Can we all pause to appreciate how lucky we are to have this content available, any time, for free? 🙌 I wish I’d known about this channel long ago. It would have allowed me to better educate friends and customers in the car world. Thank you to the crew and Professor Kelly for putting these videos together. 🍻

My Dad retired from Dana as an engineer and I never thought that was cool until now. This video was fascinating. Thanks for teaching.

No matter how much I think I've learned, The Professor always teaches much more, and in-depth. A natural teacher with great pace for complete absorption.

That was an EXCELLENT tutorial! No stupid music, perfect camera, lights, mic sound! And very sympathicly presented. Many thanks from Germany! I learned a lot from it.

60 years this has been a mystery to me. This was the most comprehensive explanation I've ever seen. I still don't own a thorough understanding but it makes more sense. Thanks for taking a great deal of your time to share this.

Man, this channel can't ever be boring with this much knowledge shared to you. Some of my friends find it funny how I talk about differentials a lot but they don't know how much goes into it 😂

You’re obviously teaching a trade school how lucky are your students to have somebody who can explain this so succinctly bravo sir.

It's a pleasure to watch a video by someone who knows WTF they're talking about. Even though it's over an hour long, it seems much shorter because he makes it interesting and informative; no stupid music or attempts to be a comedian. Classy and well done +++

Professor Kelly is an admirable instructor. His students are fortunate to have him. Thank you for sharing this information.

A truly excellent instructor. His students are privileged to have the opportunity to learn from him.

I love these videos. I've spent years (~35) trying different differentials in street cars, road racing cars, LD trucks, HD trucks, snow plows, off-road vehicles, etc. and it is nice to see someone who understands how they work. I don't know if you noticed though, the different ways that the clutch packs are applied in the limited slip differentials (LSDs) shown. In some LSDs, like the 1st one shown from the 1960's, it is the torque applied to the ring gear and the pinion cam profile that proportionally clamps the clutch packs together. For racing this is preferred over the spring preload LSD style. On the other hand for street use in icy and snowy conditions a spring preload LSD tends to perform better. Both the driving experience and how they function is unique to each LSD style. I'll explain below why. If you look carefully at the 1960's LSD you have and pay special attention to way the spider or pinion cross shaft sits in the differential housing you'll notice it rides on an angled cam machined into the housing. As torque is applied to the ring gear (and hence the differential housing), the pinion cross shaft rides up the cam and this creates the separation force that clamps the clutch pack together. There is still a separation reaction force generated by the spider and side gears, but the majority of the clamping force is due to the cam. These cam or ramp based LSDs can typically be purchased with different cam angles (not sure about the one from the 1960's). Some have replaceable cams, while others you have to machine (or purchase) the housing with the angle you want. The purpose of this is to adjust the amount of differential "lockup" you get per amount of torque applied. You can also create a different amount of lockup for the forward torque (acceleration) vs reverse torque (deceleration) by using different cam angles for each direction. The Torsen style differential bahaves functionally similarly to a cam applied LSD, but the downside to a Torsen is the forward and reverse torque induced differential coupling cannot be individually tuned as easily. Most people claim Torsen's can't wear out, but I can confirm they definitely can. Many modern limited slip differentials used in street cars (like most of the LSDs in the video) do NOT use a cam profile to apply the side clutch packs. Instead they use a less expensive spring's preload to create the majority of side clutch clamping force, combined with the spider/side gear separation reaction force. If the driver simply wants to be able to perform 2 wheel burnouts or drive in the snow, this style can work well. Detroit Lockers are another story (note in the video the LSD labeled Detroit Locker was not a Detroit Locker). They are not really differentials in the classical sense, nor are they LSDs. If you imagine taking a pair of ratchets from your toolbox, cut off the handle, and weld them to each side where the side gears normally go in a differential, you have created the function of a Detroit Locker. For straight line driving, both wheels turn at the same speed so the ratchets work fine. When you want to corner in good traction, the outside wheel wants to turn faster so that side's ratchet will allow this to happen while going "click, click...". The coolest part of the Detroit Locker is when you switch from drive to reverse and vice versa there is a bit of a clunk (and the driveshaft will turn 1/4 to 1/2 a turn). What this is doing is switching directions for the "ratchets". The direction of torque applied to the ring gear is what selects forwards or reverse for the ratchets.These work great off-road, in snow plowing trucks, and in heavy, long wheelbase vehicles. Eaton Gov-Loc's are a bit controversial and are poorly understood in general (the video did a great job explaining them). Most people I know hate Gov-loc's because if you rock the vehicle from forward to reverse to forward to reverse to..., spinning the wheels each time, you'll destroy a gov-loc in no time at all. Where I've seen them work very well is when used in a heavily loaded truck and one tire is on ice and the other is on pavement. No common LSD will lock up enough to get anywhere close to spinning the wheel on pavement. With a gov-loc, if you are gentle on the throttle, the governor will engage, the clutch packs will tighten more and more until the differential is effectively locked up, and the truck will simply drive ahead (or will start spinning the tire on pavement, even burning rubber with a heavily loaded truck, if it is stuck badly enough). With modern vehicles there is another option that most vehicle manufacturers are starting to implement with varying degrees of success (sometimes combined with viscous coupling differentials). This option is to use the ABS system to create a virtual locker. The software monitors steering angle and relative wheel speed (all 4 wheels). If programmed well, the instant a wheel starts to spin, that wheel's ABS engages and slows the spinning wheel down to the speed it should be turning. What I like about this option is every single vehicle sold today has the ability to include virtual lockers, essentially for free. All it takes is a bit of software to use the already present ABS system as a locker and by including drive by wire it becomes a seamless addition.

