# minimum radius for helix

• posted
I am designing a new layout for n-gague (never done n-gauge before).What
should I use as a minimum radius for:
1) curve on the flat?
2) Helix up?
3) Helix down?
Cheers
Phlil
• posted
Length of one turn of helix T = 2*pi*radiusR, or T=6.28*R
Grade is: vertical separation H / T*100, or G=100*H/6.28*R
Min. recommmended H is 1.5 inches, and max recommened grade is 3%. Do the arithmetic. :-)
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I'd also say it depends on what you're going to run up it. A good old fashioned Grafar 0-6-0 with a rake of coaches won't come close to the pulling power of my Jap outline Kato electrics.
Pete
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Try
there is a useful tool to do the maths mentioned above. Used it to help plan a 'OO' helix.
Chris
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Thanks for that. As it happens I have a PhD in mathematics, so I can do the sums myself. I was rather hoping to benefit from your expertise, not so much on the steepness, but the effect for the curve in a loco's ability to pull up the helix, and also on the incidents of derailments. I was also hoping for some comment on going up and down the helix, as well as what minimum radius I should use in my design on the flat.
Cheers
Phill
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Standard rule, use the largest radius that will fit in your space, it can never be to large only to small. Keith
Make friends in the hobby. Visit Garratt photos for the big steam lovers.
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In message , fghdf writes
Well in that case you can also work out what superelevation (cant, banking) to use on the curves to make it easier for your stock to go up (or down) without falling off the track because of the lack of suspension. As you are no doubt aware, you will only ever have a maximum of three wheels of a 4-wheel wagon or bogie in contact with the track at any one time.
I built a "rectangular helix" on my OO layout. The curves are 4 feet radius, and there is an 8 foot straight between the two semi-circles at the ends. More details on my web pages (URL in sig, and select "helix" in the navigation list). The incline works out at about 1 in 75, which means I should be able to pull rather a lot of coaches/wagons up it behind my favourite locomotives (Bachmann 56XX 0-6-2T, since you ask).
Derailments should only happen because of your bad tracklaying. Since you will be taking extra care over laying the track on your helix, you shouldn't have any bad track. So you shouldn't have any derailments.
You don't say how much space you have. I suggest you use the largest radius possible that fits in with the rest of your layout. Let us assume that you are going to use "my" radius (4 ft), but since you are building it in N, you halve that to 2 ft. Then you are going to build the incline at 1 in 50 (2%) which is not too steep so you should be able to get a reasonable train up it. The circumference (on the flat) will be 150 inches, which means the rise for one turn will be 3 inches. The height from the track on one level to the underside of the support of the next level will obviously be less than that, say 2.5 inches. Now you have to consider the future, and maintenance. Is 2.5 inches going to be enough room for you to get whatever tools you need in there to keep your helix going whenever things go wrong (as they undoubtedly will sooner or later). On my helix, the levels are 6 inches apart, and that is too little sometimes, especially when you want to insert a few more trackpins, or you want to solder another wire on the wrong (i.e. inaccessible) side of the track.
• posted
I read on a website that a mere 2% incline will halve the pulling power of some locomotives while the weight of cars on the descent will increase the speed and likelihood of a derailment on the curve.
(kim)
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Can't work in percentages but on my first effort I had a 1:30 incline running down to a set-track 3rd radius curve, very careful driving was required to prevent a derailment. In the end I re-built the curve, had to maintain the radius but I super elevated it to around 5 degrees. That cured he problem, didn't look at all life like though.
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During my trainset days I figured that an HO 360mm radius had about the same effect on a train as a 1:30 gradient.
You lay the track _very_ carefully! If there is a propriety curve of the correct radius then you should use that in preference to flexible track which is difficult to lay in a precise tight curve. Consider soldering the proprietry track sections together.
Going down is easy. I assume you will have the down track layed inside the up track? The combination of a tight curve and steep gradient will soon sort out the good pulling locos from the rubbish. (it's not based purely on price)
The minimum radius should be the largest you can fit in. HO works out quite well because a double track helix is just practical on a 4 foot (1200mm) wide board and the space inside the helix is large enough to get one's head, shoulders and an arm in for access. (1:44 & 1:40 gradients)
In N scale you're not going to have proportionately the same ease of gradient adjustment nor will your locos have the proportionate pulling power so no way can you convert that to a 2' wide baseboard.
Regards, Greg.P.
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Superelevation becomes a vertical curve on sharp radii curves. I've never decided if there is any real advantage or disadvantage to it for models.
On trials on my helix, a train of rather expensive rolling stock let go from the loco right at the top. I listened to the train circulating and gathering more and more speed down the helix and at the last moment removed my hand from the bottom exit (guaranteed destruction) and let it go! The train ran the length of the track along the baseboard, rounded the 180 degree bend at the end, negotiated the half dozen turnout throat and rolled to a halt in the staging yard. I wandered off for a stiff drink and wrote myself a mental note that time spent carefully laying track is time well spent!
Regards. Greg.P.
• posted
Sorry, that wasn't clear from your post.
Prototype figures of curve resistance don't scale well to models, but a good rule of thumb is that the curves add about 1 percentage point to the grade.
A 1% grade reduces tractive effort by about 25-35%, 2% by about 50%, and 3% by about 70%. These figures are based on tests done about 20 years ago, and are therefore somewhat conservative, as rolling qualities of trucks (bogies) have much improved.
I would use broad curves for the helix to keep the grade to 2% max if you want to run reasonably long trains. One often overlooked point about helices: except for fairly large layouts, the helix will be a large proportion of the total run. Some operators have reported that they get rather bored waiting for a train to navigate a helix. If the location of the helix permits it, I would suggest that you pull one of the middle turns out into the open towards the aisle and install a passing track "at a remote mountain location". Or a side-hill viaduct or similar scenic feature. In fact, I'd try to locate the helix so that this is possible, say at the end of a mid-room peninsula. (If you have enough room to tuck the helix into a corner of the space, you probably have enough to put it in the middle; it's worth a trial sketch or two anyhow.)
Also, if you need a helix at all, IMO it should be double track, else there will be too long a wait for opposing trains to start down (or up, as the case may be.)
HTH&GL
• posted
"Chris Wilson"
And I can't work on 1st, 2nd or 3rd radius curves either as they are completely nonsensical. What the hell is a 3rd radius curve, why not just state the diameter?
At least percentage of grade can be worked out. Just divide the percentage into the 100. 2 in to 100 goes 50 times, grade in UK terms = 1 in 50. Easy.
Now, how do I do that for 3rd radius curve?
As for the minimum radius of a helix? Use the largest one you can fit in but I'd suggest nothing much under 30" radius.
-- Cheers Roger T.
Home of the Great Eastern Railway

