Honda's Sci-Fi 4WD in the Acura RL/Legend Fast Cars - Now

by: John Hartley


Honda has come up with the most amazing 4-wheel-drive system, not for off-roaders but to improve the handling and stability of fast cars. It is being used first in the new Acura RL (Honda Legend).

Why is it so advanced? This SH-AWD system has the potential to increase cornering power and cornering safety by leaps and bounds. It will be ideal for supercars as well as luxury coupes.(More information about supercars at http://www.fast-autos.com/supercars.html )

First, it allows the amount of power sent to the wheels to be varied from 30% front and 70% rear to 70% front and 30% rear. True, some other systems get near that, but this allows the system to adjust itself to completely different conditions. On top of that, it can alter the torque from zero to 100% to the left or right rear wheel.

In other words, all the power can one to the right wheel one minute, and if necessary can be reversed so that all the power goes to the other wheel in an instant. This might be needed in slushy or snowy conditions, where there was a patch of bare road for a bit on one side offering good grip, but a bit later on a bare patch on the other side.

In addition to those features, the outer rear wheel can be speeded up so that it goes faster than the inside wheel! All done continuously depending on conditions. A few years ago this would have considered desirable, but science fiction.

More power to the rear wheel for acceleration

Confused? Well, let's start with the front-to-rear power. When you accelerate, the weight of the car is thrown on to the back wheels by the force of acceleration, so you want more power to go to the back wheels - if it all goes to the front you get wheelspin. When cornering, you generally want as much or more power to the rear wheels. SH-AWD does that.

On the other hand, when you are cruising along, you want more power to the front wheels as this increases stability. SH-AWD also does that.

More power to outer rear wheel for better cornering

The side-to-side variation in power is used to improve cornering power and stability. For example, when you accelerate through a corner, extra power through the outer rear wheel counteracts understeer, letting the car corner on rails. In fact, stability control systems used on many cars now do this by reducing the amount of torque sent to the inner wheel - this means that you actually corner slower than you could do. SH-AWD lets you corner faster.

On the over-run, you don't need that, and less power is sent through the outer wheel for optimum stability. But that isn't enough because when a car corners, the outer wheels have to travel further than the inner ones - by 5-10% on very tight corners, so the outer wheel normally goes slower.

Honda overcomes this with some gears in the back axle gear set which actually drive the outer wheel faster when cornering! This improves maneuverability and cornering at low speeds.

Electro-magnetic multi-plate clutches

All this is accomplished by multi-plate clutches operated electro-magnetically - a world first - and the gear/clutch set used for acceleration. In fact, these gears also serve as the rear differential.

An advantage of the electro-magnetic clutches is that the electronic controller does not need to control a hydraulic actuation system as used normally but an electric actuator - this reduces cost and weight.

Is this the best thing since sliced bread? For the person who wants to have stability and control, and doesn't want to know what is happening to the car -yes. Such a system is bound to reduce the input the driver gets from the road.

He will be less aware of whether the surface is slippery, or whether the car is understeering more than usual because the SH-AWD system is correcting things all the time.


About The Author

John Hartley is editor of http://www.fast-autos.com an online magazine devoted to fast cars and supercars. He has written from many of the world's top auto magazines, and has written about 10 books about cars and the auto industry, including 'Suspension and Steering Q&A' and 'The Electronics Revolution in the Motor Industry'. He has been writing about fast cars and testing them for over 20 years, and has been involved in race car design, and has also raced cars.