years ago I had asked this same question and gotten the answers that I expected: to help guide the locomotive around curves and support the firebox. But after some recent reading, i don’t think this exactly correct.
I think the preferrence would be to have all wheels under the locomotive be driven. But this requires that all the wheels be the same diameter.
the problem is that on some locomotives, additional support is needed both under the cyclinders and under the firebox. One solution it to additional wheels, but they must be smaller in diameter. In front, they need to be smaller to be closer to the cyclinder. In the rear, their axle needs to run under the firebox. The flexible-beam truck could help the locomotive handle tighter curves.
Of course there are switch engines (0-8-0) that have neither, but are presumably built stronger. And I thought the Reading had removed the trailing truck under its 2-8-8-2; so was it really needed.
I think the correct answer is somewhere in between.
The designers try to allow for the conditions that will be encountered in the locomotive’s anticipated normal service. They’ve been known to get it wrong, so that adaptations and improvements to the suspension are needed. Yes, RDG modified its 2-8-8-2’s into 2-8-8-0’s. They also dismantled some Mallets and reassembled them as 2-10-2’s because the Santa Fe type was more in keeping with their needs.
B&O bought some fairly new 2-8-8-2’s from Seaboard around the early 1920’s. I have heard that the locos were too heavy for SAL’s track. But B&O removed the trailing truck, turning them into 2-8-8-0’s, and kept them in service for nearly 30 more years. I’m sure B&O must have made some significant changes to the spring rigging on those locos.
Bottom line: the wheels hold up the engine, and it makes sense to spread out the weight more-or-less evenly. Trailing wheels support the rear of the loco and help to guide it into curves. If the drivers extend far enough under the firebox, and if reverse moves are not a major concern, then it may be possible to dispense with trailing wheels. A leading (pony) truck supports the “engine” (i.e., cylinder assembly) and helps lead the loco into curves. Since most locos tended to run forward most of the time, normal North American practice usually dictated a leading truck. Interestingly, our friends in England used lots of locos that had no leading truck, assigning many to mainline runs with great success for years and years. The London Midland & Scottish 4F 0-6-0’s come to mind.
One of the only reasons for more axles being added is to support/distribute the weight. There were 0-8-0 switchers. Also remember that the drives on steam locomotives are one unit and the more you add, the larger the curves need to be. Powering the leading and trailing truck would require more parts, which not only make the locomotive more expensive, but harder to work on, more things to maintain, and it would cause a headache making the speed the same.
Pilot wheels ahead of the cylinders (or surrounding the cylinders) reduce the side-to-side ‘boxing’ or ‘nosing’ caused by piston thrust. Obviously, this is greater at higher speed (double the speed, square the force) which is why most high speed locomotives had four wheel pilot trucks.
This is not immediately noticeable in model locos, since the electric motor and gearing deliver smooth power at the main axle and there are no pistons on our piston rods. On the prototype, and on live steam models, the pilot truck is much stiffer (harder to turn) than on the typical model, where it usually can be forced to the side by a hard glance. A few of my models (1960s issue brass) have centering springs, but are nowhere near as stiff as their prototypes.
I have seen a drawing of that old PRR 0-8-0 locomotive before, in an article on PRR rarities by Model Railroad staffer Bill Rau – back in the 1960s. I recall Rau quoting a railway journal to the effect that that 0-8-0 had an unequalled ability to continue going straight when the rail curved. Translation - it should have been a 2-8-0. On the other hand the British railways had large drivered 0-6-0T passenger locomotives, but perhaps their curves were easier than comparable American lines.
Adding to the issues in this discussion would be the various logging railroad 2-8-2s, a few of which are preserved in museums. These are not Mikados in the normal sense because the trailing truck plays little if any role in supporting the firebox. Rather it was there to aid in guiding the locomotive in reverse moves through tight curves, since logging engines had to be bi directional and the track was rather casually laid.
Not really. After the mid-'20s, lots of locomotives had trailing truck boosters, either included on new locos or retro-fitted to older ones. These were used mainly for starting heavy trains, and cut-out once everything got rolling.
In later years, near the end of steam, some roads removed them to eliminate the extra maintenance.
Some locomotives had booster engines. Booster engines powered the trailing truck of the locomotive or lead truck on the tender. They were used on heavy trains in starting and/or in at low speed operations. The wheel diameter was smaller than the main drivers.
Interesting to think that the very purpose of leading and trailing trucks on prototype engines is the reason for their giving us trouble on our models. Without the sprung and equalized suspension of the prototype, these are just more wheels to derail instead of guiding and supporting the locomotive.
Well here’s an example of steam locomotive design evolution that led to the Pacific class 4-6-2.
