:tb-biggrin:Hi All I like the looks of the bowl helix in terms of ease of construction and efficient use of materials. I was wondering a) Could this design be "turned up side down", ie as you go up the helix you go from a wider diameter to a smaller one. This would mean the supports would be on the inside of the roadbed, instead of the outside. b) Would it be hard to figure out the dimensions for the supports? c) One advantage I can think of would be to scenic the outside like a mountain, and could even have sections of track showing. Would there be any other advantages / disadvantages? Thanks Chris
Hi Chris; Check out this posting http://www.trainboard.com/grapevine/blog.php?b=806&goto=next To answer your questions A) Yes B) see the link above C) you basically have covered it Gary
Welcome to Trainboard Chris. The only dissadvantage I can see is access to the track. It needs to be in the open as opposed to against a wall, so there are limitations. It is a good idea but it is difficult to make comments without seeing what you have in mind. Have you drawn up a track plan, or can you give us an idea as to the size of your layout area? I have two spiral helices on my layout, and yes I found construction a breeze. Although, they are real estate hungry. But this is more than compensated for by the extra opperating potential of multi-levels.
Thanks for the replies. Sorry no layout plans, still in the contempation stage. What I was wondering was having the sprial helix going from a larger diameter at the bottom, to a smaller diameter at the top.....so more like a layer cake shape, vs a bowl shape. This would make it easier (more natural) to hide inside a mountain for example The more I think about it though, one disadvantage might be as the train nears the top, that the helix might be harder to climb, as the radius gets smaller?? Whereas the bowl shape might be more advantageous (to N scale......) What do you all think? Anyone with an up side down sprial helix? Thanks Chris
It is a very interesting question that I would be also interested in hearing views on... :tb-cool: :tb-cool: :tb-cool: :tb-cool:
Our resident Helix spreadsheet master will be here sometime in the next day. I am sure he will chime in with the pros and cons. Interesting concept cannot wait to hear what Dave has to say.
Not the helix expert here. A reverse bowl helix features a decreasing radius as you climb up it. Smaller radii require more pulling power required (or less payload). The helix would start out easier to climb but get harder the farther up you go. The opposite is true of the bowl helix; the higher you go, the easier the climb gets. It's a trade-off. The advantage of bowl helices is the ability to have longer runs at less gradient, except for where the track has to cross over itself to leave the helix. I've always wondered if it would be possible to use a crossing track to enter or exit the helix. The last spiral of a reverse bowl helix would have the least amount of grade. The first spriral of a bowl helix would have a higher rate of climb but the climb would get easier after that. It's an interesting proposition. I've always shied away from helices due to their inherent comsumption of real estate and stiff gradients. Maybe I should consider the reverse bowl helix with a crossing track at the top exit. I'll be interested in hearing what the more experienced helix builders have to say. Darrell, quiet...for now
This 'wedding cake' style of helix has been done, by no less than Mike Danneman on his N scale DRGW layout. In Kirk Reddie's N Scale Railroading magazine, there have been a couple of articles on this layout, and included in that is description of his helix, which connects the visible layout with hidden staging underneath. Mike's helix isn't in the visible scenery, but you can see the trains through the view ports in the fascia. An advantage of the 'wedding cake' style helix is that the operator can readily see the progress of the train as is climbs/descends the helix - this may not as visible in an 'bowl' style helix. Just my 2 cents worth. inverted bowl
Thanks for the feed back / comments so far. As I continue to think about this I wonder if the supports for each level (loop of track) could be designed so that as the train climbs the helix, the grade could be reduced to compensate for the tighter radius. ie, less distance between the tracks from one loop to the next, if you follow what I mean, for the last couple or 3 loops going up thanks Chris
If you had a crossing track then would that limit the length of your train to the circumference more or less of the last loop which would be the smallest? Maybe I'm misunderstanding
Absolutely, it's possible. In fact, this is one of the main advantages of either kind of spiral helix, in my opinion. Namely, that the grade can be whatever you want because the train does not need clearance from the track above. (Except for the entrance/exit, of course, which is an important caveat.) Reducing the grade smoothly just requires doing a bunch more math. (That's the disadvantage, I guess.) By the way, I would agree that the diminishing-radius-as-the-train-climbs on the "wedding cake" helix would be disadvantageous compared to the "bowl" helix. Especially because with long trains you would likely be hauling the entire train upgrade when you enter the smallest radius, and you end up with the front of the train bearing the most weight in that radius. With the "bowl", you might get the front of the train through the small radius while the second half the train still on flat grade outside of the helix, and as you climb, the least stress is always on the rear part of the train that bears less weight. I have no empirical data to suggest how big of a factor this truly is; it probably wouldn't matter unless you are already pushing your luck near the limits. In any case, my rule of thumb would still be to make the smallest radius as large as it would need to be in a traditional helix, given the trains being run. And one more thing...I'm sure that the "wedding cake" and "bowl" could be combined...to make a "diamond" helix or an "hourglass" helix...(if you follow). It would be a lot more complicated to build, of course, but it might suit that rare situation where nothing else can be made to work.
Whether the steeper gradient is at the bottom (bowl helix) or at the top (wedding cake), it is nevertheless the ruling grade. In either case, the helix should be designed with a maximum permissible grade in mind, no matter where it is located. To me the bigger issue would be access in the case of the inevetable derailments. An inverted bowl might have a slight advantage here, as the entire body would not need to fit inside the smallest portion, only the head and hand. This however brings back the minimum radius and max permissible grade question again. Considering all the above, For N scale I would design based on a 15" min radius (for human access). Given a 2" layer separation (minimum clearance) this would result in a grade of just under 2.2% (and if my lashup couldn't hack that, I'd get new locos!). A smaller radius would not provide the necessary clearance without increasing the grade, thus necessitating a bowl or inverted bowl configuration, however the inside access would be unacceptably restricted. For HO, a 15" radius is a bit tight, so assuming a 22" minimum radius and a 2.2% max grade, the separation between levels would be 3". This is not enough for clearance, so a bowl or inverted bowl design would be required.
