How a differential gear works

This great old video is a remarkably clear explanation of how a differential gear works -- the kind of thing that you could show to a young child or an adult and make the lightbulb go on for both of them.

How Differential Gear works must see 100%

I write books. My latest is a YA science fiction novel called Homeland (it's the sequel to Little Brother). More books: Rapture of the Nerds (a novel, with Charlie Stross); With a Little Help (short stories); and The Great Big Beautiful Tomorrow (novella and nonfic). I speak all over the place and I tweet and tumble, too.

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Ivan R.

Beautiful. I can guarantee that any kid would be fascinated by this or any similar concept if it was explained in such a brilliant way.
footage

The complete version of this film, called AROUND THE CORNER (in much better quality) is available as a free download from Prelinger Archives: http://www.archive.org/details/Aroundth1937
xzzy

Guy really sounds excited to be describing it, too.

I’ve built dozens of lego and model differentials in my time, and still couldn’t figure out the principles of how they worked. Simply observing them in action it’s just a whirlwind of gears.

The rod explanation is pretty brilliant.
semiotix

I want that guy following me around, shouting out explanations of how various mechanisms I encounter work. “THE NOTCHES ON THE KEY SLIDE PAST EACH TUMBLER. IF THE NOTCHES ALIGN WITH THE SPRING LENGTHS DRIVING EACH TUMBLER, THE LOCKING PIN IS FREE TO ROTATE. HOWEVER…”
- lyd
  
  Wow, yeah, totally killer augmented reality app.
- MooseDesign
  
  LOL, you beat me to it… its like my fiancee’s grandfather who is a clock fanatic (and hard of hearing) yelling at me in his workshop as he explains how the cuckoo clock mechanism works.
  
  Dig the video though… For some reason, between this and the video about the atom with Walt, it really makes me wish that Disney would get back into the nature and science videos gig (yes, I know this isn’t a Disney video).
- Grahamers2002
  
  @Semiotix: Awesome post!
  
  To OP: Thank you so much for posting this! I get the feeling that if you found one of these for each part of my car, I could save $1000′s per year at my mechanic’s!
yerbamatte

Ah, jeez, humans are clever. You go guys.
fubbs

I’ve always tried to recreate how these things worked in my head, to no avail. Seriously, thanks for posting this.
Anonymous

Here is the complete video for those who were annoyed that it cut off early. http://www.youtube.com/watch?v=K4JhruinbWc
Anonymous

I was GOING to visit my daughter’s classroom this afternoon to lead them in a soundwalk/sound mapping exercise. Now I’m thinking maybe I ought to just put this kickass video on a loop instead!
SamSam

This is great! Not only did I not know how this worked, it never even occurred to me that two motor-driven rear-wheels fixed to each other and going around a curve might cause a problem. If I had thought about it I would have realized the problem, but I never thought of it.

Question #1: When the car is going around a curve, is the slower wheel still getting as much power from the transmission as the faster wheel? Or once the differential’s inner wheel starts spinning past the slower wheel does it stop transmitting (much) power?

Question #2: I had an old motor scooter, and I was told that the reason you could still rev the engine while applying the breaks, or rev the engine with the front wheel against a wall without the wheels spinning, was because of the differentials. But in this video it would seem that if both wheels were stopped the transmission would not be able to keep turning. Am I misunderstanding the mechanism, or is the differential in a motocycle designed differently?
- xzzy
  
  #1: An annoying quirk of differentials that anyone who lives in a snowy area is familiar with is that the wheel with the least traction will get all the power (which is no fun when you’re stuck in a ditch). Pretty sure this applies to turning as well.. the inner tire is moving less, so the differential is giving more power to the outer tire.
  
  #2: I don’t think motorbikes have a differential at all. Are you thinking of the clutch perhaps?
- jasonjayr
  
  #2 — being able to rev the engine + break @ the same time is a function of the clutch or automatic transmission.
  
