Viscous LSD

I always see the Viscous LSD as an option when making a trim, but I always ignore it because I can’t see what its use is. Could someone explain any advantages/uses/examples?

A limited slip differential is a mechanical form of traction control.

When you have a lot of power, minute differences in traction between tires allow one of them to slip more than the other. And because a differential is by design supposed to allow difference in speed between wheels, this results in runaway wheel spin since once the wheel breaks loose, its traction is less, and this makes it go faster, and this means it has even less traction and so on in a vicious cycle. This ultimately results in all of the power going to just one wheel – the wheel without traction. Thus, you get poor accleration.

Limited slip differentials are designed to allow difference in speed between wheels but only to a point so that that the wheel which still has traction will get some of the power. There are varying designs for doing this.

Lockers obviously just say, fuck it, you don’t get ANY difference in wheel speed. Their on a common axle now; deal with it. This is bad for cornering and road use though. Great for offloading however because eliminates all wheelspin due to traction disparity which is common in offroad scenarios.

Geared LSDs use complicated gearing to limit torque disparity between wheels. Good because they are very consistent and will redistribute power immediately when traction is lost. Bad because they are complicated, expensive, and somewhat rough.

Viscous LSDs, which you asked about, use a wet clutch between the two half axles. When there is a disparity between the wheel speeds, the discs rotate at dissimilar speeds causing the fluid to heat up and expand which forces the clutch plates together and makes a common axle. Viscous LSDs are good because they engage smoothly but bad because they require sustained periods of wheel speed disparity to work. And also, the fluid eventually heats and cools so many times it loses its properties and needs to be replaced.

So, why would you choose a viscous LSD? When you want some mechanical traction control that isn’t rough as nails and that won’t eat tires up.


  • Audi has favored geared LSDs
  • BMW and Subaru have favored viscous LSDs
  • Lockers are seen a variety of 4 wheel and offroad vehicles from a variety manufacturers

As for real life equivalents:

  • Generation II Quattro systems (Audi 100 C3/C4 etc) use a geared center diff, a manual lock rear diff, and an open front diff.
  • Nissan S13s use a viscous rear diff
  • I have no idea what real life cars use an automatic locker. Probably offroaders.

I was actually thinking about this last night. Way back in the day, a 4X4 would have these locks on the hubs that when you shifted into 4 wheel drive, you would have to get out of your car and lock. I assume this would be manual locker.
About the mid 1980s however, they started advertising ‘shift on the fly’ 4 wheel drive, and push button 4 wheel drive. Would this be ‘automatic locker’?
And also what about, full time 4 wheel drive, such as the AMC Eagle? This was a pre-cursor to the modern all wheel drive passenger car, but had equal distribution, because it was still 4 wheel drive. Would these feature a locker, or a LSD?

Automatic lockers where used in similar applications to earlier LSDs, not really for sportiness just to reduce wheel spin.

They were an option on earlier volkswagen and some Chevy pickup trucks.

They were also a popular aftermarket upgrade for muscle cars from what I understand and are used in NASCAR now-a-days.


What you’re describing are the transmission types. Both the 'ol “Get out and lock the hubs” as well as “shift on the fly” are represented in game by the “Longitudinal 4x4” system, since they’re selectable 4x4 but usually run in RWD.

The AMC Eagle would be Longitudinal AWD since it’s on all the time.

In Automation, the biggest reason to use a viscous diff over a geared one is because you’re trying to keep an eye on cost, production units, or engineering time.

Regarding automatic lockers, the Eaton “Detroit Locker” has been used for all kinds of applications, most of which have been mentioned (except towing). Here’s a link if anyone’s curious: Summit Racing Detroit Locker

AMC used a transfer case, like a Jeep. Nowadays it’d be classified as AWD. Here’s a Wikipedia page for anyone interested in learning more about 4WD/AWD:


Its not the transmission types hes describing, but the manual vs automatic locking hubs. These are not represented in Automation, and as far as I can tell they are assumed to be automatic. With the part time 4x4 system when you put the transfer case into 4x4 it only locks the transfer case.

The hubs are are often locked in separetley, if at all as some don’t unlock and let the front axle always spin. Others will either use manual hub locks, or an electric/vacuum actuator to lock one side of the front axle to the wheel. The second system the axle shafts are still spinning but the front drive shaft isn’t.


I don’t have much RL experience with locking hubs, I looked a bit more into it and yeah you’re right.

Still, it’s not really related to differential function from what I’m seeing, as the locking hubs are there solely so the whole front axle doesn’t get driven by the wheels when in 2WD mode, reducing mechanical losses.

Volvo 940 was available with an automatic locker, it disengaged over 20 km/h or something like that, my brothers 940 has one… Actually IMO somewhat of a good compromise on something that’s far from an offroad vehicle to give the best traction in snow. Not very refined though (like the rest of the 940).


S13 restyling I think. The zenki is a open differential. At least the Spanish ones.

In plain English, Viscous is much cheaper than geared, helps reduce wheelspin while helping drivability and sportiness a little. It’s also the second most comfortable if I remember correctly, after electronic. It’s good for cheaper sports cars or if you need to get comfort up a little more without hurting other stats.