HOW I CAN CHANGE YOUR GEARS
FROM THE ENGINE TO THE WHEELS
Changing speed is generally associated with gears, the clutch and similar components but, in reality, it’s not always that simple...
Pneurama readers will have noticed there a lot of material has been published about gears and transmissions, mechanical units that are intimately linked to the wheels and tyres. Despite the numerous articles, the subject is anything but exhausted given the alacrity with which manufacturers add proposals to their catalogues.
In this article, we will examine a couple of types that we looked at previously and we will see a system that is not new but takes a completely different approach to the question of “changing speed”. As we all know, this extremely important mechanical unit has the difficult task of adapting engine output to the power and torque absorbed by the moving vehicle. The latter are extremely variable: take, for example, the difference in acceleration or gears when on the flat or a hill. To quantify this we can say that the useful revolutions of a petrol engine vary from about 6,000 to 1,000 rpm – a ratio of 6 to 1 – while speeds vary in a much wider range: 26:1 if we take the two extremes of 130 km/h on a motorway and the 5 km/h of walking pace. To adapt the range of variations in engine revs to the much range of speed we must vary the transmission ratio between engine revs and those of the wheels.
More gears, more performance
A fairly tried-and-tested way of varying the transmission ratio is to use pairs of gears: they are easy to make, durable and their mechanical output is fairly high. It is not by chance that the classic manual transmission – the kind that uses a torque converter instead of a clutch – also has gears. In these applications they are almost always epicyclical. These gears have parallel axes, generally arranged so that the smallest engage simultaneously with a central gear and an external, wide-diameter cable. What is special about this layout is that it allows for different transmission ratios simply by using suitable brakes to block one or other of the axes. The advantage of this solution is that the gears are coplanar which means the length of the gear unit can be considerably shortened. All these advantages are emphasized and, in a way, taken to the extreme in the new ZF 9HP, a 9-speed (plus reverse) automatic transmission that is so compact it can be housed under the bonnet of a car with front-wheel drive and transversal engine. The construction is fairly complicated given that it combines (for automatic gears) the usual brakes and multiple-disc clutches with Dog Clutches that are used to block the shafts without the losses that multi-disc clutches introduce even if they are “open”. There is a total of four simple and nine slanting gears, two clutches, two Dog Clutches and two brakes, all “stuffed” into a unit that is only 36.7 x 62.1 x 42.8 cm. This gear has many other interesting details: it is compatible with 4-wheel drive, Start&Stop, hybrid traction and electronic control – separated to save space – its calculation capacity can be increased by up to 30% for the more sophisticate operating strategies of the future. The torque converter, which is used only for starting and changing gear, can be blocked to minimize power loss. It is also fitted with vibration dampers that will block even if the engine is not running smoothly, a situation that could arise at low revolutions when the gear is used with engines of three or even two cylinders.
What are the advantages of increasing number of ratios? Having a lot of gears means that the engine will always be in its best range of operation, to the advantage of performance and consumption (the ninth can be very long) and less wear. These advantages are even more evident in small engines because they make better use of a non-exuberant torque.
Even more gears!
If you feel that nine gears are too few then you might be interested in a gear with a ratio that can be selected not between 5, 6 or 9, but between a potentially infinite number of possibilities. This is called Continuously Variable Transmission, or CVT, and its most widespread layout is a transmission belt with to variable diameter pulleys. The operating principle is linear: if the same belt is wound round two pulleys with different diameters, the bigger one will rotate more slowly than the smaller one. If the diameter of the pulleys can be changed, the change in the transmission ratio will be the same: by connecting one of the pulleys to the engine and the other to the wheels, the speed will change. The system is used almost universally in modern scooters, but Piaggio and Peugeot already used it for mopeds in the '60s and ‘70s. The power was so low that the rubber transmission belt was very similar to the kind used for car alternators and water pumps. Obviously, a solution of this kind cannot be used for powerful cars. Audi and Luk have developed a CVT called Multitronic, which transmits high torque and power thanks to a stout metal belt or chain. Pulley diameter is varied by an electro-hydraulic system that joins and separates the two halves of the pulley; it can also simulate normal fixed ratio transmission with the sophisticated electronic management of different driving modes, including the sports mode. Among the main problems with this system is belt resistance to wear and minimizing slipping, which are impeded by increasing the tension even at the cost of friction. The special Multitronic metal belt ensures durability and good energy efficiency as well as the very smooth operation typical of CVT.
A different system
Very, very much different is the hydraulic gear, the result of a patent by the Italian mechanic Badalini and reproposed in a modern version by Honda in its DN01 motorbike.
Its operating layout has a plate fixed to the engine shaft (nor perpendicularly but diagonally) which supports small hydraulic pistons: because the plate is diagonal, they will go up and down to pump a special oil. By means of a rotating distributor, the oil is sent to a mirror unit, another plate with more pistons that collect the oil and convert its pressure into mechanical power. This plate is also inclined with respect to the shaft onto which it is mounted – the transmission shaft – but the angle can be adjusted up to about 90°. If inclination is maximum, the shaft will rotate slowly but will transmit considerable torque and will behave very much in the same way as a manual transmission in low gear. As the plate gradually becomes perpendicular, the transmission ratio is lengthened, movement is faster even if there is less torque to the wheel. This gear also has a blocking device that minimizes losses when the transmission ratio is the “longest” achievable.