zayd

iTrader: (1)

You may change your mind after this, as from a little research I've been doing, its becoming clear that the actual driver experience has become somewhat desensitised with this diff change.

Viscous: Used as the center and rear differentials in the WRX/RS (08+ WRX has an open rear diff). Varies power applied between two axles via fluid dynamics and discs. This type of unit is filled with a silicone based fluid that becomes thicker as the difference in input shaft and output shaft speed increases, thereby increasing the viscosity of the fluid and the grip between the input and output discs, which do not actually touch each other.

Suretrac: Only available on the 2004 STi. Manufactured by AP Racing, this unit comes stock in the front differential of the 2004 STi. It is a mechanical type differential that employs a set of specially shaped teeth to intelligently transfer power, unlike the bevel gears that are used in conventional LSDs. Very similar in operation to the torsen type, though through different mechanical means.

Open differential: Used as the front differential in the WRX/RS. In a nutshell, it’s one wheel drive. The wheel with the least amount of traction will have majority (if not all) of the vehicle’s power applied to it.

DCCD: Short for Driver Controlled Center Differential. Used on the STi. Planetary center differential in conjunction with an electronically managed continuously variable transfer clutch. And as the name suggests, it allows the driver to control the torque bias of the center diff by a turn of the thumbwheel.

Torsen type differential: Used as the front (2005+) and rear differentials in the STi. Short for TORque SENsing differential. It’s worth mentioning that though Torsen is a brand name, it is the most commonly used name for this type of differential. This type of unit is also known as a helical or mechanical type. It uses gears to split power between two axles. Once one wheel is off the ground or slips, it in essence, becomes an open diff or exhibits limited traction based on the torque bias of the unit. It has the added drawback of weight. The additional torque required to rotate a heavier differential will require more energy, energy is heat, heat is friction, friction and heat are wasted energy. It requires more energy to drive and this can be shown on a chassis dynamometer if same car is measured before and after the differential change. The other downside is that if the engine is quite powerful and extra special abuse is administered (high grip launches, donuts, etc.) they will explode. These broken gears will make their way through the case and can cause considerable damage. Keep this in perspective though, as very few cars exert this kind of power and those owners tend to understand what that power is capable of breaking.

dunx

iTrader: (3)

And your point is ?

Mine (2002 STI) had a Suretrac up front, and when new the car pulled into corners like a crazed thing, on tight 90 degree junctions I found myself winding lock off, as it grew older and more tired it became less effective. At this point I added Whiteline ARBS and the handling was improved anyway.

Now run a Hawkeye DCCD box and now it handles better in extreme use.

Just my two-penneth.

dunx

zayd

iTrader: (1)

Does the front helical lsd only come with cars with centre DCCD. Its just that I've got a Maltese Import 2005 STi, which doesn't have DCCD as it was a optional extra. I believe then that it has suretrac lsd up front. The car does have the wider rear track though. Its a bit of a weird one as I've got new traction features mixed with old!

I bought the car with my eyes wide open to the fact it was imported from Malta and as it was in immaculate condition, I was happy to proceed. It does say on the V5 that it was new at first registration, which was in the UK. It had a CAT 1 alarm fitted aswell as wheel locking nuts. I know that resale value will be a little affected, but I don't plan on selling for a long while, especially as I've been bitten by the modding bug.

dunx

iTrader: (3)

Ah I see !

I can't say that the front diff in the hawkeye box is any improvement over the Suretrac the car came with originally. Over on 22B the rear diff has been described as almost non-existant, to avoid upsetting non-enthusiasts.

dunx

zayd

iTrader: (1)

Dunx, this is what I was getting at from my first post. I'll give you a prime example; a good friend of mine started off some years ago with a Blob Sti which he modded to the hills, then he went and did the same with a Mitsubishi Evo. He then chopped that in and bought a brand new Nissan GTR and as modding is in his blood, he did some on that. Crazy as it seemed to me, as the car is super quick as standard.

The problem he is stating now, is that the car feels too artificial with all its traction features. Its almost impossible to have some old school fun in this car, so he is going to sell it and downgrade to the Skyline R34.

zayd

iTrader: (1)

The following advice comes from Dave at Rallispec, one of the few people whom actually KNOWS differentials and I thank him for his contribution and advice as differential advice from people who REALLY know them is impossible to come by.

To add a couple points of information to this very cool thread:

1. Viscous LSD's are speed sensing. The work by reacting to speed differences between the two axles. They are limited in the time it takes them to respond and therefore they are not an ideal solution. They are designated by their torque reaction per difference in rotational speed. For example the stock unit is 4 kgf-m (torque reaction) / 100rpm (speed difference). Increasing this value increases how quickly it can respond because it will be able to redirect more torque for the same difference in shaft speeds. However, if the value is increased too much then a small difference in shaft speed (such as one that occurrs in a normal tight radius corner) will generate enough torque reaction as to create a noticeable understeer condition.

2. Gear type LSD's and clutch type LSD's are torque sensing. They respond to torque input and/or torque differences across the drive shafts. They do not need to be waiting for a speed difference (as in wheelspin) to be doing their job. These types of LSD's are designated by their bias ratios. Basically what ratio of the torque at the low traction side can be redirected to the high traction side.

