They say the only stupid question is the one that isn't asked, so here goes. What is meant by "trail" when referring to the front wheel/forks?

 

Extend the line of neck tube to the ground out in front of the bike, now drop a perpendicular from the front wheel axel to the ground and measure the distance between the two. That is the trail of the front wheel. The axel measurement should fall behind the extended line of the neck tube.

Even though it doesn't look like it the front wheel thinks it is following the front geometry of the bike. So it is not being "pushed" but actually is "following". Therfore making it "stable", and it won't try to swap around evertime it is disturbed.

 

If you draw two lines one straight down the forks to the ground and one straight down from the axle the distance between those 2 points is the trail.

 

They are correct. This is what it looks like.

 

Good pic Gutman; here's another:

 

can't really add much more to that...looks like your question was answered

 

No. Draw a line straight down the steering head to the ground, and the other straight down from the axle as you stated - sharkey's picture illustrates it well. The fork tubes will always be offset from the head and give an inaccurate reading... even more so with raked triple trees.

 

Correct, use the actual centerline of the shaft/pin/bolt. to where it intersects the ground.

To put this whole 'trail' thing in 'grunt' terms.
You NEED negative trail. Think shopping cart, front wheels. They have negative trail. Meaning the axle is behind the pivot point. Roll like crazy in the 'normal' position. Steers good when you negotiate past that good lookin' MILF in the 'dairy' isle. In fact, you can prob'ly run with it. Specially when the MILF's hubby comes 'round the corner.

NOW, turn the front wheels on the cart around, 180*s. (Positive trail - axle in front of pivot) Try rolling it away. WOBBLE WOBBLE WOBBLE til it spins itself 'round. That's what happens when you get too much positive trail, folks. WELL CHIT MY PANTS. And ya prob'ly jus' did if that happens to ya.
There's a ton of websites that cover it to no end.

Course, when they were teaching this stuff in Geometry class, most of us that ended up building (or trying to build) choppers, were starin' out the window at the scooter going by. Then we Knew little or nothing of this rake and trail stuff. So, it was, cut the neck, put a 3/8, or a 1/2 in that gap you just opened up. Grab the stinger and weld away. Geometry?? That chits for the 'worms'. OOPS, That's 'geeks'. Worms is what geeks used to was back then.
More than one 'bit the dust' on those contraptions.

 

Thanks a million guys! Excellent explanations from everyone!!! And Gump...You certainly
have a way with words!!! I about fell out of my chair laughin'! Y'all ride safe and God Bless!!!

 

In this photo the trail distances are shown both in metric & US standard format...