Diff types

[Brian28]

Following on from the discussion on diff types. I tried running the car with coilovers for the first time during the week, and am getting really bad rear wheelspin and inside wheel pick up. Have some ideas to improve this, so the post isn't about suspension set up. If a car is physically lifting the inside rear wheel, would a viscous LSD (from a 325ix) do anything or would a plated one be better to retain a degree of drive (25% lock I think is the BM standard LSD)? I know a fair bit about cars but diddly squat about diffs so if you start with very basics you won't be patronising me, it'd all be helpful

Cheers
Brian

[martinpallot]

Are you running with a proper coilover at the rear? If yes then it sounds like your set up is probably too hard, try using softer rear springs if this is an option. Do you have a rear ARB fitted? if yes then try removing it see how it feels for you, alot of people prefer it and it helps against inside wheel lift. Not sure how the viscous LSD works but if its anything like the torque biasing diff it will lose lock when the inside wheel lifts. Best bet is a standard E30 ZF lsd with a set of Ford atlas plates fitted. The standard 25% lock up becomes pretty useless when you start upgrading things. They are at least 20 years old by now so dont work as efficiently as they once did.

[N00b]

If the car is lifting a wheel, it's a suspension issue and nothing to do with the diff.

[Brian28]

Sorry, I didn't ask the question very well, I know that the diff doesn't effect the suspension set up. I am changing the suspension set up, that was just the trigger to asking about wheelspin/drive when a wheel is in the air eg from rattling over the kerbs.

What I'm trying to ask is, which standard BMW LSD (plated type or the less common viscous type from a 325ix) would be better in different situations, what are the relative advantages/disadvantages? Would a viscous type put any drive to the outside wheel when the inside wheel is airborne?

Thanks

[GeoffBob]

Brian,

To answer your question: Yes, the viscous differential, as used on the AWD ix, will continue to drive the outside wheel in the event the inside wheel lifts. This is due to the way the coupling mechanism works on the visco LSD. Whereas the locking action on the ZF clutch type, Quaife and Torsen diffs are torque dependent, the viscous type depends on the speed difference between the drive-wheels, and couldn't care whether a wheel is in the air or not.

In a nut-shell:
The visco LSD is much like a clutch/plate type LSD, but has two viscous coupling units (sealed) in place of the usual multi-plate clutch packs. Each viscous coupling comprises two sets of multiple plates stacked on top of each other (but not in contact). One set of plates, the outer plates, are splined to the differential cage, while the inner plates are splined to the sun gears. The inner and outer plates are arranged alternately next to each other with the space between filled with silicone hydraulic fluid.

Now, So long as the two output shafts turn at the same speed, the planet gears will be stationary about their own axis, and thus the plates will be stationary relative to each other. However, when one output shaft turns faster than the other the planet gears will rotate about their axis (same as an open diff) and thus the plates will rotate relative to each other. Now, due to the fact that the shear force (the coupling) between the plates is proportional to the shear rate (the difference in speed) between the plates, the plates will progressively couple or bind to each other the faster the inner and outer plates turn relative to each other, thus coupling the sun gears to the diff cage and progressively locking the output shafts to each other, transferring drive to the wheel with greater traction.

Note that the visco LSD is a progressive LSD like the Torsen and the Quaife, due to the fact that the percentage lock between the output shafts increases progressively. That is to say, it is also an ATB type diff. Compared with the standard ZF clutch type LSD (which delivers 25% coupling through the clutches the instant the pin mounts the ramps) a visco LSD provides reduced tyre wear and reduces stress on drive-line components.

The fact that the visco LSD can deliver power to one drive-wheel in the absence of traction at the other is the reason why the ix was so effective at ice racing.

If you curently have a choice between a ZF clutch type LSD or a visco LSD from an ix, take the visco, but be prepared to maintain the viscous couplings in the same way you would the clutches on a ZF.

[Brian28]

Just the kind of info I was after thanks.

[GeoffBob]

You're welcome.

BTW, you can download GKN's datasheet for their viscous diff here if you are looking to buy a viscous LSD.

You'd have to check with them, but I think they do one that will fit the E30 medium-case final drive, so there really is no need to track down a final-drive from an ix (note that the "diff" is actually the bit inside the final-drive that bolts to the crown-wheel). You could fit the crown-wheel and pinion gears from any number of BMW medium-case final drives to achieve your required final drive ratio. The rear axle final drive on the ix was 4.1:1 ratio only. Chances are, if you did pick up a final drive from an ix it would need attention, so building up a "new" final drive from E30, E36 and GKN bits might be worth a look.

GKN also do all the other popular types of diff, but under different names with slight variations on the original patents to avoid patent infringement. See here

Regards
Geoff

[mt1104]

lol why are LSDs so expensive , specially when my car cost me £250 to buy...

[Jon_Bmw]

It will always have a value though...

There is a chappy selling a quaife in the for sale forum for £500. Sounds about right for a 2nd hand slipper.

[DanThe]

lol why are LSDs so expensive , specially when my car cost me £250 to buy...

There are not exactly loads about, compare the price to a new LSD and you will probably buy a second hand one