4WD sytem newbie question

[Dr. Dyno]

Originally Posted by Phrede

....there is no center differential. ... the driveline is directly geared to the drive system and turns 100% of the time in sync with the front axle... The amount of allowable torque that can be transmitted thru the coupling is controlled by the ECU.

It seems to me that the spec means that up to 45% of the AVAILABLE torque can be transmitted thru the coupling before it slips. Remember, the engine never produces more torque than required by the load. In case of the front wheels being off the ground and a light total load then it is probably closer to 15% front 85% rear of the required torque. But the 85% being used by the rear does not exceed 45% of the available torque but is enough to move the car. Again, the amount of torque that "could" be transmitted thru the coupling is controlled but the ECU and the design of the coupling. As long as the car moves (the required torque is less than the available torque) there will not be any slippage in the coupling and the net rear axle speed will equal the net front axle speed. Without any slippage in the clutch the amount of torque it could transmit doesn't really matter.

Exactly!!! And this is the point often overlooked and probably not even understood by the ad-oriented inclined roller "test" guys.

Originally Posted by Phrede

The system knows if there is any slippage only when there is wheel speed difference greater than can be attributed to cornering.

Yep, the situation where AWDs need either an unlocking center diff or a viscous coupling which by its nature limits max rear wheel torque.

Originally Posted by Phrede

I expect it takes into account engine speed and load and maybe it takes into account the yaw or other sensors to get a better idea of wheel slippage and when to disengage the clutch from freewheeling 2WD mode.... Certainly the algorithm used to control the clutch took a lot of development time.

The 0-3A drive is, as you surmised, 12V PWM. On the whole system, its effectiveness prompted my comment in the 2010 thread:

Originally Posted by Dr. Dyno

The most amazing thing to me is how seamlessly the On Demand 4WD transitions from pavement to snow and from straight to turning on either.

Also from the 2010 thread came probably the most realistic real-world statement from one who's been there in spades:

Originally Posted by Plainrider

Now, on the point raised earlier by rav4two, I agree that the RAV4 4WD does not really operate like a rear wheel drive vehicle: however, a rear-wheel drive only condition temporarily occurs when there is no traction on either of the two front wheels while, at ther same time, there is some traction on one or both of the rear wheels. This situation can not be sustained for any length of time (due to loss of steering control as well as traction on the front wheels – but it can be important for brief conditions of minimal traction. (In this regard, my point earlier in this respect was that certain “AWD” vehicles that have an unlockable center differential would not be as good as the RAV4 for certain slippery-off-road driving conditions. (Of course, such AWD vehicles would not have been designed for off-road or for soft-road conditions).

BTW, did anyone say WELCOME aboard!

[Carbon]

Originally Posted by Phrede

I'm still learning the system and since that post have learned that there is no center differential. This is an entirely different situation. It seems the driveline is directly geared to the drive system and turns 100% of the time in sync with the front axle (at 3.08 times the speed). The limiting factor is an electromagnetic clutch that slips when the torque is exceeded. The amount of allowable torque that can be transmitted thru the coupling is controlled by the ECU.

I agree.

Originally Posted by Phrede

It seems to me that the the spec means that up to 45% of the AVAILABLE torque can be transmitted thru the coupling before it slips. Remember, the engine never produces more torque than required by the load. In case of the front wheels being off the ground and a light total load then it is probably closer to 15% front 85% rear of the required torque. 15% being lost in inefficiency of the gear train itself. But the 85% being used by the rear does not exceed 45% of the available torque but is enough to move the car. Again, the amount of torque that "could" be transmitted thru the coupling is controlled but the ECU and the design of the coupling. As long as the car moves (the required torque is less than the available torque) there will not be any slippage in the coupling and the net rear axle speed will equal the net front axle speed. Without any slippage in the clutch the amount of torque it could transmit doesn't really matter.

From http://www.edmunds.com/toyota/rav4/:

2.5-liter inline four-cylinder that produces 179 horsepower and 172 pound-feet of torque. ....
3.5-liter V6 makes 269 hp and 246 lb-ft of torque,

So do you think the "available torque" is 172*0.45 *gearingFactor pound-feet for the I4 and 246*0.45 *gearingFactor pound-feet for the V6?

My suspicion is that somebody came up with the 45% to make people who are used to hearing those percentages for Torsens feel more comfortable.

[Phrede]

Thanks Dr. Dyno. You've confirmed that I at least am thinking in the right direction. This type of intermittent use 4WD system has a different goal in mind than say an AWD syem seen in some high level sports cars ie Audi R8.

The bottom line is that it works well for it's designed purpose of those occasional slick conditions where a single driven axle will not go.

While the RAV4 is far from a sports car I now wonder how effective the system is when torquing thru a curve on a rainy road at 50MPH. I see an impromptu skid pad test in my future. I keep a mind in these systems regarding propagation time of the control systems. Even some of the fastest computing power is slower than some direct mechanical systems. Again, the proof is in performance.

