Pilot and Trailing Wheels

[chuff_n_puff]

Are any of the steam model trains prototypical when it comes to the way the pilot or trailing wheels are mounted? In other words, is all they support is their own weight and not part of the weight of the locomotive?

[SteamGene]

The trailing trucks support the weight of the firebox and the cab, which is the reason for the progression from -0 to -2 to -4 and even -6.  The pilot trucks are generally for leading the drivers into a curve which is why passenger locos have 4- while freight have 2- and switchers have 0-.   In fact, I've read that at the end of steam when some road engines were reduced to switchers they'd pull the pilot trucks off and an old Mike would become a 0-8-2.
Gene

[Atlantic Central]

chuff_n_puff,

To go into just a little more detail, steam locomotives have a complex suspension of springs and levers that are linked together so that irregularaties in the track still result in even weight distribution.

Each different wheel arangement is slighly different in the way it works , but the effect is the same. Shops and/or original builders can ajust the leverage points to transfer more or less of the weight to/from the leading and trailing trucks.

The suspenion linkage of the lead and trailing trucks is also built in such a way that it applies side pressure to the frame as the loco goes into a curve. This side pressure helps "turn" or "guide" the drivers into the curve.

Many large, long wheelbase locos had elaberate side motion devices that allowed the first set of drivers to move side to side and they would actually be pulled to the left or right by the pilot truck as the loco entered a curve.

A specific example of modification like Gene refered to was on the B&O. They took a number of their heavy Consolidations (similar to the Bachmann model) and made 0-8-0's out of them in the late 40's. By elimination the pilot truck, more weight is placed on all the drivers but speed and tracking in curves was reduced. Not a problem for a switcher often only going 10-15 mph at most. This made for a very powerfull switcher that could pull just as much as medium or large 2-8-2, but at a lower speed.

There is no need for our modeles to have full working suspensions, if fact they would not run well if they did. As I have explained in other threads, physics does not scale down and our models actually run/pull better with all (or at least most) of their weight on just two drive axles. And, the less pressure the pilot/trailing truck have the better our models pull.

Sheldon

[Woody Elmore]

If you designed the pony and trailing trucks on models to work like the prototype, you'd need much larger radii track.

If you ever get the opportunity to visit the Smithsonian in Washington, go to the building that houses the SRy Ps-4 locomotive. Look under the cab. There are all sorts of interesting little gadgets under there including the screw device for the automatic stoker.  Accurately modelling them and making it work on HO track would be a real challenge.

I'm not saying it can't be done but you couldn't do mass produced models very easily or cheaply.

Henry Ford told Firestone that his requirements for tires were: "Round, black and cheap." Modelers want black and cheap so, often, detailing and accuracy suffer.

[Jim Banner]

Sheldon has given you a good explanation, except for the last paragraph.  Model locomotives that have been built with full suspension on the drivers have improved traction (all the wheels are working,) improved pickup (all the wheels are touching the rails all the time,) and stay on the track better because they do not lift any wheels on rough trackwork.  But don't look for these in you local hobby shop.  The only ones I have seen were in 0-scale and larger and were all custom built.

[Atlantic Central]

Jim,

In HO and smaller scales I beg to differ. My own pulling power tests show that pound for pound locos with their weight on two rigid axles pull better than those with sprung drivers. Without the benifit of an interconnected system like the prototype, and the fact that the math of adheasion does not scale down, simple sprung drivers do not improve performance.

All the models I have ever seen, in 40 years in this hobby, with true working suspensions (not just sprung drivers) where in 1/4" scale and above.

Here is the data from my tests:

The following info is provided that it may help modelers and manufacturers alike in their quest for better steam locomotive performance.

In conducting various pulling ability tests and questioning why some locos which seemingly should pull well don’t and conversely why some pull much better than would be expected, it became apparent that a number of factors other than weight may be major factors in pulling ability.

It seems that having all or most drivers sprung does not universally improve performance/pulling power. In fact the data below suggests that suspensions that place most of the loco weight on two rigid axles are the best pullers.