  Automatic transmissions are complex beasts in of themselves — IIRC it’s able to drain the power from the engine if the output shaft is locked, but it can’t be too good for the mechanism if the different in power between input + output is too great.
  
  Clutches/Manual transmissions simply disconnect the input power shaft from the transmission when the clutch is engaged, so the internal combustion engine can continue to idle.
  
  Technically electric vehicles don’t need clutches, since they can full-stop.
  
  re #1 — I’m curious too of the loss of power, if any by going through the differential and at different turn ratios.
- Zan
  
  SamSam: #2 sounds like your motor scooter had a Torque Converter, which allows the motor to spin while the wheels stay stationary.
  - SamSam
    
    This explanation makes more sense to me than the clutch explanations. This was an old Piaggo Si, which certainly had no manual clutch and I doubt it had an automatic clutch.
    
    Thanks for the answers, all!
- ScottTFrazer
  
  Now that you’ve got the concept of a differential down, you’re ready for the advanced version:
  
  http://auto.howstuffworks.com/differential4.htm
  
  The limited-slip differential is how you get around the problem of all the power going to the wheel with the least resistance.
  - SamSam
    
    If one wheel wants to spin faster than the other, it must first overpower the clutch. The stiffness of the springs combined with the friction of the clutch determine how much torque it takes to overpower it.
    
    Getting back to the situation in which one drive wheel is on the ice and the other one has good traction: With this limited slip differential, even though the wheel on the ice is not able to transmit much torque to the ground, the other wheel will still get the torque it needs to move. The torque supplied to the wheel not on the ice is equal to the amount of torque it takes to overpower the clutches. The result is that you can move forward, although still not with the full power of your car.
    
    I think I get it! Weee!
- nixiebunny
  
  Your motor scooter had a centrifugal clutch, which ought to have another one of these fine educational films to teach you how it works. My brother had one on his mini-bike when I was a kid, so I got to watch it close-up in action. (Keep fingers out!)
  
  Basically, it’s made of a drum connected to the wheel with a rotating friction pad set inside it that’s connected to the motor.
  
  The friction pad set is held together with springs, so that as it speeds up, centrifugal force makes the pads spread apart and press against the drum, connecting the motor to the wheel.
- Anonymous
  
  #1: No, the differential shifts the input torque from the engine between the two wheels. Going straight with no slip the torques are equal. In the extreme, if the car were to pivot about one of the two rear wheels such that the wheel didn’t spin, but just rotated around a vertical axis, all of the engine torque would be transmitted to the opposite wheel tracing a circle around the stationary one. You have to differentiate between the applied torque of the engine and the reaction torques due to traction with the ground (assuming no slip). The video doesn’t do a good job in this respect.
  
  This shift of torque between wheels can cause slip and is a reason the basic differentials like the one in the video are rarely used by themselves or are replaced by more exotic variants in today’s cars:
  
  http://en.wikipedia.org/wiki/Differential_(mechanical_device)#Loss_of_traction
Caroline

This is brilliant!

I want videos like this for all the parts of my car.
Anonymous

so my drivetgrain is made from tinker-toys. never knew
mack

I found this fully instructional – and this coming from a guy who’s rebuilt a couple engines in his day.

I also found it fully inspirational:

It makes me want to a) go out and invent things b) take things apart to understand them c) disseminate my limited knowledge in Important Tones so as to Impress.

Thanks, Cory. Daymaker.
Joe Dokes

Bob Gaffney,

Your 4WD Colorado is a “Part-Time” 4WD and is NOT Designed to be used in dry weather on roads with good traction. You will experience abnormal tire wear, and could even damage the front differential or your transfer case.

This is not a design issue, it is an operator issue. The Part-time system has a couple of superior aspects including better traction in very challenging conditions. Simply put, your wife’s Subaru system is great for a street with a few inches of snow, while your colorado could actually go off-road with a fair amount of success.

If you read your owners manual you will find that your Colorado 4WD should only be used in low traction situations.

