The ultimate Limited Slip Differential thread?

[Bertroex]

Gents and girls,

After GeorgeK took the SC and me down the Anzieres/Ayens hillclimb in Swits, I really want to have an LSD in my C3.

As far as I understood, -but thats the problem: I don't think I do understand- there are 4 choices in the current market for retrofitting an LSD. It would be for a 1975 build magnesium cased 915 with electronic speedo, making it a *edit* coarse spline.


1. The ZF LSD, either 40% or 80% lock. Very difficult to obtain, all scooped up by the racing boys and NOS is rather pricey.
My first choice, budget permitting.

2. Japanese Diff which is very good, smooth in action and relatively quiet, available in 2 weeks.

3. Gripper Diff – Motorsport style diff used in many competition cars. Bit noisy but robust. This diff is the most ZF like in terms of design but does have some improved features. My favourite mechanic, the sage of the lakes, has one on his shelf. Budget and result wise a "sensible" option.

4. OS Given – very modern design has 18 plates per side and locks up very quickly and very smoothly. Used in Nissan Skylines and the like.

Now, my road car will be used for mostly long distance driving, the occasional challenging route to work cutting accross the vale of Belvoir over B roads, the occasional track day and some enjoyable motoring up and down some mountains.

What would you advice?


Many thanks in advance for your response.



Bert

[middlefour1]

Do Quaife do one??

[jwhillracer]

I would have a good look at a Quaife - not technically a LSD, but a torque biasing differential. Smoother and quieter in operation than a LSD, probably better for a road car, and readily available. Also the GT unit from the States.

Cheers!

JW

[Mike Usiskin]

obviously i can add nothing of value to this thread other than to say I reading with interest. On my kind of normal road driving what effect, if any, will i notice. On my little uber car natch!

cheers

m

[Bertroex]

Myesss, Quaife isn't doing it for me: LSD or nothing with a heavy bias towards the ZF! I know that Martin (RSRish) as well as Ferryman (Turkish delight) have one but it aint my thing.

I have read your thread re the LSD as well, Mike.
http://www.ddk-online.com/phpBB2/viewto ... hlight=lsd

I don't like Quaife, but I'm willing to listen to the voice of experience.

Best regards,

Bert

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[Mike911scrs]

HI Burt
your gearbox should have coarse splines in the diff, not a prob' which diff is fitted, as the shaft can be changed,

selecting of diff's, all LSD,



the first one is a GT, from the USA. this diff is the up to date version of the old ZF



the next is the ZF



this one is the gripper, from the UK.



the last one is from Japan?







as you can see is has carbon pads in it, how long thay last I do not know and I have not had any feed back from the owner I fitted it to his car, good or bad.

The GT and the ZF diffs have one draw back and that is you have to remove the crown wheel to do any work on the inside of the diff, (the old GT you could remove the end like the gripper).

The gripper is more serviceable, in the fact the end is removable and you have not got to fork out for new diff bolts/lock tabs.

the discussion now starts!!!

regards mike

[Mike Usiskin]

the voice of experience.

[/url]

so....not me then

hmppppfff

[MikeB]

No experience of LSDs in 911s as yet, but I have used a Gripper in a 190hp Alfa Giulia and my friend has just fitted one to a TVR V6. Very good performance, and provided gradual, controlable breakaway and better traction out of the corner than the ZF "flat plate" LSD that is normally fiitted to the Alfa.

To me the Gripper and the Kaaz look very similar designs. I would certainly be interested to hear what folk think of the Quaife ATB, I ran one in a single seater, but they have plenty of traction and grip so it was hard to tell what it was contributing.

Mike, are you saying that the fine spline output shafts can be fitted to a course spline gearbox? If so happy days

[Lightweight_911]

MikeB said:

"...I have used a Gripper in a 190hp Alfa Giulia"


- can you post some details of your Alfa here:

http://www.ddk-online.com/phpBB2/viewto ... =giulietta


Sorry for the 'highjack' Bert

[Bertroex]

Sorry for the 'highjack' Bert

You may high-, mid-, or kojak my "thread" any time, Andy!



