Truck Spring Planks
The springs selected for the trucks are McMaster-Car #9657k134 shown in drawing above, 8 springs per truck. Under no load the springs are compressed 1/8" for a force of about 55 lbs upward on the spring plank. When fully compressed at about 3/8", the spring force on the spring plank is about 160 lbs. When used under a loaded skeleton log car, the car plus load roughly equals the 55 lbs force per truck. A passenger will compress the springs somewhere between the 1/8" compression and the 3/8" compression. A ~ 200 lb passenger centered on the log car will fully compress the springs in both trucks..
Lower Spring Plank
The lower spring plank is a 11 7/8" length of 3/8" X 1 1/2" Hot Rolled Steel. A 2" length of 1/2" square CRS bar with 1/4" hole is welded to the middle underside of the spring plank. (If welding is not available a 1" X 1/2" bar with 1/4" hole can be substituted for the square bar and 4 6-32 SHCS used to attached the bar to the spring plank. ) The spring plank in then sawed in half. A 1/4" SHCS with nyloc nut though the hole in the 1/2" square bar forms a swivel.
I clamped the arch bar supports to the spring plank and after they were adjusted to the correct position used the holes in the cast arch bar support tabs to spot the mating holes in the spring plank. The holes were then threaded 4-40 and the arch bar supports attached with button head cap screws.
Four 3/8" diameter 3/8" long aluminum rods with 6'32 SCHS through the center are used as studs to hold the inner four springs in position.
The photo above shows a bottom view of a finished lower spring plank with ach bar supports attached.
Upper Spring Plank
The upper spring plank is a 11 7/8" length of 1" X 1 1/2" hot rolled Steel bar stock. The 3/4" diameter hole in the center is for a 3/4" OD - 1/2" ID bronze flanged sleeve bearing. A 1/4" thick shim washer is placed on the bearing to raise the top to the correct height of 3 7/8" above the rail head. (The wheels I purchased were nearly 1/4" oversize so I actually used a 1/8" thick shim washer.)
I first drilled the holes for the springs with a 1/2" drill with the tip penetrating about 9/16" The holes were then finished with a 9/16" end mill accurately positioned on the milling table.
The car support blocks at the ends of the plank should match the center bearing plus shim --- normally 3/8" As mentioned above, my wheels were a little oversize so I made the blocks 1/4" high. The blocks are securing with 4-40 SHCS in the bottom of the holes for the outside springs. The screws are located as close to the ends as possible. I drilled the holes in the plank first and then used those holes to spot the holes in the supports. The 1/32" deep slots in the sides of the plank permit the plank to rotate slightly to accommodate uneven track.
The studs in the lower spring plank hold the inner four springs in the correct position which in turn hold the upper spring plan in position.
A couple photos of a finished upper spring plank is shown above. Unfortunately, the fresh black paint makes it difficult to see the few details.