Automobile Transmissions

  • #1

Main Question or Discussion Point

Hi All,

Something has been nagging at me for decades and I feel that this is the place to get the real answer so here goes.

The question concerns the difference between manual transmissions and automatic transmissions regarding traction in snow.

Now it seems to me that given the same input/output gearing and the same number of gears, it would not matter if the transmission were automatic, manual, or powered by several very intelligent mighty mice. The torque at each gear delivered to the wheels would be the same in all cases.

Now anybody, including myself, who has driven a manual transmission knows that there is a sensation of more traction in snow. Is this sensation real or imaginary? If imaginary, why? If real, is it simply a matter of different gearing? If it is real and it is not a matter of different gearing, what in the world is the physics behind it?

Thanks in advance.

Bob
 

Answers and Replies

  • #2
RonL
Gold Member
1,096
215
Hi All,

Something has been nagging at me for decades and I feel that this is the place to get the real answer so here goes.

The question concerns the difference between manual transmissions and automatic transmissions regarding traction in snow.

Now it seems to me that given the same input/output gearing and the same number of gears, it would not matter if the transmission were automatic, manual, or powered by several very intelligent mighty mice. The torque at each gear delivered to the wheels would be the same in all cases.

Now anybody, including myself, who has driven a manual transmission knows that there is a sensation of more traction in snow. Is this sensation real or imaginary? If imaginary, why? If real, is it simply a matter of different gearing? If it is real and it is not a matter of different gearing, what in the world is the physics behind it?

Thanks in advance.

Bob
I think you will find the lockup of a clutch plate to flywheel, is quite a bit different than the forces moving through the hydraulic action of a torque converter, especially at slower speeds.
 
  • #3
Mech_Engineer
Science Advisor
Gold Member
2,572
171
It's probably because an automatic at low speeds unlocks the torque converter. So that paired with the fact that the engine's speed can increase when torque converter is unlocked will lead to more power being applied to the ground with a given throttle input. The available traction is the same, but the power/torque transferred to the ground isn't.
 
  • #4
rcgldr
Homework Helper
8,674
508
A fluid clutch in most automatics acts as a torque multiplier because of the layout of inner driven and outer driving vanes, minus the losses. A manual clutch acts as a torque limiter, converting some of the engine's energy into heat energy as the manual clutch slips.
 
  • #5
4,239
1
Hi All,
...Now anybody, including myself, who has driven a manual transmission knows that there is a sensation of more traction in snow. Is this sensation real or imaginary? If imaginary, why? If real, is it simply a matter of different gearing? If it is real and it is not a matter of different gearing, what in the world is the physics behind it?
Bob
Having never driven a manual in the snow, I would have to take your word for it--the apparent control, that is. An automatic selects the gears for you. With a manual, you have control over what RMP you wish to run. Just off hand, I think that running at a somewhat higher rpm than normal would give you better control, where the gas peddle can supply braking. I think I'd be asking those who race cars If a manual transmission provides better traction control, and why they think so.
 
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  • #6
driving in snow

I'm a bit surprised at you guys. What is the most apparent thing about snow? Hint: not the fact that it's white. Right, the fact that it's slippery. You don't want to apply power to a substance that will go from static to dynamic friction at the drop of a hat. You want all the wheels (this includes the wheels that steer) to keep rolling nicely along and delivering the smallest forces or they will break traction and you will lose controll. What's this talk about needing power..Oh, I can see if the roads aren't plowed yet and you're pushing snow with your front bumper...In that case you shouldn't be driving.
The automatic is the clear winner because you can meter out the power and not worry about taking the clutch up too fast. Case Closed.
 
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  • #7
4,239
1
I'm a bit surprised at you guys.
I'm suprised you didn't read my post.
 
  • #8
I have read your post. Please explain why you would prefer higher RPM and engine braking. I think you may be mistaken....or not. Remember the question was about snow, the white stuff that makes you slide around and damage your car....
 
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  • #9
Did you ever get your car stuck on a snowy surface in a rut. If you use the gas you spin the wheel and dig in deeper. Now we know that shifting back a forth from D to R and rocking the car will get you right out. But you can sometimes climb out by just creeping along up and out. This repuires a soft touch on the throttle. When I'm driving a stick shift, I have the gas and clutch to meter out and I can't really feel when I'm delivering too much power. Static friction is how much stronger than dynamic friction? I forgot I learned it in high school over 40 years ago.
 

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