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
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.
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