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      09-21-2019, 01:12 PM   #1
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What's the difference between today’s semi-automatic and automatic transmissions?

I've been wondering lately what truly the differences are between semi-automatic and fully automatic. I know that they both work a bit differently, and that back in the days only a semi-automatic would allow the driver to change gears manually, but nowadays, especially in BMWs we are also able to put our ZF-autos into manual mode, even with optional paddleshifters.

So what are the main differences nowadays between them for the driver?

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      09-21-2019, 01:19 PM   #2
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I think nowadays semi-auto usually refers to a mode as opposed to a the basic operating design.
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      09-21-2019, 01:48 PM   #3
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Quote:
Originally Posted by ScottSinger View Post
I think nowadays semi-auto usually refers to a mode as opposed to a the basic operating design.
I guess so too, but then again we have the single clutch and double clutch tranmissions, like BMW's DCT or Volkswagen's DSG.

What's the main advantages of those compared to say the ZF? As far as I know the newer F90 M5 even ditched the DCT for the ZF.
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      09-21-2019, 04:16 PM   #4
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Quote:
Originally Posted by Fentomized View Post
I guess so too, but then again we have the single clutch and double clutch tranmissions, like BMW's DCT or Volkswagen's DSG.

What's the main advantages of those compared to say the ZF? As far as I know the newer F90 M5 even ditched the DCT for the ZF.
This is a great site https://www.zf.com/products/en/cars/home/cars.html

there are so may configuration of transmissions now, some of the dual clutch banged but are capable of very quick shifts, some with torque converters handled extreme power better and creep in traffic better.
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      09-21-2019, 04:33 PM   #5
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I've always interpreted it as a semi-automatic transmission has a mode that allows you to shift manually either via flappy paddles or the gear lever while a fully automatic one's are just stuck in boring mode.
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      09-21-2019, 04:43 PM   #6
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chk out this manual with automated controls

https://www.zf.com/products/en/cars/products_29280.html
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      09-21-2019, 04:47 PM   #7
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Quote:
Originally Posted by ScottSinger View Post
chk out this manual with automated controls

https://www.zf.com/products/en/cars/products_29280.html
Dad's motorhome has one of those. It's weird. Like a slow driver driving a manual.
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      09-21-2019, 04:53 PM   #8
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Quote:
Originally Posted by dinonz View Post
Dad's motorhome has one of those. It's weird. Like a slow driver driving a manual.
Ferrari had something similar in their Mondial if I recall correctly
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      09-21-2019, 05:36 PM   #9
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Quote:
Originally Posted by Fentomized View Post
especially in BMWs we are also able to put our ZF-autos into manual mode, even with optional paddleshifters.

So what are the main differences nowadays between them for the driver?
I think technically the 'zf auto' is a manumatic and not a semi automatic.
And a dual clutch gearbox or a SMG is a semi automatic:

https://en.wikipedia.org/wiki/Manumatic

https://en.wikipedia.org/wiki/Semi-a...c_transmission

I think in the end the biggest difference is whether you let a computer choose the gear (automatic) or that you do it yourself (semi-automatic, manumatic or manual).
Because the thing a computer can't do is anticipated driving. For that the human mind is by far superior (if you know how to drive and understand vehicle dynamics of course).
computers get smarter and smarter, for instance use GPS map data to predict if a corner is coming up, but they can't anticipate on where you want to go or, most of the time, what happens on the road (obstacles, traffic lights etc), that still dictate on what gear one should choose.

Quote:
Originally Posted by ScottSinger View Post
chk out this manual with automated controls

https://www.zf.com/products/en/cars/products_29280.html
Thats basically an SMG I think?
Note that this is not similar to the mondial valeo clutch. Thats an automated clutch afaik.
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Last edited by GuidoK; 09-21-2019 at 10:01 PM..
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      09-21-2019, 08:25 PM   #10
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Fully automatic refers to a gearbox that does all the shifting for you. Think of the automatics in older cars with just the P, R, N, D, 1, 2 options. Semi automatic refers to a gearbox that also gives you the option to select your own gears but does not require you to use a clutch pedal.

Fully automatic cars with a semi-auto mode have been around for a long time, with various methods of gear selection. Some use a gated shifter like the Alfa Romeo Q-system (uses an Aisin gearbox). Nowadays nearly all automatic cars have paddle shifters to give you a semi-auto mode.

This is different from the actual type of gearbox mechanism. Traditional auto gearboxes used to be torque converter gearboxes. Then they added buttons or paddles to introduce a semiautomatic mode to torque converter gearboxes. Tiptronic falls in this category. After that came the robotised manuals (aka single clutch auto) which was basically a manual gearbox with a computer controlled actuator which worked the clutch for you. BMW SMG, Alfa Selespeed, Ferrari F1, fall into this category.

Then came the dual clutch gearboxes, pioneered by VW. BMW DCT, Porsche PDK are dual clutch gearboxes, with complex gear trains with two clutches, each operating alternate odd/even gears, to allow faster shifting.

Of late there's been a renaissance of sorts for the old torque converter automatics, led by the new ZF 8 speed that's found in many of today's high performance cars. Make no mistake, this new ZF has a torque converter like in the automatics of old, but has been tweaked to shift as quickly as a dual-clutch while retaining the reliability and ability to handle power like a torque converter.
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      09-22-2019, 12:18 AM   #11
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Semi Autos: same principle with manuals, a robotized clutch is being used, often dont have an "Park" on the selector and instead of D there is generally an A.. they re around for almost 100 years.. SMG is a "sequential" transmission with robotized clutch.. it cant change gear from 6 to 2 for example.. more like a race car sequential.. so SMG is not like Easytronic and other that lets you feel like an old nanny changing gear for you.. semi autos ll sound like manual transmission on a flyby with slower shifting speed depending on the transmission model..

