Timing chain or belt?

Gear or chain systems can also be used to connect the crankshaft to the camshaft at the correct timing. However gears and shafts constrain the relative location of the crankshaft and camshafts. Even where the crankshaft and camshaft(s) are very close together, as in pushrod engines, most engine designers use a short chain drive rather than a direct gear drive. This is because gear drives suffer from frequent torque reversal as the cam profiles "kick back" against the drive from the crank, leading to excessive noise and wear. Fiber gears, with more resilience, are preferred to steel gears where direct drive has to be used. A belt or chain allows much more flexibility in the relative locations of the crankshaft and camshafts. Camshaft drives, whether gears, belts or chains are also able to even out wear, since the chain or belt can be made such that the number of teeth on the belt is coprime to the number of teeth on the crankshaft and camshaft sprockets, thus ensuring that each tooth on sprocket does not end up on the same tooth on the belt repeatedly.

While chains and gears may be more durable, rubber belts are quieter in their operation (in most modern engines the noise difference is negligible), are less expensive and are mechanically more efficient, by dint of being considerably lighter, when compared with a gear or chain system. Also, timing belts do not require lubrication, which is essential with a timing chain or gears. A timing belt is a specific application of a synchronous belt used to transmit rotational power synchronously.

Timing belts are typically inaccessible and difficult to inspect. Engine manufacturers recommend replacement at specific intervals. [1] The manufacturer may also recommend the replacement of other parts, such as the water pump, when the timing belt is replaced because the additional cost to replace the water pump is negligible compared to the cost of accessing the timing belt. For engines whose valves extend into the path of the piston, failure of the timing belt (or timing chain) invariably results in costly and, in some cases, irreparable engine damage, as some valves will be held open when they should not be and thus will be struck by the pistons.

Some indicators that the timing chain may need to be replaced include: reduced engine power and poor economy, intermittent back-fire, as well as a rattling noise from the front of the engine.

Quilos VS Cavalos

Dá que pensar...
Hoje estava a ler uma EVO quando me deparei sobre uma carta de um leitor que dizia que a relação peso/potência dos carros muito leves não era assim tão incrível quanto isso. Que um carro mais potente mas mais pesado lidaria melhor com o peso extra em termos de performance. Tem lógica, e pela tabela em baixo vê-se que a teoria se comprova.

Só que não podemos esquecer que 500 quilos curvam com mais facilidade do que 1500.

Na tabela começamos com a tara do veículo sem passageiros, e depois passamos para o exemplo de termos um condutor com 110kg e de seguida mais um passageiro de 60kg.

BMW Active Steering

BMW's Active Steering
by Jim Kerr

Steering has come a long way from the days of the horse and buggy. Steering boxes with their large worm and sector gears have been replaced on many vehicles by more compact and precise rack and pinion steering units. Power steering is found on all but a few of the lowest price economy vehicles. Variable effort steering systems that vary the force required to turn the steering wheel are now found on many vehicles. Electric steering systems are even beginning to replace the hydraulic pumps, hoses and steering gears on some vehicles. Now a new steering innovation has been introduced by BMW, and it is so simple and works so well, the question comes to mind: "Why didn't we think of this before?" BMW calls it Active Front Steering.

Active Front Steering

Active Front Steering
Images: BMW. Click image to enlarge
The concept of Active Front Steering is based around a planetary gear set, such as found in most automatic transmissions. A planetary gear set is made up of three main components: the sun gear in the center, a set of planet gears (several gears in a unit that rotate around the sun gear, and a ring gear around the outside that has internal teeth meshing with the planet gears. Any one of the three components can be the drive input and any can be the output, as long as one or more of the components is held. This provides a variety of gear ratios in forward and reverse. In the BMW steering system, a small planetary gear set is located between the steering wheel and the conventional steering rack. BMW calls this unit a superimposing gear.

On the BMW Active Steering, the sun gear and planet gears are the input and output. The ring gear is held from rotating by a computer-controlled electric motor meshed to the outside of the ring gear. If the ring gear is held stationary by the electric motor, the gear ratio of the gear set is fixed. However, the computer can operate the electric motor to turn the ring gear at the same time the driver turns the steering wheel, providing a variable steering ratio. The effect of the system operation is amazing. In a parking situation, the computer varies the ratio so that the steering wheel needs less than two turns to move the wheels lock to lock. As vehicle speeds increase, the steering ratio increases, so it takes more turns of the steering wheel to move the wheels and increases vehicle stability.

Now the magic begins. Besides providing variable steering ratios, the computer is linked with the vehicle stability control system to aid in directional stability of the vehicle. As the vehicle is travelling down the highway, road surfaces and wind gusts can affect the vehicle directional stability. The car may wander a little or dart to one side, as many who have met a tractor-trailer unit on a windy day have experienced. Sensors on the car detect this sudden unintentional movement and the computer will stabilize the car by moving the Active Steering electric motor and steering gear. The driver doesn't turn the steering wheel at all!

If the driver experiences a skid or slide because of poor road conditions, the Active Steering will react to information from the yaw rate sensors to modify the steering angle of the front wheels to stabilize the vehicle. This occurs much faster than the driver can react. If the Active Steering angle is not enough, then the Stability Control system intervenes to help as well.

Fonte: canandiandriver.com