B-B vs. A-1-A

[bnsfkline]

Ok....I may be dumb for asking this, as I should know the answer, but I don't

When the Internal Combustion Locomotive was in its early years, the railroads specified A-1-A truck for passenger (3 axel) and B-B (2 axel) for freight.

Why did they order the 3 axel locomotives on the passenger units and 2 axel on the frieght? I figured since frieght is heavier, the locomotives would need to be heavier also. So why not use the A-1-A trucks on the freight units and 4 axel units on passenger units?

[Leon Silverman]

The A-1-A designation meant that the center axel on the passenger trucks were not powered but this arragement improved the ride of the locomotive while reducing stress on the rails. The unpowered axel still created drag which reduced the effective tractive effort of the locomotive. Consequently, three 2000 hp passenger locomotive, even if they were regeared for freight train speeds, could not handle as much tonnage as four 1500 hp locomotives, especially up grades.
The examples you might have seen, where passenger locomotives were regeared for freight use, occurred in the 50's and 60's when a severe downturn in passenger patronage of the railroads left the railroads with a surplus of passenger locomotives with nothing to pull. However, a 2000 HP ALCO PA was probably not rated higher than a 1700 hp FA unit for freight work.

[CShaveRR]

Keep in mind that at least at one time the passenger units were not lighter than freight locomotives. Your typical E unit had two 12-cylinder engines in it, as opposed to a single 16-cylinder engine on a freight unit. This also meant that there were two generators and two cooling systems--and a steam generator, which wasn't exactly weightless. They needed those extra axles just to hold the thing up.

[Leon Silverman]

I did some reserach to further illustrate my point: The 2000 HP Alco Pa (with A-1-A trucks) is rated at 33,000 lbs continuous tractive effort. The RF15-16 Sharknose (B-B) locomotive (15 to 1600 HP) is listed at 73,750 lbs, while a 2000 HP GP-38-2 (B-B) generates 52,000 lbs tractive effort.
The drag of the axel on a A-1-A truck is the reason why steam engines horsepower ratings varied so much for the same engine. The highest value was obtained with engine tested in the shop on a stationary dynamometer that measured the horsepower deliverd to the driving wheels. When the locomotive was hooked up to a dynamometer car that was part of a train, the measured horsepower was much less because of the drag created by wind resistance and the wheels of the pilot, trailing truck, and tender.

[PNWRMNM]

Leon,

It is not the drag of the extra axle, it is that the E unit at 500 horsepower per axle had a higher minmum continuous speed than did a 1500 HP F unit at 375 HP per axle holding traction motor and gearing constant. It is also possible the the E was geared for a higher top speed which also means a higher minimum continuous speed which also would lower tractive effort.

I would be suspicious of the high TE reported for the Baldwin. At minimum continuous speed and lower, TE is limited by the coeffecient of friction. At low speeds about the best you can get on dry jointed rail is .25, add sand and you could get .33. Wet rail is about .18 wet and wet rail with sand maybe you can get back to .25. If the unit weights 260,000# on the drivers, at .25 your max TE would be 65,000#. Baldwin could have got the result you reported, but no railroad would dispatch trains based on that figure becuase it is unattainable in the wet.

Mac

[Leon Silverman]

Mac;
You arwe right about thwe Baldwin Sharks and the minimum speed. I found a PRR Railfan drawing source that rate the sharks at only 48,600 lbs at 9.9 mph. A dual purpose PA was rated at 17.9 mph. I seem to recall a photograph showing a ATSF passenger train pulled by F-units slowing down to 11 mph on a grade. However, assuming the traction motors were geared to operate at the same speed, the A-1-A truck would still generate less effective tractive effort due to the wheel drag even if the axel loading remained the same (which I doubt).

[David_Telesha]

The A-1-A was for weight distribution.

Passenger diesels were heavier and longer - carrying a steam generator, water for the genny, and the prime mover(s) (2 in a DL109).

Special NH units like the FL9 and CPA24-5 had extra length compared to the factory B-B product and had the rear axle changed to A-1-A to distribute the weight - the front truck was a B - making the unit's base B - A-1-A.

[beaulieu]

The A1A truck is not only needed for carrying the heavier weight of the passenger locomotive but also had less tendency to hunt at high speeds. All EMD E-units were built with 36 inch wheels, while all F-units were built with 40 inch wheels. This alone gives the F- units more TE from the greater leverage of the larger diameter.
The lowest gear ratio installed on a E- unit from EMD is the same as the highest ratio installed on a F- unit. A few E- units (notably those of E-L) later received freight gearing and larger wheels.

[rrandb]

QUOTE: Originally posted by PNWRMNM

Leon,

QUOTE: At low speeds about the best you can get on dry jointed rail is .25, add sand and you could get .33. Wet rail is about .18 wet and wet rail with sand maybe you can get back to .25.