Full autos: they dont have a traditional clutch.. they have torque convertor.. torque convertor has a clutch inside though but different than traditional one.. so it uses wet clutch system.. they have PRND and sometimes L for low gear or gears only.. like in new M5.. even though it is much faster than traditional torque convertors but still.. automatic ll sound like on a flyby, softened dramatic rev drop while changing gears.. recent sporty torque convertors may sound like a double clutched though..

double clutched transmissions: they re around less than 20 years in traffic.. their gear layout is different and using 2 clutches for 2 different sets of gears.. they re more complicated.. you ll feel the clutch on first few meters as well but the rest you wont.. dual clutch gear box'ed car ll sound like crazy fast gear changes on a flyby..
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      09-22-2019, 12:42 AM   #12
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I have driven various automatic transmissions from "quaint" torque converter to fully electronic ones . I must say difference is minimal to me just like or in the same way the difference between minimal deep tech and tech house .
However, one car with EDC (semi auto ) seems to have a tendency of being jerky during launch .I don't take it as a gripe but this is rather something I have noticed .
There's also CRT ,I have not driven one but from what I read it behaves like a semi auto rather than fully automatic tranny

Besides, fully automatic should shift up / down when driver hits red line but semi auto when driver selects manual mode should not interfere with the shifting even driver hits red line . If a driver don't use manual mode in semi auto then it should act like a fully auto .This is my understanding as well
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      09-22-2019, 11:56 AM   #13
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Quote:
Originally Posted by Nivarox View Post
Besides, fully automatic should shift up / down when driver hits red line but semi auto when driver selects manual mode should not interfere with the shifting even driver hits red line . If a driver don't use manual mode in semi auto then it should act like a fully auto .This is my understanding as well
Not necessarily. The Alfa Q-system from the late 90s is a 4-speed torque converter gearbox with a H-gate shifter, and in manual mode it will happily bounce the revs off the limiter without upshifting until you manually do it.
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      09-22-2019, 08:40 PM   #14
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The line between semi-auto and automatic is very blurry.

A true automatic has a torque converter but BMW was able to turn that into a shiftable transmission that can hold gears and quickly change gears with a clever use of electronics.

Then we have the single and double clutch transmission like the SMG, DSG, PDK which still rely on actuated clutch plates but can automatically switch gears instantly up or down much like a motorcycle. However if you drive in bumper to bumper traffic a lot, you could end up wearing out your clutch like you would a manual transmission.

We must not forget the CVT which doesn't have a clutch or torque converted but a stretchy rubber band/metal band that moves around a cone to change ratio. To me, it's the most annoying transmission of them all since it just winds but never actually shift. Some manufactures are even adding sounds and clever engine timing to mimic a shift. Sounds familiar?

Then we have front wheel drive transaxle like in the F48 X1 which has a torque converted but no final drive.

At this point my head explodes !!
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      09-22-2019, 10:02 PM   #15
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Quote:
Originally Posted by GenXer View Post
Then we have front wheel drive transaxle like in the F48 X1 which has a torque converted but no final drive.
What do you mean the gearbox has no final drive?
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      09-23-2019, 02:45 AM   #16
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Quote:
Originally Posted by GuidoK View Post
What do you mean the gearbox has no final drive?
The final drive is already inside a transaxle since it doesn't need a separate driveshaft and a rear axle like rear wheel drive configuration.
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      09-23-2019, 08:17 AM   #17
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      10-10-2019, 11:48 PM   #18
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Dual clutch transmissions are not really more complex than torque converter automatics, they're really just 2 manual transmission molded together plus 2 clutches versus many clutches or bands in torque converter autos. They can very quickly shift up or down if an appropriate gear has been preselected, but if not, it might take a while and will be very noticeable. Basically, they are really good for uninterrupted acceleration but when the speed has to vary a lot like in traffic, they get confused. Modern autos like ZF 8spd are better in this regard because in each gear, they have to disengage and engage very few clutches to go up or down a gear. It's as if they were like 3 clutch gearboxes which would have 2 gear preselected instead of only one
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      10-11-2019, 01:41 AM   #19
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      10-11-2019, 02:34 PM   #20
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DCT is still better. otherwise you would see Porsche, Ferrari, and McLaren use autos like the ZF in their high end cars. BMW uses it to save money and its 'good enough' for most folks.
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      10-11-2019, 03:55 PM   #21
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Quote:
Originally Posted by Humdizzle View Post
DCT is still better. otherwise you would see Porsche, Ferrari, and McLaren use autos like the ZF in their high end cars. BMW uses it to save money and its 'good enough' for most folks.
"Better" is a concept that depends upon where you are coming from. A properly configured DCT will give you faster shifts but it will be less smooth and has some problems when coming to a halt. BMW has been so good at coding the ZF 8 speed automatic that it performs pretty much as well as a DCT and makes for a smoother daily driver.

As for terminology, the one I like for a DCT is an "automated manual".
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      10-11-2019, 04:05 PM   #22
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Years ago, a friend had a VW with an "Automatic Stickshift." It was also available from Porsche as the Tiptronic.

It detected pressure on the shift lever and automatically disengaged the clutch... which meant that you couldn't put your hand on the shift knob until you were ready to change gears, or it would shudder violently. Driving an early seventies VW bug wasn't much fun in the first place, but that thing made it even worse.

https://www.vwheritage.com/blog/2015...i-auto-beetle/
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