HI SOOB

Getting an STI TYPE RA next week. It's got the adjustable diff.
I was wanting to know if they ever caused problems. And is there a change in the car when the setting is changed. Will it make it more like a rear wheel drive?
Any advice would be great.

Thanks

Jamie

MorayMackenzie

Dave,

Your statement is not strictly true. A good friend of mine put this very well, so rather than wasting time reinventing the wheel, so to speak, I shall post his description.


Quote begins "

(The DCCD) is a mechanical torque distributor so can only distribute more torque of there is a speed differential, this can only happen when one axle is slipping relative to the other. The torque split is therefore only apparent at the limit of traction of one of the axles, at all other times, there will be a transmissted torque difference, but this amount will be proportional to the difference in front and rear axle speeds. IN the other case the sun, ring and planet gears are all locked together, therefore input and output shafts turn at the same speed, therefore there is no mechanical advantage therefore equal torque.

the only way of having axles turning at the same speed but with torque unevenly distributed to them is by the use of diffs which the impreza doesnt utitlise such as active hydraulic diffs as seen on the skyline and 991 turbo

" Quote ends


Ok. So basically, a DCCD equipped scooby actually has a 50:50 torque split until the point where you begin loosing traction. At that point, the rear axle will receive the larger proportion of torque through the epicyclic differential unit, so the car will exhibit a more rearwards drive bias.

Winding the diff lock on will counter the epicyclic diffs natural urge to send more of the torque backwards by limiting the _speed_ differential it allows to occur between the axles. Fully locked, it does effectively force the torque distribution to 50:50, fully unlocked, it has little effect, thus letting the diff do its natural thing. However, the fun stuff only happens when you break traction, up to that point you are driving a 50:50 scooby, like everyone else.

So, whereas the Skyline GTR operates as a RWD car up to the point that you start losing traction at the back, at which point it throws the torque forwards to help increase grip and pull you around the bend (this system potentially giving the best of RWD balance/handling and 4wd grip when you need it)...

... The guys at Subaru decided that it would be more fun to have a 50:50 bias up to the point you start sliding, and _then_ become more RWD... essentially the exact opposite of the skyline. The DCCD can certainly make life more interesting.

Moray

Dave R

Exactly

Glenn Coombs

Huh ? I thought this was how a normal scooby behaved. How does a normal scooby differ from an active center diff one then ?

I was under the impression that the adjustable center diff variants of the scooby were indeed biased to send more power to the rear than the front by default. Thus giving them a more RWD like behaviour.


[Edited by Glenn Coombs - 3/12/2003 6:00:02 PM]

Dave R

Unlike the DCCD cars, the UK cars still maintain the 50:50 even at the point of 'unsticking' etc...

DCCD uses a different torque split (see above) and a sort of adjustable version of the UK centre diff, but with different internals and no silicon fluid etc...

MorayMackenzie

When you overstep the limits of grip the regular (50:50 bevel geared diff) car will tend more towards understeer whereas a type R/RA will tend more towards oversteer, relatively speaking. If you wind the type R's difflock on it will progressively become more tame (read les oversteery, more understeery).

Andy.F

I can't quite get my head round this.....doh

When running the diff 'open' without any limited slip effect,
how does torque require a speed differential to be transmitted in proportion to the epicyclic reduction gear ratio ?
Torque is a measure of force, not power. It's sort of like saying that your 1st gear isn't a reduction ratio or torque multiplier until the geartrain rotates

Anyway, even if you accept the part about requiring a speed differential, my understanding of vehicle dynamics is that in order to create a thrust force a tyre must exhibit some slip. In this case, under 'power on' conditions the diff will have unequal rotational speeds due to the difference in weight front/rear changing the slip of the axles relative to each other. Therefore the torque split will be as per the ratio across the epicyclic gear. No ?

Andy

Edited to add
Hmmm ...The torque split when the centre diff is 'locked' is dictated mainly by the weight on each pair of tyres and the coefficient of friction between said tyres and road surface.
The split can be anywhere from 100:0 to 0:100 (it can actually go further ie 120:-20 but it gets a bit confusing ) it will be constantly varying with conditions, one of the reasons the transmission takes on such a mechanical melody when locked.



