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Home // Cars // Basic Introduction on Helical, Torsen, Viscous, Mechanical Limited Slip Differentials (LSD)

Basic Introduction on Helical, Torsen, Viscous, Mechanical Limited Slip Differentials (LSD)

The Limited Slip Differential; better known as LSD to motor enthusiasts, is an engineering marvel. Distributing torque to both wheels equally is a must for putting that power to the ground whilst also maintaining control of the vehicle’s ability to be steered in the desired direction. For the lucky few, these come equipped from factory but majority of your A to B vehicles are equipped with just a basic open differential due to production costs and driveability. I won’t get into too much detail about why cars even have differentials in the first place, but if you are reading this article I’m sure you know what the limitations of the factory open differential are.
So why would we ever need to upgrade our differentials to a LSD or a different type of LSD? Well simply because we want both wheels to turn at the same rate and distribute power equally to the ground when throttle is applied.

This provides optimal traction and in turn allows you to have more control of your vehicle, especially if it is making some decent power. This is hugely beneficial on the track when you want to improve that corner entry or exit speed for improved lap times or on the drift circuit to have more control over drifting angles and smoke output. Not to mention another advantage is the ability to do perfects ‘skids’ each time, laying perfectly parallel thick lines.
It’s not all good news though, there are some drawbacks of owning a LSD…

Advantages of a LSD

  • Controlled launches, as both wheels are spinning the same rate, reducing the chances of the car to flick out (RWD) or excessive torque steer (FWD) on power
  • No single spinning wheels as torque is equally distributed, thus same rotational speed
  • Controlled power over slides, commonly known as drifting, as both wheels are turning at the same speed when throttle is applied (applies to RWD only)
  • Power output around corners more predictable, instead of having one wheel turning excessively quick. However to the inexperienced driver the car will appear to be very hard to control as it will oversteer easily when traction is lost across both wheels

Disadvantages of a LSD

  • Increased tyre wear
  • Will create more understeer if throttle is applied around a corner, unless rear traction is broken, this will induce oversteer (RWD)
  • Inexperienced RWD drivers will not have true control of vehicle if traction is lost across both wheels. This will induce heavy oversteer and to the inexperienced driver this will be handful
  • More frequent maintenance for non viscous (mechanical)
  • Engagement noise associated (can get irritating)
  • Engagement point, slight thump and power transition off and on throttle
  • Lose ability to turn a tight corner on throttle without wheel hop or chattering as one wheel will naturally cover a greater circumference than the other

Differences

Helical:

  • Will lock and apply equal rotational speed to both wheels
  • Will distribute torque to both wheels, however not 50/50 – depending on type the ratios vary
  • Locking on throttle but requires both wheels to be on the ground
  • Helical LSDs uses gears and not clutch packs, hence do not require servicing or anything to be replaced. Maintenance would be similar to an open differential
  • Often helical differentials react much quicker than viscous LSDs and do not fade with use as they use gears
  • Great overall type of LSD but very limited aftermarket choices
Image: trdparts.jp
Image: trdparts.jp

Torsen:

  • refer to helical, essentially the same type of differential with minor differences. Both use helical gears to limit slip across both wheels on the same axle

Viscous:

Image: elitejdm.com
Image: elitejdm.com
  • Will nearly lock both wheels when throttle is applied
  • Not a true 1:1 lock as one of the wheels will be spinning a little quicker than the other in most situations
  • Unequal amounts of torque split
  • Makes use of hydrodynamic friction from fluids with high fluid viscosity to allow both wheels to turn at nearly the same rate
  • Will lose effectiveness fluid increases in heat, the effects of both wheels turning at the same rate will fade
  • Not the best for track, as it can be unpredictable on throttle as both wheels never turn at the same rate (however they appears to)
  • Perfect for street use as it engages gently, makes no noise and off/on throttle transition is smoother
  • No maintenance is required, as the viscous centre is sealed

Mechanical aka Clutch Plate (common in aftermarket LSDs):

KAAZ 2 Way LSD in a R200 R33 ABS Housing
KAAZ 2 Way LSD in a R200 Nissan Skyline R33 ABS Housing
  • Will lock and apply equal force on both wheels on throttle
  • Will rotate both wheel at the same rate regardless if one is off the ground
  • Torque is distributed 50/50 on lock
  • Uses clutch packs inside the differential instead of gears or fluid to lock
  • Requires regular oil changes and once clutch packs wear out, will require replacement and expensive maintenance
  • Very predictable once driver understands the engagement and disengagement points
  • Can be hard to drive with for the novice, for the experienced it’s a gift from the gods

3 Types of Mechanical LSDs:

  • 1 way: lock only on throttle
  • 1.5 way: lock on throttle and partial lock on deceleration
  • 2 way: lock on throttle and deceleration
Image: OS Giken
Image: cusco.co.jp

Usages

Image: cusco.co.jp
Image: cusco.co.jp

Depending on driving styles the correct differential should be selected, above is an image of how each LSD type behaves. Although it’s in Japanese, pictures speak a thousand words.

