Viscous Limited Slip Differential - Explained

How do viscous limited slip differentials work? Viscous limited slip differentials use a viscous coupling that allows for torque to transfer to the wheel with more grip. It is an alternative method to a clutch pack differential, though by design it is not as effective at locking the two driveshafts.
NOTE: Regarding the annotation at 2:30. I misspoke here. What happens is the right driveshaft and differential housing will both be rotating. Because the pinion gears are also rotating (green gears) around the left driveshaft gear, the right driveshaft gear will have to rotate faster than the differential housing. So, the right side of the diff will be attempting to decelerate the right driveshaft (or accelerate the diff housing) once it breaks free, while simultaneously the left side will try to accelerate the left driveshaft. Because of this, it is possible to just use a single viscous coupling on one of the driveshafts.
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[@tafaragadze6432]

Years later and I'm still using these videos.

[@BoostedPastime]

His channel is a blessing

[@AWDfreak]

I'm glad to see so many of these LSD explanation videos. It really does help other people to understand why most road-going cars with open-differentials aren't as effective under low-traction situations.

[@TheKoenBrouwer]

Mate you're safing my education. Without vids like this I'lld be staring blind at these illustrations in my book. These explanations of yours help me understand everything way faster and better. Thank you!

[@DonTruman]

Sounds like a great idea. Simple, durable, and somewhat effective for getting out of a slippery spot.

[@EngineeringExplained]

I have multiple videos covering this, I'd recommend starting at the beginning of the "differentials" playlist. Locked diff means both wheels always rotate at the same speed; torque distribution can vary.

[@NicMagz]

I'm not entirely sure why, but the phrase "a pile of ice" literally made me laugh out loud. You just made my day, good sir.

[@erinmills6610]

Or just some ice.

[@ryanmalin]

this comment is depressing

[@EngineeringExplained]

Different methods, there's likely a cost difference as well. A VLSD will act a little more like an open diff than a clutch type, as it will allow for varying speeds before creating a restrictive force to get the diff close to locked. A clutch type could be locked in situations like cornering (while accelerating) where you may not necessarily want it locked. But ultimately, there are numerous solutions to every problem.

[@EngineeringExplained]

Some have center viscous couplings splitting power from front to rear, yes. Not sure which makes and models, but most companies will say on their websites. LSDs cost extra money, so they're worth bringing up to the customer.

[@EngineeringExplained]

Glad to have you on-board, thanks!

[@mitoand9]

I think there is one important point to add in these video: viscous LSDs cause understeer by nature during steady-state cornering (outer wheel is forced to slow down and inner wheel is forced to speed up due to the increase of viscosity of the fluid). This is something that should be stated in my opinion when talking about clutch plate and viscous LSDs, since the transient vs steady-state operation of a vehicle during cornering is not the same.

[@wangyue9754]

Great video! Clearly explained the idea. Seems VSLD will result into some understeer when cornering, especially for sharp turn.

[@EngineeringExplained]

Haha appreciate the comment. I try to make multiple at a time and then take little breaks. Helps me keep up with the "video every Wednesday" idea.

[@EngineeringExplained]

Thanks for watching, don't know much about infinitis AWD system, but I do wish they'd put it on the coupe version; that'd be an awesome car (as an option). Probably just fine.

[@TheOhsemix]

Thanks for the vids...and dont forget to take breakes if it gets stressing trying to make them its good to get off time

[@EngineeringExplained]

No, torsen uses a system of worm gears. I'll have a video up soon enough!

[@angelalevitan8399]

Wow great video. I love all of your videos and you did this a while ago. You look so young in this video

[@turboluvr5404]

Don't want to confuse anyone, but during differentiation both viscous clutch packs are seeing relative rotation, and therefore are contributing to the torque transfer to the high traction wheel. Also, viscous diffs are what is called "Speed-sensing". They transfer torque due to a difference in the relative speed of the rear wheels, as opposed to "Torque-sensing" differentials that progressively "bind up" as they are put under more and more torque.

[@EngineeringExplained]

Yes, it's possible. I'm not sure how long the fluid lasts before needing replacement. I'd assume it'd be a long time; it's sealed, and there's not really any source of contamination. The only thing that happens to it is that it heats up and cools down.