Bert

[fetuhoe]

Comparing different types of plate type LSDs is not always easy and does need some examination of what is happening.

Obviously a conventional or 'open' differential is needed to allow wheels to turn at different speeds around corners. If a solid axle were used cars would understeer quite badly in tight corners and would wear tyres quite quickly. The problem with the open diff is that it cannot tell the difference between turning and a lack of grip and the internal gearing means it will always divert power to the whell with least rolling resistance. When a wheel loses traction the other wheel will lose all drive and the car will just not accelerate.

The locked diff or solid axle will give the best grip as one wheel will always drive even if one wheel is completely off the ground. They are used commonly on Off Roaders when slip can occur due to the surface friction conditions. The 'spool' used on some racing 911s is effectively a locked diff but does need a great deal of skill to be of any benefit.

The Limited Slip Diff is somewhere between these two extremes and there are sveral different type of device currently on the market.

It is common practice to refer to a Plate type diff as an LSD and a Torsen Differential as an ATB.

I don't think it is correct to say that an ATB isn't an LSD as it does control the slip between driven wheels just in a different manner to a plate diff.

Plate type LSDs are very common and several different designs are available in the market place.

Salisbury and ZF were the first designs to be available but now there are products from KAAZ, Gripper, Cusco, Drexler and Guard and they all share common design features. ZF are no longer producing some of their earlier designs but repair kits are readily available.

Porsche ZFs are commonly described as having either 40% or 80% locking factors but I think that this is an overly simpole way of looking at the charateristics of a plate diff. I would imagine that a diff with an 80% locking factor will need more torque differnece to make it slip than something with 40% but this doesn't say enough.

All plate LSD have 'ramps' which force the plates (clutches together) and the angle of the ramps determine the clamping force. The more power applie the higher the clamping force. This means it is imposrtant to know the ramp angle. The number of friction surfaces will also influence the amount of torque that can be transmitted os an understanding of the number of plates in the clutch stack is also part of the decision.

It is also possible to have ramps with different angles in power and coast.

The early Salisbury Powerlock had a 45 degree ramp om power and a 90 degree ramp on coast. This means it gave a limited slip effect on accel but acted as an open diff on coast.

The amount of preload applied is also an important consideration and will affect how the car moves away form rest. Early ZF LSDs often had zero preload and this gave them a fairly gentle operation with limited understeer. (It is for zero preload diffs that caused ZF to use 'Wavy Plates'. These devices were needed to apply a small preload to stop the plates from rattling when the car was driven straight. A preloaded diff using belvilles does not need Wavy Plates but they are still commonly fitted)

The number of plates, preload, power/coast ramp angles and the nature of the friction plate all influence how the diff reacts and the rate at which it operates and only by understanding all these variables can a clear decision be reached.

Standard early ZF diffs have only 4 plates tend to have moderate ramp angles and are reasonable in thier action. They have a cast end plate which can break when used aggrssively on competition car and friction plates are only located by 4 grooves. It is possible to fit more aggressive ramp sets and larger numbers of plates to enhance performance but there is no reason why this design is better than any other.

I feel they tend to be expensive for what they are.

KAAZ are a Japanese manufacturer who's main business is Lawnmowers but have also developed a range of plate type LSDs. They also use cast end plates. They tend to have more friction surface than ZF LSDs and have moderate perload. The have 8 plates per side and tend to have what is known as 1.5way operation in standard form.

This means they transmit more torque difference in power than coast. They are reasonably smooth in operation and work well in road cars and manufactured to excellent standards.

Seem quite good value.

Gripper diffs are strong and quite tough. They have Cr/Mo steel end casings, 8 plates per side typical ramp angles of 45/65 and high-ish preload. They have a reputation of being noisy but they are strong and good value.

