Oppo, school me on LSD 

"DC3 LS, will be perpetually replacing cars until the end of time" (dc3ls-) 10/02/2017 at 18:13 • Filed to: None

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But first, get your mind out of the gutter!

First of all. I’ve heard the term “locking diff” throw around seemingly interchangeably with LSD. Are they the same or different? If they are different I think that’d clear up my next confusion.

When ever I hear drift, or sometimes even truck people, talk about LSDs. They say the they “lock” the wheels together. So that when one wheel starts slipping the other one will slip the same amount even if it has grip. Getting you sideways earlier.

But when ever I here AutoXers and sports car people talk about LSD. They say the whole point is that when one wheel starts slipping, it gives less power to the slipping wheel and gives it to the one with grip. Thus giving you more overall traction, delaying going sideways, but when you do it’s both wheels that have let go.

I’m inclined to believe the second explanation since it would make since to put that on a sports car. Plus if the first explanation is true, then why would anyone put an LSD on a FWD car since you’d just understeer sooner and harder? And it’s fairly common for performance FWD cars to have LSDs.

Nibby > DC3 LS, will be perpetually replacing cars until the end of time 10/02/2017 at 18:15

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drugs are bad mmmkay

DC3 LS, will be perpetually replacing cars until the end of time > Nibby 10/02/2017 at 18:20

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NO! That’s just what the NWO wants you to think so you don’t liberate your soul with LSD!!!

Nibby > DC3 LS, will be perpetually replacing cars until the end of time 10/02/2017 at 18:20

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did someone say NWO

!!! UNKNOWN CONTENT TYPE !!!

XJDano > DC3 LS, will be perpetually replacing cars until the end of time 10/02/2017 at 18:23

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Limited Slip Differential.

benjrblant > DC3 LS, will be perpetually replacing cars until the end of time 10/02/2017 at 18:23

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LSD’s and locking differentials are two different devices. LSD’s limit the slip, as the name implies (limited slip differential) and in order for this to function, there must be some differential in wheel speed already occurring. LSD’s are reactive if you think of it this way.

Locking differentials can lock the axle shafts together to ensure that both spin at the same speed. These can be automatic but are most commonly user-engaged via an electric or pneumatic actuator.

I’m not the best person to explain what exactly happens inside an LSD.

XJDano > DC3 LS, will be perpetually replacing cars until the end of time 10/02/2017 at 18:27

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See the spring in the middle? It pushes the spider gears apart, there are clutch type discs behind either side of the axle spider gear as seen in your photo. It also takes a special lube when changing differential fluid. What I know is from a jeep guy who swapped in a 8.8 axle from an explorer, and added more discs for more limited slipping and basically more force locking it mostly.

XJDano > benjrblant 10/02/2017 at 18:30

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Spring pushes apart the axle spider gears with (number) clutch type discs behind them.

You can tell a Finn but you can't tell him much > DC3 LS, will be perpetually replacing cars until the end of time 10/02/2017 at 18:31

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HHFP has an excellent write up on the theory and operation of differentials including detail on how open, locking, and limited slip diffs work. http://oppositelock.kinja.com/your-differential-and-how-traction-works-1661277563

If that makes sense to you, then you can probably make sense of my inane ramblings on how brake based “torque vectoring” works. http://oppositelock.kinja.com/open-diffs-and-brakes-for-torque-vectoring-1792186390

DC3 LS, will be perpetually replacing cars until the end of time > You can tell a Finn but you can't tell him much 10/02/2017 at 18:32

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Thanks, I’ll check it out.

Highlander-Datsuns are Forever > DC3 LS, will be perpetually replacing cars until the end of time 10/02/2017 at 18:33

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Flex my boost > DC3 LS, will be perpetually replacing cars until the end of time 10/02/2017 at 18:34

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Magic

HammerheadFistpunch > DC3 LS, will be perpetually replacing cars until the end of time 10/02/2017 at 18:38

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Probably way more than you’d ever want to know. And hopefully it makes sense

BobintheMtns > DC3 LS, will be perpetually replacing cars until the end of time 10/02/2017 at 18:39

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Video of an Air Force LSD test subject... pretty funny.

HammerheadFistpunch > benjrblant 10/02/2017 at 18:40

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Some lsds are torque sensing (torsen) and don’t require speed bias Andsome are proactive but require sensor input.

wkiernan > DC3 LS, will be perpetually replacing cars until the end of time 10/02/2017 at 18:45

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It works like this.

Saracen > DC3 LS, will be perpetually replacing cars until the end of time 10/02/2017 at 18:47

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Differentials exist because when going around a corner, an axle’s outside wheel is moving faster than the inside. A diff means you can go around turns without the inside tire scrubbing because it’s forced to move at the same speed as the outside tire.

