Wednesday, January 30, 2013

Sizing Up The Swingarm Options

I like most things about the DR350's design, but one thing I can't get past is the rear swingarm.  It's clunky looking and those fins have not aged well.  I'm sure the three-piece design was cheaper to manufacture than a welded, boxed aluminum design that can be seen on almost every modern dirt bike, but it's also far more flexible with the open C-channels.  Most DR owners agree that the rear swing arm is the DR's biggest weak area.

I have been toying with the idea of swapping it out in favor of something else.  Since I am not too concerned about suspension curves (maybe I should be more so) and I have no issues cutting or adding tabs to the frame to support new linkage mounts, my options are broad.  So broad in fact that I put #2's swingarm side-by-side against one I have from a CRF450.  Granted, the linkage designs are wildly different, but it's eerie to see how similar they are despite their 15 year design gap and different manufacturers.

1991 DR350S swingarm in the background, 2005 CRF450X in the foreground.  The DR swingarm is at least twice the weight of the CRF swingarm.
DR350 swingarm in the background, CRF450X in the foreground.  The pivots are aligned, which helps show the added length of the CRF swingarm.

There are a couple dimensions that really give this wild idea some traction: both swingarms take a 17mm pivot axle, and the CRF swingarm is 5mm narrower at the main pivot than the DR swingarm.  That doesn't mean a swap like this one, or any other, will be remotely easy, but it does mean I won't have to modify the frame-side of the main pivot.  Just say'n...

For the fun of it, I placed my components in their places.  That's #2's frame, the DT250/400 tank, #2's forks (lowered 3.5"), an 18" CRF rear rim floating in the front, a CRF swing arm, and #2's rear 18" rear wheel.  Mentally photoshop some tires and a different, more road-race-esque rear section, and we are in business.

Sunday, January 27, 2013

Tiny War Tank

I had a CL score [impulse buy] this weekend.  I picked up a tank, fender, and seat for $100.  The tank was primered and seemed to be repaired based on the magnet test.  When I got it home and sanded it, it became more ugly-duckling and less beautiful-swan.  It doesn't have any major dents, but it does have a few minor ones that need to be addressed.  I will probably make it work.  It would be a good candidate for the rusted look.

The seller didn't know what it came off since he bought someone else's street tracker project for parts.  I'm guessing a late 70s Yamaha DT250 or DT400.

 The purchase came with a fiberglass fender and seat.  The whole reason I pursued this was for the seat, but after seeing that it has a constant curve in the sea pan profile, I might not use it.

This is the shape I am after.  Flat for the first 80%, then an upsweep for the last 20%.  I guess I could make a new seat pan with the shape I want, then steal the cover and foam from this one.  That's a pretty dirt-bag option, but it would be cheap and easy.

Thursday, January 3, 2013

Short Fork

I have been tallying expected costs and become a little concerned that my dreams may exceed my pennies; or at least eat into my "buffer" line item too significantly.  As a precaution, I decided to experiment with the stock Showa 43mm conventional forks.  If I use the stock forks instead of USD forks off another moto, I should save around $400 on my budget, so it's worth considering.  These forks do have the added benefit of already using a 15mm axle; the same bearing ID in the front wheel I would like to use.

I made these 2.5" spacers for the damping rod fork during a couple lunch breaks at my company's machine shop.  The hole and channel are for the rebound circuit.  I used two set screws to fixture the spacer in place and prevent it from covering an orifice.

The right leg (closest) has the spacers and shortened spring installed, and is moved up in the triple clamps as much as possible.  This results in 7.75" of exposed tube versus the stock 11.5".  I would like to go lower, but the spring rate increases for each coil that is removed from the fork springs.  I'll try this first and go from here.

The perks of working for a bike component manufacturing company are pretty huge.  Add to it the fact that I work in the R&D building for mountain bike suspension.  Even better, the machine shop policy is pretty liberal; as long as you aren't preventing work from being done, the machines are yours to use.  I love my job.  :D