Cam gearbox: the key to winning serious races
If a regular car and a racing car with engines of the same power come out against each other in a doubles acceleration race, the latter will undoubtedly be the winner. The key to victory is the cam gearbox.
If a regular car and a racing car with engines of the same power come out against each other in a doubles acceleration race, the latter will undoubtedly be the winner. The key to victory is the cam gearbox. The main advantage of the cam box is the speed of gear shifting. If you accelerate in an ordinary car, changing gears up as quickly as possible, almost with a blow, then changing each gear will take about 0.6 s. Approximately so much goes to high-speed clutch release/engagement.
The driver of a racing car can change gear three times faster – and he will do it without squeezing the clutch, and will win more than 0.4 seconds on each shift! This will happen due to the fact that with each shift in a conventional car, the engine speed drops and, accordingly, the acceleration intensity decreases. To find out how the high-speed racing gearbox works, we went to Udelnoye, to the base of the Red Wings team near Moscow, which competes in rallies and circuit races.
Denis Komarov, technical director of the racing team, prepares the cam gearbox for photography. He gently wipes one of the gears of the unit with a rag – a huge spur wheel. If such a gear were in the workshop by itself, one would think that it is out of the box of a big old truck. Meanwhile, it belongs to the compact Citroen C2 hatchback.
The large wheel diameter is due to two factors. Firstly, the box of a rally car transfers a solid torque from the engine to the wheels. And secondly, the wheel is spur. The advantage of the usual helical gears, which are used in boxes of “civilian” cars, is that due to a longer tooth and, accordingly, a larger surface of load distribution, they can transmit the same torque at a smaller size. In addition, they are noticeably quieter. But spur wheels are used in racing cars for a reason: they do not create axial loads on the shafts and increase the efficiency of the box.
Surprisingly, a racing gearbox is not more complicated, and 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 conventional box, large cams are used – end lugs on the gear and coupling (usually there are 5-7 pieces per wheel). To ensure the gears engage as soon as possible, the cams engage with a large clearance in width. Therefore, when switching gears on a rally car, you can hear a characteristic metallic clinking – this is the cams of the gear wheel and clutch colliding with each other.
The cam gearbox requires a lot of dexterity from the pilot – especially when shifting down: to synchronize the engine and transmission rpm, it is necessary to use the accelerator pedal delicately and feel the car perfectly. When driving carefully, the pilot uses the clutch when going downhill, during the race – especially on cars with a sequential cam gearbox – he practically does not need a clutch pedal. This is one of the reasons why rally drivers use the pedals differently than civilian drivers. Their right foot usually rests on the gas pedal, and the left is in charge of the clutch and brakes. It is very important to work accurately with the accelerator, because without a properly performed rebase, the transition to a downshift either will not occur at all, or will be accompanied by a hard blow.
That is why rally car pilots smile maliciously when I ask how popular the cam gearbox is among tuning enthusiasts. Of course, there are street racing fans who are replacing serial gearboxes with cam gearboxes. Such a replacement improves the dynamics of acceleration, but requires constant concentration of attention from the driver when shifting down, and also fills the cabin with noise from the operation of spur gears. A cam gearbox howls about as loudly as a civilian helical gearbox when there is no oil in its crankcase. Add to this the high cost of cam boxes (up to € 20,000 per unit) and low service life – and we come to the conclusion that installing a cam box on a regular car is completely unjustified.
Of course, the lifespan of a car also depends on subjective factors. Synchronizers don’t last long in harsh racing conditions. So if a maniac is driving a civilian car, the cam box, quite possibly, will serve him longer than usual. However, 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 to be replaced with worn pairs. Denis says that the cam box is dismantled for inspection after each race, and some pairs in the box have to be changed every 2-3 stages of the races. And that’s okay!
Back and forth: good and bad
There is another reason why cam boxes are not suitable for normal roads. Although these units are often equipped with a conventional search mechanism for switching, the fastest and most popular boxes among racers are the sequential ones. In rally cars, the pilot shakes a lot, so driving the shift lever back and forth is much more convenient than choosing gears, as in a conventional car. In addition, this lever kinematics saves several milliseconds with each shift.
But driving with a cam-type sequential gearbox on public roads is a terrible torment. The fact is that when we get into a traffic jam or turn at right angles from the main road to the 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 switch to fourth, third, and only then to second gear with a rebase.
Denis shows why this happens on the Citroen’s box. When the pilot of the rally car pushes the lever of this sequential box forward or backward, a special axle with numerous cams rotates at a certain angle. In this case, one of the cams returns the 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 turn on, say, fifth gear, you must turn the axle several times in succession, which is controlled by the shift forks.
Consolation of the civilian racer
It turns out that the cam box is absolutely inapplicable for civilian vehicles. 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 a cam box, a modern unit for “civil” use was even developed, which is almost devoid of drawbacks.
It happened like this. The Sportmobil company, which was engaged in tuning and preparation for the competition of the already fast Mitsubishi Lancer Evolution cars, mastered the installation of a Gemini cam box on these cars. Effective use of such a device requires superior driver skills. But since the use of a cam box radically 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 during everyday driving.
To solve this problem, electronics were involved. It was based on the Motec computer, which allows you to program the functions of the car. They wrote their own software for him, 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 gear changes in the box with the operation of the ignition and injection systems. When going down, the motor automatically re-gassed. On the one hand, this made life easier for the pilot, and on the other, it increased the service 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 movement sensor of the gearbox lever was connected to the ignition control unit. At the time of downshifting, the ignition timing increased sharply, and the revs fell, which was required to downshift. 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 hp engine made the car prepared by the company the most dynamic Evolution in the history of this iconic car. The car accelerated to 100 km / h in 3.53 seconds! Having learned about such an achievement of Russian engineers, journalists of the famous English magazine Autocar came to Moscow and were completely delighted. As a result, several companies abroad immediately became interested in the possibility of producing such boxes, and in Moscow a group of desperate guys was formed who wanted to buy Evolution with a cam box.
