High Speed Gearbox: Cam Gearbox
If an ordinary car and a race car with engines of the same power come out to accelerate against each other in a pair race, the winner will undoubtedly be the last. The key to victory is the cam gearbox. The main advantage of the cam box is the gear shift speed. If you accelerate in an ordinary car, shifting gears up as quickly as possible, almost a blow, then changing each gear will take about 0.6 s. About so much goes to high-speed disengagement / engagement of the clutch. The pilot of a racing car can change gear three times faster - and will do it without squeezing the clutch, and will win more than 0.4 s on each shift! This will happen due to the fact that at each shift in a regular car the engine speed drops and, accordingly, the acceleration rate decreases. To find out how the high-speed racing gearbox works, we went to Udelnoye, to the Red Wings team base located near Moscow, in rallies and circuit races.
Features of racing mechanics
Denis Komarov, technical director of the racing team, is preparing a cam gearbox for photography. He carefully wipes one of the gears of the unit with a rag - a huge spur gear. If such a gear lay in the workshop on its own, one would think that it was out of the box of a large old truck. Meanwhile, it belongs to the Citroёn C2 compact hatchback.
The large diameter of the wheel is due to two factors. Firstly, the box of the rally car transfers solid torque from the engine to the wheels. And secondly, the spur wheel. The advantage of the usual helical gears, which are used in the boxes of “civilian” automobiles, is that due to the longer tooth and, accordingly, the larger surface of the load distribution, they can transmit the same torque with smaller sizes. In addition, they work noticeably quieter. But spur gears are not used in racing cars by chance: they do not create axial loads on the shafts and increase the efficiency of the box.
Surprisingly, a racing gearbox is not more complicated, or even simpler than a regular civilian. There are no synchronizers here, and instead of a large number of small teeth that engage when the gear is engaged on a regular box, large cams are used - end protrusions on the gear and clutch (usually 5-7 pieces per wheel). In order for the gears to engage as soon as possible, the cams engage with a large clearance in width. Therefore, when you turn on the gears on a rally car, you can hear the characteristic metal clanging - these are the cams of the gear and the clutch collided with each other.
The cam box requires a great dexterity from the pilot - especially when switching down: to synchronize engine and transmission revolutions, it is necessary to carefully work the accelerator pedal and feel the car perfectly. When driving carefully, the pilot uses the clutch when going down, during the race - especially on cars with a sequential cam gear - he practically does not need a clutch pedal. Including therefore rally drivers differently than civilian drivers squeeze pedals. Their right leg usually rests on the gas pedal, while the left foot manages the clutch and brakes. It’s very important to work clearly with the accelerator, because without correctly performing the rearrangement, shifting to a lower gear will either not occur at all or will be accompanied by a hard blow. That is why pilots of rally cars smile maliciously when I wonder how popular the cam box is among tuning enthusiasts. Of course, there are street racing fans who replace the serial boxes with cam boxes. Such a replacement improves the dynamics of acceleration, but requires a constant attention from the driver when switching down, and also fills the interior with noise from the operation of spur gears. A cam box howls about as loudly as a civil helical when its crankcase is empty of oil. We add here the high cost of cam boxes (up to € 20, 000 per unit) and a low service life - and we conclude that installing a cam box on an ordinary car is completely unjustified. Of course, the life of the car depends on subjective factors. In harsh racing conditions, synchronizers do not live long. So if a maniac turns out to be driving a civilian vehicle, the cam box may well serve him longer than usual. Nevertheless, over time, the racing unit will begin to emit a characteristic knock, indicating that the rounded cams do not provide reliable engagement. Such a box needs replacement of worn out pairs. Denis says that the cam box for testing is taken apart after each race, and some couples in the box have to be changed every 2-3 stages of the race. And this is normal!
Back and forth: good and bad
There is another reason why cam boxes are not suitable for ordinary roads. Although these units are often equipped with a conventional search engine switching mechanism, the fastest and most popular boxes for riders - sequential. In rally cars, the pilot shakes a lot, so driving the shift lever back and forth is much more convenient than choosing gears, like in an ordinary car. In addition, such a kinematics of the lever allows you to save a few milliseconds on each shift.
