Double Helix?

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EngineerKyle

Member
Oct 3, 2005
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Troy MI
Hi Crew! :wave:


The double helix…. I could use some technical advice and general specifications for a helix…. I’m planning a new section and I’d like it to be about 12-18 inches higher than my current layout. I figured I’d put a helix in the corner, disguised and a hill or something, to get my trains up to the new level. I was wondering if anyone has built a “double” helix (and I don’t me Watson and Crick) so that a train could go up the helix, say on a 22” radius, run around the upper level and return down the same helix, on an inner, 18” radius line?….. I thought I would cut some C shaped pieces of luann (that thin plywood sub-floor) and splice them together for the base surface of the helix. I have seen some pics of helixes that include threaded rod in their design. I thank you in advance for helping me get this off the ground.

Kyle
 
We just put a helix in at the club, it is not as complex as the one you are concidering. Ours is plywood base with homosoate then cork and track. We made ours 2 percent grade max. We build a base then just used wood sticks up the sides and screwed the plywood to it with wood screws when all was at the right slope. We made a little jig, cut a peice of wood ( we used a 2x4 on its side) at the 2 percent angle, then put a small level on the narrower top flat spot..so you raise the wood base, place the jig on the base, and when the bubble is centered you have your proper grade,then secure the wood, of course work in short spans. You could make the jig to what ever grade you wish to use. With yours it sounds like you have one helix within another one. I would build two separate ones, the inner one first then the outer around it. be careful about enclosing it with a discise of a hill unless you have good access..remember that as soon as you can not reach it or inclose it you WILL have a problem.. Ron..
 
Kyle, if I'm understanding you correctly, you just need a double tracked helix, no problem there. However your radii choices are too tight. The grade needed for those radii will be quite steep, and combined with the sharp curvature, will severly limit the # of cars you'll be able to haul. There was another post about helix's last week, here is a link I provided in that thread.

http://www.the-gauge.com/showthread.php?t=1709&highlight=helix
 
Kyle: I second the post about the radii being too tight.
Careful with the threaded rod. Problem is that when you put nuts and washers on them, they tend to flatten out the roadbed of the helix. Would be really noticeable with luan.
If you can dado, cut slanted recesses in some small boards and use them to separate the levels. If you cut them all at the same time, you get equal spacing. Vertical adjustment comes after the helix is assembled.
calculation: 18" radius takes 113" of track for a circle. Close enough to 100" for this. If you go up 3.5" per circle, your grade will be around 3.5%. Can you accept this?
 
David,

3.5% is a little robust... and just about everyone I've talked to agrees that I need to plan on bigger radii. There is a little problem with this in my case however, as I use Atlas code 100 and don't really care for using flex track unless it is more or less for a straightaway. Does sectional track come in radii other than 18 and 22?

EK
 
I believe Atlas Code 83 has sectional track in 24" radius. Tru-Scale, if it's still made, had a lot of different radii (Code 100). Kato has quite a wide range of radii, and has the roadbed built in. Not sure what Walters/Shinohara has in sectional.

But even if you could find sectional track of the radius you want, it is NOT recommended for a helix because of the number of rail joints. You are looking at 16 sets of joints per circle. With flex track, on a 25" radius, you are looking at 5 sets of joints per circle.

The larger the radius, the easier it is to lay flex track smoothly - as you pointed out. So you might consider the 22" or 24" radius sectional track for the inner helix, and flex track laid to a 3" larger radius on the outer. Get a Ribbon Rail radius gauge(s) of your radius choice to help you get a smooth consistent curve in flex track.

my thoughts, your choices
 
Re: David's calculations.

With a helix, the rise is pretty much set - or at least there is a minimum. Therefore the only way to get a smaller grade is to increase the run. That means a bigger radius for the track.

NMRA minimum clearance from the top of the rail to the bottom of whatever is overhead is 3". So you still have to add the height of the rail, and roadbed if you use it. So 3.5" is a good number.

If you want to have only a 2% grade, while climbing 3.5" with every turn, you will need a run of about 175". That equates to a circle with radius of about 28" (if all my math is correct).

Andrew
 
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