Automotive Science Question

[BusaDave9]

I enjoy taking my Jeep up into the mountains on rough trails. I have gotten into an argument with some of the others that also enjoy this pastime. I was hoping you guys could settle a dispute about locking differentials.

Background
I think this is mainly a science question. You don't have to know much about cars but you do need to know the following:

Open Differential
This is the most common differential found in almost all cars and trucks.
Lets say a car with an open differential has the left tires on dry pavement. The right tires are in snow covered ice. If you have an open differential and you hit the gas your right tire would just spin in the ice and snow. Your left tire on the dry pavement will not turn. You're stuck.

Locking differential
These are not common. Sometimes they are found in off-road vehicles or in race cars.
With a locking differential both tires on the axle turn in unison.
In the same scenario, your right tires are in snow covered ice and your left tires are on dry pavement, you hit the gas. Now you can accelerate briskly. Because you have tires on the dry pavement you don't get stuck.

The Debate
Most people say an open differential will, at times, send all the torque to the one tire with the worst traction. And a locking differential will send torque to both tires equally.

I say this is almost exactly opposite of the truth.
I say an open differential will always apply torque equally to both tires on the axle. And I say a locking differential will, at times, send all the torque to the one tire with the best traction.

So what do you guys say? Am I right or is everyone else wrong?

I don't want to explain my reasoning until I hear from others.

[Ghideon]

7 hours ago, BusaDave9 said:

Most people say an open differential will, at times, send all the torque to the one tire with the worst traction. And a locking differential will send torque to both tires equally.

Interesting topic. Here is a description that may point at what you wish to discuss. (Bold by me)

Quote

The drive axles associated with an open differential are interconnected by a bevel gear set designed to divide equal torque between drive axles. This arrangement will not support any substantial torque difference between the drive axles and, as a consequence, offers very little resistance to differentiation. Virtually any attempt to deliver an increased amount of torque to one of the drive axles will result in rotation of the gear set as evidenced by differential rotation between drive axles. For example, if one of the drive wheels should lose traction, any attempt to deliver additional torque to the other drive wheel having better traction will result in undesirable 'spin up' of the wheel having poorer traction. The maximum amount of torque conveyed by the drive axles collectively is limited to approximately twice the amount of torque supported by the drive wheel having the least traction.

source http://zhome.com/ZCMnL/tech/Torsen/Torsen.htm
(also used on wikipedia at https://en.wikipedia.org/wiki/Differential_(mechanical_device)#cite_note-10)

 

Question: What does "send" mean in this context? Can I assume the differentials in this topic does not have anti slip or traction control or similar? Electronically controlled 'active differentials' (not sure about correct word) may have components that send signals but I do not think that is intended here. 

 

[exchemist]

8 hours ago, BusaDave9 said:

I enjoy taking my Jeep up into the mountains on rough trails. I have gotten into an argument with some of the others that also enjoy this pastime. I was hoping you guys could settle a dispute about locking differentials.

Background
I think this is mainly a science question. You don't have to know much about cars but you do need to know the following:

Open Differential
This is the most common differential found in almost all cars and trucks.
Lets say a car with an open differential has the left tires on dry pavement. The right tires are in snow covered ice. If you have an open differential and you hit the gas your right tire would just spin in the ice and snow. Your left tire on the dry pavement will not turn. You're stuck.

Locking differential
These are not common. Sometimes they are found in off-road vehicles or in race cars.
With a locking differential both tires on the axle turn in unison.
In the same scenario, your right tires are in snow covered ice and your left tires are on dry pavement, you hit the gas. Now you can accelerate briskly. Because you have tires on the dry pavement you don't get stuck.

The Debate
Most people say an open differential will, at times, send all the torque to the one tire with the worst traction. And a locking differential will send torque to both tires equally.

I say this is almost exactly opposite of the truth.
I say an open differential will always apply torque equally to both tires on the axle. And I say a locking differential will, at times, send all the torque to the one tire with the best traction.