[@350Zquicksilver]

nice video young man... my 350Z is fitted with a (OEM) VLSD, makes a world of difference compared to an open diff... i've had both.

[@FedericoLucchi]

Actually, the wheel on ice will spin twice as fast as the differential housing if the other wheel doesn't move, so the viscous fluid will both slow down the slipping wheel and accelerate the other one! ;) I love your work, keep it up!!

[@dannyradomski7041]

The liquid in the housing is typically a silicone based fluid, so when the plates move at different speeds, the sheer forces make the fluid turn more viscous. This helps to lock up the diff.

[@EngineeringExplained]

Perhaps, or a liquid that changes viscosity very quickly once the two shafts rotate at differing speeds. A clutch type LSD may be a better option.

[@sTeadFasT96]

Very great video and awesome educational resource. Keep doing what you're doing! You have my support.

[@EngineeringExplained]

Well VLSD does not act instantaneously, where as clutch/torsen diffs do. So if you want maximum torque transfer (instantaneously) to the wheel that has the most grip, a clutch type or Torsen (if both wheels maintain contact with the ground) are your best bet. Viscous is fine though, can do the job.

[@EngineeringExplained]

In principle, yes.

[@EngineeringExplained]

You're welcome!

[@EngineeringExplained]

Similar to a torque converter in principle, yes.

[@JohnSmith-hy1ir]

Also quite similar in principle to a Tesla turbine (boundary layer friction between discs)

[@Brancalbin]

This video brings memories

[@EngineeringExplained]

Not sure if you're saying while driving or if you just have the car lifted and spin on tire. If it's the latter that you're doing, it likely works, but the speed that you rotate the wheel isn't great enough for the liquid coupling to rotate the other tire.

[@ark-mark1]

In RC the pinnion gears and pinnion axle are slowed down with the visco liquid making them turning more with the casing. The Casing is filled with the liquid and stiffer the liquid less it lets the side gears to turn separately. So stiffer the liquid, more "lock".

[@EngineeringExplained]

Such a dedicated subscriber, thanks haha

[@EngineeringExplained]

Somewhat similar in principle, but torque converters use transmission fluid, which I do not believe changes it's viscosity characteristics much with temperature change; at least not to the degree that VLSDs do.

[@EngineeringExplained]

Great point, thanks for sharing.

[@EngineeringExplained]

As the fluid heats up (via friction) the viscosity increases, thus causing the two axles to "lock" together.

[@Cape1984]

How is possible the liquid inside to increase its viscosity when is heated up? Should not decrease??

[@Alex_EVii]

Correct all the way! So before the fluid heats up it acts like an open diff? When turning at different speeds then fluid creates friction and bias the speed. Viscous is a speed sensing unit instead of torque sensing like ATB diffs. You didnt mention though about the amount of preload the unit has. Whats happening with high or low preload and how the device acts on preload and after preload situation when breaks-away. Always great stuff from you! Thank you

[@vkotis]

I finally know how a vlsd works..Thank you!

[@EngineeringExplained]

Interesting, I would think that they should both spin together (similar speed) if you had it up in the air. Certainly the liquid shouldn't allow for much of a speed difference.

[@TucsonDude]

Can't these VSLDs leak internally?

[@EngineeringExplained]

Thanks for watching!

[@EngineeringExplained]

Probably due to the negative characteristics when the oil heats up, as far as lubrication purposes. Dunno.

[@taddyvinda]

Just pointing out its not the same speed... The weel with less friction gonna spin faster than the diff housing around 1:2 ratio... Causing the viscus liquid to resist the movement as well as the weel with less grip to move with the housing. both of the friction surfaces to work simultaneously. But i am not a fan of this diff even when basically maintenance free..

[@heinedenmark]

I had a Fiat Coupé 2.0 20VT which have this kind of diff. Works really well but is bad for the fuel effiency.

[@girlshots1]

yep

[@EngineeringExplained]

The one in it is probably fine, they likely did their math to make sure it not only reacts quickly but holds up over time.

