60 replies to this topic

[VS Legend]

For anyone who didn't realize it, this thread is mostly a joke but also a cautionary tale with a nice link.

NEU, on 30 January 2012 - 11:02 PM, said:

Yes I do mean viscous coupling system (VC), and 95%fr-5%rr is what a Volvo AWD brochure from '98 I have said it did.

I'm sorry, you're correct. The press release does say this. I did not think this was the case.

http://www.volvoclub...essDocV7-40.pdf

I also don't see how 95% of power can be sent to the rear wheels with a VC, though. Are these materials correct?

Rod, on 31 January 2012 - 12:49 AM, said:

The system would dis-engage with the E-brake. We did that when driving the car at school (it was the 1st S60 AWD car in Florida)

IIRC, there was no proportional engagement, either 0% or 100% engaged.

If the e-brake does disengage the rear wheels, that's very cool. It makes sense to me that it would.

ABS should be able to tell the Haldex computer, "Hey, we're braking these two wheels. Just FYI."

Haldex does allow for variable distribution of power between the front and rear wheels though.

[Burn-E]

VS Legend, on 31 January 2012 - 08:48 PM, said:

For anyone who didn't realize it, this thread is mostly a joke but also a cautionary tale with a nice link.

So how much damage did you do?

[NEU]

VS Legend, on 31 January 2012 - 08:48 PM, said:

If the e-brake does disengage the rear wheels,

This also applies to the Gen 1 AWD...


Freewheel Unit

Volvo has also engineered into the drive line system a freewheel component. This unit disengages the rear drive during braking and/or coasting help ensure optimum braking performance. With this feature, the ABS braking system does not need modification. Another refinement, when it comes to this freewheel, is that it also has a lock-up function. This means that Volvo's AWD system is always in four-wheel driven when it reverses.

[-Matt01]

1. if your engaged pulling the handbrake probably wont do anything because inless you modded the handbrake somehow, its not strong enough to lock the rear wheels.. Even when i clutch in and pull the handbrake it doesnt work (new pads + cables + adjusted) volvo handbrakes are crap
2. You dont need to use your handbrake if you have AWD.
3. How exactly are any of the wheels engaged if you clutch in?

source: live in canada and drive an 850 AWD Manual

[VS Legend]

NEU, on 01 February 2012 - 01:08 AM, said:

This unit disengages the rear drive during braking and/or coasting help ensure optimum braking performance.

Yes, the freewheel disconnects so that the rear wheels don't send power to the front wheels, which would occur if the front wheels began spinning more slowly than the rear wheels while braking. But how does the freewheel work? Does it still function when only the rear wheels are being braked?

-Matt01, on 01 February 2012 - 01:15 AM, said:

1. if your engaged pulling the handbrake probably wont do anything because inless you modded the handbrake somehow, its not strong enough to lock the rear wheels.. Even when i clutch in and pull the handbrake it doesnt work (new pads + cables + adjusted) volvo handbrakes are crap

See the link in the OP. It's usually the brake that blows up in Volvos. I've heard of drivetrain horror stories with people trying to handbrake drift Subaru cars and other makes though. This has been a pretty popular topic on forums for WRX and STi owners over the years.

Volvo does list the handbrake as "parking brake" not "emergency brake" or "handbrake." I don't think they expect a Volvo owner to ever need it to provide emergency functionality. How many stories are there out there about Volvo brakes failing?

[ozzimark]

Rod, on 31 January 2012 - 12:49 AM, said:

The old style viscous coupling only started to transfer power to the rear when the fluid got hot.

No, it's a viscous coupling because it relies on fluid visocity to transfer the torque through the unit. Think of a bunch of plates spinning next to each other in a thick fluid.

However, I am pretty sure that on the VC AWD systems, there is a lockup that engages if the fluid gets too hot so you don't burn up the VC unit.

VS Legend, on 30 January 2012 - 09:39 PM, said:

How so? If the clutch pedal is depressed, isn't the engine disengaged from the transmission and thus disengaged from the drivetrain preventing the engine from sending power the those rear wheels? I am not a mechanic (or anything remotely close to one) and don't know as much about how a car works as I would like, but I thought this was one of the things I understood correctly, so I would appreciate your explaining if I don't. I love to learn.

