# Bogie

A bogie (/ˈboʊɡi/ BOH-ghee) (or truck in North American English) comprises two or more wheelsets (two wheels on an axle), in a frame, attached under a vehicle by a pivot. Bogies take various forms in various modes of transport. A bogie may remain normally attached (as on many railroad cars and semi-trailers) or be quickly detachable (as for a dolly in a road train or in railway bogie exchange). It may include suspension components within it (as most rail and trucking bogies do), or be solid and in turn be suspended (as are most bogies of tracked vehicles). It may be mounted on a swivel, as traditionally on a railway carriage or locomotive, additionally jointed and sprung (as in the landing gear of an airliner), or held in place by other means (centreless bogies).
Although bogie is the preferred spelling and first-listed variant in various dictionaries, bogey and bogy are also used.

## Railway

A bogie in the UK, or a railroad truck, wheel truck, or simply truck in North America, is a structure underneath a railway vehicle (wagon, coach or locomotive) to which axles (hence, wheels) are attached through bearings. In Indian English, bogie may also refer to an entire railway carriage. In South Africa, the term bogie is often alternatively used to refer to a freight or goods wagon (shortened from bogie wagon).
A locomotive with a bogie was built by engineer William Chapman in 1812. It hauled itself along by chains and was not successful, but Chapman built a more successful locomotive with two gear-driven bogies in 1814. The bogie was first used in America for wagons on the Quincy Granite Railroad in 1829. The first successful locomotive with a bogie to guide the locomotive into curves while also supporting the smokebox was built by John B. Jervis in 1831. The concept took decades before it was widely accepted but eventually became a component of the vast majority of mainline locomotive designs. The first use of bogie coaches in Britain was in 1872 by the Festiniog Railway. The first standard gauge British railway to build coaches with bogies, instead of rigidly mounted axles, was the Midland Railway in 1874.

### Purpose

Bogies serve a number of purposes:
* Support of the rail vehicle body
* Stability on both straight and curved track
* Improve ride quality by absorbing vibration and minimizing the impact of centrifugal forces when the train runs on curves at high speed
* Minimizing generation of track irregularities and rail abrasion
Usually, two bogies are fitted to each carriage, wagon or locomotive, one at each end. Another configuration is often used in articulated vehicles, which places the bogies (often Jacobs bogies) under the connection between the carriages or wagons.
Most bogies have two axles, but some cars designed for heavy loads have more axles per bogie. Heavy-duty cars may have more than two bogies using span bolsters to equalize the load and connect the bogies to the cars.
Usually, the train floor is at a level above the bogies, but the floor of the car may be lower between bogies, such as for a bilevel rail car to increase interior space while staying within height restrictions, or in easy-access, stepless-entry, low-floor trains.

### Components

Key components of a bogie include:
* The bogie frame: This can be of inside frame type where the main frame and bearings are between the wheels, or (more commonly) of outside frame type where the main frame and bearings are outside the wheels.
* Suspension to absorb shocks between the bogie frame and the rail vehicle body. Common types are coil springs, leaf springs and rubber airbags.
* At least one wheelset, composed of an axle with bearings and a wheel at each end.
* The bolster, the main crossmember, connected to the bogie frame through the secondary suspension. The railway car is supported at the pivot point on the bolster.
* Axle box suspensions absorb shocks between the axle bearings and the bogie frame. The axle box suspension usually consists of a spring between the bogie frame and axle bearings to permit up-and-down movement, and sliders to prevent lateral movement. A more modern design uses solid rubber springs.
* Brake equipment: Two main types are used: brake shoes that are pressed against the tread of the wheel, and disc brakes and pads.
* In powered vehicles, some form of transmission, usually electrically powered traction motors with a single speed gearbox or a hydraulically powered torque converter.
The connections of the bogie with the rail vehicle allow a certain degree of rotational movement around a vertical axis pivot (bolster), with side bearers preventing excessive movement. More modern, bolsterless bogie designs omit these features, instead taking advantage of the sideways movement of the suspension to permit rotational movement.

#### Locomotives
##### Diesel and electric

Modern diesel and electric locomotives are mounted on bogies. Those commonly used in North America include Type A, Blomberg, HT-C and Flexicoil trucks.

##### Steam

On a steam locomotive, the leading and trailing wheels may be mounted on bogies like Bissel trucks (also known as pony trucks). Articulated locomotives (e.g. Fairlie, Garratt or Mallet locomotives) have power bogies similar to those on diesel and electric locomotives.

#### Rollbock

A rollbock is a specialized type of bogie that is inserted under the wheels of a rail wagon/car, usually to convert for another track gauge. Transporter wagons carry the same concept to the level of a flatcar specialized to take other cars as its load.

#### Archbar bogies

In archbar or diamond frame bogies, the side frames are fabricated rather than cast.
* Archbar bogie pool
* Japanese archbar bogie with axleboxes
* Diamond frame bogie, elliptical springs and American style journal boxes
* Diamond frame bogie, coil springs and journal boxes
* Archbar bogie with journal bearings in American style journal boxes, as used on some steam locomotive tenders. Archbar bogies (trucks) were also used on freight cars.

