I'm basing my jig off of an extrusion I-beam design. I've haven't had access to my drawing at school, so I've attached a scan of a previous printout. I know the 2nd top tube doesn't do anything, but from what I read as long as the down-tube is ample it won't hurt. For the miters I was planning on using a hole-saw to get close, and then hand filing. I'll be riding it for cross-country trails here at Bootleg Canyon and in the Golden/Denver area when I go up to school. I'm about 6'5" and 240lbs.(without gear).
I rode various 29ers at Interbike, so I based the geometry somewhat off of what I liked about those and with what I have and like, a demo 7 and a 24" bmx bike(that'd I've been using for the cross country trails). It will mostly be used for cross country but also messing around(doing wheelies around campus and what not). So what I came up with:
Head tube:130mm(68/69 angle)
Effective Top Tube: 635mm
Seattube:500mm(74 angle)
Chainstay length: 450mm
Bottom bracket drop:66mm
Wheelbase: 1160mm
I plan on using a 470mm atc fork with 38mm rake
The thing I might want to change is the chainstay length. I think it would probably be better for me shorter, but I'd like to use straight gauge 7/8" with a single bend, so I'm not sure how short I can get them like that to still have clearance for a larger tire. I plan on running a 1x9 gear setup. It will most likely be TIG welded by a professional that has been welding for a long time and has done a few bike frames in the past(the founder of Redline actually worked for him for a while) so I'm not worried about heat control but I'm not sure about tube thicknesses and changes to geometry I may want to make.
Thanks,
Aaron
 

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You should go with shorter chainstays if you plan to do wheelies or even lift the front wheel at all. Its going to be very hard to lift the front on a bike with 450mm stays. I'm at 425mm on my 650B and I wish I had gone about 10mm shorter. You can offset your seat tube forward on the bottom bracket to get a little more tire clearance. I'm sure your professional welder can give you some advice if he has done bikes before.
 

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440mm stays are just fine. Going shorter will be hard to manage in terms of tire clearance. If you like wheelies a 29er might not be your cup of tea. People get way hung up on short chainstays but 430mm to 445mm will ride just fine.

Wheelies are silly but it's not just about chainstay length...
Chad Cottom's Mundo Wheelie - YouTube

You don't need the small brace on the chainstays especially with TIG welded 7/8" stays. The little bridge does not add much strength or stiffness and is a pain to TIG. The extra top tube is also sort of silly because it makes the bike heavier and gets in the way of water bottles so I am not sure what the point is.

Good luck and have fun with the build!
 

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Discussion Starter #4
Well he's a welder, not a bike frame builder so I'm not sure if he'll have much advice in that area. I'm not sure if I'm just taking your comment the wrong way Dsaul or if my statement of "professional welder" rubbed you the wrong way, but I wasn't saying that I'm getting my frame welded better than anybody else, I just meant I know the person is a proficient welder and its not a friend of a friend of a friend who may or may not be able to do this.
To febikes, should I remove only the chainstay bridge or the seatstay bridge also? I'm considering removing the 2nd top tube, but I do really like how the bike looks with in(sorta like some of Coconino cycles) and I figure if I'm making my own frame might as well make it different.
 

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To febikes, should I remove only the chainstay bridge or the seatstay bridge also? I'm considering removing the 2nd top tube, but I do really like how the bike looks with in(sorta like some of Coconino cycles) and I figure if I'm making my own frame might as well make it different.
Loose the chainstay bridge but keep the seat stay bridge.

Also, use 5/8" round for seat stays or 16mm tapered. Use 7/8", 3/4", or bicycle specfic oval tubes for the chain stays.
 

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Aaron;

- I think you are reading DSaul wrong. He just thought your guy might have some ability to help you with the mods he suggested re; offset ST/BB, and other things to make the build easier and more successful.

- The 2nd TT is redundant, but of course you know that. Adding a pure style piece is just fine as long as you can rationalize the extra weight. You might get a piece of .028 straight gauge to pare the weight down a touch, or even thinner if you can find it.