But a “regular” differential is open. And torque always takes the path of least resistance, so open diff will spin the wheel that has less traction.

A limited-slip diff redirects a percentage of torque away from the slipping wheel. They do this through various methods. Some with worm gearing, some with clutch packs (which can be dry or bathed in a viscous fluid), and can operate purely mechanically or with electronic control.

The percentage of torque that the LSD redirects to the slipping wheel is the locking percentage. So, %3 means that %30 of the torque from the low traction wheel can be diverted to the inside wheel. The difference in torque between wheels in these conditions is called the bias ratio.

Fully locking means just that...the differential is locked at %100 and the wheels are always turning at the same speed, negating the function of the differential.

It is beneficial in offroading in many situations, and often the diffs can be locked with the push of a button.

In drifting, an LSD with a high lockup is ideal, however a lot of the kids can’t afford it for their beater S13 so they’ll weld the diff.

Urambo Tauro > DC3 LS, will be perpetually replacing cars until the end of time 10/02/2017 at 19:24

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The “slip” referred to in a “limited slip” differential is better understood IMO through the clutch-pack variety of LSD as seen in your pic. Under normal circumstances, the spring in the middle clamps the clutch-packs tightly, resulting in what is 99% of the time a “locked” diff. Both wheels will turn at the same rate, and the spider gears remain stationary inside the rotating carrier.

But when the car goes around a corner, and needs the wheels to turn differentially, the grip between the tire and the road surface overcomes the clutch-pack and allows the spider gears to turn. As soon as the car is moving straight again, the differential resumes its “locked” position.

If traction is poor, then the first wheel to start slipping will be locked with the other wheel as it fights for traction.

gmporschenut also a fan of hondas > BobintheMtns 10/02/2017 at 19:47

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My citroen won't start > DC3 LS, will be perpetually replacing cars until the end of time 10/02/2017 at 20:19

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Avoid small spaces, make sure you have a lot of room.

Bring a buddy to take care of everyone.

Water.

Make sure you are a in a healthy state of mind or else paranoia sets in.

benjrblant > HammerheadFistpunch 10/02/2017 at 21:50

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Wouldn’t sensor input and a processor make it torque vectoring?

HammerheadFistpunch > benjrblant 10/02/2017 at 22:07

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nah, torque vectoring the way we know it requires an extra step of overspeeding the outside wheel which requires a planetary gearset to do. True torque vectoring is still pretty fancy pants.

BiTurbo228 - Dr Frankenstein of Spitfires > DC3 LS, will be perpetually replacing cars until the end of time 10/03/2017 at 08:13

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To put it bluntly, the drift guys in this circumstance are wrong. LSDs (and locking diffs) are designed to ensure the rear end has more grip when powering through a corner. They do this by biasing the power towards the wheel that has more grip in various mildly-complicated ways.

The reason they’re better for drifting than open diffs is that for a controllable drift it’s helpful to have both wheels spinning, and at similar rates. So, despite giving more grip under power, it’s easier to both initiate and control a drift because if you completely overpower the tyres you get both wheels spinning, rather than one wheel spinning and the other gripping.

bryan40oop > DC3 LS, will be perpetually replacing cars until the end of time 10/03/2017 at 08:56

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They limit slip.

t0ast > DC3 LS, will be perpetually replacing cars until the end of time 10/03/2017 at 11:18

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Definitely the second description. A difference in speed of or torque required to rotate the wheels will cause the differential to engage. Depending on the type of LSD and how it’s set up, a certain amount of power can be forced back towards the tire with more grip, from some percentage to fully locking the two sides together. The ones that do fully lock (which aren’t many) differ from a “locking diff” by being able to freely ramp between partial lock and full lock depending on the various input and output forces being applied.

Additionally, some LSDs can also work during deceleration. This is what is being referred to when you hear “1 / 1.5 / 2 way LSD”, as the “way” refers to the states of accel/decel. Generally speaking, 1 will work under acceleration only, 1.5 will also work under deceleration but with a different amount of force than it does during acceleration (usually less), 2 way will work evenly both ways. The benefit in doing so is to provide stability by “evening out” the two sides of the car as it slows, but as you can probably guess, it promotes some amount of understeer.

On the autocross front, the most important part of each corner is usually the exit. LSDs help put the power down early and easily as you work through that critical area. Compared to an open diff, this can be worth a full second or more on a 60 second course. Even if it causes both tires to spin, it’s more difficult to reach that point in the first place (higher limits = faster), and easier to control when it does (less drastic transition in and out of grip). Any additional understeer encountered from an can generally be mitigated by other setup adjustments (alignment, suspension, etc.) or driver behavior and is far outweighed by the aforementioned benefits.