Larger engines on 18" radius curves?

[dknelson]

From time to time someone posts a question or comment about how their HO equipment does not run on 18 inch radius curves. The usual response is that 18 inches is a darn tight curve and that if you know your curves are going to be that tight, you should adjust what engines and cars you run accordingly.

I was looking at Model Ralroader for March of 1963 page 69. There is a very brief item reporting that Francis P. Adams (a well-known modeler at the time) had installed some 18 inch radius track and discovered a way he could make engines run on it which otherwise would have problems with anything less than 22 inch radius curve. The secret is to use transition spiral easements into the curves, and he "superelevated" the curve by the prototypical 5 inches. Easements are a rather technical subject and some people approximate them by having the tangent go into a curve which is about double the general radius -- meaning a segment of 36 inch radius which evolves (if you will) into the 18 inch radius. No sharp changes in radius. Obviously this means the curve is bigger in overall radius than pure 18 inch radius would be. Mind you this 36 inch radius idea is a substitute or mere approximation of the actual math of an easement. MR has published easement templates from time to time. I think the NMRA may offer some as well/

The idea is that straight into curve -- any curve -- is actually a more wrenching challenge to the wheels than the pure curve. As to what the superelevation adds (most sources seem to agree all it does for a model is appearance) I do not know but I guess it makes a little sense if it shifts some weight..

I offer this for what it is worth. If you are thinking of trying this out (and in no way am I suggesting that 18 inch radius curves are good - and note that Adams's engines DID run on 22 inch radius curves so we are not talking Erie Triplexes here) then this is the perfect time to try a suggestion I have made before: buying the cheapest old used track at a swap meet -- used brass curved track is often cheap -- lay some track on a piece of plywood and try out your equipment. Some people objected that I was advocating brass track. No. I do not mean to suggest using brass or cheap track on the actual layout -- this is just for a "test bed" which you might well decide to discard once your tests are over.
Dave Nelson

[Anonymous]

Here's a link to Marty McG's on-line article on easements. He notes that if you are using sectional track, you may approximate an easement by adding/replacing a section of 18" radius with a 22" radius [added] at the entrance to and exit from the curve.

Anyway, here's the article: http://www.trains.com/content/dynamic/articles/000/000/001/546jidvq.asp

And here is another (with templates):

http://www.trains.com/content/dynamic/articles/000/000/001/647dsuww.asp

Andrew

[Jetrock]

Easements are helpful in many situations--even for trolleys, an easement into the curve helps the trolley negotiate sharp corners in an absolute minimum of space.

I suppose those of us fonder of small engines have an easier time than the articulated enthusiasts...

[lupo]

jetrock, masonjar and dknelson
In an article I read about a "banked " curve: putting the outside of the curve a bit higher than the inside, do you have any opinion on that ?
Does banking a curve has any use in getting large locomotives through a curve ?

[Anonymous]

lupo - Dave touched on that - it is called superelevation. It is like banking a road. I guess it would help, but it depends on what you are running. For me, I do not have any experience with superelevation, as my locals and switchers all run at slow speed, even out on "the main".

Also, I wonder how effective it is in modelling, as our locos and rolling stock do not have the same weight distribution as the real thing.

Andrew

[Anonymous]

lupo - Dave touched on that - it is called superelevation. It is like banking a road. I guess it would help, but it depends on what you are running. For me, I do not have any experience with superelevation, as my locals and switchers all run at slow speed, even out on "the main".

Also, I wonder how effective it is in modelling, as our locos and rolling stock do not have the same weight distribution as the real thing.

Andrew

[Fergmiester]

All my main line curves are super elevated. I can honestly say it is a challenge to say the least. Does it help the engines and rolling stock? Not really, what I have found is the Heavy weight passenger cars have issues with these curves. They have to be perfect. Why do I do it? Because it looks really good watching an engine banked in a turned. Am I an expert? No way! I am still learning this aspect of the craft and have long way to go.

The real challenge is starting long consists that are on a superelevated curve. A good jolt will send a whole lot of cars into the raveen!

[dknelson]

I believe the "trick" to superelevation is that the banking needs to start a bit before the curve does -- but not too far. And as I understand it, different super elevations are chosen for different normal speeds. Back when the C&NW 400 ran through my old home town, probably around 90 mph, I suspect the curve was superelevated substantially and I assume that track was adjusted when passenger service ended in the early 1970s.
What Marty McG's article did was basically what i was suggesting -- approximating an easement -- but using 22" radius as an easement into 18" is not much of an easement -- I suspect he was just confining himself to what prefab track is or was available.
Dave Nelson

[Anonymous]

Well, my 4-8-2 is supposed to be on 22", but it goes around my 18"'s just fine.

[CBQ_Guy]

I seem to recall in an article or a book, possibly on tweaking Athearn blue box diesels years ago, some sort of modification to the middle wheelset on the 3-axle trucks. I believe this was in or put out by, Model RailroadING mag.

Maybe this will jog someone's memory.

[Anonymous]

Using spline roadbed will automatically give you curve easements. I have done the superelevation thing with mixed results. Too much superelevation and the train tends to tip and/or come off the rails on the inside of the curve. Like a previous fellow stated, the trains look super rounding a curve on superelevated track. I limit the superelevation to a 1/16" shim (max) under the outside rail. I personally do not think superelevation will help with stuffing a large locomotive through a small radius curve, but that is just my opinion.

Keep the steam up.

Tom