I have decided to just build a more or less standard leading link fork. This is the style I am following.

I am using triples from a 4V K100. Fork tube size is 41.3mm (1 5/8"). For the upper tubes, as in the photo, I will use 1.75" x .375" wall DOM tube. The length from the top of the top clamp to the bottom of the triple tree, will be turned down to 41.3mm. That will leave .250" wall thickness between the clamps. The tube remaining below the bottom clamp will be an appropriate length to weld the fabricated rear legs to. I still have some dimensions to work out. That said, can anyone get me a length measurement of the fork length from the bottom of the triple tree to center of the axle (I'm ignoring the 5mm(?) offset of the axle hole from the centerline of the lower leg)?

You are a lucky boy, I'm currently working on such a fork.
I measured 553mm from the center of the wheel axis to the side bottom of the lower triple tree (where the tube sits, not where there's the steering axis.
The fork is unloaded, on the table.
But I'm wondering how useful this measurement could be...
The very important thing in a side car leading link fork is the ground trail and the rake (angle of the steering axis). This last one should not be modified, but the purpose of such forks is to reduce the ground trail (but not to supress it!).


You first should decide how high you want your rig to be. The lower, the better, for road holding, and to preserve mechanical elements from stress they're not supposed to cope with.
Then, according to the solo bike value of the ground trail, decide what should be the sidecar ground trail, somewhere between 0 and the solo bike value.
It should be something like this:


Note that the pivot of the swing arm must be lower than the wheel axis, or the wheel will bump into obstacles, because it will have to move ahead to pass upon it.
You may create some adjustable pivot points for the swingarm (orange points in the figure), it will make easier to set the ground trail.
I strongly recommend to reinforce this swingarm, as it will twist due to the side forces applied to it.
I also recommend to avoid mounting calipers directly on this swingarm, because its torsion may broaden the space of brake pads. I always had issues with this type of solution. Always solved by mounting the calipers on oscillating plates, pivoting around the wheel axis, and linked to the fork with a rod and ball joints.

One of the effects of Earles fork is that it tends to lift the front when braking. It's not necessarly a disaventage. You can control this by misaligning the parallelogram that the calipers plate, the linking rod, the fork tube and the swing arm are making. Move upwards or downwards the linking rod pivot on the fork tube to controle, enhance or reduce the effect.

Last recommendation; build very, very solid!

jbt wrote:You are a lucky boy, I'm currently working on such a fork.
I measured 553mm from the center of the wheel axis to the side bottom of the lower triple tree (where the tube sits, not where there's the steering axis.
The fork is unloaded, on the table.
But I'm wondering how useful this measurement could be...
The very important thing in a side car leading link fork is the ground trail and the rake (angle of the steering axis). This last one should not be modified, but the purpose of such forks is to reduce the ground trail (but not to supress it!).


You first should decide how high you want your rig to be. The lower, the better, for road holding, and to preserve mechanical elements from stress they're not supposed to cope with.
Then, according to the solo bike value of the ground trail, decide what should be the sidecar ground trail, somewhere between 0 and the solo bike value.
It should be something like this:


Note that the pivot of the swing arm must be lower than the wheel axis, or the wheel will bump into obstacles, because it will have to move ahead to pass upon it.
You may create some adjustable pivot points for the swingarm (orange points in the figure), it will make easier to set the ground trail.
I strongly recommend to reinforce this swingarm, as it will twist due to the side forces applied to it.
I also recommend to avoid mounting calipers directly on this swingarm, because its torsion may broaden the space of brake pads. I always had issues with this type of solution. Always solved by mounting the calipers on oscillating plates, pivoting around the wheel axis, and linked to the fork with a rod and ball joints.

One of the effects of Earles fork is that it tends to lift the front when braking. It's not necessarly a disaventage. You can control this by misaligning the parallelogram that the calipers plate, the linking rod, the fork tube and the swing arm are making. Move upwards or downwards the linking rod pivot on the fork tube to controle, enhance or reduce the effect.

Last recommendation; build very, very solid!

My R90/6 is already a rig. It currently has a Perry Bushong offset triple tree (3.5" offset). The stock front wheel is 19". The front tire is a 3.50/19 Avon Triple  Duty sidecar tire. The stock rear wheel is 18". The pusher tire is a Metzeler 4.00/18 Block K sidecar tire. The sidecar wheel/tire is an unknown 18" Ural rim and tire. I am trying to get all three wheels to be the same size so I can run the same tire (4.00/18 Metzeler Block K) on the same wheel. I have a K100 front end (41.3mm) with dual 320mm full floating discs and dual 6 pot ISR calipers on the 18" Y spoke wheel. The 18" K100 front wheel has a 25mm axle, so I can't use that wheel in the R90/6 forks (17mm). The K forks are much more robust than the /6 38mm forks. The late K triples are also much more robust robust than the /6 tree. The current trail for my rig is calculated (modeled in CAD) to be 1". My rig steers very well. Thinking about it, I think I need the laded length as opposed to the unloaded length.
I picked up a late K100 triple tree. The 41.3mm forks are certainly more robust than the /6 trees.

OK, so if the geometry of your rig is OK for you now, don't change the setting!
Screw the front wheel in a template that bolts on the frame, remove the old fork and build the Earles fork around that.
Don't be afraid of using a /6 triple tree. OK, the upper one is thin, but the lower one is thick and very solid. As you will ad transversal links between the two tubes, it will fear no torsion.

For the loaded lenght, I'm afraid you'll have to wait for a long time 'til my bike is finished.
But I have noticed that the OEM fork springs for K4V are pretty weak, allowing only 1/4 of the total travel when loaded.
But I did not understand clearly: do you want to keep fork tubes and stanchions as shocks? Or mount external suspensions?

No. I am building a normal LL fork.