Skip to main content

I know it has been asked many times before, so please accept my apology for asking again.
I have read that the minimum radius for On3 is 36 inches.  Has anyone run equipment on a smaller radius using "transition curves" as outlined in the fabulous book by John Armstrong, "Track Planning for Realistic Operation?"  I have a MMI, K-27 and minimum space to operate and I know from my previous HO layout that "transition curves" work wonders in this regard.  I am Just curious if the K-27 work in a tighter radius.

Thanks for any assistance. Sincerely, james3751

Original Post

Replies sorted oldest to newest

Thank you Ace,  that is what I am going to do.  I'll report back when I get some results.  I had thought about trying that suggestion, but I also thought that I would take the long shot and see if any one had some information.  I am a big believer in transition curves as they worked amazingly on my previous HO layout.  John Armstrong's book shows how a transition curve into a 18 inch radius curve has less severe jerking motion on equipment than running on the tangent directly into a 22 inch radius curve...that's for HO.  I will see how it works with On3.
Sincerely,  James3751

Easements work very well. For On3, minimum radius depends on your rolling stock.

Are your locos still DC analog, or converted to DCC? Mine are all DCC and firstly, it greatly improves the slow-speed running, but also allows the stiff tether to be reduced to a single wire, which makes it much easier to turn. My MMI K27 will manage 24” radius, although it doesn’t look very good doing it.

The next problem is rolling stock. The San Juan Car Co flat cars, for example, are very detailed underneath and this restricts the travel of the trucks, which affects the minimum radius. There are ways of mitigating this, but it’s all things to attend to.

Wow, Rockershovel did you make my day, month and year...oh yes. indeed!!!  Thank you for the information.
I recently obtained some On3 items including two DCC equipped  MMI K-27's, freight cars and stock cars.  I also have some passenger cars that are metal. After doing some research, I noticed that a 36 inch radius was recommended for On3.   I also realized that a 36 inch radius plan would just not fit in the space that I had available and I didn't want a duck under track plan.  Now, having some what of a "contrary disposition," I drew a plan using 30 inch radius curves with, transition curves, because that would fit in the space that I have.  Then I got to thinking that the 30 inch radius would be too tight for my equipment. Knowing that the 24 inch radius will work,  the 30 inch radius radius curves should be great using transitions.  I don't think the grandkids will notice the overhand, if you get my meaning.

Thanks again for the information.  It is extremely helpful and I do appreciate the time you took to respond.  I am new to this forum and it excellent.

Sincerely, james3751

Rockershovel, are you sure about a 24” radius for On3? The MMI On3 K class engines have flanged drivers on every axle. The On30 models have blind center drivers. I own at least one of each loco class in On30 and my K-27’s absolute minimum radius is 26” and the curvature has to be perfect. The trailing truck does not like tight curves and will easily come off. It does pretty well on 28” but is still picky if the trackwork is not smooth.

The K-28’s on the other hand are magnificent. I have had them as tight as 24” and they track decently; again this is On30. My K-36 and 37 do not like the 28”, the pilot and trailing trucks ride up because there is not enough swing and the drivers will bind if the radius dips below 28” by a hair.
I worked at a train store from 2005 until 2015 and we dealt heavily in MMI, San Juan, and AMS. Our experience with the On3 models was that they really did need the wide radius. Our test track had a 36” radius curve and all the models did fine. Not sure what the true minimum is but it can’t be less than 30” and I am doubtful about that.

My locos are On30, I have a K27 and a K28, both converted to sound and DCC.

If you are saying from experience that the On3 versions differ in having fully flanged drivers, and this affects their minimum radius, I am happy to be corrected.

One thing I would emphasise is that both my locos are converted to sound and DCC, which means they have single wire tender connections (actually, one has two wires - one for the speaker and one for the tender pickup, the other has an insulated drawbar with current passing through it) . Neither locos has the original tether fitted and I would certainly agree that the original tethers are quite stiff, and this materially affects the riding of the trailing trucks on curves. I do also notice that my K27 has signs of minor relieving on the trailing truck, with a file retouched with paint - perhaps this is why?

I also wouldn’t use such radii on the scenic part of any layout. I’d say you need to be in the 30-33” plus range, even in On30 for that. I used to think otherwise but I’ve changed my view from experimentation. They look bad and run poorly on small radii and really, it’s only for short links in fiddle yards or off-stage stuff like that.

Definitely agree that track laying has to be  100% for any of this to work.

The On30 BIL 2-8-0 is very picky about curves, but the Bachman On30 2-8-0 has flanges on all wheels and no problems with radii down to 18”. If you WANT to build O Scale NG with that sort of radii, go to On30 and use the Bachman range, they will handle it - the 2-8-0 especially, is a super piece of kit at a good price, and the 2-6-0s are very nice locos.

