Hi
It sounds to me like you have a basement that is almost finished and you are thinking about a helix. Good time to think about it. Get the helix positioned and pretty much cast in stone now as you probably will not be able to modify it afterward. The way most MRR's think about a helix is a transition from one level to another with a *big* footprint. You were talking about 4' square which would give you about a 22 radius assuming 2 inches on the outside for clearance. Grade is calculated as rise/run * 100 for percentage. With your numbers, run is 2 * PI * radius ( 6.28 * 22 ) or about 138 inches. If you have 3 1/2 inches from railtop to railtop then your grade is (3 1/2 / 138 ) * 100 or 2.5%. You need to transition from a lower level to upper level of 20 inches. Is this height cast in stone and can not be adjusted? Now is when you think more about this. If you want to keep a certain grade maximum, AND a certain helix loop seperation ( 3 1/2 ) then you can simply calc 20 divided by 3 1/2 which gives you just over 5.7 loops @ your grade selection of 2.5% I don't know how your helix start/end point are to be. If they need to be directly above/below each other, then you will have to adjust some of the above calcs. Searching and reading about how others did their helix, I found that 3 1/2 inches is about the minimum spacing given room for trains, fingers, sub-roadbed, roadbed, ties and rail. Since is sounds like you only want one track in the helix, you can probably fudge the 3 1/2 downward a little.
Is this helix going to be hidden? If so, you might consider this also. You could have almost 70 feet of track that is not seen. I have found that when you don't see your train because it is in a hole, the throttle gets bumped up; sometimes multiple bumps. The train then shoots out of the hole; Casey Jones is to blame for that speed.
Is the train going to be stopped for some reason or another while in/on the helix? I don't know your train length, but at that grade you may see some issues.
So this brings you to building a helix before you build a helix as you need to test your grade vs train length, start and run. The helix can not fail or cause you any problems at all or it will become worthless. Well, maybe a very occasional derail would OkieDokie to deal with.
It might be better to consider another way to transition, or make the helix bigger. By making the helix footprint 6 x 6 you loose some train room space, but you gain a reduced grade and a longer radius. Assuming a 34" radius AND 4" between loops, your grade would be about 1.8% with room for train, fingers, sub-roadbed, roadbed and rail. You still need to deal with the 'train in a hole' syndrome as you now have about 90 feet hidden.
Ok, so how about this? By adding 2 additional tracks to the helix, you now have serial storage capability. Assuming 2 inch track center line separation, the inside track is about 188 inches with a grade of 2.1% which if used as 'down storage' there should not be any problems starting and running a train. Middle track is about 1.9% and should be OK also. If you assume two trains per loop, you now can store ten 14 foot trains with the center track as a 'continous running' section.
There is an additional problem here as either you or one of your operating crew would have to move trains up/down to open up a 'spot' for the next one to go into storage. Don't worry, I am currently working on that problem!
Have fun!
ctclibby
