Hi everyone, I once again need to tap into the N-scale knowledgebase here on TrainBoard.com as progress pushes forward on my CSX Dixie Line layout. With the exception of some minor details to be completed over the next few days, all of my major layout benchwork is complete...which leads me to the helix as the next project. I have searched for and read through numerous threads on this forum regarding helix construction and have learned much--although I am still unsure whether or not my performance expectations are realistic. My plan calls for a double track helix, the design of which I have mulled over in my head and on paper for months and seems to be just about ready to go. However, construction methods aside, I would like to get some feedback on what performance I can expect given the track geometry resulting from this current helix design. My typical/average train would be two six-axle modern locomotives pulling 24 coal hoppers. Specifically, I have used a pair of Kato SD70MACs and 24 Kato Bethgon Coalporters for my test runs. Each loop of my helix rises 2.5" per complete turn; the outer loop of the helix is 16.5" radius while the inner loop is 15" radius. This yields a 2.4% grade on the outer loop and a 2.7% grade on the inner loop. My question is this: If I were to magically build this helix tomorrow completely and flawlessly as described above and run that test train up and down and through the helix over and over, do you think my experience would be :thumbs_up: or :thumbs_down:? Since I have never done a helix before, I am really just trying to get a feel for where everyone thinks I am with this helix design and what reworks--if any--may need to be done. As always, looking forward to your responses! Jamie
Jamie, one BIG thing that I've learned with my helix is the locomotive to car ratio was more than I thought. My helix consists of 4 loops; track centerline is a 44" diameter (22" radius) and a rise of 3". That equates out to 2.17%. I figured I could run 3 locomotives and 30 cars up this. That did not happen. The most I can get out of ANY 3 locomotive combinations is about 25 cars. And for 30 cars, I need a minimum of 4 locomotives. Even at this, by the time they get to the last loop, they are going pretty slow compared to the first loop, and are exhibiting a bit of wheel slip. This is with a solid train of 50' boxcars with MT trucks and FV wheels. It was a big eye opener, since my passing sidings were allocated on my original thoughts of what could make it up the helix. Regarding your grades, I have my doubts that your two SD70MAC's will pull the 24 hoppers up your inner loop. 1. because of the % of grade and 2. the sharpness of your curves on that grade. With your outer loop, they might, but there might be alot of wheel slippage, which is not good on a loco. Adding another loco should do it, but you might not be able to have that as an option. The other remedy would be to lower your height between levels which would reduce the % of grade, or make the radius (diameter) of the helix larger. -Mike Pic of my helix:
Jamie, I use 18 inch radius helixes with a 2.75 inch rise, yielding a 2.45% grade. The largest helix to the upper deck is 7 turns, I got two smaller 2-turn helixes with the same basic dimensions. I model 1949, so the only 6-axle diesels I have are E-units, which are used for passenger trains: I can not judge on what modern power would do on my helixes, but 3 F-units will pull a 25 car train. I have Bachman, Kato and Intermountain Fs, all are about the same power per unit. A 4-unit F or FT will pull 30 cars. I limit 4-axle Geeps or RS to 15 cars for 2 units, but more out of operational interest. I have not tested what they would actually pull as I find I use them less and less in favour of steam (and there were not a lot of road switchers used in 1949 anyway...) I find that going down is more difficult than going up: Going up is a question of raw power: if the train does not make it give it more engines (difficult with steam pushers, but I digress) Going down a train needs to be balanced with no light or top heavy cars at the front (IM woodchip gons are the worst offenders here). Couplers need to be all working well and the trucks / wheels in good condition. If I would build the helix again I would increase the radius to reduce drag, grade and the risk that a car is pushed "out" going down. If that would mean sacrificing valuable real estate for the visible portion of the layout I would be willing to do so in favour of operational security. It should not be a "hold your breath" experience to go down a 7-tier helix.... And I would not make my helix the ruling grade again (even before drag-adjusting it is the steepest climb, whixh is a bad idea: If a train stalls it will be in the helix...) I hope that this is helpful Cheers Dirk
The common mistake when planning helixes (or curved grades) is grade % calculation. The curve itself adds up to the friction dragging the train. Wheels running on the curve's outer rail have a lot more friction than their mates; the sharpest the radius, the higher the friction. This preamble to remind helix planners to add a coefficient to their grades, based on the curve radius, to find out the grade that is "felt" by the motive power. Here is an excerpt of a table compiled by Joe Fugate. 14" -> 2.3 16" -> 2.0 18" -> 1.8 20" -> 1.6 22" -> 1.5 24" -> 1.3 26" -> 1.2 That means your 15" inner loop on a 2.7% grade should have a coefficient of about 2.1 added. The grade calculated is 2.7 + 2.1 = 4.8%. I believe two SD70MACs without traction tires would be able to pull no more than 14 cars on that grade. Moral of the story: helix needs LARGE radius in order to reduce the grade.