[Edited by Andy.F - 3/13/2003 7:59:01 PM]

Adam M

Andy,

the torque split comes from the ratio of the number of teeth of the planet gears relative to the sun gear.

I presume you know the layout of an automatic gearbox.

you have an ring gear with a toothed inner circumference, this meshes with at least planet gears which have their centres supported together on a carrier, the inner sun gear meshes with the planet gears.

the carrier, the ring gear and the sun gear all rotate about an axis (they can obviously spin at different rates about this axis).

if you consider the input to be the ring gear driven by the engine, then you can consider the output to the front wheels to be connected to the sun gear, and the output to the rear wheels connected to the planet carrier.

If the diff is completely locked then the carrier cannot rotate relative to the sun gear or the ring gear, both front and rear output driveshafts spin at the same rate as the input from the ring gear.

If you unlock the diff, and drive on a grippy surface, the front and rear driveshafts being directly connected to the tires, in the absence of slip must rotate at the same speed so its as if the diff is locked.

In a geared system, the only way you can have a mechanical advantage is in a distance mulitplier, in other words for the rear wheels to receive more torque they must travel a shorter distance. When the wheels are gripping in a straight line, front and rear wheels cannot rotate at different speeds, so torque distribution must be the same, this is because the sun gear carrier is not rotating relative to the sun gear. If there is any relative rotation of these two there wil be a torque differential.

The torque differential is naturally characteristic of the ratio of the teeth of the sun to the planet gears, in the case of dccd, its 64:36. This as moray said will be the case when the diff is completely open and the front and rear wheels are free to spin relative to each other.

When cornering, there will be a slight speed differential between front and rear axles, the degree of this speed differential as a fraction of the natural 64:36 speed differential which would if the wheels were spinning in free air, will mean there is a torque split somewhere between 64:36 and 50:50.

Andy.F

Adam

Thanks for your reply

The last time I worked on an epicyclic geartrain, the annular ring was 1.3 metres in diameter and the power transmitted was 36,000 shaft horsepower I'm having trouble adjusting to the 'micro technology' application in a car

I still dissagree with you and Moray. Forget speed and power, we are talking about the torque split.
If you have a grippy surface, car stationary, apply torque to the
(unlocked) centre diff and you will get the split as dictated by the gears, this will continue as long as you get no slip on the road surface, as soon as you get slip the distribution changes relative to the available traction at either end.......which is the exact opposite of what you guys are claiming.

Andy

HI SOOB

Thanks for your info guys.

Got a headache now. Think I understand though.

Many thanks

Jamie

Adam M

andy,

if the front and rear axles are travelling at the same speed then the carrier is rotating exactly with the ring gear and sun gear (even in unlocked mode) if this is the case, then you can rotate the whole assembly as if it was simply a rigid gear and not a diff.

if the gearing that dictates the 64:36 split is not being used (which it isnt if you can consider the diff as a solid gear) then why would the axles exhibit a difference in torque?

The key to this is that with a geared torque split there has to be a speed differential in order for a distance mutlipler to exhibit increased torque, there can only be a speed differential during slip or cornering.

Andy.F

Adam

Why on earth would you consider the free diff as a solid gear ??

It is transmitting torque to both outputs even when they are not rotating relative to each other.
Even if the planet gears have the same number of teeth as the sun gear, the output shaft connected to the planet gears will receive the greater %age of torque. It may help you to think of the gears as a series of simple levers, draw out the configuration and work out the torque multiplication.

Andy

Why why why ? What you are effectively saying is that a lever only creates a force when it is moving You are confusing torque with power.



[Edited by Andy.F - 3/14/2003 6:41:07 AM]

MelTypeR

iTrader: (4)

I just drive the thing and wind it up a bit when its wet.

Mel.

[Edited by MelTypeR - 3/14/2003 9:10:15 AM]

Adam M

no andy I am not,

consider torque as force.

when you distribute the force unevenly you must use the lever that you are referring to. A lever is a force or distance multiplier, to increase the force to a level greater than you impart to one side of the lever, you reduce the distance travelled, this is because energy must be conserved, so that force times distance on one side of the lever must equal force times distance on the other side of the lever.

Now if you substitute force in the lever example to torque in the planetary gear example you have the same problem. In order for the torque distributed to the rear to be greater than the torque distributed to the front it has to turn for a shorter distance.

