Raised on the Erie Lackawanna Mainline- Supt. of the Black River Transfer & Terminal R.R.
The plywood will form a smooth cruve (and here's the catch) for the curve YOU bend it to. If you are using 3/4 in ply it probably won't bend quickly enough, but if you are using 3/8 or 1/4" you can certainly bend it way too sharp for a Big Boy. So if you are planning that steep of grades with really long engines, you ought to mock up the vertical curve, put the engine on it and see what you can get.
Dave H. Painted side goes up. My website : wnbranch.com
We would all like to help you, but we need to know the area size of your proposed HO layout. Where are the doors and windows, etc? If you propose to run a Big Boy, the maximum grade cannot be greater than 1.5%, with a radius of curve not less than 24 inches. What is your era, and theme? Are you planning a point to point, around the room, or what ? Do you plan harbors, rivers, mountains, tunnels, urban areas, etc.etc.? Post a diagram of your room and proposed layout. If you need help in Hosting and Posting, let us know, so that we can be of help. Bob Hahn
If using 1/2" or thicker plywood, my experience is that you can't bend it sharp enough to cause problems - but I haven't used anything bigger than Mountains and Pacifics and full length passenger cars with standard over-size knuckle couplers.
I will probably have to back down to 1/4" plywood for my switchbacks with peak 8% grades to get enough curvature. I anticipate risers as close as 7"-12" to make this happen. Longest car length would be a 50ft combine, longest locomotive a 2-8-0 (HOn3). Transitions will be about 14" long (2 50ft car lengths). This will be mocked up in advance to ensure couplers don't under- or over-ride on such a transition. I also have to make sure that the rail is curved in the vertical plane to ensure it matches the planned transition.
A caution if your 3.5% grade is on a curve. Plywood doesn't like to bend in 2 dimensions simultaneously. I had a situation with a 4% grade on a 180 degree 18" radius curve cut from a single piece of 1/2" plywood. The result was some visually apparent negative superelevation on the first 90 degrees of the curve, and visually apparent positive superelevation on the second 90 degrees of the curve. It didn't cause performance problems for my short trains and cars, but looked terrible. It took the addition of several cleats and risers (with some clamping force) to "twist" the plywood subroadbed into being level across the curve instead of the defacto negative and positive superelevation.
Although my switchbacks are an extreme case, I have seen situations with lesser grades where the track was not curved vertically to match the transition. The vertical rigidity of the track caused all kinds of derailment and uncoupling problems at the vertically kinked track joints, even though the subroadbed had a nice transition built into it.
just my experiences
Fred W
The flat section is a good idea, just like the straight track between two S-curves. Beyond the flat section, you will need some length for trasnition from level to the desired grade. Three-quarter or even half inch plywood probably is stiff enough to keep you from changing grade too quickly.
But that means it will may take more running length to get into the desired grade than the published plan you are working from allowed. (Many published plans assume one can go from 0% grade to 3% grade instantaenously -- which just doesn't work.) Since you have a little more space, you may be OK, or you may end up with steeper grades than planned.
Your very best bet is to clamp the risers first along a given length of grade, try out the grades and the transitions at each end with your longest rolling stock and combinations of long and short rolling stock, then tweak and tune until everything works well. At that point you can screw the risers in place and move to the next grade.
You might want to start with the steepest grade, then work in each direction from there, otherwise all the tweaking and tuning elsewhere might build up to make the steepest grade way too steep if you tackled it last.
Byron
Layout Design GalleryLayout Design Special Interest Group
The way I lay out a cookie cutter grade (assumes a single piece of plywood for the entire grade) is to clamp plywood flat for 2 risers at both top and bottom. You would want this flat area (can be on a grade, but cannot be changing grade) to be at least one of your longest car or locomotive lengths long. This is to prevent the locomotive or car from trying to transition between 2 grades at once. The plywood should form a natural S curve between these 2 sets of risers.
If I need more risers to prevent the plywood from sagging, the next riser goes at the mid-point of the grade, at half the total rise. If need be, add more risers on either side of the mid-point, but avoid adding them in the vertical transition area if possible. The idea is to let the plywood assume a natural eased vertical curve for your transition rather than an artificial curve constrained by the riser. Bent plywood is far more resistant to sagging than flat (learned from boat building), so risers can be spaced further apart in the transition area anyway.
If you need more than one piece of plywood for the grade, you want to put the splice in the middle, flat portion of the grade. Splices in the transition areas will ruin the natural curve of the plywood.
my experiences, your choices
i've used 1/2 inch plywood for decades and never had a problem with vertical curves even while running full length passenger equipment in HO scale. potential problems are if there is a joint at the top or bottom of the grade and a long 3/4" thick splice underneath will often take care of that.
as for "twist", just check across the roadbed with a small level and adjust your risers accordingly. you can also shim under the roadbed with cardboard scraps cut from a cereal box or something like that.
grizlump