I probably shouldn’t have started watching this when I had work to do! Such a clear explanation and I now properly understand how locking differentials work. Lucky students - you have a great teacher!

CORRECTIONS: 1. The differential I called a "Detroit Locker" is not a Detroit Locker. It is just an Eaton Posi Limited Slip differential. A Detroit Locker uses dog teeth and is a truly locking differential. Thank you goes out to Keith MacDonald for catching my error. 2. I did not adequately explain how the clutch packs of a limited-slip differential are applied. The simple rotation of the side gears versus the differential pinion gears does not cause clutch packs to apply, otherwise, the clutches would apply when turning corners at low load (low torque). Torque from the engine (or another propulsion source) is necessary. The more torque you supply to the differential side gears the higher the force on the angled (ramped) teeth, the higher the force on the clutch packs. Thank you to several viewers for pointing out my omission. 3. There is more than one style of Torsen differential. See the JTEKT Corporation's website for more information www.jtekt.co.jp/e/products/lsd.html 4. I incorrectly described the operation of the original 1956 Dana limited-slip differential. The pinion pins on the original Dana differential have tapered ends. The pins sit in four V-shaped ramps in the differential case. Under a load, the differential case applies rotational torque to the pinion pins causing them to slide up the V-shaped ramps. This action applies outward force to the side gears and the clutch packs. Thank you to several viewers for pointing out my error.

You know John, that in 1947 I learned differentials in auto shop. And of course, the teacher told us they were "spider" gears. And it always happens that when you learn something when you are young, it tends to cast into cement. If'n you git me drift! LOL Thus, for 70+ yrs now, I always had a problem when someone says other than "spider" gears when talking about differentials.. But I will tell you this kind Sir; since I believe truly that you are one of THE greatest teachers I have ever had; so I will stop having" that problem"; and say something I would never have said, if I had not met you. GRRRRR! LOL Thank you John. You are truly a cut-above. As an 89 old man, I love your UKposts video's. Among THE best IMHO. Keep up the great work.

The most thorough and easy to understand primer on differentials ever.

I stopped the video at "thank you for watching"....couldn't believe an hour has passed......I am a teacher (professor) myself...totally impressed with your way of lecturing.....

I really like your "Nothing is using regular gear oil"! So truth! Great explanations!

