Why did the Berkshires have two size trailing wheels?

Smaller wheels on one axle was needed for clearance of motor and body of engine on tight 027 and 031 curves.

American Flyer just left one axle off some of their models.

Motors were much larger then.

Hopefully, Rich or Hot Water will chime in here soon, but MY understanding was that these (or many of) had boosters for starting heavy trains, and the smaller wheels were for clearence reasons. If I am wrong I Hope that someone will correct me (politely, Please)

Doug

the NKP Berkshires did not have a booster engine in the trailing truck, yet the there is about an 8" difference between the rear wheels (44" diameter) and the front wheels (36" dia) on the trailing truck.  looking at the drawing, i've got to believe it is necessary to clear the ashpan which slopes downward from the rear of the firebox.

in this case the model is actually following the prototype.

Booster engine

There were attempts to power some of these wheel sets with small steam motors.  Additional tractive effort I guess??   There were other steam power accessory motors, Stoker, most noted.   

as you can see, the stoker is well to the rear of the trailing truck and actually more adjacent to the larger rear wheels...

Bingo...give that man a cigar. Ash pan clearances were the reason and you can clearly see that in the broadside shot posted above. It has nothing to do with a booster engine.

Weight is another reason. Each of those axles carries just over 61,000 pounds.

I wonder if having smaller wheels in the front set of wheels on rear truck may have been common to all "super power" engines?? I know NYC Hudsons also had smaller front wheels in the rear truck.

No fair.  I didn't know we were playing Bingo!  Actually I think  I should get 1/2 credit for the answer booster engine.  Applying power to a smaller diameter wheel doesn't make good mechanical sense.  Hence a larger wheel.

Well maybe 25% 

Larry

But the NKP's Berks didn't have boosters.

Dag Nabit,  Foiled again.

Do we have any Berks with same diameter trailer wheels?  Or any four wheel trailing trucks of this nature?   Sure do!  There may be times when a smaller front axle/wheel set is called for, but I don't think it's set in stone.  This should turn into a most interesting thread !

C'mon man!  It's aesthetics, plain and simple.  Just look at how beautifully the long (6' 6" wheel-base) triangular trailing truck with different diameter wheels fills the space created by the sloping firebox.  Compare to the 5'' wheel-base trailing truck under an ATSF 5001 class 2-10-4.

ChipR

LLKJR - Your thesis that applying power to a larger diameter wheel is questionable. If you are correct, how come 0-6-0 switchers didn't have 80 inch drivers? Actually, just wanted to say that because an 0-6-0 with 80 inch drivers would be a pretty humorous sight! Please take no offense!

There were two Berks with the same diameter trailer wheels, Santa Fe class 4101, and L&N class M-1.

As for other wheel arrangements there were many examples in every design with trailer wheels of the same size.

Stuart

i'm sure some design decisions will never be 100% understood.  for example, ok, the front wheel on the NKP Berk trailing truck is 36", but so is the front truck wheelset.  why not make both rear wheels 36" and have one less wheel to keep in inventory.  perhaps 44" wheels were already in stock from a previously discontinued design.  perhaps the larger rear wheel added better traction to the truck.  ... or maybe it just looked better.  some people will consider it fun to analyze to death, though.

cheers...gary

I don't think Lima, Baldwin or Alco had wheels "in stock" waiting to be pulled from the shelf for the next locomotive order.   Raw material maybe, or things like wheels for the trucks from an outside supplier.

How much traction does a trailing truck provide, anyway?

Then there's always the C&O Allegheny, which had two pairs of 36" wheels and one pair of 43" wheels in its training truck...

Rusty

NKP Berks had 36" wheels in the front of the trailing truck and 43" to the rear.  The C&O 2-6-6-6 used the same sizes - 36" or 43". 

Maybe these two were from the same builder

A training truck does NOT give traction if a.) it does not have a booster, and b). the booster has to be ON!