From virtually its inception the New Zealand Railways were blessed with Locomotive Superintendants / Chief Mechanical Engineers, and Chief Draftsmen who were prepared to look at the current best practice not only from Mother England but the US and Europe, though not all the designs worked out first go.
An example of this was the U class, the first 4-6-0 tender locomotives to be built in New Zealand in 1894. As first built the boilers and tubes were too short to extract the heat from the firebox gases and were consequently bad steamers. The longer boilers of the Baldwin built Ub class saved the U class from mediocrity, in fact most of the class was still working into the 1950s, the last retired in 1959.
In some cases, especially with some older locomotives, the trailing truck didn’t so much support a larger boiler as provide guidance for reverse moves. In North American practice, locomotives without pilot wheels were generally limited to speeds under 20 mph. This is because tracklaying in America was not as meticulous as in Britain, which had more available capital and shorter distances to build.
why don’t diesels need leading wheels? Articulated wheel sets pivot.
Even for articulated locomotives, I think part of the reason is the weight of the cyclinders on an unsupported part of the frame. What if the cyclinders on an articulated were behind the wheels and next to the the cyclinders for the rear wheels?
Is it because larger wheels are more likely to derail?
There was one class of French locomotives that had the cylinders at the center - the DuBosquet 0-6-2+2-6-0. Both engines pivoted, but the pony wheels (which almost touched under the center of the boiler) were needed for weight distribution - they had to support the cylinders.
As for wheel size, some nineeenth century British and Continental locomotives had moderately humongous lead and trailing wheels, easily as tall as the drivers on contemporary American drag freight locos. It seems to me that bigger wheels get greater flange length into play to keep them on the rails. Of course, the drivers on those locos were also rather humongous - especially if built for Brunel gauge. (The drivers on the footplates, on the other hand, were normal size. [:-^])
From 1910 through 1934 the Japanese Imperial Government Railways used 0-6-6-0 Mallets as mainline power on stretches of railroad with the worst sustained grades. They were done in by more powerful simple 2-8-2s, line rerouting/electrification and their greater complexity/maintenance cost.
Greg, as you had heard before its mostly correct. To add, the pilot truck wasnt just for aiding in cornering, it also did support weight. As you have now read, it is true, you do want the weight on the ‘driving’ wheels. But not all of the weight can be put there.
The reasons are plainly evident if you follow locomotive manufacturing from its inception to the end of steam. The first locos where generally 0-4-0. Light, and had upright boilers. Track at that time was horendous! It took no time at all before the first locos where coming with a pilot truck (2-4-0). But you notice, they didnt support any (very little if any) weight. This was solely to keep the thing on the poorly laid rails. (you notice no trailing truck then-and they went both directions. remember this for later.)
As you, and the rest of us know from history, locos only got faster, but bigger. Not only in weight, but size (length). Trackwork got better too. But most was laid across the country before locomotives reached their zenith in design. 2 wheeled pilots gave way to 4 wheeled pilots on faster trains because of the inertia of the loco heading into a curve was greater. But that curve was laid 50 years ago and wasnt changed.
The weight situation concerning the pilot truck was just a given, but was just as important. Many late model steam locos moved appliances to the front like air pumps and FW Heaters (Think C&O, PM, GN, B&M etc). We both know the weight was better off being over the drivers. However, this happened. Now, not only did the front ends get a bit heavier, that coupled with the higher speeds required the 4 and in some cases the 6 wheel pilot truck (think PRR). Placing weight up front forced the truck into the curve. That equaled better handling, … ‘tracking effort’. Same with a high powered car… they use fins or wings and thrust from air. Locos dont have air, so they moved weight. Same effect.
Ya, short answer is front axles are there to help guide the engine through curves and turnouts, rear axles are there to help support weight of a larger firebox. If an engine has all drivers, the firebox has to fit between the drivers. With a trailing truck, you can build a much larger / wider firebox behind the drivers.
p.s. Given that steam engines have been around for 200 years, if lead and trailing trucks weren’t needed, I think a designer or engineer would have worked that out a long time ago…
As an addition to (or example for) this statement – got track that can only handle (for random numbers ) 70k pounds per axle, and a 150 ton 0-8-0 locomotive (75k pounds / axle)? Throw on a lead (and/or trailing) truck and pick up that 20,000 pounds of total over-weight from the drivers (likely a touch more – you’d want some wigle room with the weight).
Granted, I’m by no means an engineer; and am using back of the napkin math for a quick example. I have no idea how much weight an actual lead/trailing truck could handle (although quick googling says that ATSF mikes had approx 13,000 pounds on the trailing trucks).
