In automotive engineering , a mid-engine layout describes the placement of an automobile engine in front of the rear-wheel axles , but behind the front axle.
46-508: The AEC Reliance was a mid-underfloor mounted engined single-decker bus and coach chassis manufactured by AEC between 1953 and 1979. The name had previously been used between 1928 and 1931 for another single-decker bus chassis. Two prototypes were completed in 1953, one with Duple coach bodywork and one with Park Royal bus bodywork. Production vehicles entered service from 1954. The last Reliance entered service in 1981. Following successive changes to Construction & Use regulations,
92-401: A propeller shaft or tailshaft ) which transmits the rotational force to a drive axle at the rear of the vehicle. The drive axle may be a live axle , but modern rear-wheel drive automobiles generally use a split axle with a differential. In this case, one half-axle or half-shaft connects the differential with the left rear wheel, a second half-shaft does the same with the right rear wheel; thus
138-429: A differential (kart racing). However, other go-karts have two rear drive wheels too. Some dump trucks and trailers may be configured with a lift axle (also known as an airlift axle or drop axle ), which may be mechanically raised or lowered. The axle is lowered to increase the weight capacity, or to distribute the weight of the cargo over more wheels, for example, to cross a weight-restricted bridge. When not needed,
184-492: A front-engine or rear-engine car. When the engine is in front of the driver, but fully behind the front axle line, the layout is sometimes called a front mid-engine, rear-wheel-drive, or FMR layout instead of the less-specific term front-engine; and can be considered a subset of the latter. In-vehicle layout, FMR is substantially the same as FR, but handling differs as a result of the difference in weight distribution. Some vehicles could be classified as FR or FMR depending on
230-458: A problem in some cars, but this issue seems to have been largely solved in newer designs. For example, the Saleen S7 employs large engine-compartment vents on the sides and rear of the bodywork to help dissipate heat from its very high-output engine. Mid-engined cars are more dangerous than front-engined cars if the driver loses control - although this may be initially harder to provoke due to
276-405: A progressive and controllable manner as the tires lose traction. Super, sport, and race cars frequently have a mid-engined layout, as these vehicles' handling characteristics are more important than other requirements, such as usable space. In dedicated sports cars, a weight distribution of about 50% front and rear is frequently pursued, to optimise the vehicle's driving dynamics – a target that
322-413: A protruding hub to which the axle shaft flange is bolted. The semi-floating axle setup is commonly used on half-ton and lighter 4×4 trucks in the rear. This setup allows the axle shaft to be the means of propulsion, and also support the weight of the vehicle. The main difference between the full- and semi-floating axle setups is the number of bearings. The semi-floating axle features only one bearing, while
368-400: A single unit called a transaxle . The drive axle is a split axle with a differential and universal joints between the two half axles. Each half axle connects to the wheel by use of a constant velocity (CV) joint which allows the wheel assembly to move freely vertically as well as to pivot when making turns. In rear-wheel drive cars and trucks, the engine turns a driveshaft (also called
414-401: A single unit. Together with independent suspension on the driven wheels, this removes the need for the chassis to transfer engine torque reaction. The largest drawback of mid-engine cars is restricted rear or front (in the case of front-mid layouts) passenger space; consequently, most mid-engine vehicles are two-seat vehicles. The engine in effect pushes the rear passenger seats forward towards
460-427: A suspension and steering component. Conversely, many front-wheel drive cars have a one-piece rear beam axle. In other types of suspension systems, the axles serve only to transmit driving torque to the wheels: the position and angle of the wheel hubs is made independent from the axles by the function of the suspension system. This is typical of the independent suspensions found on most newer cars, and even SUVs, and on
506-478: A total of 44. Mid-engine design The mid-engine, rear-wheel-drive format can be considered the original layout of automobiles. A 1901 Autocar was the first gasoline-powered automobile to use a drive shaft and placed the engine under the seat. This pioneering vehicle is now in the collection of the Smithsonian Institution . Mounting the engine in the middle instead of the front of
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#1732772136786552-427: A turn at all. Several manufacturers offer computer-controlled airlifts so that the dead axles are automatically lowered when the main axle reaches its weight limit. The dead axles can still be lifted by the press of a button if needed, for better maneuverability. Lift axles were in use in the early 1940s. Initially, the axle was lifted by a mechanical device. Soon hydraulics replaced the mechanical lift system. One of