Please show me a geared system which increases the torque without decreasing the distance, your example was a gearbox, well in first gear you mulitply the torque by the gear ratio of roughly 4, but at the same time the wheels turn once for 4 turns of the engine. If you could increase the force without increasing the distance then the work done by the wheels would be more than the work done by the engine, and you would have created energy.


The reason you can consider it as a rigid gear when you are driving on a grippy straight piece of road is because (and you have to picture this) the ring gear and the planet carrier and the sun gear are all rotating at exactly the same speed. Therefore although the planet gears are meshing with the ring gear the teeth that are meshing are not moving relative to the teeth on the ring gear they are "locked" in place (only as a result of the straight grippy bit of road forcing front and rear inputs to rotate at the same speed).


Its difficult to explain and visualise, but with your experience of these boxes it should make sense and more importantly, it should make sense to everyone else reading.

In my example of viewing it as a rigid gear, if at the beginning the three planet gears are held relative to the ring gear at the 12, 4 and 8 o'clock positions, then you turn the ring gear through 180 degrees, the planet gears will not turn at all about their own axes on the cariier, but the planet carrier itself will turn through 180 degrees, meaning that the planets end up at 6,10 and 2 o'clock positions repectively.

the fact that the ratio of teeth on these gears is 64:36 is irrelevant, because they do not rotate relative to each other. In this case, where is your mechanical advantage coming from? The planet carrier and the sun and the ring are all turning at the same rate about the same axis. If there is no relative motion then the planetary gearset can be viewed as a rigid gear.

If you disagree with that, then surely you would disgaree that the torque split at full lock of the dccd could also not be 50:50.









[Edited by Adam M - 3/14/2003 10:41:33 AM]

Andy.F

Adam

The point I'm making is that you don't need any relative movement to create torque. You are still talking about 'distance' !! as in distance travelled not lever length. Force x distance = work done. We are only talking about the force here. Not work, not power.

Taking your logic on stationary gears, your car would be unable to move from standstill as your gearbox could not transmit torque due to the gear train being stationary !

[Edited by Andy.F - 3/14/2003 10:52:17 AM]

Adam M

andy, you are missing the point.

the gear train is not stationery, the ring gear IS turning, the planet carrier and sun are turning in line with it, as if they were one rigid gear, all turning with the one output shaft to the front wheels, and one to the rear.

Forget power, it has nothing to do with this, distance DOES,

the only way you can transmit a different torque without changing the distance travelled bu the output shaft, is by wasting the energy in a roque converter. you cannot do this in a geared diff.

I accepted your first gear analogy and explained why it agrees with what I am saying, in fifth, you get lower torque than you put in, because it is an over drive, but equally, the wheels turn faster than the engine because the drive ratio is lower than 1.

I dont know how many times I can say this but you cannot get a mechanical advantage in a geared (ie no slip) system without altering the distance over the which the force (cf torque when talking about rotation) is applied.

I suggest you look at some pictures:

the only models in my head that might help are the dccd. lock in the front to rear driveshafts by flicking the switch forward and imagine it rotating all together is as one gear, irrespective of planets and suns and rings, they all spin together because the output shafts are locked together.

When driving on a flat straight grippy road the diff does mind if it is locked or not because the planetary centre diff is doing the same thing as it would do if it were not locked, ie they are all turning together, thats why they dont make noises of complaint when you are driving in a straight line!

Lock or unlock its the same, locked makes it act like a rigid gear, unlocked in those conditions, and it still acts like a rigid gear. If you accept that when locked it is 50:50, why cant you accept that it is 50:50 when it is travelling on a straihgt road without slipping?

Andy.F

Adam

Do you accept that when the diff is 'open' the torque to both front and rear shafts must be transmitted by the planetary geartrain ?

Do you also accept that the mechanical advantage across the geartrain is higher for the rear shaft than it is for the front ?

If so, under non slip conditions, the torque MUST be proportional to the mechanical advantage.

Applying the diff lock eliminates the MA difference and the torque at each shaft is dictated by the available traction.

Adam M

plus andy, forgetting power still, energy has to be considered as energy is conserved.

force times distance = work done = energy.

increase force and you must decrease distance else energy goes up simple as that.

the distance here, or lever length in your example is the number of teeth of the planet and sun gears for them to yield their mechanical advanteg they must be turning realtive to eachother, which they are on a flat straight grippy road.