The OS Giken Super diff has a large number of plates and can be ordered with differnet ramp angles. It should be very aggressive and suitable for competition use but is very costly.

The Drexler units are superb in all respects but currently cost around 2400 Euros + VAT.

Guard units are also good but are costly and the Dollar hasn't helped.

The Quaife ATB is interesting but limited. The ATB should be described as a Torsen Differnetial (TORque SENsing). This design first appeared in the late fifties in the USA and was patented by Veron Gleasman who sold the design to Gleason.

In effect it is a Worm and Wheel design where the Wheel is smaller than the Worm. It needed some very complex gear cutting machinery and some very clever profiles to be able to work.

Gleasman realised that all diff designs sacrificed one side of the drive to the other depending on its basic nature.

The Torsen design uses a gear arrangement that can only work one-way. It uses worm wheels mounted on the differential housing to turn worm gears splied to the drive shafts. As the worm wheel cannot turn the worm gear, it locks on the gear and turns the axle shaft, propelling the vehicle forward. The right and left axle shafts (and right and left wheels) turn simultaneously. Each wheel then rotates at the same speed.

However, when the vehicle makes a turn, each wheel rotates at a slightly different rpm. For instance, during a left turn, the left wheel will slow down by two rpm, and the right wheel will speed up by two rpm. One axle shaft always slows down at the exact rate that the other one speeds up. This difference in rpm is transferred to the worm wheels (because the worm gear on the axle shaft can turn the worm wheel and equalize the other side via the 1:1 spur gears, which act as balancing gears). So the engine is "Locked" or engaged on the axle shafts, while allowing for differential action when negotiating turns.

The main benefit of the Torsen diff is that unlike a plate type diff which cn only transmit torque in the ratio of about 2.5:1 between the driven wheels the Torsen can manage around 4:1.

It does, however, have a couple of significant disadvantges.

It must have some torque present on both wheels or it will behave just like an open diff and it behaves like an open diff in coast mode.

Plate type LSDs with the correct ramp still operate in coast and help modify the handling behaviour of RWD vehicles.

I think ATB/Torsens are great in FWD cars but are not as good as plate diffs in RWD cars.

Like all compromises the best starting point is to define the application and duty to make the best choice. I think it is difficult to simply say A is better than B etc.

[Mike Usiskin]

Sorry for the 'highjack' Bert

You may high-, mid-, or kojak my "thread" any time, Andy!



Bert

Bas*tard never says that to me

[Bertroex]

Bas*tard never says that to me

Wishfull thinking!






Bert

[MikeB]

Fabulous information Fetuhoe :D

The ZF LSD on the Alfa was 4 plate, all plates were flat, and once used were all smooth. There was no pre-load, and the car had a real habit of kicking it's tail out when power was applied when exiting a corner. This "kick" actually slowed you down as the oversteer was excessive.

The Alfa LSD was 25% std or 48% when tweeked. The tweeking involved replacing one of the LSD case plates with an additional halfshaft splined plate, hence approximately doubling the area of the slip surfaces. However this meant that the additional h/shaft plate was now wearing against outer machined surface of the ramp plates, which were not primarily designed for this.

Is this how the 911 ZF LSDs are doubled in %age?

Because there was no pre-load on these diffs they also lost their LSD effectivness over time when used on the road. Obviously a Bellville dished plate LSD is more resistant to this, as the sprung plate maintains the required pressure for longer.

Andy just about to put some info on my previous classics to satisfy your curiousity

[Bertroex]

Thanks for your reactions....

The wonderfull mysterious DavidPPP has been pm'ming me with some concerns, of which I gratefully take note.

Now, may I please enquire what the opinion is on servicing these various choices?

Do they come with a manufacturer's recommended service schedule, just like, say, gearbox oil services?

What does this service entail?

Best regards,


Bert