Mr Kelly.... My son is not easily caught to attention... He has Mastered his Automotive Excellence classes with ONE thing... YOU... I hope you live to a Thousand years as you are 1 in a Million

I'm glad I'm not the only one who enjoys sitting down for an hour learning how differentials work, and seeing how the g80 "gov bomb works. i was disappointed he didn't have an example of a Lincoln locker, but enjoyed the subject being mentioned

Finally making it to 65 years has me relearning anew. Professor Kelly, you remind me of one very special mentor, Scott Towsly, who helped me as I struggled during my early learning years. Thank you for being so thorough. I did know some of this lesson was not exactly the correct terminology but knew viewers would be helpful pointing to that. Just the amount of info you brought was plenty tough enough. I'm impressed.

These channels should get discovery channel money, that crap on drama is nonsense and I discover nothing in one day I gain knowledge here and I’m not being harassed with commercials about self indulgent drama queens. Thank you for your content your time to produce interesting videos , your expertise . These should be in our schools .... thank you sire have prosperous new year and healthy one

It has taken me years of study to figure all this out and I still learned something! Now I have a video I can send somebody else to! Thanks!

All of the animations on the internet put together would not substitute for a great teacher like this. Professor Kelly predicted every possible question of how differentials work and left no ambiguity.

There's also the Jeep "Quadra-Trac 2" system. This uses an oil pump to force the clutch packs together hydraulically when one wheel begins to slip, I had a Grand Cherokee with this setup and it actually worked quite well!

Excellent classroom presentation. Couldn’t ask for more information with only an just over an hour of time to cover years of the evolution of the vehicle differential. Those of us that have been around differentials for decades still can learn from this as I was a “posi” guy and never really advanced any further. 2 thumbs up.

Thanks. In my o-pinion, 55 years of general lore cleared up in one enjoyable hour.

I probably shouldn't have wasted my bandwidth watching this.. but I was captive watching it for many minutes. Wow I feel like I can learn how to be mechanic from watching your videos. My friend went to ATI School of mechanics in Arizona after high School. And now he's the head mechanic for this county of Orange county California. So shout out to ATI School of mechanics. He shut it off working on forklifts. Shout out to my friend Mike Deutsch of Orange county California. I miss you Mike.

I'm a journeyman mechanic with 40 years experience, it was great to get a refresher on the principles of diffs. And see all the various types. Very well presented. Well done.

This subject has always frightened me, you made it super understandable, Thank you very much!

Thank you so much for this video. I knew conceptually about open differentials versus limited-slip differentials, but it was all a bunch of blackbox magic to me up to now. Your video goes into the exact right amount of detail to get an intuitive understanding of the different ways that the limited-slip can be achieved. I'm so happy to understand this technology now.

They should give Nobel prizes to people who can successfully explain how Torsen diffs work! I had a conversation with a coworker who's an ME once (we're both car guys) and I told him I thought the operating principle of the Torsen was that it was so complex, the torque just got confused and went in the opposite direction from usual. ;^)

Instructors like him make things simple and interesting… I bet his students go out to be excellent mechanics…. Thank you Professor Kelly!!!

This is an excellent video showing the the different differential types. There is a great old Jam Handy/Chevrolet video from the ‘30s entitled “Around the Corner” on You Tube that explains piece by piece how a differential works that is one of my favorites!

BEEN WORKING ON THEM FOR YEARS HE'S TELLING YOU THE WHOLE TRUTH AND HE IS VERY SMART TO SHOW YOU HOW THEY WORK THANKS I REALLY LIKE IT WHEN I KNOW YOUR RIGHT ill be learning on the work converter video

What is there not to like about this video,? Why would you even click on it if you were not interested in this sort of thing? This bloke explains the chosen subject so well that an imbecile could understand it. No bullshit, no waffle.... without a doubt the best video on any mechanical subject I have ever seen on UKposts and I am an old man.

Nice refresher course for us old school guys! Oh the tales of the stuff we did in our youth!!! 🤣

Perfect lecture delivery and so well presented with simple words and hands-on the 11 differentials. A bucketful of knowledge ready for transfer! Thank you Sir.

Really well explained and it's so cool that you have all the actual parts to show how they work. Makes it way easier to understand. Thanks a lot!