I also wonder a bit about wear in the clutch. Slippage = wear. But how often is the clutch really slipping? Going around a turn will cause a small amount. Wheel spin on either axle will cause more. If the system is working and the conditions are not extreme, how long does that last? IDK, something to ponder. Perhaps that is why the max torque is limited to 25MPH. Speed, force and friction are everything to a clutch. I estimate that the driveline speed is about 907RPM at 25 MPH.

[rav4two]

Originally Posted by Phrede

While the RAV4 is far from a sports car I now wonder how effective the system is when torquing thru a curve on a rainy road at 50MPH. I see an impromptu skid pad test in my future.

I don't believe a skidpad test is going to be revealing, even in the wet. I did my share of canyon carving with our Rav earlier in its life, and being an experienced road course driver, I couldn't discern any RWD characteristics at that speed. It is so predominantly FWD biased, and the handling characteristics are so biased towards understeer, that at best I was able to achieve a steady state mild-to-moderate understeer with off-throttle nearly-neutral cornering through tire pressures and corner entry techniques. Any kind of power application at that speed just generated more understeer.

Like a lot of vehicles, the Rav feels pretty darned good at 80%. Above that and the soft bushings, shocks, and roll control make handling kind of ragged. But throughout that, it really doesn't ever feel like anything other than a fairly well balanced FWD vehicle.

[Phrede]

Originally Posted by rav4two

I don't believe a skidpad test is going to be revealing, even in the wet. I did my share of canyon carving with our Rav earlier in its life, and being an experienced road course driver, I couldn't discern any RWD characteristics at that speed. It is so predominantly FWD biased, and the handling characteristics are so biased towards understeer, that at best I was able to achieve a steady state mild-to-moderate understeer with off-throttle nearly-neutral cornering through tire pressures and corner entry techniques. Any kind of power application at that speed just generated more understeer.

Like a lot of vehicles, the Rav feels pretty darned good at 80%. Above that and the soft bushings, shocks, and roll control make handling kind of ragged. But throughout that, it really doesn't ever feel like anything other than a fairly well balanced FWD vehicle.

Thanks, exactly what I would expect. A good daily driver with a smidge more acceleration, occasional slow speed traction enhancement and more cargo space. Life is a compromise.

Very close to striking a deal on a new one.

[JuneBug]

Originally Posted by Phrede

While the RAV4 is far from a sports car I now wonder how effective the system is when torquing thru a curve on a rainy road at 50MPH. I see an impromptu skid pad test in my future. I keep a mind in these systems regarding propagation time of the control systems. Even some of the fastest computing power is slower than some direct mechanical systems. Again, the proof is in performance.

Phrede, I think you'll find these documents interesting. This one gives a good overview of how the ABS, traction control, vehicle stability control, and 4WD systems all work together:

http://rav4world.com/pdf/2006/brakecon.pdf

And this one focuses on how the rear differential and electro-magnetic coupler work:

http://rav4world.com/pdf/2006/rav_diff.pdf

As a "tech geek" I find this stuff fascinating!

[rav4two]

Originally Posted by Phrede

Thanks, exactly what I would expect. A good daily driver with a smidge more acceleration, occasional slow speed traction enhancement and more cargo space. Life is a compromise.

Very close to striking a deal on a new one.

Get the V6 while you can!

[pico]

Thanks for the documents, James, and for others who explained how the drive feels in practice. My former four-wheel drives were conventional full mechanical, no-computer, and all big V-8 torque machines with manual locking hubs. I learned to drive carefully to exploit the setup. (Some of my work was dragging whole dead trees, mostly hickory, from a deep and steep valley.)

When I took the new RAV4 onto the snow and icy roads, at first I thought something was just wrong. Sometimes when I accelerated it felt like the engine was bogging, when in fact it was doing a whole lot of adjusting. I went straight, turned well enough. All is good.

But it is not a vehicle I would use for hauling trees, or even going off-road except for field work, and on non-maintained roads.

Call me a happy old man glad to be off the farm.

[JuneBug]

Originally Posted by pico

Thanks for the documents, James, and for others who explained how the drive feels in practice. My former four-wheel drives were conventional full mechanical, no-computer, and all big V-8 torque machines with manual locking hubs. I learned to drive carefully to exploit the setup. (Some of my work was dragging whole dead trees, mostly hickory, from a deep and steep valley.)

When I took the new RAV4 onto the snow and icy roads, at first I thought something was just wrong. Sometimes when I accelerated it felt like the engine was bogging, when in fact it was doing a whole lot of adjusting. I went straight, turned well enough. All is good.

But it is not a vehicle I would use for hauling trees, or even going off-road except for field work, and on non-maintained roads.

Dragging trees with a RAV4? That's just crazy!! And just to prove we've got some crazy people on this forum:

Towing - dragging trees ???

Light Off-road Trip Report