[Rollmeaway]

I am wondering what the question is in your thread.
You are correct in stating that fork length does not change rake angle or trail. Fork length is measured from the top of the fork tubes to the front axle centreline.
Raked trees of over 3* are typically not recommended by most manufacturers or retailers for the reason as you increase the degree* of rake by using raked trees you infact decrease trail. Trail is what will or will not allow for a stable ride.
The proper way to increase the rake is to have the frame neck cut, repositioned & then rewelded.
Raked trees are actually used to reduce trail on a frame raked out so handling is not lazy & more responsive

Most Softails run a 31-33* raked frame with a 3* raked tree for a trail of approx. 5.1-5.75"
Dynas 28-32*
FLH uses 26* ... while Sporster use a 29*-30*

 

[Rollmeaway]

A little while back I was in contact with RB Racing to find out from the pros what I could do to increase the rake on my bike & still have a safe & stable ride.

They stated & quote " On a 2000 FXST with a head rake of 31* you can safely go to 3* raked cups, a 3* raked tree & a 3" extension on the fork tube length"
No need to be concerned with handling at all.

By doing so I now have a 37* rake.

 

[Rollmeaway]

Increasing rake (laying the fork back at the top) increases steering axis inclination. The greater that number in degrees, the more the wheel rotates on the center-line and the less it yaws. The center line rotation is unwanted.

Adding rake to the trees increases the rotation for a given degree of rake. Example would be a scooter that had a 40 degree steering head and 0 trees having less rotation than a scooter with 32 degree steering head and 8 degree trees. This can be corrected by properly designing the tree so that the upper fork mount is behind the steering head center-line. But this introduces sweep into the equation, so is only used where steering head angles are nearly 0 to perpendicular with the ground or with difficult to steer rigs like side cars and trikes. And usually they just alter rake, lacking the space to back mount the forks.

Adding fork tube length alone does alter the rake, assuming all the addition length is below the head. The longer tubes rotate the frame so as to increase the measured rake. Your confusing advertised rake and actual rake. Advertised rake is measured with the frame at what ever the maker says is level. Usually the lower frame bars are level to the ground, but not always.

Rake and trail are for two dimensional equations and best suited for working with paper designs. Front ends or axles to be precise move in three dimensions. And while the rules of trail still need to be applied, you can have a seriously ****ed up front end with good trail. And you can have a front end that rides just fine with a seriously bad trail measurement.

When you look at the first drawing, there is no value for the distance that the fork center-line is forward of the steering head center line. It only shows the angle and the amount of difference as tree rake. The offset of these center-lines is as important as any measurement on the whole set up. But everyone ignores it. And some tree builders will increase it to gain clearance on wider trees.

That is so true & a very good point. Offset is not to be ignored. There is far more to a stable front end that handles as it should over one that doesnt.
I have always said & live by "anytime a front end is altered it must be checked before riding so you know for sure where your at"
Trail is the make or break measurement, it should never be a guess or bygolly.

 

[Rollmeaway]

I have to confess I didnt start til 1972 so you've got a few on me.

 

[Rollmeaway]

I started few years later than you, but that wasn't the question.

Its like I said I didnt ride & build til '72 so the 70's style choppers were all I rode. I would hope no further explanation is needed to grasp what has already been said.

Im not into explaing who's hammer or wrench is bigger & besides I thought this thread was about steering/front end geometry.

Maybe I misunderstood your intentions on geometry.