Minimum Radius and Wheelbase length

[Down_Under]

Good day all,

 

A couple of questions on minimum radius and wheelbase length. Modelling a small industrial layout in P4. 

 

Largest loco has a 11ft wheelbase (Austerity and YE Janus) and the real thing has a minimum radius of 120ft (from manufactures brochure). If I scale this down to 4mm/1ft this equates to 480mm/18.89" - is that correct?

A 10ft and 9ft wheelbase wagon are less than this, thus should traverse such a radius OK? And should be a doddle for the shortest wheelbase loco, a mere 6ft, so a minimum real radius of 60ft  and 240mm/9.5" at 4mm/ft scale. 

 

Turning now to track planning and templot settings - when setting/building my custom turnouts I should set my minimum turnout and curve warnings to the radius of my longest vehicle (420mm)? If that makes sense?

 

Thankyou in advance

 

James

[Miss Prism]

Presumably you will not be using 3-link couplings?

 

[34theletterbetweenB&D]

I have no documentary evidence to support this, but based on  employment experiences I have the impression that the smallest points on industrial and dock premises connected to the national network corresponded to about two and a half chains radius (165 ft). 

 

Minimum plain track radius went as small as one chain (66 ft) negotiable (with significant gauge widening) by wagons up to 10' wb. Wagons were typically moved by adapted road vehicles. Not infrequently unintentionally moved off the rails at that.

[Down_Under]

34 minutes ago, Miss Prism said:

Presumably you will not be using 3-link couplings?

 

 

Yes I was - nothing seems to tight based on what I have laid out.

 

So I have not quite understood minimum radius then?

 

Here is a picture:

 

 

When I drew up the original plan I had the image exactly under the track, but I am am just learning and must not have saved it. Layout is 740mm x 245mm so more a working diorama.

 

I have been working/making sense of the industrial turnout data - comparing it to prototype and laying it out to check I was not too astray with my measurements. Obviously I need to do a little more work/struggling with getting some alignments correct. All the turnouts above are based on the non-prototype (and are not less than 550mm radius) - but then adjusted based on the industrial pointwork on the templot website/manufacture specs.

 

The largest wagon there is a Accurascale HOU the rest 1/108 16t 9f wagons.

 

 

 

 

 

[Down_Under]

Another question that had me thinking - is there a way to calculate minimum radius from wheelbase length?

[Michael Edge]

4 wheel wagons should go round much sharper curves than this but the Austerity and Janus should be OK at that minimum. That equates to the minimum on my Mersey Docks layout but all MDHB locos had flangeless centre wheels.

Whether 3 link couplings will work has more to do with the end throwover (distance from leading/trailing axlw to the buffers) of the loco concerned but this was also a problem in full size - MDHB locos usually carried a 6ft length of chain for coupling to longer wagons if necessary. There will also be problems with buffer locking (again not exactly unknown in full size) when propelling over reverse curves - a short length of tangent (straight) track in between reverse curves helps a lot.

[Siberian Snooper]

Devonport Dockyard has /had curves down to 1 1/2 chains 99ft and the couplings had two longer than standard links on internal user vehicles, if external vehicles were being moved, they usually used a short strip to get the additional length.

 

 

[Down_Under]

Thanks for the replies.

 

I guess I wasn't asking what was the min I could make, but rather have I interpreted the term 'min radius' correctly and correctly applied the term to my design/layout. Sorry if I was not that clear before. 

 

I am sure that I will have some more questions as I progress.

 

J

Edited July 4, 2019 by Down_Under
Typo

[Miss Prism]

4 hours ago, Down_Under said:

Another question that had me thinking - is there a way to calculate minimum radius from wheelbase length?

 

For 2-axled things, the limit for reasonably free running is given by flange grinding, which is where the flange tip contacts the running rail, and where one can expect rough running to occur. For chord length c  (i.e. the wheelbase), radius of curve r, and angle a of the wheel to the rail tangent:

 

 

c = 2rcos(90 - a) = 2rsina

 

For scale wheel profiles (with a front vertical flange angle of approx 70 degrees), flange grinding is generally accepted to take place where a exceeds 3 degrees. (A large wheel is more critical than a small one owing to the longer flange section, and in extreme cases the flange tip of a large wheel could contact both the running and check rail at the same time, but forget about that for a mo.) So:

 

c = 2rsin3

 

substituting:

 

r = approximately 10c
 

Edited July 4, 2019 by Miss Prism
clarification of front flange angle

[roythebus]

On pointwork would the minimum radius really matter? It's only usually a very short section. It's when you get to constant curves that problems are likely to occur.

