From: macman+@pitt.edu (Dennis H Lippert)
Newsgroups: rec.models.railroad
Subject: Re: Helix headaches
Date: 6 Apr 92 16:21:54 GMT
Organization: University of Pittsburgh

In article <1992Apr6.084913@doodly.mcc.dec.com> swist@took.enet.dec.com writes:
>18" for whatever reason is the magic number for most HO
>equipment.  Probably 95% of the stuff on the market will
>run at this radius.   The longer stuff looks pretty ugly -
>you get not only overhang on engines, but underhang
>on long passenger cars, i.e., you need significant clearance
>on not only the outside, but also the inside of the curve.
>
>The 24" figure is probably the 100% point, i.e. there is
>nothing made that wont run on a 24" radius (again, 
>subject to aesthetics/realism constraints).

Well, here's my comments...

18" is too tight for almost anything... even small diesels (Geeps) look 
somewhat out of place here.  Another thing to consider is that an 18" HELIX
will have a much steeper grade than a larger one, so you'll be pulling a
tight curve and you'll also have a lot of side-forces from the extra weight
of the train.  The grade would be what?... 56" per loop, at least 3" rise 
per loop, so it's a 1 in 18.84 rise, or 5.3%... MINIMUM.

24" is a lot better (3.9%, plus wider curve), and the previous poster is
correct in saying that 95% of all equipment will run on it.  Just beware of
big steam engines, big diesels, and full-length passenger cars.

36" is getting grand for a home layout, but is IMHO the minimum allowable
radius for a mainline (yeah, that mean I *don't* have a layout... if I only
had enought space for an 18"-curved layout, I'd concentrate on a big shop
area, and I'd join a club!)  The new Walthers autoracks *still* look toylike
and unstable on 36" curves.  Some big steamers (PRR 2-10-4, 4-4-4-4, 4-4-6-4,
etc.) DEMAND at least a 38-40" radius.  The 36" radius also allows you to 
lower the helix grade to 2.6%, which shouldn't cause any problems until you get
over 50 cars in a train.

Our club has a two-turn 48" radius helix.  This is optimum, but unrealistic
unless you have a *lot* of spare room.  It was used to lower a staging yard
so that we could "comfortably" stack the two yards on top of each other.

This lowered the grade to a hair under 2%, which on a 48" curve is negligible
compared to the 2% mainline grade and 36" visible curves.

The best rule of thumb I know for helixes is *avoid them at all costs*. The 
second rules of thumb is *if you need them, make them as big as can fit*. 
The third rule of thumb is *always test them gradually*. If you put a 50-car
train into a 24" uphill helix, you will either stall due to lack of power, or 
if you have enough power, you'll dump half of your fleet to the floor when 
it decides to pull off the curve. The last helix rule of thumb is to put 
feeder wires on every turn... there's a lot of track in there... you *will*
get power losses.

Sorry for the length, thanks for the time!

-- 
===============================================================================
Dennis Lippert - macman@unix.cis.pitt.edu
The "Mac Maniac" operator at the Hillman Research Lab - Univ. of Pittsburgh
- all comments are mine, don't use them without proper identification.