> From: "P. BENSON" <pbenson@NMSU.Edu>
>  Date: Fri, 9 Apr 1999 13:57:35 -0600 (MDT)
>  Subject: suspension 101 (long)
>  
>  I've read the comments on rake and trail [sounds like a name for a
>  magazine on hiking and gardening :)], and wanted to add a bit to the
>  discussion.  First, what are rake and trail?
>  
>  First, rake is merely the angle from vertical that a line through the
>  steering head would produce.  This is also called castor angle.  The angle
>  from this line to horizontal (road surface) is the head angle.  Astor
>  angle + head angle = 90 degrees, obviously.
>  
>  To find trail, again find the line through the steering head....NOT a line
>  through the forks....and extend it down to the pavement.  Drop a vertical
>  line through the hub of the front tire.  Trail is the distance (in the
>  horizontal plane) between these two points.
>  
>  Rake is usually 22-28 degrees or so for most street bikes, and trail tends
>  to be 3-5 inches.  And yes, all of this is "theoretical" in that a
>  motorcycle in operation is always changing in front end geometry, based on
>  load, acceleration/braking, road surface, etc.  
>  
>  Note that most bikes have some offset between the steering head and the
>  forks, with the forks "ahead" of the steering head.  This has the effect
>  of DECREASING trail, as the line through the hub is relatively farther
>  forward, closer to the line through the steering head.  Some bikes, not
>  many, also have an offset at the lower end of the fork, often increasing
>  the offset and further decreasing trail.  While you wouldn't want to do
>  it, you can get NEGATIVE trail, or "lead", when the rake is sufficiently
>  reduced and the offset is great enough.
>  
>  Anthony Grafals, your shopping cart analogy is really very instructive.
>  Assume one builds a motorcycle with a vertical steering axis (zero rake),
>  and no offset.  Trail is zero.  Now add some offset at the end of the
>  fork, and you've got the shopping cart.  Set it up with lead, not trail,
>  and yes, any turning of the wheel will lead to torque that will try to
>  amplify the amount of turning, and extreme instability.  Trail, when
>  turned around, is merely the amount of offset.
>  
>  Can we adjust offset in a forked bike?  Yes.  Use an eccentirc bushing in
>  the headset, or change the triple clamps, or build spacers into the triple
>  clamp.  I can't think of a production bike that does this, but it would be
>  easy enough, and would add another dimension to suspension adjustment.
>  
>  Trail is needed for stability in a straight line.  Cruisers have more, as
>  a rule, and are stable.  Sport bikes have less and can be "twitchy".  But
>  too much trail makes the bike heavy to steer.  Too little and there's not
>  enough "restoring torque" when straightening the bike.  Too little trail
>  also makes a bike that is easily deflected by bumps.
>  
>  Real world.....the road surface is not smooth.  When you hit a bump, the
>  contact patch "moves" around the front of the tire, reducing trail.  Big
>  enough bump, and you have lead, not trail, and the bike tries to turn.
>  
>  Do we have rake and trail?  Yes.  It's all in the front ball joint.  Hub
>  center designs need far less rake, often only using 5-12 degrees (I don't
>  know the figure for the GTS, BTW).  But, when the swing arm moves up and
>  down, the rake changes by an amount based on the length of the front swing
>  arm.  Compression increases rake.  I suspect the GTS has more fluctuation
>  in rake and trail than a forked bike, and may be more sensitive to issues
>  of tire choice, etc.
>  
>  Kurt--a couple of comments.  First, dump the Dunlops.  Second, yes,
>  changing tires could change the trail.  With the front up (compressed)
>  you'd get more, not less, trail.
>  
>  Robt Taylor....the 60 v. 70 series on 130 v. 120 tires is not quite as
>  simple as my reading of your comments.  The aspect ratio is in relation to
>  the width of a tire.  The important number is the actual radius of the
>  tire.  A 60 series tire is only "smaller" (around) if it's compared to a
>  tire of the same width.  I'd have to do the math, but I'd bet a 120/70 is
>  pretty close to a 130/60.
>  
>  Does all of this help?
>  
>  Phil