For street use and light track:

  • Viscous
  • Helical/Torsen
  • 1 way

For track:

  • 1 way
  • 1.5 way

For drifting:

  • 1.5 way
  • 2 way

Hope this helps give you a starting ground of what is what, usually differentials are upgraded when you as the driver have reached the limits of your car and require either more control or better distribution of power to the road. Differentials are often missed on the modification list and cheap hacks are done such as shimming up viscous LSDs or welding up differentials. These cheap hacks might improve the current situation, however will translate to a poor track time or even a busted differential casing at the end of the day. Choose wisely and drive hard!

Need to get that power down!
Need to get that power down!

About Johnny

Johnny
A typical car nutter that likes to get his hands dirty. Skyline and Silvia fanatic not to mention trak-life's own personal tuner and mechanic. Doesn't mind sparing a few days at the gym to stay fit and definitely likes to eat to keep the balance right.

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9 comments

  1. Thanks for the comparison and the good info.

    If I only go the my local track when it rains, which type of a diff would be ideal for me? Please also explain the ideal diff settings for track work during rain.

    I have gathered so far that I won’t be interested in a quick power transfer speed between the driven wheels. Instead, I’d be happy with a low “blocking factor” and slightly decreased acceleration and top speed. In damp conditions, predictability will be key. Please advise. Thanks.

    • Johnny

      Hi Zain!

      In your particular situation any form of mechanical limited slip differential will be better than your current viscous or limited slip differential. For normal driving on the streets, and not applying power down around corners the factory open or viscous unit will be superior as when you do the maths for turn, one wheel will always rotate faster than the other.

      However, when the same is applied to a track environment things start to get complicated. Yes you want power down to both wheels, but then this affects the different wheel rotational speeds around a corner, but at the same time you don’t want to single peg around a corner.

      In your situation I would recommend a differential that allows you to adjust the torque requirements before engagement. If you have a Nissan vehicle, you cannot go past a Nismo GT LSD which has the exact thing I’m talking about.

      • Thanks Jay. Please elaborate more on what you meant by adjusting the “torque requirement before engagement.”

      • Johnny

        What it means is when the LSD “locks”, each LSD and also how the LSD is setup determines how much torque is required to be applied before it locks up both wheels so they rotate at the same speed. A less aggressive LSD means it requires more torque from the motor to “lock” where as a more aggressive one will require less torque from the motor to “lock”. For a daily driver and something that rarely hits the track, I less aggressive LSD is a better choice.

  2. Having ran both a 7.5″ and 8.75″ LSD on my ’95 Trans am at the strip, autocross track, and daily driving for about 8 years I’ll say a clutch system is easily the best style for drag strip and daily use. The shims can be adjusted for lockup threshold, as can the additive pack and oil weight depending on use. Use the GM additive to keep it quiet.

    That said, you probably won’t win an autocross or Solo1/2 events against drivers using Torsen/helical units. The Torsen 2R has a strong bias to get back on full lockup quicker than a T2 or any clutch LSD will.

    It also has a nasty habit of loosing torque completely when you lift a wheel (which is why you never hear off road guys even consider them, or if they do to talk about applying the brakes to induce drag on the free spinning wheel). But this is rarely an issue on all but the most severe corning conditions for race track or daily driver use.

    An expensive Torsen unit is the ideal for most daily drivers with autocross and light strip duty… but it’s pricey to get a high bias unit.

    That’s why most sportsman drivers stick to clutch pack LSDs… very durable, cheaper, and predictable feel. Excellent daily drivers and good at the strip.

    Detroit Lockers are streetable… but give up a lot when unlocked, and you definitely feel rhe engagement point which can be unnerving when at WOT and hard cornering. The more teeth the better, but it’s still pretty nasty for anything other than drag strip and circle track use. I’d never run one, but I know guys who do and theyre basically track only cars.

  3. Hi,
    I’m driving a tuned 63′ Volvo Amazon(rwd) putting out about 230 bhp. I’m looking for a differential for road use and very little track use, BUT I would like the car to be more fun and easy to control in and out of the coners, making it easy to oversteere and control. Right now the car is driving with a standard open diff and that is very unpredictable, you never know if it will go sideways or just spinning one wheel.
    I’ve been looking at a quaife atb helical LSD differential. Would this do the job for me?
    Hope you can help me 🙂

    /Mads

    • Johnny

      I would suggest a helical LSD if they’re easy to come by for your Volvo. That way you won’t need to service the LSD as much as a mechanical/clutch pack type.

  4. Hey everybody. Thank you for explaination the differences. As far as I understand, I need 2way mechanical LSD. I am going to build a 350z drift car but with legal street use that won’t be super annoying. Am I right?

    • Johnny

      Hi Sam!

      The best option is to go with an adjustable Nismo Pro LSD as they allow you to pop out one shaft on the RHS (driver side for most of the world) and adjust the initial pre-load. KAAZ on the other hand isn’t adjustable without swapping out for different parts.

      I run a 2-way myself, however it’s not as bad as some would describe. To minimize massive tramp or hops, you could add some friction modifiers to increase “slip”.

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