Mac

It is hard to get above .25 even with modern traction control and electronic wheel slip. On dry clean rail sand reduces the # because of rolling resistance. When they sit to long with a leaking sander you may have to back up or sweep the sand for the engine to rollover the sand pile. I know it sounds nuts but from a dead start is when you need TE the most. To get back to .25 would be tough. They just do not pull as well when wet no matter how much sand though the newer ones are better than before TC & EWS. I am not a mechanical engineer but maybe someone else is.[2c] ENJOY

[PNWRMNM]

Leon,

I think you are misled about drag due the the middle axle on A-1-A truck. Where are you getting this from??

Mac

[GP40-2]

rrandb said: " It is hard to get above .25 even with modern traction control and electronic wheel slip. "

No its not. CSX's C44AC's easily produce a 0.40 rating in routine service.

[petervonb]

"Axel" was my father's name. The thing under the locomotive that came in groups of two or three is an axle. <g>

[Anonymous]

I'd have to agree with GP40-2's post. The upper .20's to .30 is the max adhesion DC diesel locos can sustain. The AC diesel's big advanatage/sellingpoint is that they can easily sustain well into the .30's and even the .40's The Trains issue on AC locos (both EMD's and GE's) from maybe 18 months - 2 years ago discussed this. I'm at work, so I can't tell you the month/yr of that issue, but I'll look it up when I get home.

I'm no expert so feel free to correct me where I'm wrong, but think it's really because of the nature of the DC traction motors and electricals (which can have a starting effort in the upper .30's, but will fry if they try to sustain those higher levels of efficiency for very long), vs the nature of the AC traction motors and electricals (which don't don't have those same issues of frying.) Some of the newer SD MAC's are thought to be capable of sustaining adhesion even into the .40's all day long, whereas the late model DC SD's max in the upper .20's

I just did a google search and found this interesting link. Many here know Kevin:

http://www.trainweb.org/eastpenn/sd80mac.html

It deals directly with the SD80MAC's, but it's the same for all AC / DC locos.

Enjoy the hobby!

Greg

[cmawdsley]

Anyone have any thoughts on the Fairbanks-Morse CPA-16-5 (and CPA-20-5). These 5 axle units were trucked a B - A1A. I always assumed it was to carry the extra weight of the steam generator, which was in the rear. As far as I know, only New York Central and Canadian National ran them.

[David_Telesha]

cmawdsley: Didn't you read my post up above???

[jchnhtfd]

QUOTE: Originally posted by cmawdsley

Anyone have any thoughts on the Fairbanks-Morse CPA-16-5 (and CPA-20-5). These 5 axle units were trucked a B - A1A. I always assumed it was to carry the extra weight of the steam generator, which was in the rear. As far as I know, only New York Central and Canadian National ran them.

I believe you are correct on that. On the FL-9s, the A-1-A rear truck was there to carry the extra weight of the transformer and control equipment for the electric running.

In neither case was there enough horsepower available to justify motoring the centre axle of the A-1-A truck; in the E units, it wasn't so much horsepower, as they were geared (mostly) for higher speed, and didn't really need the extra motor for torque at low speed -- which, however, doomed them for dual service or freight service! And also which is the reason Santa Fe used F's on the transcontinental passenger service -- 11 mph over Raton Pass was quite normal (big hill there!).

[PBenham]

Didn't Axel(Axl) play in Guns 'N' Roses? I digress. [#offtopic] The A1A-A1A wheel arrangement on E units and other builders passenger units like DL109s or PAs from Alco, Baldwin's 2000 hp Passenger units, and FMs "Erie builts", was dictated by the factors others have covered. There were some road switchers (Alco's RSC2,and RSC3, Baldwin's DRS6-4-1500 and successor AS416 GMD had an A1A-A1A version of its GMD1, and Montreal which bulit RSC3s,too and had the RSC13, an A1A-A1A RS1, and the RSC24, an S13/RS23 with a VERY short hood and the A1A-A1A trucks) that had this wheel arrangement to allow dieselization of branch lines with light rail or bridges, that limited them to units with very light axle loadings. Many of the lines that were in that catagory were either upgraded for heavier equipment, or abandoned. Soo re-trucked some of its RSC3s, While CN converted some of its A1A-A1A road switchers to B-B, simply by removing the unpowered middle axle. Then they turned around and re-trucked some RS18s, to A1A-A1A trucks, to replace older RSC24s or RSC13s, used on light railed branches in the maritimes.

[Anonymous]

QUOTE: Originally posted by beaulieu

The A1A truck is not only needed for carrying the heavier weight of the passenger locomotive but also had less tendency to hunt at high speeds. All EMD E-units were built with 36 inch wheels, while all F-units were built with 40 inch wheels. This alone gives the F- units more TE from the greater leverage of the larger diameter.
The lowest gear ratio installed on a E- unit from EMD is the same as the highest ratio installed on a F- unit. A few E- units (notably those of E-L) later received freight gearing and larger wheels.

