Contest! – a problem about the “wrong” Niva

PROBLEM #177

Wrong Niva?

Nikolai tells his colleagues: “They hung my Niva on an elevator in the service. The machine is fresh, everything seems fine, they say. But I tried to spin the front wheel and the rear. And immediately felt that the front turns much harder. And they say she has symmetrical four-wheel drive. Maybe some glitch?

Help motorists to understand. The contestant who reveals more reasons wins.

Short answers will not be considered.

Send the answer marked “Competition” before February 1, 2023 to the address: 107045, Moscow, Seliverstov per., 10, building 1; e-mail: [email protected]. Please provide your contact phone number or other method of fast communication.

Below we summarize the results of the connoisseur competition published on October 1, 2022.

Readers needed to be told why an even number of gears is most common in automatic transmissions.

The winner of the competition, Sergey Rybkin from Moscow, answered in more detail:

– Until recently, a car with an automatic transmission was seen as a curiosity or a miracle. Now you will not surprise anyone with an automatic machine, and disputes arise about the structure of automatic transmissions and the expediency of the solutions used in them.

Let’s start with a simple one: how do you get different gears in an automatic transmission? Planetary gears are used in hydromechanical gearboxes of passenger cars, with very rare exceptions. They are sets of planetary gears assembled according to certain schemes. A simple planetary gear set is a mechanism with two degrees of freedom. It consists of a ring gear, a sun gear and a carrier that combines the satellites.

For the operation of the planetary mechanism, it is necessary that one of the elements of the planetary gear set be stopped. Then the second element of the planetary gear connected to the input shaft will be the leading element and the remaining element will become the driven element.

If no additional restrictions are introduced, one planetary gear gives seven gears: two reverse gears and five forward gears, including a direct gear, when all elements are rigidly connected. Otherwise neutral.

It turns out that it is theoretically possible to design a 5-speed automatic transmission with only one planetary gear set. But no one does. To simplify the design, reduce the size and weight of the output element of the planetary mechanism, they try to make it the same for all gears. Such a planetary mechanism will be able to issue two forward gears (one of which is direct) and one reverse. To get more gears, you already need to collect several rows together.

By adding another row, you can get up to 4 gears. At the input of each of the two gears, we feed the output current of the previous mechanism (there are also two gears): 2 × 2 = 4, well, plus a reverse gear.

This explains the popularity of 4-speed automatics: their design is simple, proven and reliable, and the number of gears was considered sufficient until recently. If you add another planetary gear set, you can multiply four gears by two again – you get a maximum of eight. Thus, an 8-speed gearbox is a reasonable limit for a three-row design, and is often found on expensive cars.

And what about the popular six steps? They also require three planetary gears, but fewer actuators – brakes and clutches, making the box easier to manufacture and manage than an 8-speed box. If we have already placed three planetary gears, it is unreasonable to build a five-speed gear on it – too many gears will be lost. And the 8-speed is good, but not everyone needs it. If you want easier and cheaper, then 6 gears is the golden mean.

By the way, before every new transmission in the automatic transmission was a real breakthrough, but now no one is surprised by six steps on budget cars. The reason is that the machines were initially controlled purely hydraulically, the gears were engaged slowly and unclearly. Then control electronics appeared, but only now has it become smart enough, reliable and cheap enough to allow multi-speed boxes to fit into budget cars. It is one thing to calculate how many gears you can theoretically receive with such and such a set of elements, and quite another to put it into practice.

It remains to mention the 10-speed boxes and other exotics. These include four planetary gears, but their practical use is not obvious. You can “longer” the top gears to be more fuel efficient, but there are so many gears that the gearbox starts to “jump” back and forth with every speed change, which is not good for durability. But it is more complicated than the same 8-speed gearbox.

Such boxes are installed on top models in many ways to demonstrate the capabilities of the company. Well, or in a simple way, “to show off.”

Congratulations to Sergei Rybkin on his win!

• The engine and gearbox last longer! Here are 8 simple tips.
• “Behind the wheel” can now be read in Telegram.