Choosing Our Perfect LSD

"Zachary Oberle" (zacharyoberle) 07/12/2016 at 12:55 • Filed to: Project Hoondy, LSD, Limited Slip Diff, Quaife, Torsen, Torque

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The Hyundai Genesis 3.8 Grand Touring comes as standard with an open differential. This is a total travesty in the eyes of hoonage. Any self-respecting Jalop would do well to avoid purchasing anything other than a Track Pack Genesis Coupe for that very reason. So when it came time for us to purchase our own GenCoupe, of course we went for a Grand Touring edition... Wait, what!?

There are several reasons we went for the GT spec on our car, all of which were extensively covered in a !!!error: Indecipherable SUB-paragraph formatting!!!

Of all the components that have an impact on the driving feel of modern sportscars, the differential can be among the most profound. In rear wheel drive cars especially, the right diff can make a huge difference. Which isn’t to say that other layouts don’t benefit. Indeed, there exist multiple Top Gear clips of Jezza Clarkson having a giggle over the grip-inducing efficacy of good LSDs featured in front wheel drive cars like the Ford Focus RS and Golf GTI.

Yet in a world where the OEMs have press releases about individual components that will be featured on their hotter models, the humble diff ranks low. Sexay go-faster gear such as turbochargers or dual clutch transmissions dominate the spec-sheet headlines for the latest and greatest performance cars. Transmissions especially seem to be a sticking point these days. The latest Porsche 911 GT3 is a great example of this, with much attention being called to its lack of a manual transmission option. This despite the fact that the rest of the car is among the finest odes to hooniganism ever assembled, including a killer torque-vectoring diff.

©Porsche USA

We’re baffled at the lack of interest in differentials. Maybe it’s that they’re a bit boring? Innovation is less incremental in such components and we suppose there are less frequent advances in tech to get excited about.

Regardless of popular perception, a hi-po diff was a big priority for us when it came to acquiring an amusing car of our own. This was proved out during our test drive of a Saturn Sky Redline. That car was, objectively, crap. Its control inputs were heavy and lacked depth of feel. Despite that, the Sky Redline was a hoot to drive. This was largely down to its standard mechanical LSD which gave it driving dynamics that you just don’t find in garden-variety cars. The ability to casually squirt a car sideways under power often trumps raw speed, acceleration, and grip when it comes to ‘fun factor’.

So why, then, did we end up buying a powerful, rear wheel drive sportscar with an open diff? That’s like scoring a date with a gorgeous stranger and then hanging out via web cam all night. Sure, your companion may still do much to amuse you from inside a screen but, come on, we all know it would be more fun to have them right in the room with you instead.

The simple answer is that we couldn’t find a car we liked that had a good diff. The long answer is that a lot of compromise goes into both the building and buying of cars.

Which brings us to the more recent past. Even after bleeding cash on a !!!error: Indecipherable SUB-paragraph formatting!!! we still hankered for a major drivetrain upgrade. We began to look into our options for after-market differentials, and were both pleased with and intimidated by the wide selection. As a result, we decided to re-educate ourselves on the different types of limited slip diffs.

Many cheap sportscars that come from the factory with an “LSD” are saddled with something known as a Viscous Limited Slip Differential. These types of diffs are pretty lame, with extremely limited locking factor and poor longetivity. They depend on a sealed housing full of silicone snot to create friction between a set of plates. Over time the snot breaks down and the diff stops working, especially under the high heat of abusive driving conditions. Certainly, this type of differential is not up to the rigors of repeated drifting or hot-laps.

We were totally uninterested in any sort of V-LSD. Happily, these kinds of diffs are almost exclusively the purloin of OEM car-makers and were not an option for us.

Leaving snot-filled diffs behind, we start getting into proper mechanical LSDs.

This is where the Track Pack edition of the Genesis Coupe comes in. That package included a Torsen geared LSD. These torque-sensitive, or “torque biasing”, differentials perform their limited slip duties as part of their primary design. This means they contain no spider gears like in an open diff and no clutch pack like in other mechanical LSDs. They rely on the thrust loads of their gear set to create friction against the outer case thus limiting differentiation.

