A brief comment on railhead-to-railhead clearance, grade and curvature:
For absolute minimum roadbed/tie/rail thickness, try forming a steel stud into a segmented trough and laying flex directly on the steel with acrylic caulk. With my HO code 100 ‘hidden track standard’ rail, the total thickness is under 1/4" and I can get away with 3.25" (57mm) railhead to railhead, with NMRA standard 3" overhead clearance (which is excessive for my prototype.)
Being in HOj, I’m not sure what grades N scale trains can handle. My ‘normal’ hidden track grade is 2%, visible mainline (when built) will be 2.5%, and the branchline will be 4% (and trains of four-wheel cars will have one driver axle per car upgrade.)
Going metric for curvatures, I don’t see 330mm as excessively sharp for N scale - it’s equivalent to 660 mm in HO, and that’s close enough to 26" not to matter. Wider would be nice, but on hidden track it may be an unnecessary luxury - as long as the curve is entered over a proper spiral easement. Having done the necessary testing, I’m satisfied that the teakettles (0-6-0Ts and one 0-8-0T) and the 2-6-6-2T will be able to handle the helix I’m building now - 3% grade, 350mm radius, 500 degrees total curvature - and that’s in HOj, 1:80 or twice N scale.
When it comes to track geometry, an ounce of testing trumps a ton of opinion. Mock it up, try your own rolling stock and the dictates of your chosen prototype (or home-grown) operating standards, then decide what to do based on cold, hard facts.
I am planning to use 2.5% GOS on 16 inches radius to helix edge (or 15 inches to track center). This gives me good clearance with 0.5 inch margin over my tallest car.
Here are the snapshots from my stacked helix calculator for a 2% and 2.5% GOS on 16 inches radius.
2.0% GOS
2.5% GOS
At 2.0% GOS, the effective grade on track is 2.02% while a 2.5% GOS, the effective grade is 2.55%. While 2% is recommended, pushing it to 2.5% but not more than 3.0% is probably doable. Like you said, mock it up and test out.
It seems that with minor modification, one double-track helix would be sufficient, so need for second helix could be eliminated.
There is room, particular for the helix under Algoa, for straight track sections to be added (giving the helix a semi-rectangular shape rather than semi-square) so that the grade in the helix could be reduced significantly.
Just a thought for saving headspace in your helix:
Use 1/4 inch hardboard for the road bed, perhaps covered with 1/16 inch cork to deaden the sound and cut a groove in the vertical supports (inside and outside) into which the hardboard is glued. Don’t forget to add a lip inside and outside between the vertical supports to keep derailments contained.
I’d be a little careful about using 2.5" rise per turn with N gage. I think you would want to provide side protection inside and outside to prevent derailments from ending up on the floor. Even without this, 2.5 inches is a pretty small space in which to get to and realign the occasional derailment that stays on the roadbed.
Further to this tread. An excellent Helix article in May/June 2003 N-Railroading, N scale Helix helper. Thank you to everyone for their input, have used a bit of all ideas.
Has anyone on the Forum had experience with the commercial helix systems offered by Ashlin? They had a new product announcement on page 18 of the May 2009 Model Railroader. See also http://ashlintrains.com.
This is probably going way overboard; however, to reduce distance between levels to a minimum, consider hinging the outer derailment guard to provide access when needed and install light beam sensors near the helix entrances to shut off power if the consist includes an oversized car or load, like they use on the cashier checkout counter at the supermarket. I would think Radio Shack might have the gear, which could be hidden in scenery, buildings, etc.
