Andy G,

I think scoobyboy has been through enough now.

Can any one clear up the torque split? If Andy Mc is correct with his calculations it must be a 50/50 split.

Lee

 

The more this thread goes on the more i'm glad I bought a grey...and pick a specialist to service it

woohoo!! 22b soon...

Excellent info guys...

Gethin

 

Come on guys

I can't believe there is nobody in a community this size(Other than the very helpfull Mike Wood) with the info to put this to bed once and for all .

Personally based on the info posted above by myself and others I am very comfident that the split is 50:50 on UK cars.Unfortunately I don't think the community will take my word for it despite the detailed info I have posted in this thread,we need a more well known person to confirm it or to come up with a reason why I am wrong.

Any takers?

Andy

[This message has been edited by AndyMc (edited 28 February 2001).]

 

Not wanting to put the cat amongst the pigeons I find the whole concept of front to rear torque splits quite baffling.

As I recall the Sierra XR4x4 had a torque split of 1/3 front 2/3 rear. But the gearing for wheel rotation must be the same otherwise for every turn of the front wheels the rears would rotate twice .

 

Andrew,

The Sierra had an epicyclic gearbox that splits the torque mechanically 66% rear 34% front.

They were good for getting the back end out in the wet.

Only info I can find on the web states Scoob split 50/50. Mike Wood disagrees and I'm nobody to argue with Prodrive but AndyMc's theory does hold water.

Lee

 

logiclee,

I'll not ask for an explanation of how an epicyclic gearbox works otherwise I may find myself becoming very unpopular very quickly.

I was under the impression the Scoob was 50:50 as well but as you can gather I struggle with the concept of how it can be anything else.

 

Some interesting points raised about tyre sizes and the effect on the diffs.

This begs the following question:

What happens if you've changed your wheels to 17" and you have a tyre blow out and have ti fit the space saver. Will this situation have a negative impact on the diffs, or are they the same diameter?.

James.

 

the overall rolling radius should be the same size as a 17" so it should not effect it

 

As Mike W states, the front final drive is 3.9:1, the rear final drive is 3.545:1 and the transfer reduction (front to rear) is 1.1:1. If you multiply 3.545 by 1.1 you get ... 3.899 (3.9):1. So the split is equal in rolling radius terms but because the rear is through a different gear train then the transmitted torque is differentially split as Mike says. Least thats how I see it.

 

Hello again

Here is how I think the gearbox works and why I think it is a 50:50 split

We know that when the car is traveling in a straight line there is no motion in the center diff.(as stated by M Wood)
We also know that the effective ratio to the front and back is 3.9:1.(as stated by M Wood)
This means if the center diff splits the torque by a mechanical means such as an epicyclical gear train then it would have shown up in the test I did on my car (see above) and it did not!.
This means the center diff works in the same manner as a normal bevel gear diff as fitted to nearly every car on the road ie it splits the torque equally to both of its output shafts.(when there is no differential motion that is)
We also know the output shaft to the front drives the front final drive directly whereas the output shaft to the rear drives the propshaft through an additional pair of gears.The propshaft then turns the rear final drive.

As I'm sure you all know a pair of gears with different numbers of teeth will change both the speed and torque of the output shaft compared with the input shaft,this is why cars have gearboxes.
For example if the drive gear has 10 teeth and the output gear has 20 teeth the speed of the output shaft will be halved, BUT the torque will be doubled.

Imagine if an Impreza is traveling in a straight line and the engine is turning the center diff at 3900rpm and applying a torque of 100lb/ft.
The center diff has no rotational difference between the front and rear output shaft so it will split the torque equally between them.(50lb/ft to each shaft)
This means the output shaft to the front will be turning at 3900rpm and will have a torque of 50lb/ft applied to it.The front final drive (with ratio of 3.9:1)then reduces the speed by 3.9 times and multiplies the torque 3.9 times.

So the speed of the front wheels will be 3900/3.9= 1000rpm and the torque will be 3.9*50= 195lb/ft

The output shaft to the rear will also be turning at 3900rpm and will also have the same torque of 50lb/ft but the drive now has to pass through a pair of reduction gears with the ratio of 1.1:1.This reduces the speed of the driven gear 1.1 times but also multiplies the torque the same amount.Therefore the driven gear and hence the propshaft, to which it is attached, will rotate at 3900/1.1 = 3545.45 rpm and will have a torque of 50 * 1.1 = 55lb/ft.

The propshaft then turns the rear final drive which has a ratio of 3.5454:1 so this reduces the speed 3.5454 times and multiplies the torque by the same amount.
So the speed of the rear wheels will be 3545.45 * 3.5454 = 1000rpm and the torque will be 55 * 3.5454 = 195lb/ft

Threrfore the torque to the front and the rear will be exactly the same hence why I think it is a 50:50 split.
The only difference made by the pair of gears in the drive to the rear is that the propshaft rotates in the other direction and at a slightly lower speed than if the gears were not there.

Hopefully I have explained how I think the gearbox works and why I think the different front and rear final drive ratios have no effect on the torque split as so many people seem to think.

Andy

 

Moray - where did you get your front lsd?

Thanks
Richard

 

Wow - what a thread. Haven't had one of these for ages....I think tyre contact patch was the last one!

Andy - just a few points to think about:
1) The centre diff on a Scoob is an epicyclic unit, with a "speed ratio" of 50:50
2) This does not contradict what Mike Wood is saying, because from what I gather he is saying, the effective "torque ratio" is NOT 50:50.

