Model Railroading > How tight is too tight for a helix

The biggest radius I can possibly squeeze into my layout without major alterations is 21". The question here is, would this work. Most of my trains seem to take a 21" radius just fine, but I'm worried that in a helix the physics may change and cause derailments. Any tips from those more experienced modelers?

Posted from iPhone

21 inches is awfully tight considering the additional variable of height.  I think you're asking for trouble.

Art House

Trouble, assuming HO scale.
The helixes offered as kits are in the high 20's. I went with 32. the big issue is the vertical change required to climb over the lower track. All sorts of problems start showing up. It may work in narrow guage with short trains, but that's not what most folks ask about. Don't know about N-Scale.

21" is way too tight for a helix. The grade would be impossibly steep for all but the shortest trains with small cars. I would think that a minimum of 30" would be a minimum for most trains, especially anything with longer equipment like passenger cars.

Ted Curphey
Ontario, OR

Assuming a minimun 4" separation between layers, the grade is 4/(3.1415*42)*100=3.03%  3% in a helix is relatively much steeper than a straight 3% grade because of the drag of the curvature.

My N scale double helix is 20" and 22" radius with a 3" inch rise. That gives me a grade of 2.38 and 2.17%.  I wanted everything under 2.5%. I also run trains with 3 locos, 22 cars, and a caboose, at a maximum. I have no problems running trains at these lengths, with my grades. When figuring out what radius to use, you have to consider all these variables. Train size, grade, added drag (friction) of the trains on the curves. 

Steve

Yes, it would work in N scale but not HO unless you run very, very short trains with diesels (only several cars). You might get by with 3" clearance (especially if you don't model modern railroads) but would have to add one times the thickness of helix roadbed plus tie and rail height. So if you laminated two layers of 1/4" material (like I did) to make a roadbed thickness of 1/2", your grade would not quite be 3% but still not good with that radius. Even if you can get enough power to get a reasonable train up or down the helix, it's likely the train will come off the track.

3.14 (pie) x 2 x 21" = 132"' track distance for a full circle, but your track doesn't just start with a vertical curve entering the helix so you have to add some to that. So 3.5/132= 2.7% PLUS. I've built several helixes and helped someone else build two and they are almost flawless as long as we keep the radius for HO no less than 30" and make sure that the track is level (not banked) btw the rail heads.
Remember that you are going to have to allow for outside width for whatever system you use to attach the levels and that adds to the width of the helix. They are space hogs, but I lucked out as I have a very small room in the corner of my basement I built mine in.

The bottom line is that 21" radius in HO is very, very problematic even for shorter trains.

Victor A.  Baird
http://www.erstwhilepublications.com



Edited 2 time(s). Last edit at 02/06/16 19:45 by wabash2800.

Thanks everyone. You just save me a lot of headache.

Posted from iPhone

Is it possible to begin a long grade along the backside of one long side on the layout to gain the needed elevation without a helix? This incline could be hidden behind a low false wall or scenic backdrop. If the long leg of the layout were moved out from the back wall about 4" that would provide needed clearance for the incline.

Posted from iPhone

Think long and hard about access to the track as it will be hidden so either from the underside, crawling under the benchwork or reaching over it, long arms apply in that case. And it will derail so how will you know until either it arrives on the floor or the engine apears?  In my mind bad idea I have one helix for both up and down it is about seven feet across and its going to be mostly hidden trackage. I have another place where the track is ten feet from the isle hidden, the only way to get there is to crawl not fun and yes will have a camera with a screen to monotor it. Think hard about the need for a second level before doing it. Sometimes wonder why I did it but its done and now need to finish the whole thing. Boyd in Chiloquin PS am in ON3/30 so curves are above 34" and even a 20 by 40 room gets small in this gauge.

Two pics of my 30" radius HO helix- its actually two helices stacked together (hard to see - 2 loops over 3 loops).  The slope is about 2% and it can handle about 25 cars pulled by a minimum of two powered locos.  Notice the access hole in the upper pic - works great.  The spacing between the loops is absolutely critical to good operations - the smaller the slope (gradient) the better.  I used autoracks and twin-stack container cars to test clearance between loops and there is very little clearance - about 3 5/8" from top of the rail to the underside of the loop above.  The critical clearance points are where you enter or exit the helix because you are usually crossing under a loop.  Also the total thickness of you sub-roadbed plus cork and flextrack should be as thin as possible - it affects the slope.  If possible, I'd build 32+" radius or even larger.  My helix design is right on the "edge" of reliable operation - it works well, but took a lot of planning and careful construction.

Hope this is useful   

---

---

It is nice to see the threaded rods for supports to allow micro adjustments to the grade and roadbed flatness or level.

Posted from iPhone

I gave up on the threaded rods when I realized they were making little flat spots on the grade. There's also this odd phenomona about flat plywood on grade, it makes it's own spiral but tips in towards the center. Haven't figured that one out, there was a discussion about it on TO so wasn't just my handiwork. Helixes seem to be a great idea, I have two on my plan, but I've coined them the 800 pound elephant in the train room.

fbe Wrote:
-------------------------------------------------------
> It is nice to see the threaded rods for supports
> to allow micro adjustments to the grade and
> roadbed flatness or level.
>
> Posted from iPhone



Edited 1 time(s). Last edit at 02/07/16 14:00 by TCnR.

I wouldn't have predicted the flat spots at the iron rods. I guess they need to be perpendicular to the grade angle rather than vertical to the floor. Perhaps a round pin could be inserted into the side of the roadbed to keep the grade smooth.

Maybe modeling Nebraska is not such a bad idea.

Posted from iPhone

I have used threaded rods successfully and am aware of the plywood wanting to cant in, but that is the beauty of the threaded rods in that you can correct that by adjusting the nuts and washers. It is very important not to have the track banked (it must be level across the rail tops). I made a simple but special tool with a level on it (posted on Train Orders some years ago) to make an even grade on a helix.

Ha, Ha! I've got yours beat JSnoy! I have three helixes on top of each other on my HO layout: The top one is a single track, 1.5 turn with 30" R on top of a double-track one with 30" R inside and 32 R outside that is 4.5 turns, and then,  I have another 1.5 turn 30" R underneath that (with a switching lead beside that). I also have a 4 track staging yard part way around that on the level. This is all in a small room in the corner of my basement that used to be a pantry. Trains come and go from different parts of the layout but it is a logical, double-deck, walk-around design with one peninsula.

I know some will flinch at the thought of a 4.5 turn helix, but I use the extra time to my advantage in a mostly TT and TO operation with working tower operators (the layout is still in progress).

Kids are wonderful. Someone brought their young boy over a few years ago and and he peeked inside the room and called it a "train mixer". I still chuckle at that.

I also have another 1.5 turn helix on the end of a peninsula to lengthen a siding and another planned in another room if I want continuous running.


Victor A. Baird
http://www.erstwhilepublications.com



Edited 6 time(s). Last edit at 02/07/16 19:02 by wabash2800.