598-401: Is a central shaft for a rotating wheel or gear . On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to the vehicle, with the wheels rotating around the axle. In the former case, bearings or bushings are provided at the mounting points where the axle is supported. In the latter case, a bearing or bushing sits inside a central hole in the wheel to allow
644-403: Is a group of two or more axles situated close together. Truck designs use such a configuration to provide a greater weight capacity than a single axle. Semi-trailers usually have a tandem axle at the rear. Axles are typically made from SAE grade 41xx steel or SAE grade 10xx steel. SAE grade 41xx steel is commonly known as " chrome-molybdenum steel " (or "chrome-moly") while SAE grade 10xx steel
690-500: Is a single bearing at the end of the axle housing that carries the load from the axle and that the axle rotates through. To be "semi-floating" the axle shafts must be able to "float" in the housing, bearings and seals, and not subject to axial "thrust" and/or bearing preload. Needle bearings and separate lip seals are used in semi-floating axles with axle retained in the housing at their inner ends typically with circlips which are 3¾-round hardened washers that slide into grooves machined at
736-441: Is attached to a separate shaft. Modern passenger cars have split-drive axles. In some designs, this allows independent suspension of the left and right wheels, and therefore a smoother ride. Even when the suspension is not independent, split axles permit the use of a differential, allowing the left and right drive wheels to be driven at different speeds as the automobile turns, improving traction and extending tire life. A tandem axle
782-535: Is called an axle or axle shaft . However, in looser usage, an entire assembly including the surrounding axle housing (typically a casting ) is also called an axle . An even broader (somewhat figurative) sense of the word refers to every pair of parallel wheels on opposite sides of a vehicle, regardless of their mechanical connection to each other and to the vehicle frame or body. Thus, transverse pairs of wheels in an independent suspension may be called an axle in some contexts. This very loose definition of "axle"
828-445: Is known as " carbon steel ". The primary differences between the two are that chrome-moly steel is significantly more resistant to bending or breaking, and is very difficult to weld with tools normally found outside a professional welding shop. An axle that is driven by the engine or prime mover is called a drive axle . Modern front-wheel drive cars typically combine the transmission (gearbox and differential) and front axle into
874-409: Is often used in assessing toll roads or vehicle taxes , and is taken as a rough proxy for the overall weight-bearing capacity of a vehicle, and its potential for causing wear or damage to roadway surfaces. Axles are an integral component of most practical wheeled vehicles. In a solid, "live-axle" suspension system, the rotating inner axle cores (or half-shafts) serve to transmit driving torque to
920-476: Is to date the only successful example of a true mid-engined convertible with seating for 4 and sports car/supercar performance. A version of the Lotus Evora with a removable roof panel is anticipated but no definite date is known. Like any layout where the engine is not front-mounted and facing the wind, the traditional "engine-behind-the-passengers" layout makes engine cooling more difficult. This has been
966-408: Is typically only achievable by placing the engine somewhere between the front and rear axles. Usually, the term "mid-engine" has been primarily applied to cars having the engine located between the driver and the rear drive axles. This layout is referred to as rear mid-engine, rear-wheel drive , (or RMR) layout. The mechanical layout and packaging of an RMR car are substantially different from that of
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#17327721367861012-866: The Leyland Tiger Cub and, from 1959, the Leyland Leopard as its major competitors throughout its life, even though they were built under the same ownership from 1962 onwards. After production of the Reliance ended, Leyland offered ZF synchromesh as an option for the Leopard, although the Volvo B58 and other imported buses later won a number of customers' orders. In Australia, Canberra Bus Service purchased ninety-two 470s and twenty-eight 505s while Brisbane City Council purchased three 470s and forty 590s. McVicar's Bus Service , Sydney operated
1058-427: The added weight and expense of all-wheel-drive components. The mid-engine layout makes ABS brakes and traction control systems work better, by providing them more traction to control. The mid-engine layout may make a vehicle safer since an accident can occur if a vehicle cannot stay in its own lane around a curve or is unable to stop quickly enough. Mid-engine design is also a way to provide additional empty crush space in