To make it easier to understand for the masses, consider a technic lego differential.

look at the middle picture and notice how the bevel gears in the middle are not spinning relative to each other yet the output shafts (left and right in this case instead of front and rear) are still turning but at equal speeds.

Now imagine the bevel gears in the middle are not the same size which is analagous to the case in the planetary set up. The fact that they are different sizes is irrelevant if they are not turning, but if one axle slips, then these gears come into play (click the button on the diagram to make the differential turn).

It is only when these gears are turning RELATIVE TO EACH OTHER(at any rate) that their teeth ratio can have any effect on the torque transmitted to the output shafts.


edited to correct link

[Edited by Adam M - 3/14/2003 11:30:29 AM]

[Edited by Adam M - 3/14/2003 11:37:38 AM]

Adam M

andy, the point it, the planetary gearing is only acting as such when then is a speed differential between from and rear drive shafts, when there isnt, gearste turns as one as in the picure obove of the lego type differential.

If you can accept that to unevenly distribute the torque, you have to unevenly distribute the speed of the axles, then you will see there can be no mechnical advantage on a straight flap grip road.

As I said before, look at the lego open diff. The ring gear (kind of) is turning under the engine drive, and the two output shafts are turning with it at the same speed, the bevel gear ratio could be 10000000:10, it wouldnt matter.

Andy.F

LOL at lego diff Even the Lego diff is transmitting torque to the driveshafts whilst the carrier rotates and the bevel gears don't.

I see I'm going to have to draw a force diagram for you but it will need to wait till I get home this afternoon.

Trust me on this Perhaps we should have a little wager to make it more interesting

Andy

Adam M

yes it is transmitting torque, equally, ie: 50:50 and they are both rotating at same speed which they would be if they were the output shafts of a planet gear travelling on my flat grippyu straight road.

normally I am not a betting man, but this point is not in dispute. I would bet with you, but I have a feeling I wont be able to convince you you are wrong.

draw a force diagram all you like, but let me ask you this.



what is it that defines the dccd as a 64:36 torque split? the answer is the teeth ratio of the sun gear to the planet gears.

If you can accept that they are not rotating about their own axes relative to each other, then how can they have any effect on the torque distribution to the output shafts.

If you dont think its the number of teeth, then what other intrinsic feature of the diff gives rise to that specific 64:36 torque split?

MorayMackenzie

Missed oportunity Adam. You could have won some money from a Scotsman!

Andy.F

OK - For green slips nah, I don't need a pile of bits so lets say hmmmm..... How about a case of beer far more interesting to me than your car

FFS.. AAAAAGH.. THEY DON'T NEED TO ROTATE TO TRANSMIT TORQUE
Whilst the diff is unlocked, the torque transmitted is proportional to the reduction ratio at all times. No exceptions. It can't do anything else, it's just a gear ratio.

Andy.F

Disagree, it is also defined by the effective radius of the planet carrier. ie if you have 50 teeth on sun gear and 50 teeth on planet gear, your torque split is no where near 50:50. More like 70:30

Andy.F

Already answered above, in addition the planetary carrier acts as a torque doubler so this is an 'intrinsic feature'unrelated to the number of teeth

Andy.F

Mmmmm I'm getting thirsty now

Gedi

This is a great disscussion.

What are the chances of getting a third party in to finalise, maybe somebody from the Subaru design team???? Webmaster??

As an unbiased reader, I see both view points and am now slightly confused. The addition of a crate of beer makes it all the more exciting

Andy.F

I would normally suggest 'Pat' for something like this however in this case, I believe that is who Moray and Adam are quoting.

I believe Simon is into suspension and diff 'effects' more than design philosophy.

It's really schoolboy mechanics so perhaps we will get more input after 4pm

Andy

MorayMackenzie

Andy, FYI other than the first quote I explicitly posted (and this was not from Pat), I was not "quoting" anyone. Your comment was not taken as a complement. Thanks a bunch. Moray


[Edited by MorayMackenzie - 3/14/2003 4:13:59 PM]

Andy.F

Sorry Moray, didn't mean to offend. I had seen a 'novel' from Pat on this topic and thought I recognised your quote.

Andy