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
If you have a 1950’s 3-speed transmission, you can really only use one gear at most speeds because they have small overlaps.
When you have 10 gears, you have lots of overlap. This brings an additional benefit, you can choose from among the usable gears the one that provides the most acceleration, or fuel economy, depending on what you’re trying to do.
One of the biggest factors is new regulations by the United States. This is called Corporate Average Fuel Economy. Increasing the number of gears allows the car’s control system to change the gear to the most efficient setting for the speed you’re driving at, which increases the miles per gallon that your vehicle achieves.
The thing that changed was mostly the fuel economy requirements.
They have been using 15 and 30 speed transmissions in semi trucks for decades because they needed that range to be able to haul the loads they had.
For passenger cars 3-4 was good enough for a long time, then customers started wanting smoother accel and better fuel economy and were willing to pay the extra couple hundred dollars versus a simpler transmission. Now we are up to 10 gears because the marginal cost of adding a bunch of extra gears is small enough that most people consider the fuel economy gains and improvements in power band to be worth it.
Also we have got to a point where new car buyers almost never request a manual transmission, so having a really silly number of gears to shift through isn’t considered to be a major drawback the way it would have been back when manual transmissions were more typical.
And as far as reliability goes, there is no reason that a 5 or 10 would be inherently more or less reliable. It really just comes down to the manufacturer. The kinds of things that used to break on 5 speed autos still exist in a 10, and aside from transmissions used in heavy vehicles doing towing or drag cars the actual load being on put on the individual gears was never really a limiting factor, it mostly comes down to how well manufactured they are.
Computer controlled engine and transmission. The vehicle is always in the the best gear compared to engine rpm.
Better computerized machine tooling. This technology has been improving by leaps and bounds every decade.
On the consumer side, you get really cheap 3d printers. On the heavy industrial side, you can make a much more complicated component faster, with same reliability, and slightly less metal. For cars, transmissions are the component that benefits from that extra complexity.
No one has mentioned that a lot the newfangled units are using planetary gear sets.
In the old style, each gear is a cog on a shaft, and that takes up quite a bit of space.
The use of planetary gears in modern units is a space p-saving feature that allows all those extra ratios.
The [ZF 9HP is one of the pioneering examples of this.](https://en.wikipedia.org/wiki/ZF_9HP_transmission)
It’s really complicated but the planetary gear sets perform some sort of voodoo that offers more gear rations in a fraction of the space.
It’s general progress in the entire manufacturing process. The added part count of few extra gears isn’t really much of a extra cost with the general part count explosion for comparison. Modern car has much more parts and associated assembly operations than something from even few decades ago, its on the order of 30000 parts, depending on model and configuration of course. Each and every one of those has associated manufacturing, handling and assembly costs. Yet we can still afford cars. That’s because we have figured out how to do it all cheaper, automate as much as possible etc.
> What are the downsides? Is the typical 10-speed as reliable as the old standard 5-speed gearbox?
There *are* downsides. It’s not all free – just worth doing.
– If you keep everything inside the transmission the same size, the transmission gets bigger and heavier. This can cause issues physically placing it in the vehicle – a model designed for a smaller transmission might be physically incapable of mounting a larger transmission. It also has impacts on total weight and weight distribution.
– If you make everything in the transmission smaller, you increase the risk of manufacturing defects and potentially reduce the overall reliability of certain parts – less material makes wear more impactful. You may also have to add *additional* parts and complexity to make it smaller, like how the [Ford-GM 10-speed](https://en.m.wikipedia.org/wiki/Ford%E2%80%93GM_10-speed_automatic_transmission) has a *triple*-clutch design. That’s more potential points of failure, and more things your engineers might get wrong too. These are offset by all the gains in reliability *elsewhere*.
– More parts is also always going to be more expensive. More material costs, more labour costs, more design costs, etc. Even if you can charge more, you generally need to pay for the cost of production *before* you actually get to selling the car – so you’ve got more money tied up in inventory. That’s a drawback for the companies.
– More gears closer together means more frequent gear changes, in exchange for your smoother acceleration. If your sports car offers a paddle shift, you need to find a balance between “not making the driver constantly shift” and “giving good acceleration”.
All of this is why the 10 speed isn’t universal yet. These are very real drawbacks – some markets don’t want overly frequent gear changes, sometimes the added cost is unacceptable, sometimes shrinking individual parts won’t work and sometimes making the transmission larger won’t either. Engineering is all about compromises and drawbacks, and 10-speeds aren’t free.
The engineering and technology to do this has been around for decades. It’s only a thing today because the government has forced manufacturers into an almost unobtainable requirement to meet certain fleetwide MPG requirements aka CAFE. A 10 speed transmission keeps the engine RPMs lower throughout the speed range but essentially gets you to the same gear ratio as a 5 speed at normal highway speeds. This allows for a tiny bit of increase in fuel efficiency to satisfy government regulations.
Basically, with 5 gears, your RPMs might hit 3000 at the top of each shift point. With a 10 speed you may only hit 2300 RPMs at the top of each shift point. This reduces fuel consumption.
Some American cars in the 70s had 2 speed transmissions and v8 engines to get enough torque at low revs to allow that to be drivable. Fuel consumption and emissions weren’t really an issue then.
Now we have cars with an infinite number of gears using CVT. driving one of those feels like driving an electric car; no gear changes, and better mileage.
All automatic translations suffer from the same flaw: they can’t read the drivers mind to know what gear to be in in 2 seconds time. Modern transmissions shift faster with computer control rather than the old ones that relied on engine vacuum to trigger a downshift.
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