To place the wheels of the same size is quite simple – all offset the same distance. However, for the large wheel to make contact with the ground, the offset has to compensate for the increase in diameter. To mark the position I simply place two wheels either side (note the holes are already drilled) and rest a piece of wood against them, then place the large wheel so that it too is touching, and mark the hole accordingly with a center punch.
Clamp the two rails together and drill the holes.
Now to make the clearance slot in the base – place one large in place, and mark the distance as shown in red in the photo. It is a simple matter to transfer these marks onto the base.
Cutting the slots in the base is very much like cutting a mortise. The photo shows how not to do it. Thank goodness for wood filler.
This is perhaps a better way – small shallow cuts in a slicing action with a very sharp chisel rather than the brute force way of banging away with my favourite (home made) mallet.
Once the clearance slots are cut and trimmed, and in my case, filled with wood filler and sanded, the rails can be glued in place.
-- Tony, Australia, http://www.wooden-toy-plans.com/