What Is Rx Awd?

PCM · April 19, 2015

Assuming my RX has AWD, which I'm sure it does... what does AWD actually mean?

I read about AWD, 4x4 etc... but can anyone tell me what AWD ( assuming that's the correct name ) is?

Is there a front to back drive ration, does it change when driving, what does it DO...?

vrodman · April 19, 2015

I always read it as All Wheel Drive...

...also, it always intensely annoys me how dealers have a '4x4' section where you typically find that virtually none of the cars for sale are actually four wheel drive (think Kia Sportage etc, yes, they do an awd but it's a rare beast on the forecourt). Surely 4x4 means four wheels, four of which are driven ? Could the dealers be guilty of misrepresentation ?

rant over !

:megaangry:

PCM · April 19, 2015

I always read it as All Wheel Drive...

...also, it always intensely annoys me how dealers have a '4x4' section where you typically find that virtually none of the cars for sale are actually four wheel drive (think Kia Sportage etc, yes, they do an awd but it's a rare beast on the forecourt). Surely 4x4 means four wheels, four of which are driven ? Could the dealers be guilty of misrepresentation ?

rant over !

:megaangry:

Hence my question... what IS AWD ( All Wheel Drive )

ColinBarber · April 19, 2015

AWD means drive power is/can be applied to all wheels. 4x4 is a bit more specific meaning a vehicle with 4 wheels has all 4 wheels driven. 4x4 is also used as a term to describe certain types of vehicle, like the RX, but a more correct term is SUV.

All UK based RXs are AWD but have the majority of power applied to the front wheels in normal operation. They don't have limited slip or locking diffs on the axles so one wheel on each axle can get stuck and so only the free wheel provides drive but in normal circumstances all wheel are being driven.

The series I RX300 uses a limited-slip centre diff to sense slip of the front wheels and transfer power to the rear.

The series II RX300/350 removes the mechanical centre diff and uses the traction control electronics to apply the torque distribution.

On the 400h and 450h hybrids the rear drive is purely electric and is typically only provided under hard acceleration or when the vehicle detects the front wheels are slipping.

There are RXs, and Toyota Harriers, in other countries that are available purely as front wheel drive. This is also the case for the base level NX300h in the UK.

PCM · April 19, 2015

Thank you!

( Is there a maximum % than can ever be sent to the back wheels - I assume it functions when in Reverse also ? )

rayaans · April 19, 2015

Thank you!

( Is there a maximum % than can ever be sent to the back wheels - I assume it functions when in Reverse also ? )

I read it was 50% maximum to the back

ChrisKaye · April 19, 2015

I agree with Colin, to me AWD means a car does have all wheels "powered" but usually controlled by the car/ecu.

4x4 on the other hand (at least to me) has a secondary "gear stick" (for want of a better word) to control the low ratio 4x4 and the driver can dictate when they are selected unlike AWD...

Whilst AWD is useful, it is the poorer cousin of 4x4!

Gliderpilot · April 20, 2015

I have had several discoveries, 200 and 300 TDIs, these had a centre diff but if traction was lost to one wheel then it was lost to all as the diffs were not LSD. The centre diff was lockable which meant that the engine output was applied to both front and rear axels equally so traction was taken up by the non spinning wheels on the other axel. My Jeep Grand Cherokee had LSDs on both front and rear axels with a non lockable centre diff so if wheel slip happened then the other wheels could keep driving. Used this occasionally to haul me and the carravan off a very wet field.

PCM · July 11, 2015

Nagging me...

So is there ALWAYS a bit of power to the rears, or could it be that on occasions there is none?

ColinBarber · July 11, 2015

The petrol only series III RX, not available in this country, certainly has the ability to put 100% of power to just the front wheels. I'm not sure this was the case with the series II.

rayaans · July 11, 2015

Nagging me...

So is there ALWAYS a bit of power to the rears, or could it be that on occasions there is none?

With the older RX yes, there was always 50% power to the rear wheels. This is not the case with the 3rd gen as it shuffles between FWD and AWD when detecting slip etc.

With the 4th gen, I heard that its going to be a maximum of 70% to the rear for the RX350, not sure if this may be the case for the RX200t as well to make it feel more RWD so that may be interesting

mikesands · July 12, 2015

I assumed the awd of my rx400H would get me out of most situations when pulling my caravan - which is why I bought it. But that wasn't true last year when the field was only a bit slippery, and one front wheel lost traction. It didn't look to me like any power was applied to the rear wheels once one front wheel was slipping. The only solution was to pack lots of gravel under the slipping front wheel.