But driving a cam-type sequential box on public roads is a terrible torment. The fact is that when we get into a traffic jam or at a right angle turn from the main road to a secondary one, we usually jump down several gears at once. For example, from fifth to second. With a sequential box, such a trick will not work: you will have to sequentially transfer to the fourth, third, and only then to second gear with gas relocation. Denis shows why this happens on the Citroen box. When the pilot of the rally car pushes the lever of this sequential box forward or backward, a special axis with numerous cams rotates at a certain angle. In this case, one of the cams returns the gear shift fork to the neutral position, and the other presses on another fork, and it engages the clutch with the gear of the desired gear. To engage, say, fifth gear, it is necessary to rotate the axis several times, which is controlled by the shift forks.
Consolation of a civilian racer
It turns out that the cam box is absolutely not applicable to civilian cars. This is not entirely true. British firms - the main manufacturers of cam boxes - traditionally have many requests among tuning enthusiasts who want to purchase their boxes, and in our country, on the basis of the cam box, even a modern unit for “civilian” use has been developed, which is almost devoid of drawbacks.
It happened so. The company "Sportmobile", which was engaged in tuning and preparing for the competition of already fast cars Mitsubishi Lancer Evolution, mastered the installation of a cam box from Gemini on these machines. Effective use of such a device requires excellent driver skills. But since the use of a cam box dramatically changes the dynamic characteristics, the engineers and founders of the company Alexey Chernyshev and Pavel Rustanovich decided to adapt the racing box for use by ordinary drivers in everyday driving.
Boxes and Rules
Pictured is a Subaru Impreza photo of the Krasnye Krylia team, which participates in the World Rally Championship in the offset of P-WRC production cars. Despite the fact that it would be possible to show the best time with a sequential box on the machine, the car is equipped with a cam box with the usual search gear. According to homologation, a group N car cannot compete in a rally with a sequential gearbox.
Cars with a sequential box and a traditional rygach at the central tunnel are the middle class in the hierarchy of racing vehicles. Want to change gears even faster? You are welcome! Equip the steering wheel with gearshift paddles, and entrust the hydraulics to turn the axis with the grooves of the sequential box. This solution is used on most WRC race cars participating in the main event. The same solution is used in Formula 1 and some other races.
As it usually happens, solutions from the world of motorsport over time appear on production cars. Today, many cars are equipped with sequential gearboxes with a rocking lever and shift paddles. In combination with conventional non-cam gearboxes, such a mechanism practically does not increase the shift speed, but drivers find it more convenient compared to the traditional search engine.
To solve this problem, electronics were involved. Based on the computer Motec, which allows you to program the functions of the car. They wrote their own software for it, which, together with the developed electronic unit, became the basis of his system, called SGSM (Sequental Gearshift Management). Employees of the Sportmobil company were able to connect the gear changes in the box to the operation of the ignition and injection systems. When going down, the motor automatically rebased. On the one hand, this made life easier for the pilot, and on the other, it increased the life of the cam box due to guaranteed smoother shifting. It is worth noting that a similar system was previously used on racing motorcycles - in them the motion sensor of the box lever was connected to the ignition control unit. At the time of switching down, the ignition timing increased sharply, and the speed fell, which was required to switch to a lower gear. But the Sportmobile system, which automated the injection system, became the next level of development of the idea.
The use of a cam box along with a tuning 420-horsepower engine made the car prepared by the company the most dynamic Evolution in the history of this iconic car. The car accelerates to 100 km / h in 3.53 s! Upon learning of such an achievement by Russian engineers, reporters from the well-known English magazine Autocar came to Moscow and were completely delighted. As a result, several companies immediately became interested in the possibility of producing such boxes abroad, and in Moscow a group of desperate guys was formed who wanted to purchase Evolution with a cam box.
The editors thank the Red Wings team for their help in preparing the materialThe article was published in the journal Popular Mechanics (No. 10, October 2008).