[@genieohnehirnspaziern3819]

One question, just for my understanding: imagine a viscous-coupling used as longitudinal centerdiff. There is silicon oil (e.g. 90%) and air inside, not mixed. In operation with different speed the oil transfers torque, but the rotating plates also start to mix oil and air, so viscosity decreases, less torque is transmitted up to a certain point (hump). During normal operation (no hump) with higher speed difference more and more air bulbs will "hide" in the special shaped blades so that more torque can be transmitted? I always thought that viscosity of silicon oil increases but read somewhere that this oil is not dilatant...

[@padraiggalvin2807]

Great videos, very well explained

[@billmunro3533]

Good explanation in general, though I'm not sure if the late Robin Price, who did so much work on VC diffs, LSDs and viscous transmissions for FF Developments (And who I interviewed at great length for my book on Ferguson Formula 4WD) would agree that the plates are 'essentially a multi-plate clutch' as they do not touch unless something goes wrong

[@TheAefril]

+1, Explained perfectly 👍, Thank you.

[@nikhilkash92]

really nice videos!! keep them coming!!

[@danielvought5935]

youre the best man. thanks

[@jhalkoski]

to complete the differential series, you should make a video comparing Viscous vs clutch type LSD, which one is better in which applications.

[@damiangorski6347]

explained perfect

[@Nike10Soccer787]

love the explanations!

[@oldfashionedwrx3574]

just checked the rally cars the evolution and the STi, they both have LSD's, and the lesser model the plain WRX has open diffs. interesting that it states if the hand brake is used it does something to prevent damage to the VC

[@aaronbritt2025]

Old video, but, that's the driven shaft, or axle shaft, not the drive shaft. The drive shaft has the pinion at the end of it.

[@jchoops7yt]

Super helpful video. Thank you. Is the fluid in the diff such that its viscosity increases with friction? Or, perhaps that it counter intuitively becomes more viscous with an increase in (friction induced) heat? My other idea is that it is similar to the fact that when I move my arm slowly under water I feel little resistance, but when I move it quickly under water I feel a lot of resistance. [continued in next comment]

[@TameToothSGA]

correct me if i’m wrong (because i want to understand better) but it sounds like it works very somewhat similarly to a torque converter. sending torque to the tires using fluid

[@calibredki2852]

just an FYI 350z use a VLSD system for their OEM cars, with the nismo diffs just being harder, stronger materiel for the gears... Now, why is the VLSD non-repairable/serviceable other than just changing the gear fluid in the differential???

[@s.r.global7912]

I have an 04 350z and want to put a 370z vlsd but idk what to get to fit any advice

[@sidharms7206]

Hey know you commented like 4 months ago but all you would need to do for ideal fitment would be to run 370z shafts and driveshaft

[@tinkerbell69714]

Love your videos ;). Thanks

[@EngineeringExplained]

Check out my video on gearing "car gears" and "first gear torque." This should help you start. Just multiply across the gear ratios. :)

[@tonyspark1071]

Really no one else was confused about the drive shaft vs axle? his drive shaft should be labeled what connects to the pinion right? and the " driveshafts " are the cv shafts. right?

[@nicholasxuu]

Like your videos. Just wondering why not just put the oil/grease inside the diff gear area? both lubricating the diff gears, and creating the VLSD effect. is that because those gears are so heavy duty they need special lubrication to work. so those oil need to be used with the clutches?

[@badtzlotus]

I was spinning it in gear with some gas at rate of about 10km/h speed so it was not fast, just so I wanna check. anyways, thanks for your input!

[@fred16773]

Great explanation.

[@slickstretch6391]

Can you change the amount of slip by changing the viscosity of the fluid?

[@bypreparador5683]

In finality of liquid is proporcity of generaty the atrict forçe from the plates and realises the proximity locking differentials, and not ghe principy of in fuction of flux ghe liquid corretly?

[@fightingfalconfan]

the oil does both. acts as transmission fluid and gear oil

[@asianwithoutasianeye]

really detailed thanks. what is a locked diff?

[@mohamedidris9086]

hey could explain about roll center...........i hope that will your next video

[@derekmatthew6383]

Hey do they make a magnetic limited slip differential? I feel that would be one of the easiest ways to make one and it would last forever.

[@kierenalvarez]

Here I am in 2024 learning how my awd diamante diff works!