1104-400: The axle is lifted off the ground to save wear on the tires and axle, and to increase traction in the remaining wheels, and to decrease fuel consumption. Lifting an axle also alleviates lateral scrubbing of the additional axle in very tight turns, allowing the vehicle to turn more readily. In some situations, the removal of pressure from the additional axle is necessary for the vehicle to complete
1150-426: The bearings, housings, and only a short section of the shaft itself, that also carries all radial loads. The full-floating design is typically used in most ¾- and 1-ton light trucks, medium-duty trucks, and heavy-duty trucks. The overall assembly can carry more weight than a semi-floating or non-floating axle assembly because the hubs have two bearings riding on a fixed spindle. A full-floating axle can be identified by
1196-521: The case of the Ferrari FF taking power from both ends of the crankshaft with two separate gearboxes. These cars use a traditional engine layout between driver and rear drive axle. Typically, they're simply called MR; for mid-rear (engined), or mid-engine, rear-wheel-drive layout cars. These cars use mid-ship, four-wheel-drive , with an engine between the axles. These cars are "mid-ship engined" vehicles, but they use front-wheel drive , with
1242-461: The early manufacturers was Zetterbergs, located in Östervåla, Sweden. Their brand was Zeta-lyften. The liftable tandem drive axle was invented in 1957 by the Finnish truck manufacturer Vanajan Autotehdas , a company sharing history with Sisu Auto . A full-floating axle carries the vehicle's weight on the axle casing, not the half-shafts; they serve only to transmit torque from the differential to
1288-438: The engine in front of the driver. It is still treated as an FF layout, though, due to the engine's placement still being in the front of the car, contrary to the popular belief that the engine is placed in front of the rear axle with power transferred to the front wheels (an RMF layout). In most examples, the engine is longitudinally mounted rather than transversely as is common with FF cars. Axle An axle or axletree
1334-413: The engine placed between the driver and the front axle. This layout, similar to the above FMR layout, with the engine between driver and the front axle, adds front-wheel drive to become a four-wheel drive. An engineering challenge with this layout is getting the power to the front wheels past the engine - this would normally involve raising the engine to allow a propshaft to pass under the engine, or in
1380-562: The factory-installed engine (I4 vs I6). Historically most classical FR cars such as the Ford Models T and A would qualify as an FMR engine car. Additionally, the distinction between FR and FMR is a fluid one, depending on the degree of engine protrusion in front of the front axle line, as manufacturers mount engines as far back in the chassis as possible. Not all manufacturers use the Front-Mid designation. These cars are RWD cars with
1426-498: The front axle (if the engine is behind the driver). Exceptions typically involve larger vehicles of unusual length or height in which the passengers can share space between the axles with the engine, which can be between them or below them, as in some vans, large trucks, and buses. The mid-engine layout (with a horizontal engine) was common in single-decker buses in the 1950s and 1960s, e.g. the AEC Reliance . The Ferrari Mondial
AEC Reliance - Misplaced Pages Continue
1472-481: The front of many light trucks. An exception to this rule is the independent (rear) swing axle suspension, wherein the half-axles are also load-bearing suspension arms. Independent drive-trains still need differentials (or diffs), but without fixed axle-housing tubes attached. The diff may be attached to the vehicle frame or body, and/or be integrated with the transmission (or gearbox) in a combined transaxle unit. The axle (half-)shafts then transmit driving torque to
1518-404: The front of the automobile between the bumper and the windshield, which can then be designed to absorb more of the impact force in a frontal collision in order to minimize penetration into the passenger compartment of the vehicle. In most automobiles, and in sports cars especially, ideal car handling requires balanced traction between the front and rear wheels when cornering, in order to maximize
1564-508: The full-floating assembly has bearings on both the inside and outside of the wheel hub. The other difference is axle removal. To remove the semi-floating axle, the wheel must be removed first; if such an axle breaks, the wheel is most likely to come off the vehicle. The semi-floating design is found under most ½-ton and lighter trucks, as well as in SUVs and rear-wheel-drive passenger cars, usually being smaller or less expensive models. A benefit of
1610-437: The heavy mass of the engine is located close to the back of the seats. It makes it easier for the suspension to absorb the force of bumps so the riders feel a smoother ride. But in sports cars, the engine position is once again used to increase performance and the potentially smoother ride is usually more than offset by stiffer shock absorbers . This layout also allows the motor, gearbox, and differential to be bolted together as
