When I first started driving in the 90s it seemed like the 4 and 5 speed transmission was the best, modern transmission you could get. They were relatively reliable, and had enough gears to make acceleration, and fuel economy good in my Honda CRX.
Today, I can buy a Honda Odyssey with a 9-speed automatic transmission standard, and 10-speed on the high end model.
**What has changed in transmission engineering, materials science, and technology to allow more gears in the gearbox?**
What are the potential downsides? E.g. is the typical 10-speed as reliable as the old standard 5-speed gearbox?
Edit: To be more clear, this is an ENGINEERING, MATERIALS SCIENCE, and TECHNOLOGY question. I understand the benefits of having more gears for ride quality, and fuel efficiency.
In: Engineering
In addition to the things that others have mentioned, it’s also necessary to consider the SCALE of the effort that produced the 10 speed transmissions. GM and Ford developed them in a joint venture, because it was too complex and expensive for either to do alone. They spent literally BILLIONS of dollars on that project. It’s easy to think “oh, they went from 5 to 10, ho hum” … but the scale of the effort needed to accomplish that deserves some recognition.
1) Presision of automatic tools. It allows to make finetuned details with almost perfect fit (it can be made perfect and you can find gifs of so fine made details that they look like solid one when connected. However machinery **needs** to be almost perfect)
2) better metal production. That allows to make sturdy details smaller in size.
In the end of the day automatic transmission has more than 200 details. 50 years ago it would take 5x volume to be placed and 10x weight.
First and foremost advanced control. The sensors relaying data and the computers to control operation have all advanced greatly. This allows manufacturers to MUCH more tightly control engagement windows leading to much less heat being generated and much quicker gear changes.
Second is advances in metallurgy. This allows smaller gears to handle more load reliably. This combines with greatly improved manufacturing tolerances and advanced controls that ensure smooth engagement, which also allows smaller gears to handle greater loads, as your most likely point of mechanical failure of the gears themselves is from a sloppy, hard engagement.
The other major factor is better multi-planetary design. A planetary design allows smaller gears to handle higher loads, and having multiple planetary systems allows you to create more final output ratios. This is all at the cost of control complexity, but we are finally achieving that level of control.
A lot of the 8 or higher speed transmissions now have two intermediate shafts. So they can put 1-3-5-7 on one, and 2-4-6-8 on the other, then the valve body moves synchronizers like a manual trans but much, much faster than any human ever could.
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