How do I mount a TORTOISE™ on very thick (2” - 4”) subroadbeds?
The TORTOISE™ design was optimized for about a 1" total subroadbed and roadbed combination such as one would find with 1/2" plywood and 1/4-1/2" cork or homasote roadbed. It will work fine with greater and lesser thicknesses, but you have to be aware of certain issues that are raised.
Ideally, we want the total throw of the wire at the points to be just a little more than the point's movement themselves. In this way, the spring wire will provide modest tension on the points to keep them closed at each side. Lesser thickness really doesn't create a problem because the fulcrum can be moved down a bit to compensate. Greater thickness can be a problem because the extra length of the wire above the fulcrum will create a situation where the end of the wire is trying to move a much greater distance than the points are able to. Although this just creates longer tension zones on either side, what will happen is that the arm on the TORTOISE™ will start moving down below and this will then start releasing the tension on the points. Then, at some point, the points will move rapidly across and contact the other stock rail and then the further motion just adds more tension on that side. Where the problem can occur is that if you are using the internal contacts to power your point rails in any way (this was common with the older Walthers/ Shinohara turnouts but is no longer an issue with their DCC-friendly models), the points need to break contact with the stock rail BEFORE the TORTOISE™ changes connections internally at the half way point of its throw. Thus the switch machine needs to be mounted PERFECTLY centered in order to insure that a short circuit does not occur. As mentioned, this is not a problem with newer designed turnouts that are "DCC friendly". These turnouts are designed to have isolated frogs and only the frog polarity needs to be changed. You will not get a short circuit using the TORTOISE™ with these. So really the greatest limitation may just be that you lose the slow smooth action at the points that the TORTOISE™ is capable of and it is replaced by a rapid traverse. This is not much of a big deal and we know of many modelers using the TORTOISE with 3" of polystyrene foam as a layout base. We DO recommend the addition of a 1/4" or 3/8" plywood plate fastened to the foam with adhesive to give a good solid mounting base for the switch machine. You will also need to substitute a heavier gauge spring wire to compensate for the additional length. Try 0.032" or 0.039". This heavier spring wire is available from K & S Engineering through Walthers or through their on-line retail division, Special Shapes (www.KSMetals.com).
Another possibility with thick subroadbeds is to use our Remote TORTOISE™ Mount (RTM). The RTM uses a similar final connection to the throwbar as the old Earl Eschelmann link but the tube and wire we provide are adequate only for a couple inches of thickness maximum. You can get your own tube and wire from KSMetals on line and then there is no real limit to thickness. So that is another way to go, but the Remote mounts, extra brass tube and wire will add extra cost to the install.
Here is another possibility, never tested. Drill down from your throwbar hole location from the topside of the layout. You are going to have to get a very small but 6" long drill bit for this. Make sure the hole is as perfectly vertical as you can make it. Now make a mark of some sort on the shaft of a 1" diameter wood spade bit. This mark should be calculated so that when you drill up from underneath, you will have about 1.5 inches of foam remaining above the flats of the bit when the mark is at your plywood. Drill the large hole up going very slowly in the foam to try and create a smooth flat bottom hole. Remove your drill. You will then have to go the rest of the way with a 5/16 or maybe 3/8 bit up to the throwbar. Be careful with that part but going just through foam, you may be able to do that by hand. Now, find some 1" diameter discs of some sort or cut them out of styrene or thin metal..... whatever. You want a round disc to match your larger hole. The disc will need a small hole (just a little larger than the wire diameter) in the very center for your wire. If you use thin plywood, we would recommend saturating the center hole with thin Super Glue and shoot it with "kicker" to form a more rugged plastic surface. Use some foam compatible adhesive (white glue would be fine with a wood disc) and glue the disc up into the flat bottom of the big hole. Then just mount the TORTOISE conventionally without using our plastic fulcrum. As mentioned elsewhere, you should first mount a 1/4” plywood pad to the underside of the foam for the TORTOISE™ to actually mount to. Use 0.039" wire and you should have pretty good action topside.
Circuitron could probably cut those discs on our laser engraver out of thin plywood for you if you want. Since this would be a custom design we would charge $20.00 for up to 50 pieces. You would need to send us a check or money order, we are not set up to take credit cards.