Mike

ColinBarber · July 12, 2015

With the 400h these is no engine power applied to the rear wheels, there is no prop shaft, it is all done with the rear electric motor. Not sure why it didn't apply that. If one wheel looses traction the system will apply the brake therefore transferring power to the other one, maybe you needed to give it a bit more welly to achieve that?

ChrisKaye · July 12, 2015

Or disable traction control/VSC so the slippy wheel continues to slip but the car doesn't knobble the power to the other wheels.

I know this is a totally different scenario, but I had a later Mini with a CVT gearbox and when stuck in snow, I had to disable traction control and manually select 2nd gear to get myself out of a slippy situation!

It took several attempts as without disabling the traction control, as soon as I gave it a little welly to keep a constant level of engine power, the traction control kept kicking in and dropping any minimal momentum I had managed to gain!!

Since then, I have always kept a collapsible snow shovel and half a carrier bag of grit in the boot of whatever car I am using, comes in really useful at least once a year whether snow/ice related or not!

mikesands · July 12, 2015

can you disable traction control/vsc on the 400H - I can't remember seeing a button for it

Mike

PCM · July 13, 2015

I assumed the awd of my rx400H would get me out of most situations when pulling my caravan - which is why I bought it. But that wasn't true last year when the field was only a bit slippery, and one front wheel lost traction. It didn't look to me like any power was applied to the rear wheels once one front wheel was slipping. The only solution was to pack lots of gravel under the slipping front wheel.

Mike

... which makes me think of the 'ECT SNOW' button.

I always thought it meant, starting off in 2nd gear with traction control off, but maybe I was wrong. Posts suggest that it just means, start off in 2nd gear.

ChrisKaye · July 13, 2015

To be honest, the RX manual was a bit crap at not offering any explanation...!

If I recall rightly, it says something like "use this button when there is snow" but no clues as to what it does to help!

Mike: Come to think of it, I can't say I've seen the VSC button in the cockpit anywhere but our '53 plate Celica has one to turn it off! :/

The plot thickens!

mikesands · July 13, 2015

...... nor is there an ect snow button (on my 2009 RX 400H). The only element of control over what the engines and gearbox are doing to the drive is a position on the gearbox called 'B' which is for holding the car in a lower gear when for instance towing a caravan downhill.

Mike

mick4545 · July 13, 2015

Not any help to you, but the 450 does have a traction control button to the rear of the mouse hand rest.

rayaans · July 13, 2015

Not any help to you, but the 450 does have a traction control button to the rear of the mouse hand rest.

Yep and the Hybrid snow mode definitely works in winter. The whole car seems heavier and smoothes acceleration out quite well to avoid any slip.

DanD · July 13, 2015

You cannot disable traction control on the 400h.

Also, there is no SNOW or ECO mode.

ChrisKaye · July 14, 2015

Hi all,

I stumbled across a U.S. LoC post that tells RX400h owners how they can disable traction control?

Any one brave enough to try it?

1. Switch the car off, then switch it on without (ready) light.

2. While on P, press the gas pedal 4 times.

3. Shift to N, press the gas pedal 4 times.

4. Shift to P again, press gas pedal 4 times.

5. Switch the car to ready and drive (at your own risk) without computer controlled wheels!

Original post is here:

http://us.lexusownersclub.com/forums/topic/64588-traction-control-rx-400h/?p=481904

PCM · July 18, 2015

... and if it's AWD, how does that affect things when one ( rear ) wheel spins.

Do both stop, just one, or...?

ChrisKaye · July 18, 2015

I believe the power transfer is just front to back, so if a rear wheel loses traction and spins, with traction control/VSC on, it should theoretically apply the brake to the spinning wheel / or back the power off by electronically numbing the throttle (depending on driving conditions from wheel speed senso per wheel, steering angle and yaw sensors etc).

With traction control/VSC disabled, the slipping wheel would still spin at the rear and I assume the power would be sent 50:50 front to back.

Power isn't split or transferred between wheels (left and right) on the same axle BUT braking can be as is independently controlled for each wheel by the ABS computer.

It's all clever stuff but is not fully automatic (only safety features are) so understanding how and when to use/enable certain features could mean the difference with getting stranded or not!

This is where I believe the car's manual really sucks explaining how the AWD works and how to cover common scenarios (stuck in mud or pulling away in snowy or icy conditions for example!).

Chris

ColinBarber · July 18, 2015

It seems with the RX350 Lexus reverted back to using a viscous coupling centre diff, as with the series I RX. The series II RX300 just relies on applying the brakes to individual corners to stop a spinning wheel.