Differences in diffs

[Chris Wilson]

DIFF DIFFERENCES

 

 

 

Clearly, both worm gear and clutch pack differentials have their

adherents, and both are used successfully in racing.

 

However, the situation is actually a bit murkier, because the behaviour

of both types of differential can vary according to design and tuning

details. Both types are similar in that they generate a locking torque

in response to the total torque being transmitted. In both types, the

locking torque depends on pressure angles. In a ZF style clutch pack

design, the angles are those of the ramps on the spider shaft and the

housing halves. In a worm gear design, it’s primarily the helix angle on

the gear teeth, and secondarily the pressure angle of the tooth profile.

Lubricant choice also influences behaviour.

 

Consequently, all clutch pack diffs don’t act alike and neither do all

worm gear diffs. A lot depends on how a specific example is tuned.

 

That said, the clutch pack design probably offers a greater range of

tuning options, and probably greater wear resistance. With the worm gear

designs, we are trying to make gear teeth act as a friction device.

Clutch discs are designed to be a friction device; gear teeth can be

made to act as a friction device, but they are less comfortable in that

role.

 

 

 

This affects the ability of the differential to maintain consistent

properties over time, and its longevity.

 

The pressure angles determine how rapidly locking torque builds as

transmitted torque increases. The preload in the diff determines how

much locking torque there is when no torque is being transmitted. A

clutch pack is easily preloaded, and it maintains its preload relatively

well, especially if the preload is applied by springs or some other

compliant system such as dished clutch plates. Worm gears can also be

preloaded, but because they are not very compliant, the preload rapidly

goes away as the teeth wear.

 

One limitation in worm gears is that the pressure angle is generally the

same for forward torque and rearward torque (as when engine braking, or

when transmitting brake torque from a single rear brake, as seen in FSAE

cars). In a clutch pack diff, we can use different ramp angles for power

and decel.

 

Another peculiarity of worm gear designs is that because power and

decelleration apply force to opposite sides of the gear teeth, preload

doesn’t have identical effects in both directions. If we preload the

gears in the direction they’re loaded under power, what happens under

decel is that we have diminishing friction with increasing reverse

torque, until the preload is overcome, at which point locking torque is

zero. As reverse torque increases beyond that point, locking torque

builds again. With a clutch pack, preload has similar effect in both

drive and decel modes.

 

This does mean that we can make a worm gear diff act different in drive

and decelleration, but not in a manner that’s independent of preload.

 

One interesting, though uncommon, trick we can use in a worm gear diff

is to use plain thrust washers to absorb the thrust of the worm gears in

one direction, and needle thrust bearings to absorb the forces in the

other direction. This can afford us some limited measure of difference

in friction depending on torque direction. Last year’s North Carolina

State University FSAE car had a diff like this.

 

It will be clear, however, that using these tricks is not nearly as

straightforward as varying the ramp angles in a clutch pack diff.

 

Finally, neither option is ideal, because neither is speed-sensitive.

Both clutch pack and worm gear diffs rely on Coulomb friction, which is

largely dependent on normal force and not speed. We would rather have

the locking torque vary with the speed difference between the wheels,

either entirely or at least in part. This argues for either a pure

viscous limited-slip, or a design that uses a pump, driven by relative

ouput shaft rotation, to load a clutch pack, or a design that combines

viscous effects with a clutch pack.

[bignum]

hi chris good info there, now i know a bit more about diffs, i learn something every time i come on this forum, what i was going to say was when i lift off the loud pedal i can here a slight whining noise coming from the rear of my car, is this normal? i only noticed it after changing the diff oil i put in some motul 75/90, oh and the cars a uk tt 6 chears simon.

[Chris Wilson]

The Torsen LSD in it makes a bit of a whine. If you track day it you NEED to buy a decent LSD. If it were mine I'd also change the CWP for a lower ratio, drop the top speed and make the acceleration stronger.

[Pete]

Your diff worried me when we set off. Thought the back end was falling off! clunk thud clunk clunk clunk.

 

I see why you're encouraging me to do track days now - so you can sell me endless upgrades!

[Chris Wilson]

It's set with a LOAD of preload, sounds absolutely horrendous when manoeuvring it about. The race car is far worse though. Track days don't have to cost a bomb, well not until you NEED to go faster each time you do another. It's a bit of a drug thing with me, I'm afraid.

[JohnA]

Chris, do you think that there is a fundamental reason in the torsen design that it makes the supra lose the tail sometimes so easily in slippery conditions?

[Chris Wilson]

Not at all, compared to a plate type they are very benign. Most cases i see are caused by either driver ignorance or error, the autos need an understanding before they are pushed hard, as when you lift off they will oftn change up a gear, if you are into track days or gooning around you MUST drive them manually, so YOU decide when it will or won't change gear. I found my auto version superb, and very easy to hang the tail out and retrieve it from quite lurid angles. The car was stock, no LSD at all. The Torsen ones are so lightly set as to be little different from an open diff. A proper plate diff makes it grip better, but breakaway is more sudden, as, uinder power, both wheels will lose grip together.

[JohnA]

OK, just checking...