Re: The confusion surrounding diffs.

So just how does the viscous coupling in my RR work then ??

Graeme

Your theory with regard to axle diffs is correct, but the post wasn't correct in it's entirety as I pointed out because he was criticising Bob Cooke when indeed it was Samual McGregor who was confusing a part-time 4WD system with no centre diff-lock with a full-time 4WD system with centre diff-lock. Bob Cookes description of the workings were over-simplified, but not wrong.

"Graeme" wrote in news:bf4gmj$aur3p$ snipped-for-privacy@ID-198594.news.uni-berlin.de:

I think its all done with mirrors (and treacle).

Terry

sorry exit, but you are the wrong here. a vehicle with selectable 4wd, such as my series 3, does not have a centre differential. when in 4wd,the front and rear propshafts rotate at the same speed. vehicles with ful time 4wd have a central differential and (in the instance of defenders etc) can be locked, which causes the front and rear propshafts two rotate at the same speed. therefore, a series 3 in 4wd has an identical setup to a defender with the centre diff locked! if i have to go over the torque splitting argument again to prove my point, please tell me!

When in 4wd in a selectable 4wd vehicle (series 3) the front and rear propshafts rotate at the same speed. this means that either end can recieve the bulk of the torque while the other recieves only enough to rotate an airborne / slipping wheel! an open differential will always split torque evenly which is precisely what you do NOT want in a 4wd situation!!!!

samuel.

"Graeme" wrote in news:bf4iku$as5rk$ snipped-for-privacy@ID-198594.news.uni-berlin.de:

has lots of intersting info on diffs including Viscous and Torsen (as used in the new new RR) Terry

Read my original posting, not the reply to Terry.

You quote the following from Bob Cooke:

"In cars with selectable four-wheel drive the engines torque is split securely between front and rear axle." (this describes a situation identical to a locked centre diff)"

This is correct - selectable 4WD does not have a diff-lock but does split torque to both axles. What in the above quote do you think Bob Cooke has got wrong? You state that anyone worth his off-road salt will tell you a diff does the exact opposite of Bob Cookes statement above - tell me what he said that was wrong with his statement?

But there is no open diff between the front and rear axle of your Series 3 when in 4WD so the effects you describe of having an open diff between the front and rear axles are irrelevant. Thus if the axles have open diffs you will need an airborne wheel simultaneously on both axles to stop forward motion.

Am I correct in saying?

Differential Locked - Equal SPEED. Differential Unlocked - Equal TORQUE.

And surely this will apply to either differential (irrespective of what is at the other ends of the output)?

Jase.

"Jase" wrote in news:bf60q8$nk7$ snipped-for-privacy@pegasus.csx.cam.ac.uk:

Yes. when the diff is locked the output shafts (propshafts for a centre diff or halfshafts for an axle diff) MUST turn at the same speed.

Normally yes, which is why when one wheel on an axle spins with no grip there is no torque to the other wheel.

Some vehicles however use an epicyclic diff (Schuler RR, Jensen FF and XR4x4 are exanples). These provide an unequal torque split while keeping the output speeds constant.

Yes

Terry

a locked diff will not split torque equally!!!!!!!! a selectable 4wd, in 4wd has an identical setup to a fulltime 4wd with the diff locked!

I agree, but you still haven't grasped my simple point - put your *SERIES* motor in 4wd, jack one whole axle off the ground and then try driving forward or backwards. Guess what - it moves!!!!!!!

Because the torque from the transferbox does not go through a diff it is forced to both axles which then in turn distribute the torque through axle diffs. This means if you are in selectable 4WD with either the front or rear axle off the ground, it will still drive providing both wheels on the axle touching the ground have traction. In order for it not to move you need one wheel from each axle not to have traction, if both wheels on any axle have grip it will move.

On or around Fri, 18 Jul 2003 15:45:08 GMT, "Exit" enlightened us thusly:

you're both right, and both wrong. the locked diff (or no diff, a la SIII) in normal conditions puts equal torque to both ends. As soon as one end has more grip than the other, that end gets all the torque.

Hence the fact that one wheel in the air won't stop it, and hence too the fact that it'll still move with one end jacked up.

The open diff does the same as the locked one in normal conditions, while allowing the 2 ends to go at different speeds, thereby preventing transmission wind up and trye scrub and all that. But one wheel in the air will stop it, because then all the torque buggers off through the non-gripping wheel, exactly the opposite of the locked diff situation.

that's what he said. one wheel in the air doesn't stop the vehicle, because the torque goes to the other end.

it simplifies it to look at a single axle - normally, torque is supplied equally to both wheels, via the diff. Lift one wheel, and it stops, because all the torque is going to that wheel. Lock the diff, and all the torque goes to the other wheel.

On or around Fri, 18 Jul 2003 07:43:45 +0000 (UTC), Terry Lyne enlightened us thusly:

ah, now they're clever. Wonder what cunning person realised that an epicyclic gear set can be used as a torque-biasing diff?

it works by effectively having different gear ratios on each side of the diff. Suspend it all in mid-air, and the one end goes faster than the other, but having all wheels on terra firma constrains them to go at equal speeds, thus you get the torque biased instead.

The XR4x4 has an LSD in it as well :-)

I don't have any problem understanding the concept but it was Samuel's contention that with a Series transmission or full-time 4WD with centre diff locked and open axle diffs on both, losing traction at one wheel would stop forward motion, which is not the case.

In message , samuel mcgregor writes

In a no traction on one axle situation, almost 100% of the torque is supplied to one axle only so you are correct.

However, that torque split is a secure one (it's going where it is meant to) ... so in fact Bob is correct.

You are reading 'securely split' and probably inferring 'equally split' which is not valid. 'Secure' is being used to mean 'controlled' - it's just odd semantics ... and I believe you are actually both correct.

I'll get off my fence in a minute ... :-)

no where in my post did i claim that a locked centre diff vehicle would stop moving if one wheel lost traction.

i would argue that "torque is split securely" means that the split of torque remains constant. and that "exactly the same porportion of the engines output" means either equal torque, or equal watts. Bob cooke used these two terms to describe a locked differential, and they are both wrong.

Austin Shackles wrote in news: snipped-for-privacy@4ax.com:

Oh No it doesn't :-)

It would be very unusual for the torque to be equal from a locked diff. In the 'on road' situation both axles would have very good grip and 'wind-up' would occur. This would mean that one axle would be driving one way and the other in the opposite direction! Half the torque from the engine/geabbox would add to one propshaft and subtract from the other, thus causing movement in the desired direction. Once grip is lost, as Austin says, the other end gets all the torque.

Terry

On or around Sat, 19 Jul 2003 18:46:42 +1000, "samuel mcgregor" enlightened us thusly:

well, yeah, I was trying to keep it simple. But of course, only in optimum conditions do you get equal torque and no wind-up. Most of the time it doesn't happen like that.

personally, I've a lot of time for the viscous coupling, which gives you the best (or worst, maybe) of both worlds. It doesn't have the same all-or-nothing approach of a locked diff. In an ideal world, the 110 would have on on the rear diff as well as the centre diff (as the XR4x4 has), but that'd require more money.