It isn't the larger wheel diameter that gives a higher tractive effort, it is the lower gearing. A larger driving wheel actually lowers the T.E., just as it does on a steam loco.To compensate for this leverage effect, on a steam loco, the piston stroke would be lengthened, on a diesel loco the axle mounted gear would be larger, or the drive gear smaller, to give a lower gear, to raise th T.E..

[Anonymous]

QUOTE: Originally posted by jchnhtfd
[I believe you are correct on that. On the FL-9s, the A-1-A rear truck was there to carry the extra weight of the transformer and control equipment for the electric running.

What transformer? Why would you need to change 600 Volts DC into anything else to be able to operate 600 Volt DC traction motors? Some extra relays sure, some resistance grids probably, but no heavy transformer.

[David_Telesha]

There was some additional equipment but no "transformer" that I know of - and I've been inside.

The additional control and related equipment simply took up enough space to need added length and pushed the steam generator back, making the rear A-1-A necessary.

[oltmannd]

A few facts:

Adhesion - EMD did quite a bit of work and developed something they called "all weather adhesion". That is the adhesion you could count on day in and day out in the heat, cold, wet, dry, etc. It INCLUDES the use of sand (when sand helps).

1st generation wheelslip - 15-18% all weather adhesion
IDAC/WS10 - 18-21%
SuperSeries - 24-27%
AC - 35% (That 45% in the SD80MAC article is a typo)

FM CPA20-5 and CPA24-5s:

The LIRR had 4 of the 24s and a bunch of the 20s. The NH has some 24's, too.

You couldn't have 5 powered axles run from one main generator because there is no transition scheme that would work. (series parallel to straight parallel). Nowadays you could do it straght parallel - the main gens have enough oomph.


Transformers in FL9s:

No transformers in FL9s! The FL9s used dyanmic brake grids with taps and a rotary program switch (just like a straight DC MU car or PCC car) to regulate the voltage to the voltage to the traction motors while running off 3rd rail. An FL9 weighs the same as a regular DB equipped FP9. The A-1-A trailing truck was used to keep the axle weights down low enough for the NYC's Park Ave Viaduct.

Why A-1-A ever?

Those who have thought about HP and TE needed are right on! You needed twin diesel engines in order to get enough HP per unit. At that HP, you only needed 4 powered axles, but the loco weight required 6, so A-1-A was a close to optimum solution.

Nowadays, you can squeeze a similar ratio of HP to TE into a 4 axle design ala F40s, but it's a tight squeeze on weight. Those steam genny equipped GP40Ps that the CNJ had for commuter service were over 290,000# with full supplies.

[CSSHEGEWISCH]

The FL9 had its B-A1A wheel arrangement due to the additional weight of the control equipment and the axleload restrictions of the Park Avenue elevated approaches to GCT. I'm not sure why the various passenger versions of the C-Liner, which operated on several roads (CN, NYC, LIRR, NH, and others) had the same wheel arrangement.

[wccobb]

Kindly remember that the first "E" unit came in 1937 and the first "F" unit in 1939. We're talking technology that is nearly 70 years old and that had to "work" on the physical structure (bridge loadings, etc.) of the railroads of that day.

Locomotives are weighted per contract. The weight of a "F" unit might be around 242,000# = 62,000# per axle. (For comparision, the weight on drivers of a USRA Heavy Mikado 2-8-2 was 240,000#). The weight of an "E" unit might be around 339,000#. On B trucks, that's 84,750# per axle = a very big NO-NO. On A-1-A trucks, that's 56,500# per axle. (For comparision, the weight on drivers of a USRA Light Pacific 4-6-2 was 162,000# = 54,000# per axle).

Those who have cited weight (and, by inference, weight distribution) as the reason for using the A-1-A trucks are quite correct. And those who cited riding qualities and horsepower-per-axle are likewise correct.

[Tulyar15]

In Britain in the 1950's a number of manufacturers buitl diesel locos with A1A - A1A wheel arrangement rather than Bo - Bo in order to spread the weight over a greater number of axles. At that time there were still a lot of branch lines where axle weight was a critical factor. Most of these lines were closed in the 1960's and improved constructional techniques have meant that locos have got a lot lighter. (The new rebuilt HST locos weigh 75 tons and are 2,700 hp).

One curiosity was the 1,400hp Co-Bo locos built by Metropolitan Vickers in 1958. Quite why they adopted this wheel configuration remains a mystery. These were built just after they'd built some 1,400hp Co-Co's for the Irish railways. Only 20 of the Co-Bo's were built and they only lasted 10 years. (But one has survived and is now being restored, with the aid of parts obtained from the Irish locos - they remained in service until the mid 1990's, albeit having been re-engined with EMD power units. The character "Bo Co" in the Thomas stories is based on a Co-Bo!).