The most important figure for this topic is the overall weight of a car, which will be measured in either Kilograms (Kg) or Imperial Pounds (lbs). It matters little in this case whether this is biased to the front or rear, left or right, or with a high or low centre of gravity. For the purposes of calculating a power-to-weight ration, it is the total weight of the car that is used. If being calculated for rules and regulations in a race series, there may be stipulations as to whether this is the 'dry weight' of the car, or if it should include fuel and driver.
Given that we are not interested here in different weight measurements, these can still be explained briefly as the terms are likely
to be encountered when looking into chassis and vehicle performance calculations.
One frequent distinction that is made is between 'sprung weight' and 'unsprung weight'. These figures relate firstly to the weight of the main mass of the
vehicle that is carried by the suspension, and secondly to the weight of the wheels and suspension components that move under bump and rebound travel.
In suspension design, it is generally a target that unsprung weight is reduced as much as possible, to improve the ratio between sprung and unsprung weights
as that will improve the effectiveness of the spring and damper units when working to keep the wheels and tyres on the ground.
A further measure will be that of 'corner weights', which is a measure of the vehicle weight measured at each wheel. The total weight of the vehicle will be carried by all [four] wheels, with the weight distribution dependent upon vehicle design and configuration. Ideally, the weight carried by each 'axle pair' of wheels will be the same, to avoid any side-to-side imbalance in weight distribution. This can be improved by suitable positioning of heavy items such as batteries, fuel tanks and fire extinguishers - not forgetting that the driver affects this balance in most cars (other than single-seaters and the McLaren F1 road car). Further adjustment of weight loadings at each corner of the car can be made by adjusting the height of the spring platform, as raising or lowering this will decrease or increase the pressure on that wheel/tyre combination. There is a lot of fine tuning involved in correctly setting-up the corner weights of a car, which is not covered any further here.
A final consideration on the weight of a car relates to the distribution of that weight within the overall body mass of the vehicle.
The position of the engine and transmission will usually have the greatest effect here, with front-engined cars generally being
front-heavy, with more weight carried by the front wheels than the rear. Rear-engine cars have the reverse effect, with a much
larger bias to weight over the rear wheels, which gives increased traction for acceleration from rear-engined cars such as the
Porsche 911. Mid-engined cars can provide the best overall balance, although packaging of the components in the vehicle may lead
to other technical problems and compromises.
Besides the issue of whether a car is front-heavy or rear-heavy, which mostly affects accelerative grip, the position of the
centre of gravity has a big impact on the handling of a vehicle. It is clearly best if the C-of-G is low, as that translates into
less transfer of weight when cornering, accelerating or braking. As well as the centre of gravity, the distribution of weight between front
and rear of the vehicle can also have a noticeable effect on the dynamic responses of the car. This is described under the theory
of 'polar moments of inertia', with cars having higher weights at the outer areas of the vehicle having increased moments of inertia.
This essentially means that it will take more effort to change the direction of travel of the vehicle, whether that is a change from
straight-line running going into a corner, or exiting a corner and aiming to return to straight-line travel. In this way, the
aforementioned Porsche 911 will exhibit basic dynamics that allow the front of the car to be moved quite easily (giving good corner turn-in
and steering response) but more effort (and care) is needed to move the rear of the car around. Once the weight of that rear engine is
moving out, it also takes significantly more effort (and responses) to 'catch' it and get the inertia travelling in the desired direction.
Clearly, the Porsche engineers have performed wonders over the years in creating effective suspension designs to make the car handle
so well, but they did start with something of a disadvantage in the theory stakes. Conversely, a mid-engined car has lower moments of
intertia at front and rear, so the direction of travel can be more easily and rapidly changed. This does also depend on driver skill,
as a short-wheelbase mid-engined car such as the Lancia Stratos is inherently unstable - which was the design aim for a rally car.
It does not take much to get the Stratos to turn into a corner, but it also takes very little for the driver to get it wrong and see
the car change direction much faster than intended/expected, resulting in a rapid spin if not corrected in time!
Note that measurements used here mix Imperial terms (bhp, lbs) and metric measures (Kg) as these are more commonly encountered.