Buyer uncertainty for Lexus UX all-wheel drive

Lexus will promote its new small SUV with an all-wheel drive solution that could be worth less than the sum of its parts when the Lexus UX arrives in Australia late this year.

The AWD option — also known as the 'E4' option in some markets — is only available for the more expensive hybrid model, the Lexus UX 250h.

A 5.3kW electric motor drives the rear wheels from standstill up to 70km/h. That sort of output represents just enough supplemental power to offset traction loss at the front wheels when the vehicle is moving away from a standing start.

Lexus also insists that the motor can help adjust the car's handling on low-friction surfaces, supplying torque to the rear wheels if the UX 250h begins to understeer or 'braking' a rear wheel in an oversteer situation.

But the fact is this, with such low power available, the electric motor will have a negligible effect on the vehicle's handling on anything but the greasiest road surfaces.

Even then, the car's handling problems are just as likely to be sorted out by the stability control system with little aid from the electric motor.

During the launch of the Lexus UX, motoring.com.au spoke with both Yoshiaki Ito, the test driver for the project, and the UX chief engineer Chika Kako about the all-wheel drive option.

Firstly, Ito-san explained through an interpreter that the decision to opt for the electric motor was based mostly on conserving fuel.

A conventional final-drive system with a driveshaft to the rear wheels and either a centre differential or a multi-plate (on-demand) clutch system would add weight and burn more fuel.

“The concept for this particular all-wheel drive system too is for traction really off the line [in an] urban setting... [and] good turning radius,” Ito-san confirmed.

“Let's say you're on a snowy road, and you're taking off. In those types of conditions that it was designed for the motor has plenty of power enough to give you the traction where you need it.”

When questioned about the argument that the electric motor can compensate for understeer or oversteer, Ito-san acknowledged that the engine may not have enough kick – and in fact none at all beyond 70km/h – in which case the stability control would take up the slack.

“The fundamental thought process is the distribution of power is in two – [rear-motor torque] decreasing as we go up to 70km/h. So depending upon that curve... of course the motor has enough power to make a certain amount of difference, but if it's not enough, then we have other systems like full VSC [stability control] to start kicking in and take over.”

The beauty of the system is it can overcome the potential for traction loss on slippery surfaces without that extra litre per hundred kilometres that would be the result of going to a full, mechanical all-wheel drive system.

“There's almost a minimal amount of difference in fuel economy with this system on and off, which shows how efficient it is,” Ito-san explained. “That's why Kako-san decided to put it on... not too much compromise.”

But there's no all-wheel drive system for the Lexus UX 200 – the conventional-powertrain model.

And although the hybrid all-wheel drive variant presumably runs a rear differential to supply torque at varying rotational rates for cornering, there are no plans to link that differential – or one heavier in construction for a higher torque application – via a driveshaft and coupling to the transmission of the Lexus UX 200.

“Technically it could be done,” Kako replied through an interpreter, “but if you look at the total package with our engine, it's obviously questionable whether it's good to have a mechanical [system], between the weight and [consequent] fuel economy loss; whether it's appropriate is questionable, which is why we opted to introduce the 'E4' all-wheel drive [system].”

So unless Lexus UX buyers spend a lot of time in the snowfields, it may be hard to justify the extra cost of purchasing the all-wheel drive system.