Why do many vehicles used for heavy work, like trucks, bulldozers etc. use manual transmission instead of auto?
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Manual transmission is a lot more robust and easier to repair if something goes wrong while also being a lot cheaper, and in the right hands will tend to outperform an automatic transmission in most metrics too.
The only benefit to automatic transmission is that it’s a little easier to use, which just isn’t that meaningful when you are talking about big expensive pieces of equipment that should only be operated by a professional in the first place.
Manual transmissions are more durable, tougher and easier to repair than automatic transmissions. The durability allows them to last a long time-a million miles for our trucks. Because they are simpler, there are fewer moving parts so those parts can be larger (for a same sized housing) to handle more torque and break down less.
In addition, you can use manual transmissions as an engine brake and “lock” the gearing for long descents. Engine braking reduces brake overheating, fade and allows the operator to keep the 80ton vehicle safely under control.
In addition to both of those great answers, manual transmissions experience less “slip”, or torsional power loss between the engine output and the transmission input. This is due to the fact that an automatic transmission uses a fluid to trace this power bergen the two, while the manual tranny has a friction clutch that “grips” onto the engine output.
The premise of your question is wrong. Heavy duty vehicles can (and do) use automatic transmissions.
I’ve worked on many large mining machines, including Sandvik Lh514 loaders, Sandvik Th430 trucks, as well as Epiroc Mt436b trucks which have Dana 6000 series transmissions in them. 4 gears forward, 4 gears reverse, full on powershifting.
Increasingly, they don’t. Most new transport trucks, for example, use automatic transmissions. Fuel economy advantages outweigh maintenance costs.
It’s generally the cheaper option, and up until recently, drivers preferred the manual transmission option, especially for large trucks. The early automatic trucks were slow to shift and had limited gearing options. They would hold gears too long and not shift when you wanted them to. They were often the slowest thing on any hill. More automatic options are available now, and I heard drivers actually like them more than not.
Off-road equipment often benefits from automatic transmission and hydraulic driven motors. The smoother power transition is less likely to dig into soft surfaces. Some stuff is powered by a manual transmission still, but it’s usually geared stupid low to limit speed and maximize torque. The last tractor I drove had a 4 speed that could be split twice and topped out at a blistering speed of 25mph. The lowest gear was slower than a gentle walking pace.
Easier to work on. Which kind of matters on construction sites where you’re bouncing all over uneven terrain and smashing your equipment up against earth walls or maybe pushing the equipment way harder than you would, like, a car.
That’s it.
Anyone in the comments saying “manual transmissions give better performance” or anything of the sort have been regurgitating greatly outdated info that aren’t true anymore. Basically, a modern automatic is superior to manual transmissions in terms of performance, top to bottom, from driving granny to the supermarket, to tearing ass on gravel county roads, to racing.
There was a long stretch of time where automatics lagged significantly behind, but tech has come a long, long, long way and that gap closed year by year until now the only reason to drive a manual is A) you’re thinking about fixing something that’s going to get beaten up regularly, or B) you just like the feel of manual.
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