- 17.5-ish CS is just fine. You can go shorter of course, but at the possible expense of some ultimate climbing traction. A 74* ST would shift your weight forward more, and that with the "longish" stays might make it harder to loft the front. However, I am the same size as you. I've got a 20mm longer TT, 454mm CS, and a 40mm longer WB, and I can still loft the front pretty much at will. I'll give you a traction for-instance; even riding my Kroozer in 6-8" of snow, I can climb a pretty steep grade, feel the front wheel floating off the ground a bit, and not lose traction at the back. Impressive.

- I think .875 is overkill for the CS. I use .750 and it seems to work just fine. You can probably also use .625 for the SS without trouble, and save a bit more weight to offset your 2nd TT. Smaller tubes will also free up some tire space for you!

- I wish that I had ditched the CS bridge on my Kroozer. It really catches a lot of muck. While it did add a noticeable amount of stiffening, that is when bending stays together with no wheel attached, and I'm not sure that is a valid test for any realistic action. My next frame will not have one.

This from a two-frame-wonder with about 25% of a clue on any of this, so... :)
 

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TM is correct. I was just thinking your guy might have some experience that could help you figure out whether you should make changes to your design. With regard to chainstay length, I'm just sharing my experience. My frame also has a slack 69 degree head tube angle that lengthens the front center and puts the weight of the front wheel further forward. I ride fairly aggressively and like to lean back and loft my front wheel over obstacles while descending. I found that very difficult with the sliding dropouts at 435 and considerably easier at 425. I think it would be perfect for me at about 420.
 

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The second TT will be welded into a thinner section of the down tube. No biggie, just harder to tig thin stuff but do it for the look if you'd like.
cheers
andy walker
 

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Discussion Starter #12
tubing

I'm dropping the chainstay bridge, and I didn't realize this printout didn't include the disc brake bridge. The second top tube will go for now, and if its all done and I decide I still want it, it can be added on without much trouble. I don't think I'll need to offset the bottom bracket for this one. The things I was unsure about were the effects of the seat tube angle and the fork trail.
As far as tubing goes, I think it'll be;
Head Tube:
1.625" x .156" 4130 tube (Aircraft spruce), turned down to 1.6-1.7mm wall
Top Tube
31.8 x .7 / .6 OX Plat (BikeLugs)
28.6 - 8/5/8 Ox Plat (Bike Lugs)
31.8 x .9 Stress Relieved(VERHT1?)(Bike Lugs)
Downtube
34.9 x 1.2 / .8 x Stress Relieved 4130(Bike Lugs)
Seattube:
28.6 1.2/0.6/0.9 4130(NovaCyclesSupply)
Bottom Bracket:
1.5" x 73.5mm 4130(Paragon)
Chainstays:
3/4" x .035" 4130 tube(Aircraft spruce)
Dropouts:
Boomerang plate dropouts HD(Naked Bicycles)
Seatstays:
5/8" x .035" 4130(Aircraft spruce)

Thanks for the tips and help everyone,
Aaron
 

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Discussion Starter #14
Didn't come across the Boomerang dropouts until just now, so I didn't realize I wouldn't need the bridge(correct?). As for buying a head tube, I don't want the 44mm Paragon one and I don't mind making one.
 

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I find it interesting that big Riders keep shelling for smaller Dia downtubes. If you are spidery like me, I can get away with it, but boy!! 250 pounders....38mm min, if you like to play, 42mm. The strength increase of a couple of mm is huge for little weight gain when, now that the redundant tube is out of the picture, won't add diddly squat to the big picture weight wise. The riding quality reward for the bigger tube is just too much to compromise on. Top tube = 31.8mm with a .9mm butt. No problem with turning a head tube, its not mythical, Paragons are turned. 74* ST = about 10mm at the seat for a longer leg over 73*. Its tuneable for seat adjustment, but if a more aggressive ride position is adopted, it uses the riders leg power better, though compromises the ability to wheelie. Also, that 635mm TT will keep the wheelie balance firmly planted at the front end. Big bikes will always be a challenge to get all the ticks of the boxes. Looks good otherwise.

Eric
 

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Discussion Starter #19
So would a 38.1mm 1/.7/1 ST downtube and 34.9mm 1/.7/1 ST top tube work? And would it be worth getting a .9/.6/1.22 S3 seattube too than? Or is it not worth the extra trouble for reaming it?
Thanks for helping me
 
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