So what do you guys say? Am I right or is everyone else wrong?

I don't want to explain my reasoning until I hear from others.

From @Ghideon's post it seems you are right. 

[studiot]

I remember my first Land Rover had manually set bosses on the front wheel hubs to activate / deactivate a 'differential lock'.

I never properly understood it so I am looking forward to lots of explanation in this discussion.

[pzkpfw]

59 minutes ago, studiot said:

I remember my first Land Rover had manually set bosses on the front wheel hubs to activate / deactivate a 'differential lock'.

I never properly understood it so I am looking forward to lots of explanation in this discussion.

Sorry, that probably wasn't a differential lock.

That was probably for free wheeling hubs. It disconnected (or connected) your front hubs to the front axles.

It was so when you were driving along in 2 wheel drive, the front wheels could turn without turning the front diff or the front drive shaft. Saves fuel and wear and tear (including tyres).

One of my Father's several Land Rovers (this one was a Series II soft top) had such manually controlled front hubs. If I was out with him, it was my job to hop out with a short bit of broom handle, with a slot cut in the end, to activate/deactivate the hubs.

More modern vehicles do this automatically.

[exchemist]

1 hour ago, pzkpfw said:

Sorry, that probably wasn't a differential lock.

That was probably for free wheeling hubs. It disconnected (or connected) your front hubs to the front axles.

It was so when you were driving along in 2 wheel drive, the front wheels could turn without turning the front diff or the front drive shaft. Saves fuel and wear and tear (including tyres).

One of my Father's several Land Rovers (this one was a Series II soft top) had such manually controlled front hubs. If I was out with him, it was my job to hop out with a short bit of broom handle, with a slot cut in the end, to activate/deactivate the hubs.

More modern vehicles do this automatically.

Yes, I had the same feature on the 4 wheel drive I had when I lived in Dubai in the 80s.

I remember I was also told not to engage 4 wheel drive until I went off road, to avoid "winding up" the transmission on surfaced roads when turning. The front wheels turn through a longer arc than the back so without a centre differential, which such vehicles don't have, the front and rear drive shafts are trying to turn by different amounts.  So the procedure was when going off road to engage 4 wheel drive and then jump out and connect the front hubs.  By the time I left in 1987 many of the newer vehicles did it automatically.

(The vehicle I had was what is called in the UK the Mitsubishi Shogun. However in Dubai it was called the Pajero. Many years later, I learnt from a S. American colleague that "pajero" is Spanish slang for wanker. Another example of the Japanese instinct for picking brand names that don't work in Western culture.)

 

Edited July 11, 2021 by exchemist

[studiot]

3 hours ago, pzkpfw said:

Sorry, that probably wasn't a differential lock.

That was probably for free wheeling hubs. It disconnected (or connected) your front hubs to the front axles.

It was so when you were driving along in 2 wheel drive, the front wheels could turn without turning the front diff or the front drive shaft. Saves fuel and wear and tear (including tyres).

One of my Father's several Land Rovers (this one was a Series II soft top) had such manually controlled front hubs. If I was out with him, it was my job to hop out with a short bit of broom handle, with a slot cut in the end, to activate/deactivate the hubs.

More modern vehicles do this automatically.

Yes that is the description the LR specialist gave me, thank you for the correction. +1

 

2 hours ago, exchemist said:

Yes, I had the same feature on the 4 wheel drive I had when I lived in Dubai in the 80s.

I seem to remember a similar feature on some Suburbans and/or Blazers we had in Saudi in the 1970s.
But others had an automatic method.

[Ghideon]

16 hours ago, BusaDave9 said:

Most people say an open differential will, at times, send all the torque to the one tire with the worst traction.