Regards,

Joe Dokes
Enormo

The title of the video was astoundingly accurate. It was a must see 100%.
GreenJello

Wow! Super cool! Learn something new every day I suppose.
Anonymous

Maybe one day it will be possible for mankind to put their differentials aside…
Caroline

Also, I just love “IT IS CALLED….THE DIFFERENTIAL!” I feel like there should be swelling triumphant music behind that announcement.
terp

A car needs wheels on the left and right that can rotate at different speeds to keep one or the other from skidding around a turn. A train, however, relies on having its left and right wheels rigidly connected to a common axle (plus some taper to the wheels themselves) in order to keep it moving straight down the track. Here’s a video in which Feynman explains the details:

http://www.youtube.com/watch?v=WAwDvbIfkos
bartoncasey

Lego has a marvelous little differential gear part:

http://groups.csail.mit.edu/drl/courses/cs54-2001s/differential.html

My 8-year old had no trouble understanding it when I showed it to him.
Anonymous

All this talk of motorcycles forces me to point out that the URAL motorcycles which can engage and drive the wheel under the sidecar for added traction do NOT have a differential. So they turn VERY poorly when driven on pavement with the sidecar wheel engaged.
Anonymous

To answer a couple of questions posted by others.

1. An automatic transmission uses a device called a torque converter in place of the clutch allow the motor to spin without spinning the wheels. It is a fluid coupler device. To get an idea of how it works imagine two fans facing one another, the air from one fan pushes against the fan blades of the other fan. When the fan is on low (i.e. the car is at idle) it is not enough power to spin the opposing fan, as the speed of the fan increases the blades on the other fan begin to turn, thus turing the wheels. Rather than use air between the fans, you car uses oil between the fan blades. If this sounds inefficient, it is, this is why manual transmission cars get slightly better mileage, as automatics aren’t as efficient at transmitting the power to the differential. Modern automatics try to overcome this with a lock-up torque converter that locks at highway speeds.

2. Most normal differentials are known as open differentials, they are similar to the one in the video in that they send power from the motor to fastest spinning wheel. As others have pointed out this means that the wheel with the LEAST traction gets the most power, bad for icy conditions. Which ever wheel is spinning faster gets more power. There are a number of ways to overcome this, one is with a series of clutches that try to maintain power to both wheels, this is called a limited slip differential. On Chevy’s they are called a Posi-Traction, Ford calls them Trac-Lock and Mopar calls them Sure Grip. In the 1970s, a new kind of limited slip differential was created that doesn’t use clutches, but is purely through gears, the genius of this system is the it sends power to the wheel with the MOST traction. This type of differential is called a Torsen.

Regards,

Joe Dokes
Anonymous

@jasonjayr, it gets even better with electric vehicles: if you have 4 motors rather than one central one, you not only don’t need all this extra linkage/weight/complexity, you get the best possible all-wheel drive (the mechanical linkage can be replaced with a central programmable controller).

extra credit: 4 small batteries, one on each wheel, mean less transmission loss from power cabling, and redundancy.
Anonymous

This makes my nerdy little heart flutter! I wish they made more films this clear and this simple for other little understood things (maybe automatic trasmissions?). I never knew how these worked until now, or even suspected they were more than just a simple redirection of power. I love mechanisms that have passive functions like this, there is no need for a controller to tell it what to do it just does it naturally.
Nadreck

The full film is here:

http://www.youtube.com/watch?v=K4JhruinbWc

More motorcycle stunts and plugs for the “cars of today” from Chevy!
Anonymous

In a standard differential, the power is transferred to the wheel that is turning faster. If one tire slips on an icy surface, it will receive all the power and the car won’t go anywhere. Most differential have limited slip to prevent this. With a worm-gear differential, the power is transferred to the wheel that is turning slower. When a tire slips, the other wheel receives the power and the car can move.

