A Transmission Primer

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BK Vissers
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A Transmission Primer

#1 Post by BK Vissers » 25 Apr 2017 16:11

Maybe someone here with more knowledge of transmissions and gear rations can help clarify things for me (and others; I'm sure I can't be the only person wondering a bit about all this stuff):

So I understand some basics: first gear has a high ratio to give maximum torque, while final gear has a low ratio for low engine revs at speed.

I get confused by the differential ratios: a higher number means more engine turns per wheel rotation, or more wheel turns per engine rotation? A low ratio is better for speed or for torque?

I also struggle to determine what transmission is suitable for what purpose - epecially when one also has different differential ratios. Too many variables in the equation! :lol: for example, (just throwing some numbers not specifically a particular transmission) I see things like this:

14.86-1 transmission with a 2.55 differential

Vs

A 11-0.78 transmission with a 3.08 differential.

How do I choose between them? It seems to me as if the differential is cancelling out the benefits of the transmission. Obviously I'm missing something.

Throw in Allison transmissions for a real spanner in the works!

6 gears, and it often says 2.42x torque or some such thing, and say, 6-1.0 gears. So what does that mean? At low end I treat it like a 2.42x6 but at high end I only count the 1?

I'm at a loss as to compare that to some 18 speed, or even 12 speed.. clearly these Allisons are useful, or they would not be an option, but I don't know how to make a determination as to what to choose.
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galimim
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Re: A Transmission Primer

#2 Post by galimim » 26 Apr 2017 10:41

I get confused by the differential ratios: a higher number means more engine turns per wheel rotation, or more wheel turns per engine rotation? A low ratio is better for speed or for torque?
Higher ratio=More engine RPM for 1 wheel rotation
I also struggle to determine what transmission is suitable for what purpose - epecially when one also has different differential ratios. Too many variables in the equation! :lol: for example, (just throwing some numbers not specifically a particular transmission) I see things like this:

14.86-1 transmission with a 2.55 differential

Vs

A 11-0.78 transmission with a 3.08 differential.
In real life the first transmission would be better for situations where you need to have low speed maneuverability and torque, where as the second transmission would be more suitable for long range highway trucking.
How do I choose between them? It seems to me as if the differential is cancelling out the benefits of the transmission. Obviously I'm missing something.
In the game the aim is to get around 1250RPM at 90km/h, so the ratios are geared towards that. In the real life you usually pull very similar loads on a more consistent rout so you can get the truck specifically for that purpose.

Here is an example of what is available for Volvo trucks.
[ external image ]
6 gears, and it often says 2.42x torque or some such thing, and say, 6-1.0 gears. So what does that mean? At low end I treat it like a 2.42x6 but at high end I only count the 1?
Allisons have torque converter instead of a clutch. Torque converters can multiply torque from the engine so they don't need as many gears. They are very good for stop-start traffic. But also they have their disadvantage.
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FlyingPigeon
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Re: A Transmission Primer

#3 Post by FlyingPigeon » 06 May 2017 07:41

A rule of thumb would be to say that small ratios (2.55) is short gears and big ratios (3.08) is long gears, small ratios for big heavy loads, and big ratios for highway trucking with box trailers.
That is how i use those numbers, what is under that on the technical side i dont know.
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Blackspots
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Re: A Transmission Primer

#4 Post by Blackspots » 06 May 2017 17:34

A low differential number (like 2.78 or 3.08) good for fuel mileage, meaning engine RPM is lower at a given speed, but less torque available for pulling, while the opposite...
A high differential number (like 3.55 or 4.08) good for pulling heavy loads, engine RPM is higher at a given speed, which gives you lower fuel mileage.

An overdrive gear in the transmission (like 0.78) lowers the RPM at highway speeds to give you better fuel mileage.

Some of our smaller 5 yard dump trucks and crew trucks (both Freightliners) have a very high differential ratio, something like 7.11. You can out accelerate some cars from a stop (with no load), but max speed might be 75 or 80MPH.
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stb155
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Re: A Transmission Primer

#5 Post by stb155 » 31 May 2017 17:07

In ETS i have the problem that the overdrive gearboxes mostly do not shift in to 12th gear, so i end up going with higher rpm than with shorter geared standard box.
(Even on that i sometimes have to command upshift to 12th manually sometimes when going 75/80kph)
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DWVTLtd
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Re: A Transmission Primer

#6 Post by DWVTLtd » 14 Jun 2017 13:43

As long as it's in the green zone, if the vehicle is naturally in 11th at say 80km/h then that's the best most efficient gear to be in in that situation. I used to worry about it but not any more.
bluejamouk
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Re: A Transmission Primer

#7 Post by bluejamouk » 06 Jun 2023 14:31

I found this thread just today while looking for some info regarding the renault magnum with the zf16.
is it normal that I cannot shift into gear 13-16 (7L/H - 8L/H)? I noticed that at 90km/h (which is the auto limited max speed) I'm already at 1.000/1.500 rpm in 12th gear (6H) so I guessed it's because those higher gears are out of range hence not in use.
is this a normal behavior or am I missing something? is this also true in real life trucks?
note: yes, the gearbox model is correctly selected
thank you in advance

EDIT: meanwhile, I found a mod that adds some gearboxes to the basic ones (according to the modder, they are all available and tuned as the real ones), and with those I get the full 16 gears range, but with the base ones, still 12 out of 16.
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H-G-B
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Re: A Transmission Primer

#8 Post by H-G-B » 10 Jul 2023 09:51

I just tested it with the setting "true automatic". For me, the Renault Magnum 440 DXi (without trailer) at 77 km/h shifts up to 16th gear when I accelerate at full throttle.
If you shift manually with a H-Shifter and can only shift up to 12th gear, then something is wrong with your settings. I'm betting on a wrong selection for "Shifter layout", there you can u. also select "ZF 16 gears".
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