I've actually thought this through before and you don't gain anything by it. Every loop of the helix has to cross the track that brings you into the inside loop(or go under it for an inverted bowl), not just the first loop. Clearance is not a problem for any loop other than the first one because each loop builds on the first loop and the first loop had to have enough clearance. If you use a crossing on the first loop, then the second loop has to rise enough to clear the entrance track. The second loop has a larger radius, so it's grade will be slightly less, but now the first loop does not gain any elevation so you have to add another loop to the helix. If you have enough real estate to add one more wider loop, then you could have started with the radius of the second loop to begin with and not bothered with the crossing. The last loop will still have to have enough of a grade to cross itself to exit the helix.
Ok, great comments and lots to consider !!! With a sprial helix, wedding cake style, the upright supports would be on the inside of the roadbed. Will the sticky on this forum calculating measurements for a bowl sprial helix be the same for a weeding cake style?? thx
Calculations for the footprint and number of loops would be the same but the height of each support would be quite different. It would be possible to determine the wedding cake helix support heights with the bowl-shaped helix calculator, if you subtract each support height from the total rise. If you are rising 14 inches between upper and lower decks and have 3/64ths, 5/64ths, and 8/64ths for your first 3 bowl shaped helix support heights, then your smallest radius support heights at the top of a wedding cake helix would be 13 and 61/64ths, 13 and 59/64ths, and 13 and 56/64ths.
The circumference of each loop is determined by its radius, which, in a spiral, is constantly increasing or decreasing...so the circumferences of each loop increase or decrease. The grade within a helix is determined by how much rise there is for the length of run: 2 inches of rise in 100 inches (about 16 inch radius loop) yields a 2% grade. Once the track has passed over itself on the first loop, you can make the grade as steep or shallow as you want. If you want to rise 14 inches with a 1% grade, you will have to keep looping until you have 1400 inches of track. At a 2% grade, you will only need enough loops to provide 700 inches of track; and at 3%, you will only need 467 inches. You may choose to have a relatively steep grade for the first 1/2 loop (while half of the train may still be on level track), and then reduce the grade to present less challenge to the loco(s) as more cars are being pulled up a grade instead of on level ground. You may set the height of the supports so the track is on the same % grade from the bottom to the top of the helix. As the radius increases (in a bowl-shaped helix), the track will travel over a larger circumference loop so (at a fixed grade) will rise more in the larger loops than it does in the smaller radius loops. If you want to rise the same amount of height each loop, but the loops get larger as you rise, then you will need to reduce your grade as the loops increase in size. With a wedding cake helix, the loops will get smaller as they rise, and the grade will have to increase if you want to rise as much in the smaller/shorter loops as you do in the larger/longer loops. However, if you want to keep the grade the same, then you don't have to keep rising as much as you make each loop. Since each loop is outside (or inside) the next loop instead of over or under it, you only need 2 inches of clearance where you enter the bottom of the bowl shaped helix or top of the wedding cake helix. Everywhere else you can have well under 2 inches of rise (or fall) from one loop to the next.
very well explained and easy to follow. thankyou!! 1) can someone put the calculations into a spreadsheet for the wedding cake sprial helix? thanks again Chris
The grade throughout this oval bowl-shaped helix is a constant 2%. Looking carefully at the space between the loops, it is possible to see how the lower loops are not as far apart from each other as the upper loops. This is because the lower (and smaller radius) loops are not as long as the upper (and larger radius) loops so the track on the bottom loop rises 2% of 120 inches (IIRC), but the top loop rises 2% of 160 inches.
The practical limit on minimum radius in a helix for an N-scale layout is determined by an interaction between several variables: 1. the maximum grade you are willing to have through your first (smallest) loop 2. the height of your tallest loco or rolling stock 3. the thickness of the subroadbed that supports the track where you exit/enter the helix 4. the minimum radius that permits reliable operation of all locomotives or cars that will use the helix. (Note: This is the minimum radius for reliable operation, not minimum radius for best appearance...a loco that looks terrible on a 19 inch radius curve may still function quite reliably on an 18 inch radius curve within a 2% grade.) On my layout, I have limited the grade within all four of my helixes to about 2%. If I want 2 inches between the top of the railhead and the bottom of any subroadbed that passes over the railhead, then I will need at least a 100 inch long loop to yield enough clearance to pass under the next highest loop (of a bowl helix)...and an additional 12 inches if there is 1/4 inch from the bottom of the subroadbed to the top of the railhead of the higher track. If the subroadbed is thicker (e.g., 1/2 inch, instead of 1/4), then my lowest loop will have to be at least 125 inches long...which means about an 18.5 inch radius with a 2% grade. If I can tolerate 1.75 inch clearance and and .25 inch thick subroadbed-plus-rail, then I can gain sufficient clearance with a 16 inch radius loop and 2% grade. On my Kellar Helix, I had to cut the 1 inch thick subroadbed down to 1/2 inch to give sufficient clearance. This view shows the track and cut subroadbed at the lower entrance to the helix (looking from inside the helix). I put trees both outside and inside the helix because operators standing in the aisle on the outside of the helix could easily see into the helix through the backdrop that covered that portion of the helix until I used the trees to block seeing into the helix interior. If I increase the grade to 2.25% for the 1st half of my lowest loop and only 2% for the 2nd half of the lowest loop, and still want to rise 2.0 inches from ground 0 to the top of the 1st loop's railhead, then I'd only need my first loop to be about 94 inches long (about 15 inch radius).