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A novel "layout" that I've seen at Ontario shows consists of a vertical cylinder which itself revolves, and has a small plane flying around it in the opposite direction. A short freight train (HO IIRC) ascends two revolutions of a helix, passing over trestles and through tunnels, disappearing into a tunnel mouth at the top and reappearing a short time later at the bottom, having descended a helix inside the cylinder. The motion of the cylinder and the aircraft holds the attention while the train is out of sight.
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In message , Chris Wilson writes
Divide the "in" number into 100: 1:30 is 100/30 = 3.333%.
Obviously. That is because the third radius curve isn't at all life-like. Make it a much larger radius and you can reduce the superelevation. It'll look much better.
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It's a setrack measure. If someone has a circle of track using 2nd radius curves they know they can double the circle by adding either first on the inside or third on the outside. In that instance, as a measurment device it does its job perfectly.
Pete
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Yes, but how do you know what size board it will fit on? Do the Brits sell boards in the local shops labelled 1st board, 2nd board, 3rd board to match?
VBG...
John Dennis
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I went that way with the Mk2 layout, end to end, it's not quite finished (see previous posts) and regrettably it has to be torn down but what I found was that although I kept the layout "busy" son and daughter's interest waned. They still enjoy making little models to go on the layout and whatever but driving a train from end to end bores them ... even put in a turntable, electric points throughout but shunting and going backwards and forwards just doesn't hold them. Trains going round in circles, even the occasional race kept them happy for hours. I would imagine that for many if not most small children the story would be the same. Hence tight curves on snap together track are a necessity.
Still I should get the best of both worlds with the Mk3 layout. Twin track around the garage for continuous running and a couple of stations for me to play with.
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:-)
The set track thing comes from the train set side of the hobby and to that extent train sets do have instructions and guides inside with regards to base sizes yes. As it happens board sold by timber merchants in the UK are 8'x4' (or nearest metric equivalent nowadays) which surprisingly enough are just the right size to accommodate the plans available to build using set track components. What a co-incidence. ;-)
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FWIW my old train set with oval of track and 2nd radius curves has the minimum space required printed on the front of the box and they are 37"x57" although I would recommend at least 39"x60".
(kim)

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