Those with traction tires and this type of suspension pull dramatically better with weight only being a secondary factor as observed with the Proto2000 0-8-0 which easily pulls 40 cars at only 9.1 oz. and the Mantua General which pulls 36 cars at only 7.2 oz. both have all their weight on two axles and have traction tires on at least one wheel. 

The Bachmann 2-8-0, while not a poor performer, seems to actually suffer a little from having three fixed axles and the Athearn 2-8-2, even with added weight and better balance actually seems to suffer from having too many axles sprung.

And based on this data, the Bachmann 4-8-2 Heavy would be an even better performer with a traction tire on driver #3.

This chart notes the configuration of each driver from front to back - see the cahart at the end of the list.

LOCO               DRIVERS      PULLING ABILITY

LIFE LIKE PROTO 2000 0-8-0   1-F, 2-S, 3-S, 4-F,T    40+ CARS
WEIGHT - LOCO 9.1 OZ   TENDER 3.0 OZ   TOTAL 12.1 OZ

ATHEARN GENESIS 2-8-2       1-S, 2-S. 3-F, 4-S   35 CARS
WEIGHT - LOCO 14.5 OZ   TENDER 4.3 OZ   TOTAL 18.8 OZ

PCM T-1 4-8-4         1-S, 2-F, 3-F, 4-S,T   65+ CARS
WEIGHT - LOCO 24.0 OZ   TENDER 10.0 OZ   TOTAL 34.0 OZ

BACHMANN USRA HVY 4-8-2   1-F, 2-S, 3-F, 4-P                   38 CARS
WEIGHT - LOCO 20.0 OZ   TENDER 6.6 OZ   TOTAL 26.6 OZ

BACHMANN 63” DRIVER 4-6-0   1-F, 2-F, 3-P      17 CARS
WEIGHT - LOCO 9.5 OZ   TENDER 5.3 OZ   TOTAL 14.8 OZ

BACHMANN 2-8-0         1-F, 2-F, 3-F, 4-P   23 CARS
WEIGHT - LOCO 13.2 OZ   TENDER 6.6 OZ   TOTAL 19.8 OZ

MANTUA 4-4-2         1-F, 2-F      17 CARS
WEIGHT - LOCO 14.2 OZ   TENDER 5.3 OZ   TOTAL 19.8 OZ

MANTUA 4-4-0 GENERAL      1-F, 2-F,T      36 CARS
WEIGHT - LOCO 7.2 OZ   TENDER 4.0 OZ   TOTAL 11.2 OZ

DRIVER KEY
F = FIXED, MINIMAL PLAY
S = SPRUNG
P = LOTS OF PLAY, VERTICAL AND LATERAL
T = TRACTION TIRE

Those locos with more than two axles, but having two rigid and the others sprung or with play pull the best.

Sheldon

[ebtnut]

To directly answer the original question (which can be inferred from the previous detailed answers), no, the pilot and trailing wheels on models are essentiallly "decorative".  There are a few high-end, custom built models out there that have been built with something resembling the prototype mountings.  But as has been noted, they aren't going to go around our typical model curves.  Most moderate size steam locos would squeak around a 20 degree curve (in yards, or on tight wyes, etc.).  That works out to about a 40" radius curve in HO. 

[Virginian]

  I am not aware of any models built with anything approaching prototypical type suspension on trucks, for all the reasons already noted, the primary one being money and lack of demand I am almost certain.  If it took 100" curves some collectors would want it if it was available.
  To vastly improve the traction on a multi-axled, fully sprung, HO scale model steam locomotive, get a lot softer springs.  I have not done any comparisons with sprung and unsprung combinations or such, but I can vouch for this change on many brass locos in years past.  As delivered, most sprung axles are sprung so stiff they may as well be fixed.

[ebtnut]

I concur with Virginian that almost all sprung model locos have springs that are too stiff to do much good.  OTO, finding the right springs for each situation would be near-impossible, since it depends so much on the characteristics of each model--weight, center of gravity, etc. I have heard of some folks who have used the springs from Kadee couplers, but they seem awfully soft to me.

