[Thermodyne]

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.

 

[Thermodyne]

just out of curiosity ....
how many of you have built or ridden a long 60's style chopper?

Back in the early to mid 70's I rode a few. And had a Sporty with 10 over forks. That's all most people did to em back then. Put some forks tubes in based on the length of our legs. Geometry, what geometry? We pulled the front brakes to keep from spilling them, be we didn't really think too hard on it. Stretch the forks, chrome the frames, put a four foot sissy bar and king and queen seat on em, then wonder why the rear fenders and struts would always crack. Those scooters were all show and no go. If you rode em someplace where looks mattered, ya needed to get there an hour early. So you could wipe the oil off the shinny parts.

To give an example of how little handling was an issue, we ran car tires on the rears. Didn't take much of a lean to make them get real stupid and evil.

This was back when Harleys were right side shift. So you put a guy off a Honda or Yammie on a Sporty with long forks and right shift, they were usually a little overwhelmed at first.

Where I lived back then, Triumph was king in the late 60's early 70's. Mostly because the Local AMF dealers were only interested in selling new scooters and a guy named Sonny Routt had a local Triumph Shop in the area.

Any of you old enough to have seen one of these on someone?