[Poor Old Bruce]

11 hours ago, roythebus said:

On pointwork would the minimum radius really matter? It's only usually a very short section. It's when you get to constant curves that problems are likely to occur.

 

Another thing to consider as far as buffer locking is concerned (apart from overthrow) is not the minimum curvature but the rate of change of curvature. Where you have two similar wagons on a circular curve, the buffers should be OK, it's on the changes in curvature where buffer locking can occur.

[Gordon A]

Another way along similar lines to Miss Prism's is to take a straight edged piece of card and mark on the edge the location of the loco's three axles, using your longest wheel base loco.

Draw on Templot and print the minimum radius curve you wish to use.

Place the card with the first and third axles on the inside curve, and mark the position of the middle axle.

Remove the card. The distance from the middle axle mark to the adjacent curve is how much side play you will need in the middle axle. 

As already mentioned keep an eye out for buffer locking.

 

Gordon A

[Poor Old Bruce]

Another thought on minimum radius for six-wheel (model) locos is the lateral side-play allowed to the wheels. The Electrotren 0-6-0 chassis will baulk at curves which the J94 will sail round because of a lack of side-play (I,m working in 16.5mm btw). Fitting check rails and giving a bit of gauge widening could solve your problem. Going to 17mm gauge with a check rail will allow Bachmann MR 1Fs to haul trains of 4wh wagons round a 10 inch radius curve without any problem. 3F tanks still don't want to know but 57xxs might play OK. Propelling with 3-links is a different matter but OK with T/L couplings.

[Down_Under]

Thank you very much for all your replies. I will bear in in mind and do as suggested printout and and do some test alignments etc.

 

im new to this, so learning, not knowing much about the intricacies of trackwork/permenent way. Any hints, tips and advice are welcome.

 

will keep plugging away and post up for critical analysis as I got.

 

thanks again

 

James 

[garethashenden]

There are two related but separate issues with minimum radii track, they’ve both already been mentioned. There is the minimum radius a given vehicle can traverse on its own, and there’s the minimum radius that a coupled train containing that vehicle can traverse. The second is always larger, often considerably. 

 

Prototypical couplings don’t actually provide much in the way of lateral freedom, which is the limiting factor. With three link couplings, the chain itself can go wherever, but it doesn’t take too sharp a curve before the buffers come in contact on the inside of the curve. If it gets any tighter, one or more axles will be forced off the track. American knuckle couplers don’t have the issue of buffers, but they don’t pivot as far either and ultimately have the same problem on tight curves. I’ve seen US diesels given two minimum radii, one alone and one coupled. 

 

A factor in coupled minimum radius is the difference in end overhang between the two coupled vehicles. The wheels are never at the extreme ends of the wagons, therefore the coupling itself is outside of the track centreline in a curve. It needs to not be too far outside I feel we are to avoid trouble. The issue is most apparent when something very short is coupled to something very long, say a 16T mineral and a 22T tube wagon. The coupling on the tube wagon is going to be much further away from the centreline  than the coupling on the mineral, because of the differences in wheelbase and overall length. Bogie vehicles are a bit more complicated. The bogies let the vehicle negotiate tighter curves than a similar length fixed axle vehicle, but the same rules apply with different length vehicles. 

 

I hope that that helps a bit

[Hitchin Junction]

Another radius related problem that has not been mentioned, but which could be a bad surprise, is the reverse curve.  This can have a more serious an effect on longer vehicle end swing as  the ends of two adjacent vehicles are swinging in opposite directions at the crossover point. Having ashort straight section between the two curves helps, but the minimum safe length usually needs to be determined by experiment.

 

Tim

[Michael Edge]

There should always be a length of tangent (straight) track in a reverse curve - full size and model.

[Miss Prism]

12 minutes ago, Michael Edge said:

There should always be a length of tangent (straight) track in a reverse curve - full size and model.

 

It isn't always possible.

 

[jim.snowdon]

9 hours ago, Michael Edge said:

There should always be a length of tangent (straight) track in a reverse curve - full size and model.

Try telling that to a tramway engineer. Although it may be desirable, it is by no means always possible and street track geometry is distinctly constrained by the urban environment.

 

Jim

[dhjgreen]

On 19/10/2019 at 10:56, Miss Prism said:

 

It isn't always possible.

 

This topic  is in the handbuilt track section.

[sharris]

9 hours ago, dhjgreen said:

This topic  is in the handbuilt track section.

 

I bet the (1:1 scale) trackwork in the linked picture was put together by hand.