The VC unit transfers torque from front to rear when the fronts are spinning faster than the rears, regardless of clutch state. If the rears are locked and the fronts are spinning, that'll cause the VC unit to do its thing.

NEU, on 01 February 2012 - 01:08 AM, said:

Volvo has also engineered into the drive line system a freewheel component. This unit disengages the rear drive during braking and/or coasting help ensure optimum braking performance. With this feature, the ABS braking system does not need modification. Another refinement, when it comes to this freewheel, is that it also has a lock-up function. This means that Volvo's AWD system is always in four-wheel driven when it reverses.

Yes, it freewheels when the output from the VC unit is spinning faster than the input from the prop shaft. I'm not 100% on this, but I bet the lockup unit just senses input rotation direction for the reversing lock-up function.

[andyb5]

Here's some freewheel unit info, provided by lookforjoe in this thread



The actual unit:

[Deimos]

VS Legend, on 30 January 2012 - 09:39 PM, said:

How so? If the clutch pedal is depressed, isn't the engine disengaged from the transmission and thus disengaged from the drivetrain preventing the engine from sending power the those rear wheels? I am not a mechanic (or anything remotely close to one) and don't know as much about how a car works as I would like, but I thought this was one of the things I understood correctly, so I would appreciate your explaining if I don't. I love to learn.

The clutch disconnects the engine from the drivetrain, transmission and beyond. But that's not where the potential for damage in AWD appears, it's between the front and rear sections of the drivetrain. This is also why you have to flatbed tow almost all AWD cars.

Problem is, even with the engine connected, the drivetrain is all connected to itself still, and the vehicle's speed actively drives the drivetrain. So a major speed differential will cause, in the old AWD, the viscous to seize up, drive the rest of the wheels, and if it can't it will go pop pretty quick.

When someone refers to the old AWD system split 95F/5R they mean that the viscous is in it's open state because the axle speeds are the same. It does not actively control split like a Haldex does, but it's done passively by the center viscous, which is basically an LSD.

VS Legend, on 01 February 2012 - 06:31 PM, said:

Yes, the freewheel disconnects so that the rear wheels don't send power to the front wheels, which would occur if the front wheels began spinning more slowly than the rear wheels while braking. But how does the freewheel work? Does it still function when only the rear wheels are being braked?

It says it prevent the rear wheels from driving the front. Not the opposite, which would occur in handbraking, and prevent normal AWD operation if it was set up that way.

And it can theoretically send 100% to either axle, IF the other axle has lost traction. What I'd like to see would be a quaife/audi style gear LSD center.

[VS Legend]

ozzimark, on 01 February 2012 - 10:51 PM, said:

No, it's a viscous coupling because it relies on fluid visocity to transfer the torque through the unit. Think of a bunch of plates spinning next to each other in a thick fluid.

Well, to be fair, the heat, generated by the shear/friction of the spinning plates, is what increases the viscosity/thickness.

So it is true that the heating of the fluid is what causes power to be transferred.

ozzimark, on 01 February 2012 - 10:51 PM, said:

However, I am pretty sure that on the VC AWD systems, there is a lockup that engages if the fluid gets too hot so you don't burn up the VC unit.

Is there? I thought that if the fluid rises to a certain temperature at a normal rate, it becomes almost solid and the shear/friction is eliminated because the plates beginning to spin at the same relative speed again. In other words, there should never be enough shear/friction to cause overheating unless you are dealing with an extremely powerful engine or locked-up wheels.

ozzimark, on 01 February 2012 - 10:51 PM, said:

The VC unit transfers torque from front to rear when the fronts are spinning faster than the rears, regardless of clutch state. If the rears are locked and the fronts are spinning, that'll cause the VC unit to do its thing.

Deimos, on 02 February 2012 - 03:21 AM, said:

The clutch disconnects the engine from the drivetrain, transmission and beyond. But that's not where the potential for damage in AWD appears, it's between the front and rear sections of the drivetrain. This is also why you have to flatbed tow almost all AWD cars.

Problem is, even with the engine connected, the drivetrain is all connected to itself still, and the vehicle's speed actively drives the drivetrain. So a major speed differential will cause, in the old AWD, the viscous to seize up, drive the rest of the wheels, and if it can't it will go pop pretty quick.

