What is the definition of a Mid-Engine vehicle?
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The engine is located in the center of the chassis directly behind the passenger compartment (normally these are only 2-seater vehicles) but ahead of the center line of the rear wheels. In most cases, the fuel tank is directly in front of the passengers. This provides much better weight distribution and handling. There are three basic chassis configurations used today. The front engine configuration with the engine in the front, the passenger compartment in the middle and the trunk in the back. There are rear-engineered cars (like the Volkswagen Bug and the Porsche 911) where the trunk is in the front, the passengers are in the middle and the engine is in the back, behind the center line. And the mid-engine configuration described above.
Most competition cars use this drivetrain design, and so do many high-performance sports cars.The MR layout was first widely used in single-seat racing cars. In a narrow, rear-wheel drive single-seater, the driveshaft of a front mounted engine must pass underneath or beside the driver. This means that the body will be higher or wider than otherwise. For a single-seat racing car, decreased frontal area for less aerodynamic drag is an advantage of the mid-engine design. When mid-engined single-seat racers gained acceptance in the late 1950s, two-seat sports-racers were developed from them. Mid-engine high-performance road cars followed. The
MR design can also decrease the frontal area of a two-seat sports car,
and allow other aerodynamic advantages. But perhaps the most important
advantage of the design for a high-performance vehicle is that it
concentrates the vehicle’s mass near the center of gravity. This
decreases its inertial resistance to quick changes of direction,
increasing its maneuverability. Because of this maneuverability, a
mid-engine car can be set up to make full use of its tires and corner
faster by having just a small amount of oversteer designed in. That can
make it very nervous and
tricky to drive at the limit. It takes skill and fast reactions to be
able to drive such a car well, but a skilled driver can take full
advantage of a mid-engined
cars cornering abilities. Excerpt
from pg. 10 of the Reference Manual for the Playstation game: Gran
Turismo 2 |
Although
the mid-engine configuration with its attendant rear weight bias is universally
accepted as optimal for racecars, a 50/50 weight distribution still has the
magic ring when it comes to high-performance street machines.
Pontiac has chosen the racing layout for the new Fiero, so it’s
appropriate to take a hard look at the conventional wisdom.
Since
overall performance in any car is dependent on the forces that can be exerted on
the pavement, an examination of any aspect of vehicle dynamics must center on
the tires and their loading. The simplest case is acceleration.
Hard acceleration depends on traction, and traction is a result of
vertical forces pressing the tires to the pavement.
Obviously, rear-heavy, rear-drive cars and front-heavy, front-drive cars
have the advantage. But rear drive
holds the edge once the car is in motion, because load transfer during
acceleration increases the vertical force on the driving wheels. Conversely, front-drivers lose vertical loading and traction
with acceleration.
During
braking, optimal traction-and therefore the shortest stopping distances-result
from equal loading of all four tires. Although
50/50 seems like the obvious solution, dynamic load transfer must also be taken
into account. In the case of the
Fiero, with its 93.4-inch wheelbase and 19.5-inch-high center of gravity, about
seventeen percent of the total vertical tire load is transferred from the rear
to the front tires during a hard (0.8g) stop.
Thus vertical loading changes the weight distribution from the static
43.5 percent front/56.5 percent rear to 60.5/39.5 percent.
Although this transfer results in far from equal loading, it’s much
better than the 67/33 distribution a 50/50 car would have under the same
circumstances. Obviously, a
nose-heavy, front-drive car would suffer even more.
All other parameters being equal, a rear-heavy car has far greater
stopping potential.
An
extensive GM study has shown that 50/50 distribution is best for steady-state
cornering in a rear-drive car, although slight variances from perfect balance
(say, 51/49 to 49/51 F/R) were not
found to be significant. Front-drivers,
of course, do not fall within this narrow window, since packaging necessities
place too much weight up front. Most cornering is not done at steady speeds,
however, but is combined with acceleration or braking.
The result is far more complicated vehicle dynamics.
According to the handling experts we consulted, tire and suspension
characteristics are more important than static weight distribution under these
circumstances. The engineers report
that excellent performance can be achieved over a wide range of weight
distributions in both front- and rear-drive configurations.
While the evidence seems confusing at first, certain conclusions can be drawn. The optimal static weight distribution for simultaneous braking and cornering cannot be determined. A strong rear bias with rear drive is far and away the best design for acceleration and braking. Equal front and rear distribution is best for pure cornering, no matter which wheels are driving. When we add up the pluses and minuses, our conclusion is that the best layout for overall performance is rear drive. The ideal weight distribution is impossible to specify to the third decimal, but it lies somewhere between a small rear bias and the long-revered 50/50.
Excerpt
from Brooklands Books
MID-ENGINE
The positioning of a car's engine partly or wholly within the wheelbase. Long favored for racing and very-high-performance cars because it results in more even front-rear weight distribution than front- and rear-engine layouts; this benefits handling and roadholding. A mid-engine car is most commonly thought of as one in which the engine sits immediately behind the passenger compartment and ahead of or over the rear wheels' centerline; but some front-engine cars (such as the Mazda RX-7) are considered front mid-engine designs because their engines are positioned aft of the front-wheel centerline.