Engines Requiring Large Radius Curves--why?

"I am in the process of including  2 loops of 072 just so I can run some of the beautiful 072 models."

"But I'm curious to know what is driving the need for big curves when the sales incentive would seem to go the other way." 

#1 answers # 2.

Equipment isn't generic. I have 072 curves, except in certain yards where the big stuff (like the real world) never goes. Loco B will not substitute for Loco A when you want Loco A. I do not want "a loco/train/car"; I want "that loco/train/car". The incentive is indeed to make the biggest as flexible as possible; the cost of development is about the same, big or small, but the retail price we will pay will go up or down based partly on size. 

"Does the issue relate to scale versus traditional or near-scale? Is it the swing of the trucks? The overhang of the ends of the machine hitting tunnel portals and other track-close stuff?" 

All of that, and more.

"But I won't dare let the 072 guys stray into 054 territory when this is done."

They won't be able to (mostly; there's often fudge in the ratings - not always, though).

I think you answered your own question. IMHO it relates to scale appearance, both the engine, itself, and also how it looks on the track (over-hang, etc.). It seems to me that the minimum diameter an engine can run on is primarily, but not exclusively, related to the spacing of the trucks/wheelsets. So, you can't have a scale VL Niagra and use trucks that are spaced a short distance apart - it doesn't look right.

A good example is school buses. Years ago the rear wheels were well back near the rear end of the bus. Today's buses have the rear wheels much closer to the front with a lot of overhang in the back. I believe the reason is that the closer the front and rear wheels are to each other, the tighter curve the buses can negotiate. OTOH, they do look a little funny.

Having said all that, I think both Lionel and MTH have made great strides in the last couple of years to make larger engines be able to run on smaller diameter curves (witness the Big Boy in the 2019 Lionel catalogue running on O-31 curves).  A lot of people won't like the way it rounds curves, but it does give smaller layout operators the opportunity to run larger name equipment.

A number of steamer that require large radius curves actually only need them because of either the tender or the coupling between the tender and the cars behind the locomotive.  I know the coupler on the TMCC Three-truck Shay is the limiting factor for that locomotive, the actual locomotive has no issue making O31 curves, however that all changes when there's a car coupled to it.  There's actually a K-Line extension piece for the couplers that allow it to actually bring it's cars on O31 curves.

A friend that's into "O" scale fell in the same dilemma when he built his layout in the 90s. Short of restarting, when he wanted bigger engines, he started looking into many of the scale K Line engines which would negotiate the curves. Many of K Line engines equal some of the newly highly detailed engines and can still be found at reasonable prices. I believe his biggest radius is 048, but he has K Line scale  Hudsons and Berkshires running.

For the OP, it gets even more complicated!

Many times when one runs scale equipment, the limiting factor becomes the curve diameter for the passenger cars.  For example, if one runs scale passenger cars behind a scale Pacific engine, the cars may need O-72 curves while the engine only needs O-54 (or maybe O-42) curves! 

Any equipment always looks better on larger curves. My new layout is a carpet layout in our rental house and I have opted for Lionel O-84 and O-96 diameter Fastrack.  Looking forward to finally be able to run some of my scale passenger cars!

Adding on to John's comment, this is also why there are often two holes drilled into the drawbar connecting the engine and tender, with the one hole allowing the engine/tender to run on tighter curves.

Jim

Well large, scale size locomotives can't physically make it around sharp curves. In real life or model form. My MTH Premier Santa Fe Texas is not going to go around an 031 curve. The wheel base is too long to even fit. It's rated at 072 but will go around 054 curves without issue. 

My layout isn't massive (12x16 or 16x17, depending on how you measure it) but I have 072 curves on one line and 054 on the other. I could have had a more involved track plan with smaller curves, but then I couldn't run scale equipment. So I compromised to run scale equipment. You're not going to be able to run a scale Big Boy on a 4x8 layout. It's just physically impossible to have something that large go around the tight curves required. 

As for what's driving large curves it's people wanting scale equipment, which require large diameter curves to operate. 

Don Merz 070317 posted:

In another topic on the forum, a member is wondering why 072 is required for a specific loco he is shopping for. I have often wondered about the issue of curve requirements. Correct me if I am wrong, but as the required curve goes up, sales of that engine or car set must go down, right? Because fewer layout owners have layouts with those big curves., right? If that is true then manufacturers must have a strong incentive to make their gear run on the tightest of curves. Or maybe the collectors who never run their gear make up for the lost sales?

It bugs me sometimes when I see, for instance, a SHAY that requires 072 curves. What the heck? The prototype  was designed to go around a dime and give you 9 cents change. Now the model needs (what I consider to be) luxurious, mainline curves? I don't get that.

Does the issue relate to scale versus traditional or near-scale? Is it the swing of the trucks? The overhang of the ends of the machine hitting tunnel portals and other track-close stuff? 

Most of my layout is limited to 054. I am in the process of including  2 loops of 072 just so I can run some of the beautiful 072 models. But I won't dare let the 072 guys stray into 054 territory when this is done. And that leaves out most of my track plan. 

So anyway, I know this isn't world politics and I am just whining. But I'm curious to know what is driving the need for big curves when the sales incentive would seem to go the other way. 

Don

Well, what you perceive should be happening is not necessarily what really is happening.  A misperception.  Don't feel bad, happens to me sometimes, too.

Depends what size of prototype you are talking about.  The largest Shays and other geared locos needed a fifty cent piece to go around and only gave back 4 or 5 cents in change.

One other thing to think about, O72 curves, while you may think they're generous, are pathetically small compared to the prototype.  O72 curves translate into 144 foot prototype radius, and most full-sized locos couldn't even begin to operate on curves that small.  If a manufacturer produces a large, scale-sized loco and says it needs O72 minimum, we better thank our lucky stars we can run it on curves that small!