This is about the most complete explanation on the web.

As a 16 trying to learn all that he can for a hopeful career in automotive in the future, I thank you for being the first person who made me understand differentials

I've rebuilt engines, but this video highlighted to myself just how little I know about the variations Differentials come in. The video exceeded my initial expectations, thank you. Most definitely subscribing.

THE most interesting and educational video I believe I have ever had the pleasure watching on youtube! Thank you Dr. Kelly! Have always wondered about differential basics. I sent this to several of my colleagues. So much appreciated.

If you’ve watched 20 different videos on the function of diffs and still aren’t clear, THIS is the video for you! Excellent explanations.

When the professor starts explaining its very difficult not to understand. Thank you Sir..

Thank you for fulfilling my request from some time back. You did cover the majority of the varying differential styles. I was looking forward to the actual Detroit Locker with dog-clutches and also the cone-type limited slip (Auburn). I’ve even seen planetary gear sets used as differentials to vary torque split front to rear in transfer-cases. My compliments on the broad coverage and the history of the better differentials. Your videos make my job easier. Thanks.

Still love this man for his excellent presentation and thorough knowledge. A real teacher I would like to have during my studies.

What a BRILLIANT video! I had a 65 GTO (with "Posi" - in fact I put a 4.56 diff in the car for drag racing and boy was that fun) as a teenager but a couple of my friends had Hemi Roadrunners with Dana rear ends. Thanks very much - now I know what those are some 40-50 years later!

I'm 75 years old and I've heard of all these different types of positraction rear ends But it is really nice to have the differences Explained in such detail.. Thank you.

Professor, thanks for posting another excellent video! Two things are constant when enjoying your well made clips. I never have a single regret for the time I spend watching them. I always learn something. Two huge wins by my accounting! Only an elite pantheon of content providers like yourself are worthy of this distinction. Please keep raising the bar.

It's great to have a source of information that specializes in educating. I've watched other videos about this kind of stuff in which the host is either pimping their product or doesn't understand what they're talking about.

Dr. Prof. Kelly, Perhaps it is a "regional " thing, - but we always called here in the Midwest "Spider gears" for all the " Miter Gears" but identifying the side gear as you do with the axle spine, and the side case called the " Carriage " . Also, for those the enjoy 4 wheeling the Australian company ARB makes a very popular "Air Locker" which many here in the US use as a aftermarket Diff. Thank you very much for your presentation !

This vid took me back to my first year of Voc school in ''77! LOL I would like to add, when pulling the preload springs on a Eaton style LSD you can use a "head on block" valve spring compressor. These cost only few bucks and the jaws are adjustable to spring diameter. Works pretty good.

Great video!! What makes the side gears pushing on the clutch packs is NOT the rotation, but the force applied: in other words the higher is the input torque and the higher will be the force on the clutches. This force basically acts as a preload: in case of a wheel on ice, the wheel with traction will still transmit the torque that is generated by the clutches (the bias torque) which is needed to "unlock" the differential The same occurs in "torsen" diffs, but that is achieved by the high friction involved in worm gears. More input torque and more "unlock torque". By changing the pressure angle of the gears it is possible to obtain different ratios between input and unlock torque F

I believe that the original LSD operation mode (in my opinion) is different from the explanation given. The clutch packs are squeezed by action of the ramps in the casings and not the bevel of the gears (although it might be part of action). Specifically, the pinion pins are floating on the ramps/cams (heat treated angled areas in the case) and when there is a speed difference and torque applied the pins will ride up the ramp thus squeezing the clutch packs. This particular case would be identified as 2-way LSD (having effect in both directions). Note: I might have misunderstood due to possibility that the original LSD was not the same LSD used for demonstration.

Finally got to see an open differential working before my eyes! It's like solving a puzzle whomever came up with it. The various methods of limiting the slippage and locking up was a fascinating tour. I feel very learned after watching.

John Kelly is an engineering genius and humble to boot. He got there by many years of hard work and dedication. If you ever want to match him in his genius then there's no time like the present. I'll see you in a few years!!!

