If I wanted to push the limits on a single track HO helix in terms of radius and grade, what might that be?
Looks like an 11.5 in radius at 4% is a elevation change of 2.89"
The biggest things on this layout would be an RDC or a GP-7
-
NMRA recommended clearance from rail to overhead obstruction is 3". Less is at your own risk. Add to the 3" (or 2.89") the thickness of your roadbed, subroadbed, and track to get the rail-to-rail rise needed for each level. Then add room for fingers to rerail box cars. Typically, minimum in HO between helix levels is 4".
-
I’m guessing here, but I think the minimum practical radius for your GP-7 to remain coupled to a generic 40ft car is around 15". Sharper radius curves will create two problems - 1) the locomotive yanks the trailing car off the track 2) odds increase dramatically for stringlining for trains of more than a few cars.
15" radius with a 4" rise gives a 4.25% grade. For 3-5 car trains, this might work, and is probably all your GP-7 is going to pull up the helix, anyway. I would do some serious testing before 100% commitment, though, because it’s at the ragged edge of the practical operationally. The helix would be about 34" across. Putting the necessary twist in the subroadbed and/or making smooth subroadbed joints will be a challenge with that radius/grade combination. You needn’t ask how I learned this.
A different solution would be the use of 34" long train cassettes, which would be used to manually get a GP-7 plus 4 cars from one level to the other. Same 34" of wall space, but only 4" of depth instead of 34".
A variant of the train cassettes would be a train elevator. Much more complex to design and build, but has nearly the same space savings of the cassette.
yours in
I am not sure that an RDC will navigate a 11.5" radius - especially if you have the longer Proto 1000 model and you have 2 or more in a consist. You can always set up a temporary test track to try it out and at the same time play with the radius and grade. Good luck.
Alan
Newcomers to model railroading often hope a helix somehow magically negates the laws of physics. Sadly, such is not the case. Besides the steepness of the grade itself in such a tight helix, the extra friction of the cars on the tight curve radii adds to the effective grade, making it nearly impossible to pull any length of cars without stringlining (pulling and derailing to the inside of the curve).
See this thread for similar discussion. In it, I mention the example of Joe Fugate, who found his 24" radius helix was not sufficiently broad and did major layout surgery to tear it out and replace it with a much larger radius.
http://cs.trains.com/forums/1473626/ShowPost.aspx
Byron
As others have said you really need 4" railhead to railhead so lets figure the radius for various grades:
1% = 1 in 100" of length /pi = About 33" radius (doesn;t meet our 4" requirement)
4% = 4 in 100/pi = 33" radius also
Now lets change the radius and hold the 4"constant
30" radius = 90" = 4.4% grade
28" = 4.5%
24" = 5.3%
22" = 7%
18" = 7.1%
12"= 10.7% At this point you are probably overcoming the factor of adhesion for a model train. In my opinion anything lower than 28" better have perfect trackwork and grades including the tranisition into and out of the helix. Even so the weight of the train is going to be limited going up and may derail cars coming down. I think your idea is a bad one in my opinion.
I kinda figured those answers would be what I got… thanks to all.
My problem is I’m a HO guy with an N scale room. [8]
Aren’t we all… [;)]
My present plans include a couple of single-turn helices, 360mm (14") radius, 4% grade. Trains will be short (three to five 4 wheel cars, with one glaring exception), locos will be 6-coupled steam tanks. Railhead to railhead clearance of 90mm gives me plenty of room for my preferred subgrade and roadbed system (flex on 10mm foam inside steel stud trough) while still clearing the NMRA clearance gauge.
The glaring exception? A unit coal train, powered by a 2-6-6-2 or equivalent. The articulated is a Mantua, the prototype of whch was designed to take curves equivalent to 12" radius in HO. The cars are modified US-style hoppers, shortened or articulated, not Bethgons.
Note that the longest car that will ever traverse the line is only 8" long - an old wooden coach.
Note also that I do NOT consider this a reasonable design for US-style rolling stock. While I model in a scale slightly larger than HO my carlengths are similar to US N scale.
Chuck (modeling Central Japan in September, 1964)
You forgot the factor of 2. Circumference = 2 time pi times radius. So the actual grades would be 1/2 of what you listed for a given radius.
An 18" radius helix in HO would be practical (for rolling stock that are happy with 18" radius curves) except for the combined curve drag and grade drag is such a heavy penalty. John Allen once estimated the effective grade of a curve as 30/r times 1.3%. (I could be slightly off on the constants in the formula.) Which means a grade on an 18" radius curve adds about 2% to the actual grade for an effective grade on a helix of 5.5%.
A couple of other issues that have been mentioned by other with helii experience:
-
an accidental uncoupling of a respectable length train at the top of the helix is both an awe-inspiring and a tear-inducing event. Gravity still rules after all these years!
-
the length of time a train spends in the helix compared with the rest of the layout is disconcerting to operators. If the helix is hidden from view, the tendency is to speed up the train until the speed le
It would not be fun to try to pull a train around a helix with those dimensions. When it derails–and it will–you’d better have good access to get in there and fix the problem.
Yep! [sigh]
Even if you can not increase the radius of the curves in your helix, can you add straight sections–basically turning the circular helix into an oval? This is the solution that I am moving forward with. In my case, the straight sections reduced the steepness of my grades from about 2.7% to a much improved 1.4% and also reduces friction since only part of the train will be on the curved sections of track at any time. Jamie
The length of train compared to the length of the straight sections would have a big influence on any grade change effectiveness.