That makes sense. Would love to know more about how the freewheel works to understand if clutch-in engages "coasting mode" / freewheel. Thank you, andyb5, for posting what you did. I had only seen an older document on the same systems with less info.

ozzimark, on 01 February 2012 - 10:51 PM, said:

I'm not 100% on this, but I bet the lockup unit just senses input rotation direction for the reversing lock-up function.

Would love to find out if this is how it works or not. Currently, all I know is that a vacuum actuator is involved. Looks like it should be right after the cut off in that image from andyb5.

[Deimos]

Viscous couplings don't have much clearance and will transfer some without heating up, just from the same fluid spinning principle as a torque converter. And as far as them locking up, they don't have a lock up mechanism exactly, but when they get hot enough, the fluid essentially won't allow the plates to move... but if you hit that point and still have slippage, either from handbraking, power, or just because it's been slipping too much for too long, the fluid will expand enough to crack the VC's case.

Ford has had problem with soccer moms blowing up the VCs on explorers.

[Rod'sT-5]

Found an interesting web site.

http://www.awdwiki.com/en/volvo/

It has some general info on the viscous coupling and Haldex

[yangotang]

dude i just got dumber by reading one of legend's posts.

[VS Legend]

Deimos, on 02 February 2012 - 08:17 PM, said:

And as far as them locking up, they don't have a lock up mechanism exactly

The coupling may not be locked up, but there is a Centrifugal Lock-up Mechanism that performs some sort of lockup involving the freewheel. I posted on swedespeed looking for more info on it.

[Deimos]

VS Legend, on 07 February 2012 - 06:21 PM, said:

The coupling may not be locked up, but there is a Centrifugal Lock-up Mechanism that performs some sort of lockup involving the freewheel. I posted on swedespeed looking for more info on it.

We already discussed that. The freewheel unit has a lock up function that allows power to be sent to the rear... in reverse. That's all that locking thing does. And that's after the VC anyway, doesn't lock the VC, just locks the freewheel.

[VS Legend]

Sure, that's what it does, but I would be curious as to exactly how it works and exactly how the freewheel is engaged/disengaged (or whatever the proper terminology for a freewheel would be).

[andyb5]

VS Legend, on 10 February 2012 - 07:02 PM, said:

Sure, that's what it does, but I would be curious as to exactly how it works and exactly how the freewheel is engaged/disengaged (or whatever the proper terminology for a freewheel would be).

See my post further up the page. It describes how the freewheel mechanism works.

[Deimos]

VS Legend, on 10 February 2012 - 07:02 PM, said:

Sure, that's what it does, but I would be curious as to exactly how it works and exactly how the freewheel is engaged/disengaged (or whatever the proper terminology for a freewheel would be).

It works using the principle of http://en.wikipedia....ntripetal_force

[VS Legend]

I wasn't following, but I guess I just didn't realize that a freewheel, by definition, is a mechanical device which disengages when one shaft is spinning faster than the other. It's all in the basics - which I does not know. TY guys.

[Deimos]

VS Legend, on 14 February 2012 - 10:23 PM, said:

I wasn't following, but I guess I just didn't realize that a freewheel, by definition, is a mechanical device which disengages when one shaft is spinning faster than the other. It's all in the basics - which I does not know. TY guys.

Indeed, their primary use if bicycles so the pedals can idle without it stopping.

[Jardim]

VS Legend, on 30 January 2012 - 09:39 PM, said:

How so? If the clutch pedal is depressed, isn't the engine disengaged from the transmission and thus disengaged from the drivetrain preventing the engine from sending power the those rear wheels? I am not a mechanic (or anything remotely close to one) and don't know as much about how a car works as I would like, but I thought this was one of the things I understood correctly, so I would appreciate your explaining if I don't. I love to learn.

Even with the clutch depressed it would still monkey your stuff up.

The motor might be disengaged from the drivetrain but its not the motor we are worried about. Your 4 wheels are still moving. Those 4 wheels are attatched to a differential both in the front and the rear and the VC. Locking the 2 rear wheels while the front diff is still rotating with the driveshaft, etc will just cause disaster. Engine rotation doesnt matter. If you pull the e brake in a RWD while moving without depressing the clutch you just simply kill the motor (you cause the rpms to go to 0 thus killing the engine)


This isnt a rally spec evo where you can just pop the ebrake in every hairpin and bounce her off the limiter and drop the clutch.