What is the difference between the "speed ratio" and "Torque ratio"? Well, if you cast your mind back to some of the early performance fwd cars, such as Saab 900 turbo, etc. These commonly suffered what was called "torque steer", which basically caused the car to vere off one direction when accelerating. The reason for this is simply because the torque split on such cars was not equal under heavy loading. This unequal torque split is nothing to do with unequal split in the diff, but due to the unequal torsional rigidity of the drivetrain from diff to wheels caused by unequal length driveshafts. That in essence is how you can get un-equal torque ratio split even when you know the speed ratio split is 50:50.

Therefore your assumption that 100lbf.ft into a 50:50 speed ratio split gives 50lbf.ft + 50lbf.ft is wrong (sorry ).

Put another way, if you torsionally grounded all the wheels, and replaced the rear propshaft with a very elastic material and applied torque through the transmission - which wheels would you expect to get the most torque? obviously the front ones would.

Torque split is complex, and I'm no mechanical engineer (but a know a man who is!), but basically I think Mike Woods is saying that since the final drive ratios are different between front and rear there is a torque ratio imbalance between the

At the end of the day, every force has an equal and opposite re-action. If everything was infinitely stiff then it would be simple and we'd solve for speeds, but since we have a compliant system we need to balance torques, not speeds.

Sorry if the rantings of a software engineer has confused things even further - but I do know that at least some of it IS fact.

Cheers,

Andy

 

Don't start slaging off electricians, because I'll start to cry and come round and turn off your power.

 

AndyP

I've only just stopped having nightmares about this thread and you have to go and write a post like that.

Now as I see it the subject you have raised relating to torque steer effect on Saabs etc is a separate topic and is also a transitional effect ie it has a small effect under certain conditions ie when the torque load is changing.

I don't think it is an issue when the transmission is carrying a constant torque load which is when I am saying the split is 50:50.I will try to explain what I mean.By the way I liked your example of the elastic propshaft,I wish I could explain what I mean so clearly.

Anyway the amount of distortion in the drive train cannot be that great and I think the viscous center diff will be able to absorb the slight rotational difference that this distortion causes without its locking characteristic having much effect.Remember the center diff needs quite a large difference in rotational speeds to transfer any meaningfull amount of torque and there is also a delay in locking while the silicon fluid heats up and thickens.Therefore I think the center diff will be acting more like an open diff,ie it will split the torque at very close to 50:50 in this situation for the above reasons.

So in my opinion you are dead right in that there will be a torque imbalance as a result of any distortion,however this imbalance will be no way near as big as in your example with the elastic propshaft.

Now lets say the engine is producing a constant amount of torque.The transmission components will have distorted by a given amount,the center diff will have rotated a couple of degrees to absorb this distortion and the system will be back to equilibrium.It is in this situation that I think the torque split is 50:50 compared to Mike Woods 10% bias to the front.

Another small point is that you cannot put a set figure on the torque effect you have described because the size of the imbalance is related not only to the amount of torque being transmitted but also the rate at which the torque load is changing.The reason I never mentioned this effect in any of my posts is because I was having enough trouble trying to get people to understand the basics issue let alone such a complex related topic such as yours.

And yes I still think the split is 50:50

Have you shown this thread to you mech engineer mate.I wouldn't mind hearing his opinion.

Andy

 

Okay,a quick question. I remember hearing somewhere recently that the P1 has a 60:40 split. Can anyone confirm if this is true and if so, is it 60 front, 40 rear or vice versa?

 

Nooooooooooooooooooooooooooooo

My Dr has only just managed to cut down my tablets after this thread. LOL

Was it realy 9 months ago.

Scoobyboy is now my best mate LOL

Some of the early pics have now expired.

PaulMc
Doesn't the P1 have the adjustable centre diff from the TypeR? It would have variable torque split dependent on the setting of the centre diff.

Lee

 

No, the P1 doesn't have the adjustable centre diff. I am assuming this is because it has ABS instead. I think the quote might have been on the P1 video I got when I purchased the car. I shall check. I might even recall them stating the split was 60% front, 40% rear.

[Edited by PaulMc - 10/18/2001 9:09:57 AM]

 

Always thought the power split was 60 rear 40 front on uk cars?

 

I GIVE UP

 

Sorry

Richard

 

sorry andy, but opinion, thoughts and recalling do not constitute hard facts in my book. isnt this spec written down somewhere?
I know for a fact that the power splits for the sti5typeR is
36% front
64% rear
while the diff is free. As it says this in my sti handbook, cant find similar stats for the my P1 in handbook

 

Every piece of official Subaru literature I've seen has said 50:50 for the UK cars.Your right that some of my ramblings in this thread are not backed up with hard evidence but there is a lot that is such as the test I did to prove the center diff has a speed ratio of 50:50.Also if I all the calculations I did about the gear ratios is wrong then it is one hell of a coincidence that my figures add up.

Do a search on the net using the key words 'torque split' and just see how much Subaru stuff there is that says 50:50

At the end of the day if people still don't believe me after all the info posted in this thread thats OK by me, if a little dissapointing

Andy

 

I Believe. LOL

Lee

 

The Aussie brochures say 60/40 split 60 front 40 rear, for the MY00 WRX which is similar to the UK turbo. I have it right here in front of me.

Wrexy.

 

this is a fun thread.

Guys, if things are getting heated just bang a mail to me rather than slagging each other off. If apropriate i'll tidy it up a bit If not i wont but lets resist the personal slagging.

I havent been as active as i would like of late due to family issues but i have my beady eye on you again. OK, on with the slippy diff we go. LOL