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Centre Diff Problems / Questions / Suggestions
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(33 posts, started )
Quote from jtw62074 :
After doing a bit of checking, it appears the Torsen diff, mathematically speaking, is identical to the clutch pack diff without the preload. So for all practical purposes, until the preload was added to the diffs recently, everyone using a clutch pack diff has already been driving a Torsen diff.

Except that the locking% on coast / power would have to be identical on a torsen diff right?

I would still like to be able to use LSDs for centre diffs in LFS
Quote from jtw62074 :Imagine you and I are holding on to a shaft. You're the front or rear wheels and I'm the diff, if I twist with __ torque you'll get the same torque.

yes before the front/rear diffs ... but those have a final drive setting to convert that torque

Quote :After doing a bit of checking, it appears the Torsen diff, mathematically speaking, is identical to the clutch pack diff without the preload. So for all practical purposes, until the preload was added to the diffs recently, everyone using a clutch pack diff has already been driving a Torsen diff.

The Torsen appears to do exactly the same thing as the clutch LSD, it's just doing it through a beautiful arrangement of gears without any clutches or preload. Mechanically it's a great design, but for simulation purposes at this level it's identical.

shouldnt there be a difference considering a torsen diff cannot slip ?
I thought the difference was:

Viscous begins to lock after slip has happened
Clutch pack locks up as slipping occurs
Torsen works instantly when slipping begins to occur, so if the torque transfer is enough, excess slip never even happens?
Quote from Ball Bearing Turbo :Except that the locking% on coast / power would have to be identical on a torsen diff right?

Right, yes. That's an important distinction to make.
Quote from Shotglass :yes before the front/rear diffs ... but those have a final drive setting to convert that torque

It didn't seem to me that that's what he was suggesting, but yes, that sounds good

Quote :
shouldnt there be a difference considering a torsen diff cannot slip ?

The torsen does "slip" in the sense that the wheels are allowed to differentiate in order to maintain a given torque bias ratio, which is precisely what a clutch pack differential does. The mechanism for doing this is very different of course, using gears instead of allowing anything to literally slip internally, but the end result is the same from what I've gathered so far.
Quote from Bob Smith :I thought the difference was:

Viscous begins to lock after slip has happened
Clutch pack locks up as slipping occurs
Torsen works instantly when slipping begins to occur, so if the torque transfer is enough, excess slip never even happens?

Really the main thing you have in any diff is a locking torque that's trying to bring the wheel speeds back together into a solid axle. Whether it has literally locked or not doesn't really matter all that much. The maximum torque bias ratio is not allowed to be exceeded by the mechanism. Sometimes that means it will be locked, other times it won't.

The clutch pack and torsen effectively work the same way in the end from what I've gathered so far. The locking torque is a function of the two road reaction torques described in another post I wrote a couple of weeks ago or so. The only difference I'm aware of is that the torsen does it through a clever gear arrangement instead of having clutches sliding, which could be subject to changes in friction coefficient (which effects locking torque) due to wear, heat, and so forth. I'd expect the torsen to maintain its torque bias ratio virtually perfectly. It also likely produces the same torque bias ratio under power and coasting, but I'm not 100% sure on that. There's also no preload at all apparently given the description of what happens when you lift one wheel into the air. It then acts like a fully open diff and the car doesn't go anywhere.

Viscous diffs produce a locking torque as well. However, the locking torque isn't a function of the input torques like it is with the other two diffs discussed here. Instead, it's a function of slip velocity, or how much faster one shaft is spinning than the other, as well as rather strongly dependent on the temperature of the fluid which usually changes quite significantly during all this.
Quote from jtw62074 :The torsen does "slip" in the sense that the wheels are allowed to differentiate in order to maintain a given torque bias ratio, which is precisely what a clutch pack differential does. The mechanism for doing this is very different of course, using gears instead of allowing anything to literally slip internally, but the end result is the same from what I've gathered so far.

Whoops turns out I expressed myself wrong in many ways there but still.

From the earlier discussion about preload I´ve gathered, that a clutch pack has intermediary states in which it adjusts itself back into its 'safe aera of operation' by slip. So if I caught the drift, on the transient behaviour, correctly a clutch pack can step out of the allowed area of static output torques, which from my understanding isn´t possible with a torsen.

To get back to what I originally posted about torsens not slipping. What I actually meant was, if I didn´t misunderstand the torsen it limits the maximum amount of slip (as in difference in output shaft speed).
Quote from Shotglass :Whoops turns out I expressed myself wrong in many ways there but still.

From the earlier discussion about preload I´ve gathered, that a clutch pack has intermediary states in which it adjusts itself back into its 'safe aera of operation' by slip. So if I caught the drift, on the transient behaviour, correctly a clutch pack can step out of the allowed area of static output torques, which from my understanding isn´t possible with a torsen.

It can step out during transient periods. For instance, the car might have been in a tight turn with lots of weight transfer and loads of inside rear wheel spin. If you suddenly straightened the car out, eliminating the weight transfer, the inside rear tire will remain at a very high slip ratio, producing a lot of traction force. The torque bias ratio will determine the torque that is bringing the wheel speeds back together, but it's finite and doesn't result in the wheels immediately locking together.

The diff's locking torque is acting to slow down the (formerly) inside rear and speed up the outside rear. So for a brief period of time the torque bias ratio is indeed being exceeded. This is one of the transient situations. Here, after you straighten the car out, you get a kick to the right as the inside wheel slows back down.

The driver says, "the inside wheel suddenly regained its traction in a big hurry and almost spun me back the other direction."

Anyway, the Torsen should be the same way just by virtue of the fact that they describe it as having a fixed torque bias ratio just like a clutch pack diff has. So it shouldn't do anything different as far as what is happening at the tires. Internally of course it's handling things its own unique way, but mathematically speaking the diff can be considered to be a magical black box unless you really want to model every shaft and gear in the device. That's not really necessary for vehicle dynamics sims any more than attempting to model every molecule or square inch of rubber in the tire. Once you have the characteristics, you're good.

Quote :
To get back to what I originally posted about torsens not slipping. What I actually meant was, if I didn´t misunderstand the torsen it limits the maximum amount of slip (as in difference in output shaft speed).

It doesn't limit the speed difference. If it did, as soon as the speed difference hits its maximum, the torque bias ratio is free to go to infinity just like a locked diff can. Torsen states the torque bias ratio is constant, though, so this can't be the case.
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Centre Diff Problems / Questions / Suggestions
(33 posts, started )
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