Just a thought, is it possible that there is misunderstandings* about torque, power and energy? In the case of an open differential: If one wheel slips and spins up while the other wheel stops spinning then energy will be required to increase the angular momentum of the slipping wheel. When only one wheel is spinning the load on the engine comes from the spinning wheel, not from the wheel that is stuck and not spinning. So that one can say something like "open differential will, at times, send all the energy to the one tire with the worst traction". Similarly, since there is friction and other losses, "open differential will, at times, send all the power to the one tire with the worst traction". Not sure about the word "send" in this context, I'm paraphrasing the statement in opening post. 

 

*) not @BusaDave9 but in the group referenced in opening post (or in my understanding!). 

[BusaDave9]

7 hours ago, Ghideon said:

Just a thought, is it possible that there is misunderstandings* about torque, power and energy? In the case of an open differential: If one wheel slips and spins up while the other wheel stops spinning then energy will be required to increase the angular momentum of the slipping wheel. When only one wheel is spinning the load on the engine comes from the spinning wheel, not from the wheel that is stuck and not spinning. So that one can say something like "open differential will, at times, send all the energy to the one tire with the worst traction". Similarly, since there is friction and other losses, "open differential will, at times, send all the power to the one tire with the worst traction". Not sure about the word "send" in this context, I'm paraphrasing the statement in opening post. 

 

*) not @BusaDave9 but in the group referenced in opening post (or in my understanding!). 

Yes, people misunderstand torque, power and energy .... and I would like to add speed. Sure it takes energy to spin a tire but that doesn't mean it's applying that energy (or torque) to the ground. For that reason I say for this discussion it is best to imagine the weight of the wheel can be disregarded. Imagine it doesn't weigh anything.

I disagree with your sentence I underlined.

Lets say we've got an open differential and one tire is on glaze ice. You can't apply much torque to this tire before it starts to spin. Lets say only five foot pounds of torque can be applied before it spins on the ice. The other tire that is on dry pavement is also getting exactly the same amount of torque (5 foot pounds) but this is still not enough to move the jeep.
This is why I say an open differential ALWAYS applies the same amount of torque to both wheels.

On the other hand a locking differential will, at times, apply all the torque to the one tire with the best traction.
For example lets say a tire on a jeep comes off the ground due to very rough terrain. (Imagine he's in 2 wheel drive, I don't want to think about what the other axle is doing). In this case the tire on the ground is getting all the torque and can still propel the jeep.

I say no torque can EVER be applied to ANY tire that is off the ground. This is true if you have an open differential or a locker.

All my off-road buddies disagree with that last statement. If the jeep has an open differential the tire off the ground would be spinning fast. My friends would say the spinning tire would have lots of torque. "Go grab that tire, Dave. You'd go flying up into the tree tops."
1. A spinning tire of any weight will be hard to stop. This is why I prefer to say it has neglectable weight for the sake of discussion.
2. If the spinning tire isn't pressing against the ground it CAN'T be applying any torque. To disagree would be to disagree with Newton's 3rd law, to every action there must be an equal and opposite reaction.

To explain my last point it would be like me going into a gym and bench pressing an empty bar bell. If I were to yell out "I am pressing up with 300 pounds of force." Everyone would yell "BS!" It's not possible.

Then lets say a couple of guys were to put 300 pounds of weight on the ends of my bar bell while I am holding it above me. Now I sure as hell better be able to hold up 300 pounds or that bar bell will come crashing down on my chest.

On the other hand lets say I can bench press 300 pounds (a lie) and when that much weight is put on my bar bell I continue to do repetitions. Now everyone would say "WOW he can press up that hard. I guess he was always pressing up with 300 pounds of force like he said." NO NO NO just because I CAN bench press 300 pounds doesn't mean I was when there were no weights on my bar bell.

This is all hypothetical, I'm a wimp and can't bench press 300 pounds.

Edited July 12, 2021 by BusaDave9

[Ghideon]

9 hours ago, BusaDave9 said:

I disagree with your sentence I underlined.

I agree with the disagreement; "load" is not a good word to describe. Probably "work" is better"?  When only one wheel is spinning the mechanical work performed by the engine is related to the spinning wheel, not related to the wheel that is stuck and not spinning.