I dunno if you have thought about this, but 2.5" rise isn't much. When a derailment happens, do you have easy access to fish out errant cars and locos? What if you need to replace a section of track in the future? That tiny clearance will give you fits when it comes time to do maintenance. Cleaning? Same deal. My solution was a spiral helix. It rises at a constant 2% grade, continuously-increasing radius. No track is above the other in the helix. Think of it as a bowl. Minimum radius ia 15.5" for a afoot or so, then it continues to get bigger, all the way to 21" rad. I can't think of a reason these cannot be stacked, to gain more rise.
Jamie, On the old Oakville the helix radius was 24 inches; the grade was around 2%, and I was able to pull 60 fifty foot cars with four six axle units. If I were going to do a helix again, I would make it a "nolix" with a shape other than circular. An oval racetrack type nolix would allow for a lesser grade with the same radii. It looks like you could do that on your peninsula. Also, be sure the upbound track is on the outside. This last MAY be the most important part of the equation.
Another option to add to the mix would be to extend the helix to an oval (which is my plan) thus reducing the amount of side friction on the wheels. Only the parts of the train in the curves would be experiencing the maximum friction.
Greg, This is actually the "ace in the hole" that I can play. When I built the peninsula, I specifically built it about 18" shorter than what was originally planned so that I could convert the circular helix into an ovular shape with 18" straight sections between the two semi-circular curved sections. This would reduce the "inner" grade to 1.9% and the "outer" grade to 1.8%--combined with the reduced friction by introducing the straightaways, this seems like a "no brainer" for me to do. Question is, even though I am better off, am I now in safe waters where I will not be constantly stalling/derailing trains? Jamie PS: I have briefly read through all of the previous replies as well but my internet has been down at home all day (thank you bulldozer working up the street!). I hope to have connection restored when I get home today and can respond to the other posts also.
Jamie, after reading and replying to your question, I did a test this morning on my helix using the closest locos/cars that I have to what you will be running - 2 SD60M's and 24 coalveyor bathtub gondolas. And I had no problems. If you build your helix with the straight sections like you talked about, then I feel you'll be just fine, even with the radius of your curves. As far as derailments are concerned, if your trackwork is superior, then don't even worry about them. Of course you can't be slamming the throttle down to "stop" when a train is going down the helix either. That might cause some cars to bunch up and jump the track, thou it depends on how fast you are going. -Mike
Dirk, In all my years of reviewing trackplan design, I have always heard this rule "not to make the helix the ruling grade" but it has never been explained to me...until you just did. The primary reason my helix is the ruling grade is because my original prototype was the mountainous CSX former Clinchfield route, and the helix fit in right where Sandy Ridge Tunnel would be located, which is the real-life ruling grade and requires helper service. Now that I have switched prototypes, I am sorta locked in since none of the grades on my current prototype really approach 2%. Denny, Thanks for this handy reference. I have not seen it before but I will be factoring it in to my future helix calculations. Hemi, I absolutely LOVE the spiral helix design. In fact, I may build one to test how my trains perform on one (I believe I already have all of the materials to do so) although I don't see a way for this to fit into the final design. Also, I don't believe I will have any clearance issues (knock on Masonite)--my roadbed is only 1/4" thick and I am using the threaded rod method, so in a pinch I can raise/lower levels to address a track bug. Also, the helix will be completely accessible from the outside and the inside in the event of a derailment or other more typical mishap. Jim, I believe the oval nolix idea will in fact get my grades and curves into an acceptable configuration and get me to where I will not have any problems other than the usual ones that occur on the non-helix flat parts of the layout. Of course, only time will tell, and I am building the helix so it will be strong and sturdy but also using inexpensive materials in the extreme event that I need to have do-over. Mike, This is GREAT news and I really appreciate this info. Gives me a good sense of confidence that the oval design is the way to go and will work out just fine for me. Thanks everyone so far! I'll post updates on this thread as my helix design and construction progress and will continue to look for feedback & suggestions. Jamie
Hello Jamie, have a look over at the "Layout Design Discussion" forum here on the Trainboard. The third topic (it is "stuck" up there for ease of reference) titled: "Tables for Calculating Spiral Helix Dimensions " will explain the thoughts behind the spiral helix in detail. Dave did a great job there! Cheers Dirk
http://www.trainboard.com/grapevine/blog.php?u=4195&blogtype=recent That's who inspired me to build my spiral helix. Dave did just about everything but cut it!:thumbs_up::thumbs_up:
Expand the minimum radius Jamie, I recently dismantled a helix built with the curve radius right at 18". My C44-9s and Sd70MACs were constantly pulling Kato coalporters off the track because of the overhang of the locos on the curve. Add to that the fact that I had trouble getting a heavy mixed freight train (35-40 cars) down the grade, I decided to can the idea and start over. I recommend a minimun radius of 22" or so. Couple that with the oval "nolix" idea, and you may have a winner. As for pulling power, if you stick to Kato locos, you won't have a problem there. I could get 40+ coalporters up my helix with three six-axle units. Jim