### Tramway
#### Modern

Tram bogies are much simpler in design because of their axle load, and the tighter curves found on tramways mean tram bogies almost never have more than two axles. Furthermore, some tramways have steeper gradients and vertical as well as horizontal curves, which means tram bogies often need to pivot on the horizontal axis, as well.
Some articulated trams have bogies located under articulations, a setup referred to as a Jacobs bogie. Often, low-floor trams are fitted with nonpivoting bogies; many tramway enthusiasts see this as a retrograde step, as it leads to more wear of both track and wheels and also significantly reduces the speed at which a tram can round a curve.

#### Historic

In the past, many different types of bogie (truck) have been used under tramcars (e.g. Brill, Peckham, maximum traction). A maximum traction truck has one driving axle with large wheels and one nondriving axle with smaller wheels. The bogie pivot is located off-centre, so more than half the weight rests on the driving axle.

### Hybrid systems

The retractable stadium roof on Toronto's Rogers Centre used modified off-the-shelf train bogies on a circular rail. The system was chosen for its proven reliability.
Rubber-tyred metro trains use a specialised version of railway bogies. Special flanged steel wheels are behind the rubber-tired running wheels, with additional horizontal guide wheels in front of and behind the running wheels, as well. The unusually large flanges on the steel wheels guide the bogie through standard railroad switches, and in addition keep the train from derailing in case the tires deflate.

### Variable gauge axles

To overcome breaks of gauge some bogies are being fitted with variable gauge axles (VGA) so that they can operate on two different gauges. These include the SUW 2000 system from ZNTK Poznań.

## Radial steering truck

Radial-steering trucks, also known as radial bogies, allow the individual axles to align with curves in addition to the bogie frame as a whole pivoting. For non-radial bogies, the more axles in the assembly, the more difficulty it has negotiating curves, due to wheel flange to rail friction. For radial bogies, the wheel sets actively steer through curves, thus reducing wear at the wheel's flange-to-rail interface and improving adhesion.
In the US, radial steering has been implemented in EMD and GE locomotives. The EMD version, designated HTCR, was made standard equipment for the SD70 series, first sold in 1993. The HTCR in operation had mixed results and relatively high purchase and maintenance costs. EMD subsequently introduced the HTSC truck, essentially the HTCR stripped of radial components. GE introduced their version in 1995 as a buyer option for the AC4400CW and later Evolution Series locomotives. However, it also met with limited acceptance because of its relatively high purchase and maintenance costs, and customers have generally chosen GE Hi-Ad standard trucks for newer and rebuilt locomotives.
* EMD HTCR radial steering trucks on an EMD SD9043MAC
* GE radial steering trucks on a GE AC4400CW
A 19th century configuration of self-steering axles on rolling stock established the principle of radial steering. The Cleminson system involved three axles, each mounted on a frame that had a central pivot; the central axle could slide transversely. The three axles were connected by linkages that kept them parallel on the straight and moved the end ones radially on a curve, so that all three axles were continually at right angles to the rails. The configuration, invented by British engineer John James Davidge Cleminson, was first granted a patent in the UK in 1883. The system was widely used on British narrow-gauge rolling stock, such as on the Isle of Man and Manx Northern Railways. The Holdfast Bay Railway Company in South Australia, which later became the Glenelg Railway Company, purchased Cleminson-configured carriages in 1880 from the American Gilbert & Bush Company for its 1600 mm (5 ft 3 in) broad-gauge line.

## Articulated bogie

An articulated bogie (aka Jakob-type) is any one of a number of bogie designs that reduce weight, increase passenger comfort, and allow railway equipment to safely turn sharp corners, while reducing or eliminating the "screeching" normally associated with metal wheels rounding a bend in the rails. There are a number of such designs, and the term is also applied to train sets that incorporate articulation in the vehicle, as opposed to the bogies themselves.
If one considers a single bogie "up close", it resembles a small rail car with axles at either end. The same effect that causes the bogies to rub against the rails at longer radius causes each of the pairs of wheels to rub on the rails and cause the screeching. Articulated bogies add a second pivot point between the two axles (wheelsets) to allow them to rotate to the correct angle even in these cases.

## Articulated lorries (tractor-trailers)

In trucking, a bogie is the subassembly of axles and wheels that supports a semi-trailer, whether permanently attached to the frame (as on a single trailer) or making up the dolly that can be hitched and unhitched as needed when hitching up a second or third semi-trailer (as when pulling doubles or triples).

## Tracked vehicles

Some tanks and other tracked vehicles have bogies as external suspension components (see armoured fighting vehicle suspension). This type of bogie usually has two or more road wheels and some type of sprung suspension to smooth the ride across rough terrain. Bogie suspensions keep much of their components on the outside of the vehicle, saving internal space. Although vulnerable to antitank fire, they can often be repaired or replaced in the field.