[Jim Banner]

Sheldon, that is a very interesting table.  To properly analyze the data, we first have to remove the locomotives with artificially enhanced traction, that is, the ones with traction tires.  To compare locomotives with traction tires to ones without traction tires would be like comparing locomotives with sanders on to ones with sanders off.  That still leaves us with 5 good examples.

Now we know from the laws of physics that the more weight applied, the greater the friction (and yes, this does scale.)  An ideal data set would include different locomotives with different suspensions but all with the same weight.  Lacking that, we have to normalize out the weight differences.  The easiest way to do this is to rate the locomotives not by the number of cars they can pull but by the numbers of cars they can pull per engine weight.  In the table below, the locomotives are arranged in order of the number of cars they can pull per ounce of engine weight.

Bachmann 2-8-0              1-F  2-F  3-F  4-P     1.6  cars/oz.

Mantua 4-4-2                   1-F  2-F                   1.7  cars/oz.

Bachmann 4-6-0               1-F  2-F  3-P           1.8  cars/oz.

Bachmann 4-8-2               1-F  2-S  3-F  4-P    1.9  cars/oz.

Athearn 2-8-2                   1-S  2-S  3-F  4-S    2.4  cars/oz.

As you can see, your data indicates that the more sprung wheels there are, the better the locomotives pull.

The following article goes into the details of model locomotive suspension in greater detail and is worth reading:

http://www.clag.org.uk/41-0rev.html

Incidentally, all wheels sprung still does not "full suspension."  For that, you also need equalizing bars and levers.

[Atlantic Central]

Jim,

I stand corrected or at least more completely anylized. I hope to test more locos in the future and to continue to evaluate this data. I may have picked up on that if I had tested a larger number of different locos, but not counting articulated locos, that list is most of my fleet.

I have began new tests where I am measuring pulling ability measured in total oz of train weight, not mumber of cars. And have found some discrepancies in my orginal data. Small ones, but discrepancies none the less.

Fore my money, all these locos could/should have traction tires and get into that 4+ cars per oz range.

When I said the adheasion math did not scale down, I was refering to the fact that our model wheels and rail have a totally different factor of adheasion than the prototype. Yes it is proportional to weight has you discribed but it is not the same as the prototype.

If it was all our models would pull much more. Our locos weigh as much or more than the prototype scaled down and our freight cars weigh about the same as the prototype scaled down but the locos pull much less.

Thanks for the additional info and insight.

Sheldon

[Jim Banner]

Sheldon, I think this is a subject many of us are interested in and I hope you will keep us informed about any further testing.  I like your idea of using train weight (including, I assume, tender weight) as a measure of performance.  I used to be very anti traction tires because of their effects on electrical pickup, but now that additional pickup from tender wheels has become so common, I think that objection has gone out the window. 

[Bill Baker]

This is a great thread.  That's what this forum is for....keep it up and thanks to all.

Bill

[Atlantic Central]

Jim,

A few other thoughts.

The Athearn Mike was a dog until I disassembled it and added close to 4 oz of lead to weight and balance it. The test results reflect the weighted version. The driver springs are way too stiff for the factory weight.

It would be nice to measure a much larger sample of different locos, especially properly weighted fully sprung ones. I must admit I never bought brass do to what I considered a price/value issue. Too expensive for something you still had to work on to get running well, most of the time.

Setting the Athearn Mike aside, my data also suggests that the design used on the Proto 0-8-0 and the PCM Reading T1, is a very sound concept. That is on a 4 axle loco, having two rigid and two sprung with the traction tire. And, the Bachmann Heavy Mountain is similar, but with no traction tire and pulls very well also.

The T1 is heavy, but I did have to add some weight in the smoke box to get the best balance/tracking. Theyu left a big space in their for a smoke unit - why?

I will report back as testing proceeds.

Sheldon

[Terry Toenges]

I guess my question would be why the tender weight was included with the loco stats?
Unless the tender is powered, shouldn't it be treated like a car being pulled?