This is a question from a newbee, how does a booster work?  Is it a turbine?

if you had read and understood my reply, i was referring to the traction of the trailing truck itself.  traction is merely the grip of the tires to the rails which would obviously be effected by weight.  you are confusing traction with tractive effort or tractive force which is a completely different thing.

Wes gives a good explanation here...

http://www.steamlocomotive.com/appliances/booster.php

While on a visit to the Owosso shop about 2010, the trailing truck for the 1225 was out from under the engine and was apart.  I was surprised to see that the big axle (43" wheels) had a large gear pressed on it from when it was built at Lima in 1941.  Obviously it was intended for the day that a booster engine was added to the 1225's trailing truck but as I have gathered - that never happened?  I assume that it is still that way as she returns to service.

The PM 1225 is also an Advisory Mechanical Committee locomotive, same as the NKP Berks and the C&O 2-6-6-6.  Just last month a former NKP machinist passed away.  Howard Lewis, 93, of the Cleveland area was assigned to the Lima Locomotive Works for 4 months in 1942 to help assist with the inspection of new AMC  locomotives which included PM, NKP and C&O steam locomotives.  He helped inspect the second order of C&O 2-6-6-6 locomotives and as the "low man" on the team he did not understand until over 60 years later why they had him weighing all sorts of parts on the Allegheny order but not on the other locomotives.  It was of course the fact that the first order was horrendously overweight.  Howard was interviewed in October for the new WBGU Television (Bowling Green, Ohio, State University) documentary on the Lima Locomotive Works.  The following day he was stricken and went to the hospital, not to recover. 

I'm going to venture a guess here....and that's this.  Could it be that some mechanical officers and designers may have thought in the early years of four wheel trailer development, that a smaller lead wheel/axle might better guide the truck along, especially entering curves at speed?  IIRC, the original applicants of these trucks; AT&SF 2-10-2(4) and Lima 2-8-4s were low speed machines. First high speed app. I'm aware of would be first 4-8-4 or 4-6-4s, a bit later.  The large wheels on the booster axle makes sense when you consider the machinery needing to be installed on it.

Good grief! What part of this explanation did you not understand?

It has nothing to do with what wheels were in stock, booster engines, smaller wheels guiding the truck or anything else. LOOK AT THE PICTURE!

As a matter of interest, the Berkshire was not alone. My MTH model of the French Chapelon, a Pacific, also has two different sized wheels on the trailing bogie. The size differences even greater I think, than the Berk.

Its the increased size of the boiler and balancing the weight of same.

Well actually, it is the increased size of the FIREBOX!!!!!

Dave,

Pacifics have a 4-6-2 wheel arrangement, meaning only a single axle beneath the firebox. If yours has wheels of different sizes on the last axle, then I would return the engine to MTH for repair! 

Chris

LVHR

The reason a 4-wheel trailing truck came to be in the first place was the Lima "Super Power" design. They enlarged the firebox (not the boiler) beyond what the Mikados (2-8-2 wheel arrangement) of the era had, in order to build in more steaming capacity. That necessitated another axle back there to carry the weight. Thus was born the 2-8-4.

The design of the larger firebox/ash pan combination required two different diameter wheels on the trailing truck to clear the ash pan and carry the weight.

That's it. It wasn't complicated.

I do not get it...

...could you provide 27 - 8×10 color glossy pictures with circles and arrows and a paragraph on the back of each one explaining what each one was used for?

<snicker>

LOL...now that's funny.  Although the photo does clearly demonstrate exactly what Rich is saying.

Of course it is Chris, my mind must be going. Now I have to look for the loco I have with the different wheels. It was what came to my mind.

Aw, c'mon now...

Everybody knows they just put too much air in the rear wheels.

Rusty

Hmmm, I always thought the heat made it "swell right up"...

I always considered the firebox and boiler two sides of the same coin.

i finally got it figured out.  Everybody has the wrong answer.  yea Rich even you.

It's because it makes the locomotive "look pretty".  My wife said so and she is never wrong.  

Larry