Last edited by Rockershovel

I suspected your models were On30 and that the On3 statement was a typo. That’s interesting about filing on the K-27 trailing truck. I’ll have to look at mine and see what kind of adjustments I can do to it to improve tracking.

Totally agree with you on 30”+ radius for the any of the Rio Grande prototype stuff. The San Juan cars are particularly stiff. The On30 cars actually have a worse turning radius than the On3 cars since the wheels are closer to the center beam of the car.

Hi Ryan, thanks for the clarification and information in the On3 minimum turning radius in your post. You had mentioned that you had experience in a hobby shop that did substantial business in On3 equipment.  Did anyone mention the fact that they had used easement/transition curves to allow for a smaller radius curve than a 36 inch curve, that did not hinder the operation of the On3 equipment? 

None of our customers had mentioned laying transition curves although I think many of them were more collecting these models as opposed to building On3 layouts.
I’m sure a transition curve would help siginificantly with easing these engines into a tighter radius, the question is how tight will they go before derailing. My instincts tell me the driver wheelbase will be the limiting factor with the K class engines. I’d suggest experimenting with some flex track and tacking it down to see what they can do. One of the O gauge 3 rail topics had some good recommendations about using a piece of wood lath and pinning it down to lay out the geometry for a transition curve.

Flangeless drivers make a big difference, too. The BLI 2-8-0 isn’t a large loco but its fine flangers and long wheelbase make it quite light-footed over smaller radii. I’ve seen articles describing switching the drivers - exchanging the two rear sets, so the 1st and 3rd pair are blind - to shorten the effective wheelbase and improve this.

Referring to the MMI, On3, K-Series engines, I wanted to know if the wheel base spacing is all the same on each of the engines?  The reason being is that I am going to set up a series of different curves of different radius.  I will then use easement/transition curves from the tangent to each different radius.   Maybe, I'll find a suitable smaller radius than the recommended 36 inch minimum .  I have the MMI K-27, On3 and if the wheel base spacing on the other K-series engines is the same then a smaller radius would also work for them.
Now, as I write this I came to this brilliant epiphany...what's the point.  So Jim, you do a transition curve into a 35 inch radius turn and it works.  Hello, the lights are on, but nobody's home.  Honestly, sometime I wonder if my alter ego isn't the Scarecrow on the Wizard of Oz.  Out of sheer stubbornness,  without regard to logic, I'll try a significant radius reduction just to see what happens, say a 30 or 32 inch, just for the heck of it.  Yes, even though, through some quirk of luck, the engines run through it these curves, the over hang of the consists would undesireable.

Anyway thank you for indulging me.  It's enjoyable to have a place to comment with other modelers.  This forum has been helpful to me and I do appreciate it.


The MMI K-27 and K-28’s have a very similar wheelbase length for the drivers. The pilot trucks have very similar travel. The K-27 has less lateral travel but this limitation really only affects the On30 models. The K-36 and K-37 have a longer wheelbase for the drivers and will not go as tight as the 27s and 28s. I can barely squeak my On30 K-36 through a 28” radius but my 27 and 28 will handle it like a champ.
I don’t think you will encounter these issues with On3 models though. The wheelbase is the same for On3 and On30 models. The only reason that the On30 models can push the envelope on turning radius is because of their blind center drivers. The On3 models should all have the same minimum curvature (unless there is a limitation on lateral travel with certain models). I believe PSC advertised a minimum 34” radius for the MMI K’s. I would bet with careful track laying and using transition curves, you might get that down to 32”.

Joe, I haven’t seen anyone convert On30 to S however converting S gauge equipment to On30 is a popular thing. Some of the AF rolling stock adapts really well when On30 trucks are installed. I have a tank car that someone made by using a postwar AF gulf tank car and installing On30 trucks. It was then custom painted for Conoco. It looks really nice and the proportioning is great; on my first inspection I didn’t realize it was made froma Flyer car.

I’ve been a long time lurker on the OGR forum until now. Finally decided get off my butt and make an account. I don’t have that particular car handy at the moment. A lot of my train collection is still at my parents’ house since I don’t have room yet at my new house. I’ve brought some of it home to use for testing trackwork on an overhead O/On30 layout I’ve been building.

Most of my On30 collection are Colorado prototypes. I love SP narrow gauge and the SP in general as evidenced by my profile pic. I wish there was more available that is actually accurate to SP prototype.

Add Reply

OGR Publishing, Inc., 1310 Eastside Centre Ct, Suite 6, Mountain Home, AR 72653
Link copied to your clipboard.