Minimum degree of curvature for Big Boys

Just because the BB could go on a 20° doesn't mean that UP laid track that tight.  I'm sure most of the main line the BBs ran on were much more broad then 20°, hence the light swing you see in photos.  Min requirements doesn't mean ran at requirements.  It's just to let operators know where it couldn't go.

First, there is no way a UP 4000 class locomotive will negotiate a 20 degree curve, no matter what the current manager says!

Second, throughout the various UP Employee Timetables of that era, you will find specific locations that the 800s, 9000s, 3900s, and 4000s were restricted from operating, i.e turnouts, sidings, curves, etc..

Alex,

Kratville's book, "Big Boy" discusses this, see pages 13 and 14.  The Big Boy would not clear a 12' wide load on a double track, 10 degree curve with standard 13' track centers.  There is also a diagram of a 12'6" load on a 50' car on a 13' track center curve with an accompanying table showing that this would foul on an 8 degree curve WITH 13' TRACK CENTERS.  The solution was to realign the curves, usually by jacking up and moving the track over 3 or 4 feet.

ChipR 

Here are a few conversions from degrees of curvature to the more familiar O scale diameters.
  5 degree curve = O-575
10 degree curve = O-287 (considered a SHARP curve on a main line.)
15 degree curve = O-192 (rarely used on a main line)
20 degree curve = O-144 (NEVER used on a main line)
25 degree curve = O-115 (NEVER used on a main line)

NKP 765 (a Berkshire) is rated to go around a 20-degree curve. It doesn't like it, but it will do it without derailing.

The limiting factor with a Big Boy is NOT the locomotive. It is that long, rigid wheelbase on the TENDER that is the limiting factor. I seriously doubt that a Big Boy tender will go around a 20-degree curve. Up front, the boiler overhang on a 20-degree curve would likely not clear the next track.

Not considering the overhang, the 4014 should be able to go wherever the 844 has been in terms of curvature...is that correct?

So far, that sounds logical since the 3985 could go places that 844 could not. 

I am in the process of trying to find out what the curve negotiation numbers are for the Union Pacific "big four" steam locomotives, i.e. 800s, 9000s, 3900s, and 4000s.

Thanks for the conversion.  My largest loop is O-148 in 3-rail parlance, or 74" radius.  Big Boys and large Northerns can go around it, but the tail beam must be tapered to the rear.  The real thing did not have a tapered tail beam, and its track gauge is narrower.

I have more trouble with 80" drivered Northerns than with articulateds, and model centipede tenders - scale ones - easily go around 60" radius curves.  I think the limits on the Big Boy will be tail beam clearance, pilot wheel clearance, and brake shoe clearance between rear drivers of the front engine and the rear cylinder block.

Thank you all for your replies - my suspicion was valid.

Chip,

I'll look up that book - thanks.

Rich wrote, in part:

"The limiting factor with a Big Boy is NOT the locomotive. It is that long, rigid wheelbase on the TENDER that is the limiting factor. I seriously doubt that a Big Boy tender will go around a 20-degree curve. Up front, the boiler overhang on a 20-degree curve would likely not clear the next track."

Yes - even I have experienced this with my MTH Big Boy tender - it doesn't like some turnouts, such as Ross curved turnouts No.6's and No.8's.

Thanks!

Alex

To me, the 'pede tender seems to be a real pain in the caboose to operate and maintain.  UP had 6 axle tensers for the early 800's, and I think early chanlengers and the 9000's.  Why change the tyender configuration?  Looks like NYC used the same outline on some of its power.  Not all of the NYC or the steam era UP was not exactly straight in places!

If possible can you post the numbers for the centipede tender as well.

Excellent, that information will be greatly appreciated.  Thank you.

I think the Big Boys are awesome!  The new model coming out is the greatest ever.  But even in real life those things were so limited because they were huge!  I wish that I had the space.  I've only got a basement.  You need a barn!

It's been my understanding that articulated locomotives were design to allow larger locomotives negotiate sharper turns and lighter rails. (Though the Garratt is a more efficient design.) So, I would think that the UP articulates were more versatile than the 4-12-2 and the like.

But speaking of track curvature, reminds me of my favorite narrow gauge railroad: The Uintah Railway. Their grades up to 7.5% and curvature as much as 65 degrees at Moro Castle, plus their #50 and #51 Mallets fascinate me. You could model them with 044 track (albeit was 3' gauge track)

As promised, here is what I have found so far:

Curve negotiation, moving forward, maximum (sharpest) curvature

800 class (4-8-4)        14 degrees 

3900 class (4-6-6-4)   17 degrees

4000 class (4-8-8-4)   16 degrees

Curve negotiation, moving in reverse, maximum (sharpest) curvature

All three types listed    12 degrees

I have not been able to locate information on the 9000 class (4-12-2), but I'm still looking. Also, note that none of this information has been published in any books, nor "publicized" by the Union Pacific. These maximum (sharpest) curvature numbers were the result of "learning it the hard way" on the UP system.

Here is a C. E. Drawing from BIG BOY, by William W. Kratville, 1963, p. 14.

Fine, but that does NOT discuss curve negotiation. That drawing/report was about overhang on curves, which resulted in many locations where the outside track or siding, required wider spacing for the 4000 class locomotives upon delivery.

Excellent information HW, thank you for taking the time to track that down...I appreciate it.

Gentlepeople,

In the three Kratville books, Big Boy, The Mighty 800 and The Challenger Locomotives, the Union Pacific's mechanical drawings are reproduced.  All of the drawings for the Jabelmann locos are annotated as "Locomotives Designed for 20 Degrees Maximum Curvature".  

ChipR

All well and good, however NONE of those locomotives would actually negotiate a 20 degreed curve in actual everyday operation! I don't care what my late good friend Bill Kratville published.

Hot Water,

I'm sure you are correct on the everyday, actual track on the ground, however my qute is from the UP's own mechanical drawing and says "designed".

ChipR

Designed is one thing, but actual performance is another.  20 degrees just seems so tight for the 4000's.

But it is the same for all 3 locos.  Makes me wonder if the tender is the limiting factor.

ChipR

I'm not sure what you mean by "But it is the same for all 3 locos.", because what I posted above was for "all 3 locos". However, you are indeed correct, that that darned "centipede tender" is the real problem with the 3900s and the 4000s. The 800s, which their 22 foot ridged wheel base, as a result of the 80 inch diameter drivers, makes things even more restrictive.

The tender IS the limiting factor! I thought I said that above.

Centipedes are a pain....be they steam or Diesel !