9 hours ago, BusaDave9 said:

For that reason I say for this discussion it is best to imagine the weight of the wheel can be disregarded. Imagine it doesn't weigh anything.

That is ok, and in an ideal situation with no losses and massless wheel my description with energy required (my second post) does not apply. 

9 hours ago, BusaDave9 said:

I say an open differential ALWAYS applies the same amount of torque to both wheels.

Note my first post, I believe we agree that you are correct. My use of incorrect terminology in the followup may have added some confusion, sorry for that.

 

 

[BusaDave9]

6 hours ago, Ghideon said:

I agree with the disagreement; "load" is not a good word to describe. Probably "work" is better"?  When only one wheel is spinning the mechanical work performed by the engine is related to the spinning wheel, not related to the wheel that is stuck and not spinning.

With an open differential, when one tire is spinning and the other is not. BOTH tires are helping push the car. You can say push. You can say torque. You can say load or work. You can say whatever you want but both tires help the car get moving. People see the one tire spinning and they think this one tire is the only one doing any effort to move the car. This is not true. Both tires are helping out EXACTLY the same amount.

Thought experiment: Lets say someone is trying to turn the tire on dry pavement. He has a long torque wrench on the hub of the wheel. He may be pushing with all his might and the tire is not turning. He is still applying torque. If he had just a little more help he might be able to move the car. This is not how the engine applies torque. It applies it through the differential . With an open differential it applies torque equally to both wheels no matter what.

Edited July 12, 2021 by BusaDave9

[Ghideon]

37 minutes ago, BusaDave9 said:

With an open differential, when one tire is spinning and the other is not. BOTH tires are helping push the car. You can say push. You can say torque. You can say load or work. You can say whatever you want but both tires help the car get moving. People see the one tire spinning and they think this one tire is the only one doing any effort to move the car. This is not true. Both tires are helping out EXACTLY the same amount.

Does the bold part differ from what I said?

 

(Note that torque and mechanical work is not equivalent; the mechanical work required for or applied during rotation is the torque times the rotation angle. An open differential that applies same torque to both wheels and more mechanical work is required for or applied on the wheel that spins fastest. This is just a clarification and does not change the conclusion; an open differential applies same torque to both wheels.) 

[exchemist]

1 hour ago, BusaDave9 said:

With an open differential, when one tire is spinning and the other is not. BOTH tires are helping push the car. You can say push. You can say torque. You can say load or work. You can say whatever you want but both tires help the car get moving. People see the one tire spinning and they think this one tire is the only one doing any effort to move the car. This is not true. Both tires are helping out EXACTLY the same amount.

Thought experiment: Lets say someone is trying to turn the tire on dry pavement. He has a long torque wrench on the hub of the wheel. He may be pushing with all his might and the tire is not turning. He is still applying torque. If he had just a little more help he might be able to move the car. This is not how the engine applies torque. It applies it through the differential . With an open differential it applies torque equally to both wheels no matter what.

You are both in violent agreement. 

A stuck wheel does no work, even though it applies a force, since it is not moving.

But you are talking about equal force being applied by both wheels, which no one here disagrees with.  

[BusaDave9]

1 minute ago, exchemist said:

You are both in violent agreement.

Yes, I agree. When the vehicle is stuck no work is being done.  When it starts to move lets not try to talk about which tire is doing the work. Then I'd like to talk about torque applied to the wheels.

[dimreepr]

Isn't it more a question of grip? A tank track is an expression of an enormous wheel or a large number of wheel's; if the torque is distributed evenly then every wheel has an even chance to move.

[BusaDave9]

38 minutes ago, dimreepr said:

Isn't it more a question of grip? A tank track is an expression of an enormous wheel or a large number of wheel's; if the torque is distributed evenly then every wheel has an even chance to move.

Yes, if a tire can't grip it can't apply torque and propel the vehicle. But when one tire has grip and the other doesn't that's when the design of the differential makes a difference.