Helical diffs like the Torsen are a great compromise for street cars. They work well for providing extra grip. They’re low maintenance and have no clutch disks or special viscous fluid to wear out. They just need regular oil changes, like any other diff, in exchange for long service life.

Sadly, a helical diff is not a perfect solution for us. While these diffs will bias drive torque towards the wheel with more grip, they’ll only do so when they’ve got a bit of momentum to work with. Helical diffs bias torque across the differential, not to each individual wheel. This means that if the load on one wheel is low and the other wheel doesn’t have enough grip/power to accelerate the car then the whole thing just acts like an open differential. In this situation a geared diff provides no extra grip whatsoever.

This comes down to the helical diff’s lack of “preload”. Other types of LSDs have some kind of spring that provides a bit of clamping force between the drive wheels. Preload force ensures at least some drive torque will make it to the drive wheels even if one of them is completely off the ground.

Furthermore, helical diffs are limited on the amount of maximum locking force they can provide. This is why some higher-end Torsen diffs also include a clutch pack in addition to their gearset. Without some kind of outside aid, a geared diff can’t even get close to %100 locking force between the drive wheels. In fact; a purely geared LSD can’t “lock” at all, but merely biases the torque towards the drive wheel with more grip...

...at least as long as the other drive wheel doesn’t have too little grip and- ugh, screw it, let’s just move on to Salisbury differentials.

Salisbury diffs are the sledgehammers of the performance diff world. They utilize a stack of clutch plates to provide locking force between the drive wheels depending on the amount of drive torque provided. They often feature heavy cone springs to produce tons of preload force. This makes them extremely tight and therefore prone to breaking traction on the drive wheels when steering input is applied. Stiff Salisbury diffs are great for drifting and drag racing, but can make a car unpleasant and impractical to drive on the street; slower around a road course as well.

If geared LSDs can be criticized for acting too much like a fully open diff, then Salisbury diffs can be criticized for acting too much like a fully locked diff.

Clutch-based diffs have other problems as well. They are mechanically complex and therefore expensive. Because they rely so much on friction to work they can generate a lot of heat. This means they tend to cook their oil, necessitating frequent oil changes. High heat makes for high wear which means that aggressively sprung Salisbury diffs can obliterate their clutch disks in short order.

These myriad problems are why the OEMs have gone to electronically controlled clutch type differentials. Added cost and complexity yeilds a host of benefits, namely in improved drivability and reduced wear on internal clutch components. Unfortunately, this tantalizing tech still resides mostly in the realm of the OEMs. Very few companies in the after-market have even begun to offer an “E-Diff” solution, let alone anything like the torque-vectoring wizardry that the likes of Porsche and BMW are getting up to these days.

So where does all this leave Project Hoondy? Were we foolish to forsake the Torsen diff available in the Track Pack version of our car? It is, after all, one of the best compromise solutions and by far the cheapest LSD option for us.

Quaife offers an alternative helical  diff option that is arguably better with its horizontal, rather than vertical, gear set. We could go with that, not spend too much money, and save face for having at least wound up with something “superior” to the Track Pack’s Torsen unit.

However, as you may have figured out by now, that’s just not our style. We think Project Hoondy is begging for a drift-tacular diff that can provide plenty of locking force for lots of sideways shenanigans. This goal is certainly at odds with our desire to maintain the Hoondy’s quiet cruising credentials. We don’t want a car that’s gonna cramp our style while rolling slow, especially if we’ve got a hot date riding shotgun.

We had some sneaky tricks up our sleeves, and a great big pile of tax return cash for good measure. See what solution we cooked up in !!!error: Indecipherable SUB-paragraph formatting!!! of !!!error: Indecipherable SUB-paragraph formatting!!!

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LOREM IPSUM > Zachary Oberle 07/12/2016 at 13:08

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I love a good limited slip differential.

