rake and trail specs

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ahh, now knowing that the 850 yokes have a 1.342 degree built in angle. one can calculate the trail using the Atlas yokes.
 
Then there is the seat of pants school of engineering. As in, IF you stick early commando yokes *Atlas style) on an 850 frame, does it work, or does it display bad habits?
According to a Roy Bacon book I was reading last night, you can put 750 yokes on an 850 frame. Course he doesn't specify which 750 yokes, and all he may be saying is that you can bolt them on, which doesn't speak to how they will function.

Stephen Hill
Victoria, BC
 
madass140 said:
thanks for the drawings, as i expected and I think your drawings confirm this , that if you fit the Atlas type with large offset yokes to the 850 frame then the trail will end up very close to a 70-72 or 73 750. which is less than the standard 850 .
Click to expand...

The way I undestood it was that the trail was nearly the same between 750 and 850, the 1 1/2* offset on the 850 is there to bring the trail back to the same figure as the 750
 
Found this in the Service Notes. Not sure what to make of it. Sounds like the rake on an 850 is greater than a 750, thru a combination of frame and yokes.
Also sounds like if you put a 750 yoke on an 850, it would increase the rake more. And this is considered desireable. Sound Correct?

1973 Steering head angle (rake) change from 27 to 28° (yokes also changed).
In any case where a new frame is needed 1 would recommend the early 850 type frame (although the
change-over is a bit complicated for a pre-1971 model with Featherbed-type yokes and adjustable
steering bearings). The steering is more positive at high speed with 850 geometry, but to get the greatest
benefit you also need the 850 yokes (but the 750 type will fit and give the desirable increased trail).

Stephen Hill
Victoria,
 
madass140 said:
ahh, now knowing that the 850 yokes have a 1.342 degree built in angle. one can calculate the trail using the Atlas yokes.
Click to expand...

Im back at work tomorrow so if you give me the offset of the Atlas yokes I can do a picture of that as well. Sag and tire diameter have a reasonably big influence on all of this as well.
 
Stephen Hill said:
Found this in the Service Notes. Not sure what to make of it. Sounds like the rake on an 850 is greater than a 750, thru a combination of frame and yokes.
Also sounds like if you put a 750 yoke on an 850, it would increase the rake more. And this is considered desireable. Sound Correct?

Stephen Hill
Victoria,
Click to expand...
You can see from Cheesy's diagrams the actual rake is only defined by the angle of the headstock. If you draw a line through the stem to a point on the ground it is the starting point for measuring the trail. The other point being directly below the axle. The distance between them is the trail. The axle position can be moved, for any given tire size, and rake, by moving the fork tubes forward and backward by changing the offset of the yokes, or by angling the tubes in the yokes. Either way moves the axle so it changes the trail. On the 850 they tilt the tubes backward in the yoke, moving the axle backward and increasing the trail. You wouldn't think so to look at a bike with angled yokes where the tubes angle is different that it would have the same rake as it does with straight yokes, but it does, as long as the angle of the stem remains the same. Putting a 750 yoke on an 850 won't change the actual rake but it might look that way.
 
?Putting a 750 yoke on an 850 won't change the actual rake but it might look that way."

Maybe not, but it will change the trail and that is what I'm trying to ascertain, by how much.
 
I spent nearly an hour in the archives on this and can't find any definitive consensus about whether it is possible to mix and match earlier and later triple trees on earlier and later frames.

Stephen Hill
Victoria, BC
 
The second picture is a 750 frame with atlas offset

rake and trail specs
 
thanks for the efort, a little confused though in the 2nd picture , it shows the rake as 28 degrees does it not?
 
madass140 said:
thanks for the efort, a little confused though in the 2nd picture , it shows the rake as 28 degrees does it not?
Click to expand...

You can ignore that if you like, it is the effective rake with respect to the ground, the frame rake hasnt been altered, its still 27deg. Basically as the back of the bike gets higher that angle gets smaller or conversely if the front gets higher the angle will increase.
 
ok, thanks, so we now know what the trail is with the Atlas yoke on the 750 frame is, now the trail of using the Atlas yokes on the 28 degree 850 frame is????
 
did we determine the what the trail would be using the early Atlas type yokes with the 2-1/4" offset on the 850 frame???
 
using Foales calulator with these measurements
28 degrees (850)
2-1/4" offset (Atlas or early Commando)
4.10 x 19 K81 345mm Radius ( I just measured it)

ground trail 118.71
Real trail 104.81
 
I see someone mentioned 850s have alittle over a degree built into the yokes. Im assume using a straight yoke would yield quicker steering response?
 
Did some measuring a while ago on the yokes and fork of an 850. Also some tigonometry to figure out trail formula.
Forgot about it, than saw thread on tweeking steering head, now tabulated it all in excel.
Below a printscreen, including the formula of cell L5 used for the trail. Added some lines for random changes and effects.
Measurements were done with not too sophisticated tools like digital vernier caliper, angle block and tape measure, so don't hang me up on it.

rake and trail specs
 
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