3. Torsen, Quaife, and Helical LSD's are all very similar...all gear types that involve complex interactions between the gear teeth to do their job. Quaife and Helical LSDs both use helical type gears. Torsen uses a worm gear arrangement. These LSD's typically have bias ratios in the 1.5 to 4 range I think. However, most are not set up with any preload (although it is possible to build in some preload with these LSD's by shimming the end play of the gears against the case to a certain amount of interference fit). Therefore when the torque at one wheel is zero or very near zero (wheel off the ground or on ice) then the LSD cannot transfer any torque to the other wheel (anything multiplied by zero is still zero!).

4. Clutch type LSD's (Salisbury type...also known as plated diffs) use stacks of clutch plates alternately splined to the axle and the diff case. Pressure to apply the clutch packs comes from a set of pressure rings. The spider gear cross shaft rests on sets of ramps cut into the pressure rings. When torque is applied to the diff the shaft rides up the ramps and forces the pressure rings apart thereby applying the pressure to the clutch packs. Clutch type LSD's set up for racing generally have very high bias ratios (upwards of 6:1 I think). They also can be set up with significant preload if desired. The preload on the clutch plates ensures that the LSD can still do something even when there is no zero traction at one wheel.

5. The DCCD's open torque split is determined by the tooth arrangement of its planetary gear set. Unlike a typical diff's spider and pinion gear arrangement which gives a 50/50 split as long as available traction on both axles is equal, the planetary gears provide a torque split biased toward one end (be it 35/65 or whatever it is designed for) as long as available traction is equal. Attached to that planetary gear set is a clutch pack and a second clutch pack controlled by an electromagnet. The pressure on the clutch pack (which biases torque back toward a 50/50 split) is determined by the current flowing through the electromagnet. Therefore it is a clutch type LSD that can be electronically controlled to alter its operating characteristics.

In terms of tuning the clutch type LSD, your options are as follows:

A. Preload setting -- increasing preload increases its effectiveness in very slippery conditions. However, this preload must be overcome in order to go around a corner (which is why these LSD's when set up with any measurable preload will make noise). It will generate understeer. Preload is set both by a preload spring (some types use a bellville washer but Cusco's RS type uses coil springs) and also by the stack height of the clutch pack (the thicknesses of the clutch plates).

B. Ramp angles -- there are two ramp angles, one that influences on-throttle operation and one that influences off-throttle (decel) operation. When an LSD is described as 1way, 1.5way, or 2way they are talking about the off-throttle ramp angles. 1way types only offer LSD action on acceleration and have no ramps on the decel side. 2way types have equal ramp angles for both accel and decel. With 1.5way have steeper angles on the decel side vs. the accel side so it takes more torque to create the same pressure (basically on decel the LSD is working but less effective). Some manufacturers for some reason use other values such as 1.6way or 1.7way but they are just describing the relative angles from one side to the other.

C. Active friction surfaces (lockup percentage?) -- By altering the arrangement of clutch plates (outer splined vs. inner splined plates) you can alter the number of "active" friction surfaces in play. Reducing the friction surfaces reduces the bias ratio.

In terms of general recommendations....

Front LSD: for track, autox, and street use most people find that a gear type front LSD is most predictable and easiest to drive near the limit. It does not generate much understeer on corner entry due to the absence of preload and the low bias ratio under decel. A 1way clutch type LSD can be just as, if not more, effective when set up properly for the conditions and the driving style of the user. But if it is not set up well then it will cause difficulties for the driver. For slippery conditions such as rallyx, iceracing, rally, etc. it will be beneficial to run a 1.5way plated front LSD as it will help with braking stability. Preload should be set according to available traction and driver preference. For tarmac drivers having difficulty under braking it can also be worthwhile to try a 1.5way LSD with low preload.

Center LSD: I am very interested to try the PPG Torsen center LSD but have not yet had the chance. I suspect it may be an excellent track option. I would avoid using a 20kgf center viscous except for gravel, snow, or ice...or maybe straight line drag racing. On high speed tracks with no tight corners it may be OK but it will be counterproductive on tight tracks or autox. The Cusco Tarmac gear is worthwhile only when you have good front and rear LSD's to back up the fact that it is an open diff.

Rear LSD: There are very few instances for racing applications where I would not recommend a clutch type rear LSD. They are so tuneable that there is no reason not to use one (other than the need to rebuild it every season or every other season). The stock viscous unit used in the WRX and 2000-2001 RS and also in the Legacy GT models is essentially useless (the unit is so thin as its packaged off the side of a standard open diff and its capacity is very limited). The factory R180 clutch type LSD on the STI is set up extremely loose from the factory and should be rebuilt with appropriate preload and possibly different pressure rings if you want to use it for racing. For a street application the stock Torsen or a Quaife unit is usually preferrable from the standpoint of not needing to be rebuilt and creating very little noise.
__________________
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dunx

iTrader: (3)

But a GT-R is all about the electronics, not the diffs...

dunx

P.S. Your posts may be cool, mine are just niggling

T5NYW

iTrader: (4)

I drive a JDM Hawk and with its Type20 setup handles very good compared to my old Bugeye STi with a sureTrac diff.

but i did drive another JDM Hawk with a Quaiffe front diff fitted and it felt much better on turn in

Iain litcho re-callibrates the DCCD when he programs the ECU, IIRC he said the DCCD "speed" was wrong compared to "road speed" !!!

Tony

lockheed

iTrader: (1)

Nice one Tony just what i was looking for.

Great thread.