I am planning a new HO layout to feature two levels and there are two questions that have come to mind so far. The first concerns using a helix to get from level one to level two. I need to know waht grade to make the helix and about how many turns it would take to gain 2 feet of climb? My second question concerns track grade through mountains. To gain 6 to 8 inches of climb, what would be the maximum incline that I could use?
The general “rule of thumb” is to keeps grades to 2% (I think lots of people might stretch to 2.5% on occasion). So, to climb 2 feet you need 24"/.02=1200" of climb, or 100 feet. (Suprisingly enough, 2% of 100 feet is 2 feet, funny that!) So, then you need to know your radius that’s available for the helix. The wider the better, since the curve has the effect of increasing the pull it takes to go up the hill. So let’s say you have a little over 6 feet available, so you can get a 36" radius. Each time around the helix will be 72*pi inches, or about 226 inches. So to get the 24 inches of rise you need about 5 and a third turns. Sharper radius, more turns. You can’t go too sharp, since you have to have eneough clearance between turns, that also means you can’t redure the grade too much.
As far as the mountain calculation, it is easier. At 2% you get 2" rise per 100" run, so you need 300-400 inches of run to get the 6-8 inches climb you want.
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Here’s a link to a Helix Calculator. You can download the program or use a quick link.
That takes all the fun out of it!
The important thing to remember is that the helix is always a compromise of grade, clearance, and radius.