Frame #3: short chainstay 29er

I started frame #3 after a break from building anything last week. It’s good to sit back and digest, recover from (over)thinking things about frames and forks.  I took down a few beetle-kill trees, cut them into pieces that’ll fit in the stove next winter, walked the pups and my girl, rode my bike with some friends, did some GIS contract work, and reflected on how in the hell we’re going to move to norcal by fall with 4 dogs and 8 horses (…!).  The rest paid off because when I welded today for the first time in over a week it was almost like I knew what I was doing.  These are my best welds yet!

This is after i scrubbed the weld with a wire brush to clean it up a bit before stopping for the night.

Some history first.  Me and some friends did a supported trip of the Colorado Trail in 2004 (after failing miserably on a self-supported attempt in 2001 after 2 feet of snow fell on our first night out of Denver…).  If you’ve never done the CT in its entirety, it is a must.  On that ride, I used a (gasp) carbon fiber hand-me-down frame — Trek 9800 — and a steel Wily unicrown fork with disc mount with a 29er front wheel.  So it was pre-69er but other than the higher bottom bracket, it was pretty similar in geometry to what was to become TBrown’s 69er a few years later. It cornered like no other bike I had ever ridden, was super fun to ride downhill, and climbed ok for what I expected.

When the actual 69er came out from Trek (yes, i know Carver back east has had a 96’er too) I got a bro-deal on one and rode it around for a few years.  To be honest, I didn’t like the hydro-formed aluminum frame, way too harsh, and the rear 26″ wheel made it corner tight and was good for wheelies, manuals, and other times when you wanted to lift and ‘place’ the front wheel, but I hated how the bike climbed and how harsh the rear end was.  Especially after riding my steel fillet brazed Hunter 29er for a couple of years before getting the 69er…I’m just done with 26″-wheeled bikes.   I haven’t tried 650b’s yet but that is on the list of bikes to make.
ANYWAYS…

Because I loved the carving ability (69 degree head tube angle) and the short stays of the of the 69er I will try to incorporate those features into this next 29er.  It’s definitely NOT a new idea — to try these features out on a 29″ wheeled bike — but it’ll be my first try at making such a bike and riding it.   Waltworks has one he loves, and many others are building them as well.

Front end:  I believe that the 69 degree HT angle is part of the ‘carving’ equation but the front end trail along with the HT angle is what creates that feeling.  My 1st 29er frame has a 70.5 degree HT angle and about 80mm of trail when I put the Fox F29 100mm (51mm offset) that was on my 69er.  It feels nice on the corners but it just isn’t the same.  So for this SS29er bike I am going to make a segmented fork with 51mm of trail and a 470 axle to crown, giving it 86mm of trail.  It’s amazing what a mere 5mm of trail will do to the feel of a bike! I’ve had my adjustable rake fork set to give me 82mm of trail and i like it but it’s not all that.  I’m thinking the sweet spot for trail is around 85-90mm for me.  On that same bike, I could set the trail to be 72mm and although fun to ride, it was just too twitchy for my likings and felt unstable.  Everyone has their own style and bike feel they prefer, and I just prefer bikes with slacker head tube angles and more trail than some.

Rear end:  It’s cool to see how people are creatively solving the problem of shortening stays on a big wheeled bike.  For this next frame I’m trying 425mm (16.7″) chainstays but since I’m building it as a singlespeed with horizontal dropouts (cheaper than sliders, and since this is only my 3rd frame…) will make the stays more like around 430mm so that I can get the chain off and take the wheel off.  Using horizontal dropouts will also let me play with the effective chainstay length and dial in exactly what I like.  Shortening the chainstays to 17″ or below should help lighten the front end to pick up the front end and wheelie easier than a ‘normal’ 29er with 17.5″ chainstays.  There are other factors, like BB height and bar height (among others), that influence the ride characteristics I’m talking about, but before I go changing too many things at once I want to try shorter chainstays first.

Offset seat tube by 5mm

In order to shorten the chainstays to something similar to what’s found on 26″ wheeled bikes without getting the rear wheel to rub on the front derailleur mount or even the seat tube, you need to either push forward the seat tube.  You can do this by introducing some offset (see above pic), or something more drastic like bending the seat tube so it curves where the tire would hit it.  I don’t have a big tube bender that would allow me to do that so I’m trying what I first heard about on Walt’s bikes – an offset seat tube.  Other frame builders have welded the seat tube to the downtube instead of the BB for extremely short chainstays (or just to be able to make a 36″ wheeled bike work), or created a seat tube yoke like Wolfhound Cycles.  With a singlespeed, it’s much easier to make a short-chainstay 29er since there is no front derailleur mount to get in the way.  On my current bike with 445mm (17.5″) chainstays, there’s maybe 1cm between a regular low clamp XT front derailleur and the 2.25″ tire.  Any larger a tire I’d be in trouble.  A ‘direct-mount’ derailleur (mounts on the side of the seat tube) is one way to fix that problem (if you offset the mount foreward on the seat tube a bit), or just don’t use a front derailleur and run 1 x 8, 9, or 10.  There are now several types of front derailleurs that you could run for a 2 or maybe even 3×9 like a Shimano LX571 or XT M771 because of their svelte clamp and cable pull mechanism.

Anyways, singlespeeds are close to my heart so that’s what this bike will be. Oh how I loved my first SS – a bright orange Ionic Johnny Rotten with a 2nd gen Fat Chance Yo Eddy fork!  It’s amazing how so much has changed since then (1998) in bicycle components but not too much in frame and fork design.

Phew, long rant!  Until next time…

EDIT (12/19/11):  Having ridden this frame for awhile now and having posted THIS i feel even more strongly that short chainstays are just one piece of a very large pie.  The front center on this bike is 720mm…HUGE! It’s difficult to wheelie and bunny hop even with 16.75″ chainstays.  My 5th frame that I really like has 17.2″ chainstays and a front center of 685mm and it is way better in those departments most people attribute to just short chainstays.

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