When one tire on an open or regular differential has no grip then neither tire can propel the vehicle. (Torque is limited by the tire with the worst traction)

When one tire on a locking differential has no grip then the one tire with grip can propel the vehicle. (Torque is delivered to the tire that has traction)

[BusaDave9]

10 hours ago, dimreepr said:

A tank track is an expression of an enormous wheel or a large number of wheel's; if the torque is distributed evenly then every wheel has an even chance to move.

I'd rather not confuse this thread with talk about how a tank works but since you brought it up.

The engine of a tanks turns the drive sprockets at the rear of the track (marked in red here). This is where the engine applies torque to the track to propel the tank.
All the rest of the wheels are idle wheels. The yellow are road wheels with suspension.
The blue is tension wheels for the mechanic to adjust how tight the track is.

and that's all I have to say about that.

Edited July 15, 2021 by BusaDave9

[dimreepr]

14 hours ago, BusaDave9 said:

I'd rather not confuse this thread with talk about how a tank works but since you brought it up.

The engine of a tanks turns the drive sprockets at the rear of the track (marked in red here). This is where the engine applies torque to the track to propel the tank.
All the rest of the wheels are idle wheels. The yellow are road wheels with suspension.
The blue is tension wheels for the mechanic to adjust how tight the track is.

and that's all I have to say about that.

Fair enough, but your diagram confuses my point, look at the contact point of the track rather than the number idler wheel's contained in it's presumed circumference, you asked for a scientific answer to your question, why is grip not the answer? 

[BusaDave9]

2 hours ago, dimreepr said:

why is grip not the answer? 

Grip alone is not always the answer. A 2 wheel drive car may have a problem with grip, especially off road. A locking differential will help and four wheel drive helps. You are saying a tank has great traction. That's true, tracks help tremendously. A tank has such good traction (grip) that it almost never gets stuck. In a tank grip alone is good enough.

I say grip alone is not always the answer because many vehicles don't get excellent grip. When a tire starts to spin that's when the type of differential is important. Off-roaders like to have locking differentials to apply torque to the wheel with the best traction.

[dimreepr]

18 hours ago, BusaDave9 said:

Grip alone is not always the answer. A 2 wheel drive car may have a problem with grip, especially off road. A locking differential will help and four wheel drive helps. You are saying a tank has great traction. That's true, tracks help tremendously. A tank has such good traction (grip) that it almost never gets stuck. In a tank grip alone is good enough.

I say grip alone is not always the answer because many vehicles don't get excellent grip. When a tire starts to spin that's when the type of differential is important. Off-roaders like to have locking differentials to apply torque to the wheel with the best traction.

"A tank has such good traction (grip) that it almost never gets stuck. In a tank grip alone is good enough."

Indeed and that's the reason why I included it, it has no differential.

The only reason to include a differential, however sophisticated, is to go around corners quicker and easier. 

"Grip alone is not always the answer."

To what, automotive, question?

[BusaDave9]

My question pertained to torque and differentials. Yes, a tank's track gets such good traction it doesn't need a differential but that's not what I asked. It's like someone asking about how an airplane flies and you say "don't ask about airplanes, helicopters are much better because they can land in more locations." We're on a different topic now.

I don't know what more to say. Yes, a tank gets good grip. I try to get good grip with my jeep by putting on larger tiers with an aggressive tread but I can't put a track on my jeep and even if I could it's not allowed because they tear up the trails and roads.

[dimreepr]

7 minutes ago, BusaDave9 said:

My question pertained to torque and differentials. Yes, a tank's track gets such good traction it doesn't need a differential but that's not what I asked. It's like someone asking about how an airplane flies and you say "don't ask about airplanes, helicopters are much better because they can land in more locations." We're on a different topic now.

Do you not see the problem with that logic?

15 minutes ago, BusaDave9 said:

My question pertained to torque and differentials.

Indeed, and my answer pertains to grip in the corners... 😉

F1 struggle's with that question, every day...