16:30 I believe Ford's incarnation was known as Traction-Lok™ in 1958-59, but even earlier Edsel and Mercury models had a version called Equa-Lok™ in 1958.

I have never clicked so fast! Thank you professor!

Probably the best and most educational video out there for automotive differentials

I heard, watched, read a lot of documents. I found that differential mechanism is magnificent. But, now, the way you teach through video is fabulous. Thank you for your teaching.

This was a great tutorial and cleared up a lot of the mystery surrounding diffs for me. Thank you!

Something that often goes unmentioned is that an open differential in it's elegant simplicity always delivers equal torque to both axles. The amount of torque may be limited by the axle with the least traction but it is always equal. :)

Thank you Prof. Kelly for introducing different differentials.

Wow really great video. Explains a subject which has been as clear as mud for years. Thank you for clearing up a lot of misunderstandings.

Great video, thanks for taking the time. On the Gleason-Torsen helical diffs, I recall the selling point being that they are torque sensing, thus sending torque to each wheel based on the amount of traction available at that wheel.

This is phenomenal and precisely what I was looking for. I'm having an LSD installed in my Ford Focus ST and wanted to educate myself further on them. Truly awesome. Thank you!

I've been trying to understand differentials for a few days now but couldnt find a good explanation anywhere but this video really made it click. Thanks so much!

Those are long videos but I can’t stop watching. I am an engineer myself but the detailed explanation keep me interested. Ing. Bernhard List, Germany

No no, thank you for making! Another insightful, informative and thoroughly enjoyable video.

Great stuff. The only thing missing was a diff with side gears that have cone clutches. Same principle, very different clutch theory. Thanks!

Thank you for taking the time to make this video. Not many people would.

Thank you for breaking down the different types of differentials. Great job giving knowledge in a simple to understand way. Wish you were my teacher. You totally explained the mechanics and made me understand how & why they work. Thanks for the knowledge so I can get the right diff for a particular application. Time to rebuild and rework my crawlers.

Ah, sure wish you were my shop prof back in high school, would have been a much different experience I think- thanks for putting these videos up!

Now I know why 'positrac'/LSD's wear out a lot... them clutch packs. Thank you again!

Thank you sir. I spent 47 years knowingly how it works.

Articulate, Informative. Well Done Professor!

a lot of interesting and relevant info , answered many questions I had

Great video! The best part starts at 34:00 mark for Gen5 Camaro owners. Note: You don't need to mark the case halves since it's impossible to assemble wrong, there is only one hole for the roll pin that holds the spider gear pin.

We live in a fascinating era with amazing new technologies but looking at automotive inventions of the past is equally amazing.

I can't believe I sat through an hour long video !

Very interesting video! I was wondering, which is more economical for manufacturers these days; installing a LSD or use a regular open diff and use the ABS system to individually brake the wheel that has lost traction in order to transfer power to the wheel that grabs? Also, what is a "torque vectoring" system on an AWD vehicle? Does it use the ABS system to do what I've described, but for all four wheels? Thank you for sharing, professor. Always very clear and easy to understand explanations and demonstrations.

Professor Kelly, thank you! What a amazing and in depth explanation. I now understand how these work so that I feel much more confident in working on them.

Great tutorial, but I wished you would have included the real, two clutch packs posi, which I had in my '68 Javelin. Luckily, I found a (real) Detroit Locker to replace the posi, because NOBODY tells you that the housing bolts on the posi are LEFT HAND thread, and I promptly killed two of them thinking they were loc-tited in place! Now try finding replacement parts for the posi. let alone those dead left-hand bolts!!! As a note, I started to rebuild my diff with a lot of trepidation, because this was my first diff in 45 years of car crafting. Also, I WORK ALONE! I do have the complete '68 AMC service manual, which without, would have made the rebuild too daunting. In the end, with the right information and tools, putting the Detroit Locker in, getting all the numbers right AND installing single piece axles, the rebuild was a piece of cake!!!! Possible videos to follow, as I've documented just about everything. CORRECTIONS BELOW...points 2&4 - I'm not sure about the limited slip in #2, but in #4 you mention again limited slip......#4 ALMOST perfectly describes my POSI-TRACTION, not the limited slip! SORRY...the spider gear pins (pinion shafts - AMC) are NOT tapered, but instead are MACHINED AT THE ENDS ONLY to match the "V" in which they sit and slide up. Hope this helps, und guten Tag WzW.