[@genmaxpain]

you need a video with cone type lsd's

[@jchoops7yt]

[Cont. from below] Is it true that as the difference in velocity between the two wheels increases, the friction in the diff increases causing it to allow less slip? One final related question: Is a viscus LSD's utilization of fluid similar to that of a torque converter? Thanks so much for helping out!

[@nrgrao1991]

the weight distribution ratio is 54%-rear and 46%-front...i hope you can help me out here...

[@frostbite1991]

ok I think I understand this. A lot of newer cars just use the brakes for this (my 08 Jeep Patriot for instance). This is called BLD, Brake Locking Differential. The vehicle has speed sensors on all 4 corners. if it notices 1 wheel is spinning significantly faster than the other on the same axle, it applies the brakes on the faster wheel. Same thing happens here except instead of using the brakes, it used clutch pads inside the diff. So no, not a real LSD. it's cheating really and robs you of power. I'm glad the auto industry has strayed from VLSD's and went to BLD's. my G35 coupe, for example, has a VLSD but it's 100% open due to the fact the VLSD clutches wear out at around 60k miles. So it's either replace the clutch discs and have semi-decent traction at performance cost, or slap down 2K and install a true LSD. Kind of BS when you realize the g35/350gt skyline/350Z never came with real LSD's.

[@cybertree]

+Zac Price *VLSD's are MUCH better than BLD's*, and they *don't use clutches btw*, you only need to replace the Viscous Oil, and that's super cheap and lasts FOREVER. (Sometimes if your doing REALLY extreme offroading, or racing the viscous oil can overheat and needs changing, but a normal G35/Patriot shouldn't need it unless it leaks.) *VLSD's are essentially a smoother LSD system*, and you can get thicker oil to increase the slip limiting torque too, perfect for high performance GT cars and such.* (If I owned a G35, I'd personally look into getting a good *CLUTCH TYPE LSD*, because that's the best type for both speed and safety.)

[@cybertree]

+Zac Price *"BLD's" are just a way to prevent wheel spin.* Brake-based systems don't actually send the greater torque to the slower wheel. Plus they *REALLY wears on your brakes* if your somewhere slippery where they need to activate on & off constantly. *It's much more costly to replace than Viscous Oil, especially if you do go offroading.* (Still a good safety system to prevent people from going sideways and sliding though, I personally think Jeeps should come with LSD's, and the BSD's just a way around it.)

[@raybankes7668]

BLDs rob power from the engine to the tire with braking, why would u want that. its the same issue with an old Cat bull dozer and braking the one track to turn. u lose lot of power with by dragging the brake. LSD does not loose as much power when in a turn. adn locks up when under power. thus not loosing anything

[@heartkiller93]

it seems like a fraction of the power would be wasted with a system like this because neither side is ever directly connected, the diff housing plates would always be sliding through the liquid slightly faster than the plates connected to the axle receiving power from the liquid. am i right?

[@alanphillips4431]

Would this be more efficient by using a non-Newtonian liquid that doesn’t follow the laws of viscosity?

[@Qkfltpdls777]

Can you use normal GL-5 gear oil to VLSD (Nissan’s R200V) instead of GL-5 gear oil with LSD additives?

[@Brenby]

Will you do De Dion suspension in the future?

[@oldfashionedwrx3574]

so cars like the STi, evolution, GTR, lambo have awd systems that use this?

[@a6thomas574]

Does this type of diff require limited slip fluid?

[@0bigantney0]

Both of sides of the housing are filled with liquid, but have no pressure plates? When the tire on the ice rotates at the faster speed, the liquid transfers to the other side so torque can be transferred to the side initially rotating slower and create its own pressure to the clutch? With that being said, how does the liquid transfer? Via small turbine????

[@badtzlotus]

So that means it works? I had my car in the air with 4 corners on jack stand, 1. spinned one of the rear wheels with hand, the other doesn't spin 2. turn the car on, put in gear, gave gas, it still have one wheel spinning and the other barely moves one wheel fast + one wheel slow/barely moving = still working viscous? So getting the speed up or sudden accellerate will trigger the lsd to work? cause I still don't know if my viscous is still alive thanks!!!

[@imantisocial3179]

one can notice the VLSD being less effective after 80000-100000km in a car that is driven hard, then it needs changing.

