torque bias/split

[doctorstupid]

even google can't answer all of my questions, and my auto tech instructor is at a loss as well. so how the fuck can an awd car have a torque split other than 50/50? i understand a torsen center diff could bias torque when the wheels slip. so how can it be all time? there is no system i or any one i know knows of that can increase torque without changing speed, be it VCU, diff, clutch, etc.

i dont want to hear "i heard form my best friend's little brother's retarded classmate's third cousin's babysitter's mechanic" that it's possible, i need to know how the fuck it works before i lose anymore sleep over it. i can't decide if it really exists or if it's a collective misunderstanding of a torsen differential...

edit: there is no mention of such a system in my 2000 page auto tech book either.

[stu]

Have you tried www.howstuffworks.com yet?

[doctorstupid]

i beleive you meant howstuffworks.com ;)

and they have nothing to offer on the matter.

[stu]

Thank you, I knew that sounded funny.

[doctorstupid]

ok, i see discussion of it here: http://www.highaltitudeimports.com/showthread.php?p=276654#post276654
yet i still fail to see how a vcu can manipulate torque while maintaning wheel speed.

[hotrod]

ok, i see discussion of it here: http://www.highaltitudeimports.com/showthread.php?p=276654#post276654
yet i still fail to see how a vcu can manipulate torque while maintaning wheel speed.


It is probably easier to explain it first as if the viscous clutch unit acted like two solid disk clutches.

Visualize a single flyweel with a clutch disk pressing on both sides. The clutch disk on the left sends power to the front wheels and the clutch disk on the right sends power to the rear wheels.

If both clutches are fully engaged you have equal split, each axle gets equal torque (up to the point one axle looses traction) then the axle that still has traction suddenly gets 100% of the torque.

If the right clutch is disengaged all the torque goes to the front wheels.
If the left clutch is disengaged all the torque goes to the rear wheels.

Now suppose your left clutch is fully engaged and the right clutch is slipping so that it can only transfer 100 ft lbs of torque. If the engine is making 300 ft lbs of torque you have a 66 / 33 torque split.


Now mentally replace the solid clutch disks with a coupling, on the rear wheels only. In the solid disk analogy above, the left clutch is replaced with a solid link, and the right clutch disk is now a viscous coupling.

The coupling will vary its ability to transfer torque depending on the temperature of the viscous fluid. When everthing is moving at the same speed the viscous clutch transfers very little power. When the powered portion of the coupling is moving faster than the driven side (rear wheels) of the coupling, then the viscous clutch fluid gets hot, it expands and the increased pressure, causes more drag between the two parts of the the coupling which are moving at different speeds, ie a variable clutch but using fluid rather than friction material.

That is the basic principle, but the mechanical details differ by manufacture.

Now imagine that when the relative speed difference between the driven and powered portion of the clutch is small, the viscous fluid acts like a thick oil, considerable drag but some slippage is allowed. But if there is high relative motion between the driven and powered ends of the coupling the viscous fluid suddently takes on the consistancy of pine tar, and becomes very thick and sticky. Now you have very high friction between the two parts of the coupling and lots of torque can be transfered through that part of the coupling.

The trick is to come up with a mechanical configuration so that you normally apply power to say the front wheels, but connect them with a viscous coupling to the rear wheels. If both sets of wheels spin at the same speed most of the torque goes to the front wheels and the rear wheels are along for the ride except for a small amount of power transfered due to the low speed drag in the coupling.

Now if the front wheels hit a patch of ice, and start to spin faster than the rear wheels the viscous clutch suddenly locks up and trys to spin the rear wheels up to the same speed as the front. To do that, it has to transfer a lot of torque to the rear wheels. As they begin spinning at the same rate, then the coupling relaxes and most of the power goes to the front wheels again.

By setting the stiffness of the coupling at low slip rates -- the stiffer it is, the more torque is transfered to the rear wheels under no slip conditions.


Hope that makes sense?
This is a generic description of the concept and may not match a specific car in all details. It is in essence how I understand the subaru system works.

Larry

[doctorstupid]

sheesh, i wake up at 4:30 craving a hamburger (on the foreman right noe :cool: ) and look at this... all of a sudden i have to think :(

i think i understand what you're saying, so let me be sure i 100% understand by making up a sceanrio.

let's say you have a car with this torque split going on up on a 4 point lift (so the wheels/drivetrain are just kinda chillin in mid air), and you get in it and "drive" it on the lift, all 4 wheels spinning. if this car has a 25f/75r torque split, and no load on the wheels, the front wheels would be spinning slower, correct? but on the road the car actually moving would force the front wheels to match rear wheel speed and in turn increase torque transfer through the vcu. right? or should i just shut up unitl i've had more sleep?

[hotrod]

The wheels would spin at the same speed on the lift, but the rear wheels would spin up to speed slightly faster. As I understand it, you only really see the torque split when there is a difference in wheel speed between the axles.

Here's another analogy. You have two people lifting an object. If each person can easily lift their own end you have no way of knowing which one is stronger. If the object is very heavy the stronger person will shift his grip and take more of the load to keep the object level, if the weaker fellow can no longer lift his end at the same speed.

In your analogy, where your driving the car on the lift and if you apply the brakes, the rear brakes would have to do 3x the work to stop the rear wheels spinning as the front brakes would to stop the front wheels.

Its really hard to explain in absolute terms you kind of have to get a feel for the concept. The system adapts instantly to changes in load, It sends what ever fraction of the power is necessary to keep the wheels moving at the same rate. so it only reaches the theoretical torque split when the low torque wheels can no longer carry the load.

Like the two guys lifting an object the strong guy only applies enough force to keep his end moving in unison with the weakers persons end. He only has to take a larger share of the load if the weaker guy can't keep up to the required load.


Larry

[stu]

If you kept your foot on the gas to keep the wheels spinning at 25mph, and only used the e-brake, you would see a noticable difference in the power required to stop the wheels if you tried it at 25/75 split, then a 75/25 split.

Great info Larry.

[doctorstupid]

ok, in theory it sounds like a great big cluster fuck, but i guess i can accept that's how it works. i'd really have to mess around with it to truley understand it.

are there other systems than VCU to bias torque like that?