An interesting use of differentials is the South-Pointing Chariot, https://secure.wikimedia.org/wikipedia/en/wiki/South_Pointing_Chariot It uses two differentials and always points south, provided the wheels don’t slip.
Anonymous

What I love about this era are the beautiful castings and machine work. I miss that.
retrojoe

Regarding Clutches (and a pedantic pet peeves of mine): When a clutch is “engaged” power is transmitted to the wheels (it’s “clutching” like you clutch a ball). When you push in the clutch pedal the clutch is disengaged. Think of it as an always on switch that you turn off by pressing.
splint

If you want a great lesson, go rock crawling in your auto. I learned all about my differential when I busted the rear one on my jeep.
Anonymous

@lyd make it so number one.
Anonymous

this is your man:

http://en.wikipedia.org/wiki/Henry_Jamison_Handy

more of him here:

http://www.archive.org/details/jam_handy
MrJM

More, please.
Muse

Brilliant and lucid demonstration. I am enamored with the approach of constructing a simple physical model and then gradually adding layers of complexity with each subsequent model. I would have learned so much more in school if more teachers took this approach.

It reminds me of an automotive engineering class I took. The teacher was trying to explain how transmissions work. The first day he was just describing it verbally and writing diagrams on the chalkboard. Almost no one understood. In frustration, he said ” Forget it!” We will discuss it next class. The next time he brought in a motorcycle transmission and took it apart in front of us. We all got to play around with it and at the end of class, everyone understood the basic principles.

Only some people learn from hearing, others can learn from a diagram, but everyone can understand a physical demonstration.
AGC

Why where educational videos much better 40 years ago? Seriously now we have all this computer graphic visual stuff, but it can’t even compare. Best part of the video ‘Now we have the DIFFERENTIAL’ said like it was echoed by the gods.
- Alan
  
  No joke. Nowadays, there has to be trendy music, sound effects, some funny character and too many edits to be considered interesting.
  
  I’m not gonna lie – I learned something from this viddy. Thanks, Cory!
  
  This brings up something I’ve long advocated – mandatory auto shop in high schools. Hardly anyone but gearheads understand how cars work, but almost everyone uses one. Hands on experience, like Muse above states, is a great learning tool. By learning how a car works and how to do basic repairs, there are a number of applied sciences at work, including physics, chemistry and math. Once kids have gone through a one semester auto shop class, they’ll be more self reliant on taking care of their vehicle, have a better understanding of some basic science concepts, and we’ll have safer, more well maintained vehicles on the road.
kossmikman

I didn’t know Hank Hill made educational videos.
MollyMaguire

We need more spokes!
Anonymous

For those wondering who did this video, it’s the Jam Handy Organization (logo seen at begining of full 10 minute video).

They produced a great number of training videos over the years:
http://en.wikipedia.org/wiki/Henry_Jamison_Handy
MadMolecule

I’ve wondered how those things work for decades (though apparently not enough to search youtube for it). How cool; thanks for posting.
John Farrier

That was simply fascinating! It was explained rather well.
Anonymous

Gosh thank you!!!!

Usually when these things are explained to me, there is step A and step B (which are easy to understand) and then there’s suddenly some magic to get to step C which I cannot follow. This explanation is wonderful, there is no ‘magic’ phase.

Thanks Cory!
Drew from Zhrodague

You mean, noone else has an inner monologue of a science announcer explaining things? What’s strange is that I do sound like a science announcer. Duck and Cover!
lorq

Wonderful clip. Could not be clearer.
chgoliz

Thank you.

Continuing education is a wonderful thing.
Bob Gaffney

My 4WD Chevy Colorado has “undifferentiated” drive for the front wheels – they’re both driven at the same rate, which makes parking in 4WD on dry pavement virtually impossible. They’ve had 50 years to solve this problem, and just haven’t bothered. Yet my wife’s Subaru has full-time 4WD and you don’t even notice it. Same problem with the Fords. If Detroit wonders why they’re getting their clock cleaned by imports, it isn’t just health care costs.