February 7, 2011

The Main Chassis

Engines are neat.  Without them, motorcycles would simply be called "cycles" and their audience would be limited to trendy city kids with one pant leg rolled up.  However, one cannot overlook the significance of the frame and suspension when building a motorscooter.
I am told that frames are dynamic parts of the machine.  Flexing and moving with the myriad forces of accelleration, braking, and manouvering.  I've never witnessed any of this, because I'm having too much fun driving (or crashing, in nominally less- fun moments).  However, I was forced to get up to my elbows in geometry when evaluating how I was going to 'do' the frame for my CB550.

Fig. 89: To the right, a complete CB550 frame, brackets and all.  To the left, a frame with the centerstand brackets ground down...and pretty much stock besides that.

If you've ever taken apart a UJM (Universal Japanese Motorcycle, and my chosen medium) you'll see that their frames have a lot of brackets.  We're talking dozens.  And with good reason.  Every component needs to mount to the frame in some manner.  The idea behind a 'racer' is to strip off as many of those components as possible, hence, many of those brackets go bye bye.  I'm impatient, and I have a welder, so I wasn't about to catalogue each bracket and calculate how many needed to stay.  No sir, I went typical Demoto and hacked 'em all off with an angle grinder!  Of course, I left a few (I think they're called 'engine mounts') but most came off.  I smoothed out the scar tissue with abrasive disks, and the result was a very clean looking frame.

Fig. 90: In the foreground, a clean frame, all unwanted brackets removed.  In the background, that same stock frame, sucking up space in my garage.  Future hardtail?

The grinding being done, and the messieness gone, it must be stated that many of those brackets also doubled as gussets.  Gussets are basically any piece of metal that is welded in the corner of a junction in the frame.  They provide added strength against the frame flexing at the angular moment of the blah bla whosm whutsleflartz. 
They're important. 
I  found a googleplex of great information on frame stiffening from a fellow named Tony Foale:


He sells a program where you can do all these calculations and run simulations and whatnot.  Some folks are into that, and I have mad respect for them, it's just not my style.  I have two standards for weight and strength: "Can I lift it?" and the ever important "Can I jump on it?"  You don't need a computer program for these tests.  You don't even need a brain, strictly speaking.

So some frame stiffening will be required now that the brackets are off, I just haven't gotten around to it yet (see Fig. 90).  I'll probobly put a a pair of gussets beneith the frame on either side of the oil pan, and some cross bracing on the downtubes (the pair of pipes heading down from the head tube and looping around the front of the engine).  Tony Foale also talks about the many faults of the oil dampened fork suspension system used on...pretty much everything that's not a springer or Bimota Tesi 3D.

If you can afford this motorcycle, you are not reading this blog.

A fork brace can mitigate some of the unwanted flex found in fork type suspensions, but they're expensive.  Basically, it's a big piece of metal (usually aluminum) that clamps the two forks together closer to the wheel than the triple tree.  However, I have seen (rarely, so it might be a horrible idea) folks who have used an extra triple tree clamp low on the forks to act as a fork brace.  I've got an extra tree clamp so it's worth a shot.  What's the worst that could happen?

Ahh yes...This....

Hope you folks enjoyed this article.  The is the special commemoritive edition celebrating my first Follower and First Comment.  Special thanks to Demoto's Sister, Hister, owner, rider, and builder of a sleek KZ400 we bought off the side of the road for $29 cash.  Thanks sis, you rock my world!

Fig. 91: Hister, keeping it real.


Heather said...

Gee thanks!

You know, from a follower standpoint, you can probably post links to new blogs on your facebook status. Then you could reign in some other moto-enthusiasts :)

Demoto said...

I should probably do that...