Honda's Sci-Fi 4wd In The Acura Rl/legend Fast
Cars - Now
Author: 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 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.
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 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.