[@thesetruths1404]

Hey man thanks. I bought my teen son a 2004 Impreza Outback 5-speed, 156,000 miles. AWD of course. We didn't notice a whine noise, but it just started clicking or tapping around curves and acceleration. I can feel the tapping in the gear shift handle. And it seems it sometimes loses AWD grip when it taps. So I think the center diff is binding the output shafts sometimes. Is this the noise that they make when the vicous diff is broken?

[@wheraboo5013]

hey man don't know what happened in the 8 months, but the gears in the diff are probs grinding and destroying themselves. get it checked out.

[@thesetruths1404]

@@wheraboo5013 it was just the center differential. $300 eBay unit and back together.

[@wheraboo5013]

@@thesetruths1404 nice 😊, glad you’ve resolved your troubles.

[@coltenphillips8174]

My Miata has one of these

[@bengleckl1877]

What is cs gl5 fluid?? Controlled slip??? That the same as limited slip fluid ?

[@nrgrao1991]

Hi Jason. Sorry this post is completely out of context but I really wanted to know how is it that the starting torque of a car can be calculated....the mass of the entire car (inc passengers) is 800kg and the wheel radius is 10inches. Please help me as I'm trying to make the car run on an electric motor with gear reduction mechanism..

[@ipodfreak93gh]

so it would be better to use a thicker liquid for say drifting?

[@michaelparker4830]

explain how the bmw m Visco Lok differential works fitted to.the E46 E90 E60 M SERIES

[@m.morininvestor9920]

Wich one is the best in a 2020 370Z ??? Open diff. or Viscous LSD and WHY ? thx all

[@Grno1984]

Hi, can you tell me, my 2005 Subaru forester has rear VLSD, but it dos not lock up anymore, can i change the liquid inside the viscous coupling ? and what kind of silicon oil i should use and what viscosity?

[@nixonwphoto]

Is the the viscous coupling kinda like torque converter?

[@GianLombardo]

So... Basically works on the same principle of automatic gearboxes?

[@silentbob9797]

Great Vid! I have a question: I have a viscous rear diff on my awd 3000gt vr4. With the car in the air, I spin one rear tire and the opposite wheel should turn the same way, but it doesnt spin at all. Does this mean my rear diff is not working? thanks!

[@Jimmy.O.]

So there's a non-Newtonian fluid in the viscous diffs?

[@vignesh410]

What kind of differential can i use if i use Motor cycle engine and chain drive to transmit power to rear wheels

[@tudorsor91]

do you think torsen is better than viscous lsd(like haldex)?

[@EngineeringExplained]

Sorin Tudor For longevity, yes. They're somewhat different though, a Torsen diff will act essentially like an open diff with one wheel in the air, a VSLD will be a bit different.

[@tudorsor91]

ok, which kind of diff is the best for all purposes?

[@alexbeau348]

+Sorin Tudor torsen is better. both torsen and viscous are long-lasting - they live the same time as gearbox or even longer. clutch-lsd - every 20-30k km u should check preload and tune it or change friction plates - all because of wear. so it's for sport and petrolheads. torsen works from differents of TORQUE VLSD works from Differents of SPIN So torsen better when when u need torque on both wheels but there is no differense in wheelspin - on tarmac in the corner . VLSD need different wheelspin so it will work in mud or other slippery surface. BUT!!! It depends a lot from initial tune of both when they were designed. Example - pulsar vz-r VLSD is so weak that car cannot jump down from lift jack. torsen can jump down but u need to press both brakes and acceleration pedals

[@gehtdianschasau8372]

there is a difference in response. torsen reigns supreme. hard to say numbers, but in corners that slight delay makes a huge difference.

[@mwmorph]

Is Viscous the principle behind the Torsen line of diffs?

[@tomknud]

Are you an engineer by any chance?

[@Cambo866]

Is this similar to a fluid coupling for say a conveyor belt drive? I need to understand this for a mechanical engineering assignment. My understanding is as the motor starts up the power gets transferred gradually to the rest of the system. Is this correct?

[@niklasxl]

so do you have different oils in the middle and between the plates with diffrent wiscosity?

[@emmanuelarthur1263]

hi, i was wondering how can this mechanism allow the ties to spin with different velocity in a curve, since the viscous flow may be acting anyway. How can this viscous differential act like an open differential when needed?