1656-461: The inner end of the shafts and retained in/by recesses in the differential carrier side gears which are themselves retained by the differential pinion gear (or "spider gear") shaft. A true semi-floating axle assembly places no side loads on the axle housing tubes or axle shafts. Axles that are pressed into ball or tapered roller bearings, which are in turn retained in the axle housings with flanges, bolts, and nuts do not "float" and place axial loads on
1702-536: The maximum length of the Reliance was increased twice from the original 30 feet: firstly, to permit an overall length of 36 feet from 1962; and later, to permit a length of 39 feet. Various AEC engines were fitted during the chassis's production, including the 7.7-litre AH470, 8.1-litre AH505, 9.6-litre AH590, 11.3-litre AH691 and 12.4-litre AH760. Transmissions fitted to the Reliance include an AEC synchromesh gearbox , AEC Monocontrol semi-automatic epicyclic transmission, and ZF 6 speed crash-gearbox. The Reliance had
1748-406: The possible speed around curves without sliding out. This balance is harder to achieve when the heavy weight of the engine is located far to the front or far to the rear of the vehicle. Some automobile designs strive to balance the fore and aft weight distribution by other means, such as putting the engine in the front and the gearbox and battery in the rear of the vehicle. Another benefit comes when
1794-412: The superior balance - and the car begins to spin. The moment of inertia about the center of gravity is low due to the concentration of mass between the axles (similar to standing in the middle of a playground roundabout, rather than at the edge) and the spin will occur suddenly, the car will rotate faster and it will be harder to recover from. Conversely, a front-engined car is more likely to break away in
1840-458: The two half-axles and the differential constitute the rear axle. The front drive axle is providing the force to drive the truck. In fact, only one wheel of that axle is actually moving the truck and trailer down the road. Some simple vehicle designs, such as leisure go-karts, may have a single driven wheel where the drive axle is a split axle with only one of the two shafts driven by the engine, or else have both wheels connected to one shaft without
1886-413: The vehicle puts more weight over the rear tires, so they have more traction and provide more assistance to the front tires in braking the vehicle, with less chance of rear-wheel lockup and less chance of a skid or spin out. If the mid-engine vehicle is also rear-drive the added weight on the rear tires can also improve acceleration on slippery surfaces, providing much of the benefit of all-wheel-drive without
AEC Reliance - Misplaced Pages Continue
1932-417: The wheel or gear to rotate around the axle. Sometimes, especially on bicycles, the latter type of axle is referred to as a spindle . On cars and trucks, several senses of the word axle occur in casual usage, referring to the shaft itself, its housing, or simply any transverse pair of wheels. Strictly speaking, a shaft that rotates with the wheel, being either bolted or splined in fixed relation to it,
1978-400: The wheel positions steady under heavy stress, and can therefore support heavy loads. Straight axles are used on trains (that is, locomotives and railway wagons), for the rear axles of commercial trucks, and on heavy-duty off-road vehicles. The axle can optionally be protected and further reinforced by enclosing the length of the axle in a housing. In split-axle designs, the wheel on each side
2024-456: The wheels at each end, while the rigid outer tube maintains the position of the wheels at fixed angles relative to the axle, and controls the angle of the axle and wheels assembly to the vehicle body. The solid axles (housings) in this system must also bear the weight of the vehicle plus any cargo. A non-driving axle, such as the front beam axle in heavy-duty trucks and some two-wheel drive light trucks and vans, will have no shaft, and serves only as
2070-403: The wheels, usually via constant-velocity joints . Like a full floating axle system, the drive shafts in a front-wheel-drive independent suspension system do not support any vehicle weight. A straight axle is a single rigid shaft connecting a wheel on the left side of the vehicle to a wheel on the right side. The axis of rotation fixed by the axle is common to both wheels. Such a design can keep
2116-400: The wheels. They "float" inside an assembly that carries the vehicle's weight. Thus the only stress it must endure is torque (not lateral bending force). Full-floating axle shafts are retained by a flange bolted to the hub , while the hub and bearings are retained on the spindle by a large nut. In contrast, a semi-floating design carries the weight of the vehicle on the axle shaft itself; there
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