Really enjoy the thorough nature of your presentation. I wish you had explained the concept of the ball plate. I assume this is similar to the brake apply mechanism of my massey-Ferguson industrial tractor, and is a self-energizing expansion device that can apply crushing amounts of pressure to the clutch mechanism. Far in excess of what is possible in the Posi setup. I also expect that it may be necessary, in certain instances, to reverse slightly to release tthe pressure, but I may be wrong. Also, in the case of welding the spiders into a spool, the gears are definitely not cast iron. They are high-carbon alloy steel. This detail aside, the dynamics of welding such material is indeed similar to cast iron as electric arc welding introduces such a localized high heat, that the portion of the gear not directly exposed to the arc remains cool enough that it removes the heat from the weldment fast enough to produce a quenching effect that manifests itself in the formation of a brittle boundary layer at the weldment root, similar to cast iron. This boundary layer causes fractures under stress, sometimes during cooling, that many refer to as "tearing out". The solution to the tearing out problem is an understanding of what is actually taking place, as well as special alloy electrodes and a willingness to sacrifice any heat treatment present in the gears. In short, as you say, welding spiders is an extremely bad idea, if for no other reason that the assembly cannot be undone, and will likely be destroyed in the process. However, like the old Aamco ad used to say "let me try boss, I always wanted to work on a transmission." I'm sure Goober is alive and well.

Thanks for making this video! Can you comment on which category Honda’s SH-AWD rear differential falls into?

Wow, what an amazing lecture. Thank you so much Professor Kelly. You are definitely in the right profession as you are very easy to understand and your lecture flows so well. This is easily the best coverage of this topic I have ever seen.

Sir, you’re a true master of your craft. You helped clear some misconceptions I had, especially at the beginning of the video. Thank you and God bless the work of your labor always.

I remember asking my dad when I was about 6, for a month how does a clutch work?, Then I hounded him asking him how spider gears worked this is special

aaaaaah yeah! Thanks John.

Thank you for this excellent content and for sharing it publicly. I've just swapped out a Torsen diff for a locking differential for offroading. I understood the Torsen diff was a fantastic piece of kit, but now I understand how it and the others work. Great presentation of information also

Amazing to have such high quality video, teacher and information for free! Thank you!

The only one you didn't have was a Lunchbox Locker, it's basically a mini spool but with dog teeth and springs. It "can" have slip from one wheel to the other but it sounds like a ratchet when it does, as the dog teeth slide over each other

The pinion pin on the Dana Differential, instead of going in a round hole in the case, it goes on V, creating a ramp , as the case applies the torque to the pin it rides on the V forcing it to the side and against the side gear applying the clutch, thats ingenious, because in a daily low torque to the wheels maneuver, the clothes don't engage as much has in a high torque situation, making the differential work as an open diff when limited slip in no necessary.

This presentation is excellent!!! The Professor's ability to convey the information is right on target. Thank you!

I've always loved differentials because they really are an ingenious design and I just love learning about all the different types of LSD, lockers, etc. One of my favorites and the one I installed in my Ram is the Torsen type or helical LSD. The only downside being if you get one wheel in the air, you may have to set the parking brake or apply the brakes to sort of fool the differential into thinking there is load on the axles. Very informative video as always! Weber Auto is always one of my goto channels for accurate, easy to understand information.

This give new meaning to the term “differential equations”!

Thanks a lot!

Great video. There are many diff videos on youtube, this was the first one that covered several types, with all the theory, and physical disassembly on the bench. Just brilliant.

Thank You Sir. You are a good man. You are admired by the many students you have taught and continue to teach. You share your time, your knowledge, to teach those with a willingness to learn. Thanks again for these tutorials.

Things get real interesting when that retainer bolt breaks inside the case and the customer says “well you guys broke it”!!