Topic: Caster on 78F versus 77K

[doobiebro]

I am currently working on putting a dual disk 78F front end on my 77K and have a question about the difference in caster.  The 78F has 62.5 degrees caster as compared to 62.0 for the 77K.  Is this difference in the triple trees steering stem angle or is it in the frame neck?  I don't want to change the steering geometry so I need to know if I need to keep my 77K triple tree or use the one from 78F.  I prefer to use the 78F since it has the mounts for the dual horns and brake line splitter.  Thanks in advance for your help.

[zeus87]

I would like to know also....Ayone?

[eurban]

I don't know enough about suspension theory to tell you if .5 degree difference in caster will make any noticeable difference in handling or not.  My gut tells me that it is pretty insignificant, maybe as significant as slight variations in "same size" tire diameters from different manufacturers?  A .5 degree difference is going to be pretty small to observe / measure but you have all of the pieces in front of you, so why not take a stab at it and answer a question that no one seems to know the answer too?  On the work bench you should be able to slide the fork tubes through the upper and lower trees of the k and f.  Tighten the pinch bolts and make sure they are essentially free of twist.  Keep the conditions are the same (same fork tubes, same exact spot on bench, etc etc) with each set of trees and measure the angle of the steering stem.  Perhaps a precise measurement of distance from the end of the stem to the work bench or a piece of metal laying on the work bench?  Is there any difference?  If not then you know the difference is in the steering head on the frame. . . .Or you could just not worry about it and install the F pieces in your k frame.  It has been done a number of times by others and seems to work just fine.

[rbmgf7]

i believe the correct term for a bike is "rake" 

[eurban]

The factory manual uses "caster" angle in its specs.  Here's some info from Wiki . . . .


"The steering axis angle, also called caster angle, is the angle that the steering axis makes with the horizontal or vertical, depending on convention. The steering axis is the axis about which the steering mechanism (fork, handlebars, front wheel, etc.) pivots. The steering axis angle usually matches the angle of the head tube.

In bicycles, the steering axis angle is called the head angle and is measured clock-wise from the horizontal when viewed from the right side. A 90° head angle would be vertical. For example, Lemond[1] offers:

    * a 2007 Filmore, designed for the track, with a head angle that varies from 72.5° to 74° depending on frame size
    * a 2006 Tete de Course, designed for road racing, with a head angle that varies from 71.25° to 74°, depending on frame size.

Example of a chopper with an unusually large rake
Example of a chopper with an unusually large rake

In motorcycles, the steering axis angle is called the rake and is measured counter-clock-wise from the vertical when viewed from the right side. A 0° rake would be vertical. For example, Moto Guzzi[2] offers:

    * a 2007 Breva V 1100 with a rake of 25°30’ (25 degrees and 30 minutes)
    * a 2007 Nevada Classic 750 with a rake of 27.5° (27.5 degrees)"

[doobiebro]

Thanks for the clarification of terminology.  I am sometime unsure of the exact wording as I know just enough to be dangerous, as some might say.  The Wikipedia description is very helpful in describing this for someone like me.  And the last sentence of the first paragraph that states "the steering axis angle usually matches the angle of the head tube" seems to indicate that the angle is indeed in what I called the frame neck.  I will consider trying eurban's suggestion of testing the difference between the two on the work bench, because I am somewhat anal about the little things, but I agree that it may be too small to observe and/or measure.  After all, the factory manual shows the difference in trail is only 1.5 mm.  I doubt that difference could cause me to loose control or fall over in corners lol.

[dusterdude]

eric,caster is the term for you k8 owners.

[eurban]

Naw, it's "Master" of the K8, or did Eldar already do that??

[dusterdude]

yea eldar beat you to it