The problem is, 90% of people buying new cars have terrible car control skills. An open diff is essentially a safety feature, which helps avoid situations like these:

Also, an open diff is much cheaper than an LSD, and automakers will save a buck whenever they can because most buyers are woefully ignorant about what they’re actually purchasing.

Same rationale of everything going front wheel drive back in the day. They told everyone fwd was superior in the snow. The real reason was it was cheaper for the manufacturers to produce thus increasing their profit margins.

LSD all the things though, and let God sort em out. Imho.

Out, but with a W - has found the answer > Zachary Oberle 07/12/2016 at 13:09

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Great overview! Where do street-oriented clutch-based LSDs come in, such as the Cusco Type-RS ?

By the way, if you’re looking for a manufacturer that made a lot of noise about their LSD, VW did just that with the VAQ in the GTI Performance Pack.

Daily Drives a Dragon - One Last Lap > LOREM IPSUM 07/12/2016 at 13:12

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Because one wheel peels are awesome.

notsomethingstructural > Zachary Oberle 07/12/2016 at 13:18

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Some of the issue with diffs is they’re a black box to all but the most enthused, the other is people genuinely don’t understand the effects of one diff over another without a huge primer on it - far more than can fit in a marketing pitch. So most people’s understanding is “Torque goes in, ?? happens, ?? comes out”. I have a vague idea of how various diffs work but even I get lost keeping them all straight in my head to remember how each impacts driveability, so it’s hard to notice a difference (GET IT?!) from A to B on a test drive.

HammerheadFistpunch > Zachary Oberle 07/12/2016 at 14:09

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With LSD, as you probably know, its not about the “best” one, its more about tailoring the profile of the lockup to the goals of the car. if its drifting you want crazy high proload and TBR...basically a locking/not locking diff...that also doesn’t release with throttle off. The Torsen is a good unit because it reacts quickly, acts like an open if a shaft breaks and doesn’t require throttle inputs to be active. Viscous coupling LSD’s aren’t....bad...they just aren’t long term solutions as you mention plus they only work with speed bias so no throttle off application. Without a dedicated controller, a Torsen type is the best bet (in my mind) though there is a new type thats supposed to be like a torsen but more tunable...can’t remember what its called though, I will have to hunt for it.

Zachary Oberle > Out, but with a W - has found the answer 07/12/2016 at 14:11

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“Great overview! Where do street-oriented clutch-based LSDs come in, such as the Cusco Type-RS ?”

Hah! Very topical question you’ve got there. We’ll be covering those types of diffs in excruciating detail exactly one week from today because, spoiler alert, a clutch-type diff ended up being the only option that was aggressive enough for our liking.

“By the way, if you’re looking for a manufacturer that made a lot of noise about their LSD, VW did just that with the VAQ in the GTI Performance Pack.”

Yes, we saw VW’s fanfare over that diff at the time. We remember Clarkson getting very excited about that one. That’s why we mentioned the Golf along with the Focus when talking about Top Gear coverage of FWD diffs.

Stay tuned!

Out, but with a W - has found the answer > Zachary Oberle 07/12/2016 at 14:19

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Haha, I was kinda expecting it would be something like that. Looking forward to it!

Zachary Oberle > HammerheadFistpunch 07/12/2016 at 17:01

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“With LSD, as you probably know, its not about the “best” one, its more about tailoring the profile of the lockup to the goals of the car.”

This is exactly right, and really what this article was supposed to be about: exploring different diff technology and comparing it to our stated goals for Project Hoondy.

“Without a dedicated controller, a Torsen type is the best bet...”

We actually felt the same way up until late in our research on differentials. We found that, like you mention here:

“...there is a new type thats supposed to be like a torsen but more tunable...”

tunability is the future of differentials. A few of the more advanced models currently available have at least some degree of adjustability. That is our real ‘secret weapon’ on this build and the key to getting idealized performance for our specific platform.

Stay tuned!