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January 2003

What is the minimum radius for a 6 coupled loco?

One of the first considerations when designing your garden railway is the minimum radius. This will basically determine what sort of locomotives and rolling stock can go around your railway.

If you've got really tight corners, locomotives with more than 4 driving wheels may struggle on the corners. If you imagine a loco with 6 driving wheels (an 0-6-0, for example), going round a tight corner, then as the outer wheels on the front and rear axles rub against the outer rail, and the inner wheels on the centre axle rub on the inner rail, they cause friction which slows down the loco. Ultimately, with a radius which is too tight, the flanges bind to the sides of the rails, and the loco doesn't go anywhere.

As you can imagine, the more driving axles you have, the more pronounced this effect will be.

We don't need to worry about leading or trailing pony-trucks, as these are articulated. It's only the main set of wheels we need to worry about.

So, to answer the question, what is the minimum acceptable radius for a 6-coupled loco in 16mm/ft? In 2003, I conducted a straw poll on the 16mmngm mailing list to find out. Here's what various 16mm modellers said:

Tag Gorton
Most close coupled six wheelers can pass the checkrailed 2' 9" curve in my cutting but a Pearse Leek & Manifold needs a 3' 6" curve to be comfortable.

Cyril Rutter

 Tag Gorton 2' 9" L&M needs 3' 6" Cyril Rutter 3' 9" John Rogers 2' 0" (because I have to) Barry Reeves 3' 0" Sam Evans 4' 6" Roundhouse minimum 2' radius (according to their website) 45mm gau. 'Russell' 2'6
Just a point that Tag did not mention, a lot of 6 coupled locos in our scale do not have flanges on the centre pair of driving wheels thereby allowing them to negotiate sharper curves without jamming - you are quite used to this in 4mm scale ! ( In fact it applies on 9F 2-10-0's standard gauge ) I have two 6 coupled locos like this ( a Roundhouse Lady Anne and a Friog diesel outline Harlech Castle) and they will both negotiate Mamod curves - approx 2'6" radius - but not to be recommended !!! The overall wheelbase of both locos is 98mm (just under 4")
My minimum radius is approx 3'9"

John Rogers
Good point about the radius. I use a minimum radius of 2 foot because I have to. If I owned half of Surrey, 4 foot 6 would be my minimum.
John also mentions on his website that ...With hindsight, I should have built the appropriate jigs in my workroom and bent [the rail] there.

Barry Reeves
My line has 3 ft. radius curves.
Go for the largest radius you can but tight curves are alright as long as you run your railway accordingly. Coaches need to be relatively short. I've had an 18 coach 50 axle train around my 3 ft. radius reversing loop but I had to arrange the stock with care.

Sam Evans
I 'own' a tiny piece of Hertfordshire and in the (approx) 20' X 20'(probably less) I manage 4'6" min. It goes round the outside of the garden.
As for Surrey, if you are fool enough to pay for somewhere south of the Thames......

John Busby
...suggests LGB R3 radii (for 45mm), which is 117.5cm diameter. He also recommends adding 2.5cm outside the kinematic envelope for longer rolling stock to swing out on the corners.

Dominic Greenop
For 6-coupled locos, I recommend 4ft radius, with 4 inches clearance beyond the ends of the sleepers. 4-coupled locos can manage 2'6"

Off-the-shelf track
Peco SM32 set track curves are 2'6" (762mm) radius, as are Mamod curves. (16 Mamod sections to a full circle).
Somebody commented that locos slow down and skid a bit round corners because, unlike cars, they don't have differentials.

### Prototypes?

Surprisingly, these radii are realistic in the prototype. The sharpest curve on the Darjeeling Himalayan Railway has a radius of 69'6" (21.2m), which would be 3'7" in 16mm. This curve used to be 59ft 6in (18.1m), which would be 3'1" in 16mm scale, but has been eased.

The curves on the Welsh Highland Railway are considerably more generous. Here, a 50m radius curve is installed on the tramway section in Porthmadoc (on a road crossing), which would work out as 8'7" in 16mm scale. The three height-gaining loops in Beddgelert forest have a radius of just over 60m on a 1:49 gradient, although an abandoned curve on the original "1923" line was going to be 2 chains or 40.2m (on a 1:21 gradient!). That's still quite generous compared to the DHR, and would be 6"10" radius in our scale.

John R wrote on the 16mm e-group that... the Byers geared logging locomotive, the prototype for the Cricket , was designed for 25 foot radius curves (7.62,) or - 15¾" in our scale. That's tight!