Minimum Radius

[peterjg]

Being new to the hobby I recently bought my first loco a Bachmann Standard Class 3MT Tank but was disappointed to find in the instruction small print that all Bachmann locos are designed to run on a minimum 2nd radius, as I have a double oval with 1 and 2 radius will this mean I can only run it on the outer track, what would happen on the inner, derailing or other problems?
What locos should I be looking for to run on the 1st radius?
I have the track but it’s not wired yet so can’t try it out.
Any thoughts on this would be appreciated by this newbie.

[stuartp]

On radius 1 curves you will find two basic problems with modern steam locos - either the leading wheels on a six-coupled loco (0-6-0, 2-6-0, 4-6-0 etc) will ride up over the outside rail and derail, or leading bogies will catch on the backs of cylinders etc. You will not be able to fit some detailing parts - front steps, drain cocks and the like. In the past manufacturers used dodges like flangeless wheels or slightly smaller wheels on the centre axle, effectively turning the loco into an 0-4-0.
With deisels you may find that on modern drive trains using cardan shafts from a central motor driving gear towers above each bogie, the bogies will not turn far enough to accommodate r1 curves. Again, in the past, with bogie-mounted ringfield motors, the bogie could turn as far as the loco bodywork would allow with often spectacular overhangs on curves.
R1 curves are the reason that lots of older models, particularly Hornby, are too high - the body is lifted a couple of mm to allow coach bogies etc to turn without the wheels catching the back of the solebars, and coach wheels are often 12mm instead of the correct 14mm.
Stick to locos with a shorter coupled wheelbase - 0-4-0s or short 0-6-0s like 08s or similar, or older models like some of Hornby's Railroad range. I'm reasonably sure the Railroad Black 5 will go round r1 curves (it certainly did when it was a main range model, not sure now it has a loco drive chassis) whereas their current main range scale version would be a bit marginal if it fitted at all.

[Mike Parkes]

The only locos in the current Hornby catalogue stated as being okay on first radius curves are all 4 wheel locos, the Railroad 0-6-0T, the Peckett B2 0-6-0ST, Terrier 0-6-0T and BR 08.

[Bigmet]

There's a decent number of locos that will manage R1, despite what the manufacturers state. The layout builders part in this is first making sure that the track is laid dead flat, the rails have no dips or bumps and are a smooth constant radius, and all the track joints on curves are unstrained. And then secondly making sensible model selections, and then being prepared to make small adjustments to the models as required, to allow trucks and bogies to swing sufficently, and driven wheels to move side to side. This may involve shaving off a little more plastic on external detail close to the wheels and truck or bogie frames and removing or correcting any internal constraints. (On the latter, I have seen wiring on twin bogie locos 'tidied' so aggressively as to limit the bogie swing to prevent R2 being negotiated.)

Try your 3MT with the pony trucks off, as an 0-6-0, to assess if the coupled wheelbase will work on R1 (you will need a loose piece of R1) and view from the underside to ensure that the rods will not foul as the wheels throwover to take the curve. Some gentle and slight reshaping of the rods may be necessary to ensure all the moving parts clear each other. Once satisfied, then add both pony trucks and see if they need help to swing sufficiently for the R1 curve.

[Dad-1]

Hi peterjg,

The simple answer is try it, there is a good chance it will de-rail, but there are some
surprising exceptions. A warning note here, just because MY model will go around 1st
radius doesn't mean yours will.
The second Radius claim is simply to make the Manufacturers life easier. Set one basic
minimum that you publish in all your literature.

An example of this is my most recent purchase, a Hatton's Andrew Barclay 0-4-0. It clearly
states minimum 2nd radius. Rather a laugh as there's spare lateral movement when going
around 1st radius curves and could work somewhat tighter if wanted !!

There is a section here https://www.newrailwaymodellers.co.uk/F ... 16#p633067
where I've been running some quite long locomotives around my garage oval which has some
1st radius curves.

