Driving In Snow

[bonjo]

I was looking for something else and found this: http://www.toyotagb-press.co.uk/protected/vehicles/releases/2009/180809toy_rav4_pack.htm which might be of interest.

very interesting. This confirms what I found out at the time I was considering buying a 4.3 and totally matches my experience of the car's handling after a test drive.

At the end of the day, I stayed with my 4.2 as I preferred the handling and the 4WD system and didn't need a larger car.

Now If Toyota brings out a RAV with a Audi Q5 quatro drive system, they would get my attention

Now if you could find similar article on 4.2.

[anchorman]

I'm trying to think of the best way to explain..........

........on a 4.2 there is no slippage - it is all mechanical. If you imagine a conventional drive axle, front or rear wheel, if one wheel gets on a low friction surface it will spin and the othere will grip. This is the action of the differential (diff) which is there to allow the vehicle to go around curves where the outer wheel needs to go faster than the inner wheel as it has further to travel

In a 4WD both axles are driven so if one wheel gets on a low friction surface and spins the drive to the other wheel on the same axle is lost. However, the other axle will still transmit drive and the vehicle can keep moving. There is a problem with this situation and that is there must be some provision for all four wheels to turn at different speeds when you go around a curve. For this reason a third diff is employed. The third diff shares the drive between the front and the rear and each axle has its own diff so now they can all turn at different speeds. If one rear wheel were to spin on ice all the drive would be lost because the rear diff would spin and so would the third diff. Provision is made to lock the third diff manually and drive is forced to the front. You can still get stuck if one front and one rear spins in this case and true off roaders can lock up the axle diffs as well.

Now back to our humble 4.2. There is no third diff. The transfer box is actually plumbed into the front diff so this caters for all front rear variation and the front and rear diffs cater for left/right variation. The split is 50:50. If one front or rear wheel spins the associated diff will be responsible. If one front AND one rear wheel spins you are stuck. The difference is that in this design you cannot have both rear wheels stuck and both front wheels driving like you could with a third diff. In other words if you jacked both rear wheels up and put it in drive it would drive off the jack like a third diff was locked, you can only have one front and one rear spinning if you are jacking wheels up or letting them spin on low friction surfaces.

The 4.3 has a 55:45 max front to rear ratio. This is because there is a small amount of slip built into the drive unit and you cannot completely lock it up. By doing this it ensures that there is always a small amount of slip when it is locked and it stops the unit taking excessive forces on high friction surfaces. Because it is controlled by an ECU it is easy to make t drive or slip as required and what is nice is that you can do it by any amount variably. To maximise drive a 4.3 always starts from rest in 4WD. Anyone who has driven a large 2 litre FWD will know that starting off on a steep hill in the rain nearly always causes the front wheels to spin. This never happens with a RAV 4.3 unless the surface is very slippy like when it is snowy. The ECU takes care of this and when it senses the speed increasing and the need for torque decreasing it simply backs off the drive until eventually it becomes a front wheel drive. Remember this is steppless so when it is backing off the drive starts reducing from 55;45 then 56:44 then 57:43 until it is 100:0. It is achieved by that little clutch slipping on the input of the rear axle. It doesn't end on starting off either. If you are travelling at say 40mph then change down and floor it to pass something it will come in again and then back off after achieving whatever torque you are asking - that might be when you are up at 80mph!

If you start off with 55:45 and you were to turn the steering to full lock you now need all four wheels to turn at different speeds because they all have different distances to travel. This would impose huge strain on the transmission and the result would be that be that you would get what is known as "tight corner braking". This is where all the wheels are trying to turn at different speeds but they are being resisted by the transmission being locked up. It would feel like the vehicle was holding back and wanting to stall. In a third diff design this would simply cause the third diff to operate - that is what it is there for! However, on a 4.3 we have the all knowing ECU looking after us so he (or she!) simply recognises full lock through the steering angle sensor and backs off our super clutch to allow a high amount of slip but at least some drive to the rear wheels so as not to bind up the transmission. I don't know the ratios but imagine that on full lock the front/rear ratio might be 90:10 and on half lock might be 75:25 reducing to our max of 55:45 in a straight line. In the snow you would lock the 4WD and this protection would be over ridden and that is why it drops out at 25mph just to avoid strain on the transmission especially on high friction surfaces. It is inevitable that it would get forgotten.

What you have to remember is that the little clutch is constantly being engaged and disengaged depending on how we are driving. It is a like a fiddlers elbow for most of the time unless cruising on the motorway.

[*srjp*]

The most comprehensible explanation of this that I've read on the various sites, thanks anchorman.