Here is a visual about trail - and how rake and angled triple trees affect trail. You can see in the image how trail is measured.
More trail results in the reduced ability for either variances in the road surface, or the rider pushing on the handlebars to change the direction of the bike. Choppers have high-speed stability - you can easily do "Look Ma, no hands".
Less trail means enhanced ability for the road surface or rider to change the direction of the bike. Motocross bikes are very manuverable, but you must keep your hands on the handlebars at all times.
- Trail - effect of reduced frame rake and angled triple trees
- trail.jpg (92.64 KiB) Viewed 16100 times
Look at the image, and notice what rake and angled triple trees do not only to the trail measurement, but also to the wheelbase of the bike.
The 1952-1953 K had a reduced rake, and angled triple trees. Both things reduce trail, and doing both together make this a very maneuverable bike. Too maneuverable in fact, so in 1954, they increased the frame rake to 29-3/4 degrees. This increased the trail, making the bike less maneuverable, but more stable.
In 1955 (very late 1954) they changed to parallel triple trees. This made the bike even more stable, and even less maneuverable. But these are road bikes, not motocross, so this configuration of rake and triple trees was the best, and pretty much what was used from then on. In 1956, they lowered the frame a little, and increased the rake 1/4-degree to 30 degrees.
One thing you can't easily see in my crude images is the effect on the front of the frame that the rake/triple trees have. The reduced rake causes the front of the frame to rise upwards a little bit. (To visualize, imagine shifting the rake to 0-degrees). The frame is no longer parallel to the ground. The angled triple trees have the opposite effect.
In 1955, the change from angled triple trees to parallel triple trees caused the front of the frame to rise about 1 inch. To bring the frame back to parallel to the ground, they reduced the length of the fork tubes 1 inch.
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This is a very complicated subject, since every little change has an effect on everything else. Changing the frame rake changes the wheelbase and changes the height of the front of the frame. Changing the height of the front of the frame actually changes the "effective frame rake" a little. To visualize, imagine a bike with 6-
foot over fork tubes. The front of the frame is now so high that if you measure the angle between the frame neck and the ground, it's about 85-degrees !!!
Frame rake and angled triple trees have a big effect on trail. Other things have an effect too. The change from 19" wheels to 18" wheels had a small effect on trail. Other things such as longer/shorter fork tubes, taller/shorter tires. Using a taller front tire than the rear. Not applicable to us are things like offset triple trees - where both the top
and bottom of the fork tubes are closer or further away from the frame neck.
I don't know how modern engineers design steering geometry. But back in the early 1950s, Harley engineers used the trial and error method. After lots of errors with the K-models, they finally got back to the same geometry they used with the VLs, Knuckleheads and Panheads. Duh.