Geoff T.

[Mike Parkes]

A Bachmann Std 3 tank will not go round 1st radius curves - it is pretty close to its limit on 2nd radii. Bachmann have never designed a loco for use on 1st radius - its possible that some bogie vehicles (diesels and multiple units) might manage it and I would expect the WIckham Trolley to be okay but certainly not locos like the Std 3 with outside valve gear that limits the throw of the driving wheel axles.

[Bigmet]

...The second Radius claim is simply to make the Manufacturers life easier. Set one basic minimum that you publish in all your literature...

But why was that selected as the minimum radius? Because it is essential for the current UK set track system, as it is the substitution radius of the curved road of the standard (and for a long time the only) point. I have heard dealer estimates that 80% of their customers with layouts will have set track points on that layout, so RTR OO models have to work on it.

Note that 'substitution' radius: the curved road actually goes tighter than R2 as it comprises an initial straight section, a curved section, a straight section throught the crossing, and another curved section. In order to make an equivalent to R2, the curved sections of the point have to be tighter than R2: overlay an R2 track piece aligning the ends, and you can see this.

[peterjg]

Wow! I have got a lot to learn I will be much more careful in my choice of loco in future, thinking back I remember my Dublo locos didn't have flanges on the centre driving wheels but that was 60 yrs ago.
When the track is wired I'll give it a try and only use it on the outer loop if I have too.
Thanks all.

[GeoFF03]

A Bachmann Std 3 tank will not go round 1st radius curves - it is pretty close to its limit on 2nd radii.

I have one, and it runs perfectly well on my 1st radius loop. In fact, very few of my collection, Hornby or Bachmann, will not run on 1st radius, steam or diesel.

[Bigmet]

Another thing to look for on six (or more) coupled locos that you might want to try on R1, a hinge or hinges in the side rods. Does aid flexibility, and is now usually found on RTR OO models.

Wow! I have got a lot to learn I will be much more careful in my choice of loco in future, thinking back I remember my Dublo locos didn't have flanges on the centre driving wheels but that was 60 yrs ago...

There's a treat awaiting you if current twin bogie models might be of interest. The smaller BR pilot scheme BoBo diesels, EE type 1 (20), Derby type 2 (24) which would overlap in service with your 3MT are both available from Bachmann, and will work happily on R1. These have the considerable asset of the now standard centre motor all wheel drive and pick up mechanism combined with a body filling metal casting for weight, and 'perform'.

Dapol and Heljan have offered further BR pilot scheme BoBo types (TOPS 15, 16, 17, 21, 22, 23, 26/0) with equivalent mechanisms, but I haven't tried any of these on R1 because the friend's sometime layout with R1 on it has now gone, replaced by a vast domain with easy curves.

[Bufferstop]

It seems that 2nd radius is another "safe figure" for manufacturers to use, like the 100 hour motor!. RTR models are mass produced, mass production relies on tolerances, specified figure equals target plus or minus tolerance. All plusses on the pin and all minuses for the hole are still expected to be assembled so all minuses for the pin and all plusses for the hole means "it ain't half gonna' be a sloppy fit"
Some very close to the lower limit locos are going to get round the tighter curves in the right circumstances. We've already said smooth and level helps, a bit wide on the track gauge and a bit tight on the back to back is going to help as well. Some formations are just asking for it, reverse curves are going to be dodgy if there isn't a straight bit in the middle. A curve that starts as second radius and tightens down to first before easing back out to second might just get that longer wheelbase through, provided the driver does his bit. A real killer is a reverse curve formed of two "first radius" (ie Settrack) points in a crossover. We compress the sort of dimensions that railway track uses to degrees never intended to work, look up the minimum curvature (they rarely call it radius) that prototype locos are expected to traverse. A settrack layout wouldn't be acceptable round most dockyards and they are notorious for tight curves.

[Dad-1]

I'm going to add a few more - All small, to that thread in FAQ's.

Geoff T.