Development
On a very gross level, the SH-AWD system is combination of the Honda VTM-4 automatic all-wheel-drive system and the ATTS variable torque distribution system. Honda introduced the Active Torque Transfer System (ATTS) technology in the 1997 Honda Prelude Type SH. Honda introduced the Variable Torque Management 4WD (VTM-4) automatic all-wheel-drive system in late 2000 on the 2001 model year Acura MDX sport utility vehicle (SUV) and later on the 2002 Honda Pilot SUV.
The ATTS equipped front-wheel drive Honda Prelude was labeled Type SH - for "Super Handling". The ATTS was able to distribute torque across the front drive wheels in order to reduce understeer in cornering, and provide more neutral handling for the front-wheel driven car. The ATTS garnered high praise for the 5th and last generation of the Honda Prelude, and earned the Type SH Prelude the best handling car under $30,000 award from Car and Driver magazine. However, the real usefulness of this high technology differential, which came at a more than $2000 premium, was debatable in a car that was already well known for its handling prowess. The ATTS was only available in the highest trim level, manual transmission form, or about 5% of the Preludes offered for sale.
The VTM-4 system claimed to be different from other all-wheel-drive systems in that it "anticipated" wheel slippage, rather than waiting for the vehicle to sense wheel slippage, after the vehicle may already be in trouble. The VTM-4 system consisted of a pair of computer-controlled wet clutches in the rear axle that worked in concert with the front wheels regardless of road conditions. On start-up, the VTM-4 system would send torque (power) from the front wheels to the rear wheels to supplement the normal front-wheel drive mode. With all four wheels being powered on start-up, regardless of conditions, more tire adhesion is provided from four driving wheels rather than two front driving wheels. In the dry, this served to reduce front-wheel drive torque steer, and provide more front tire adhesion for cornering. In the wet, snow or mud, power to the rear wheels moves the vehicle from rest with a minimum of wheelspin because all four tires are working together to move the car, rather than just two front tires. If conditions are really slippery and wheel spin is unavoidable, VTM-4 instantly reacts by adding rear-wheel torque in proportion to both the slippage and the rate of increase of slippage. VTM-4 was also designed with a special lock mode to help extract the vehicle from a stuck condition, such as a snow bank. This lock mode automatically engaged power to all four wheels at low speeds. As speeds increased, the power to the rear wheel would decrease, and by 18 mph, the system would revert to front-wheel drive.
SH-AWD combined both the automatic all-wheel drive VTM-4 system capability with the "super handling" torque shifting ability of ATTS on the rear axles.
In the North American market, Honda introduced the original SH-AWD system introduced in late 2004 with the second generation 2005 Acura RL. Two other SH-AWD variants were introduced in late 2006 in the then all new 2007 Acura RDX and the new second generation 2007 Acura MDX Sport Utility Vehicles SUV. In late 2008, an improved version of the Acura RL SH-AWD was introduced in the significantly revised Mid-Model Change (MMC) of the 2009 Acura RL. This improved version provided earlier SH-AWD intervention (first gear vs second) and more precise use of the rear differential electromagnetic clutch system to provide an improved limited-slip differential function.
Yet another version of SH-AWD was introduced in late 2008 with the fourth generation 2009 Acura TL. The Acura TL implementation of SH-AWD is actually mechanically more similar to the SH-AWD layout in the Acura MDX and RDX in that the rear differential is over driven at a constant 1.7% faster than the front wheels, unlike the Acura RL, which, since its introduction in 2004, added an acceleration device which can over drive the rear wheels up to 5.7% faster than the front wheels.
As of 2010 the Acura RL remains the only SH-AWD configuration with the variable speed rear differential acceleration device. The newly introduced 2010 Acura ZDX four-door sports coupe maintains a mechanically similar fixed 1.7% over driven rear differential configuration to the other Acura SH-AWD equipped vehicles
Acura announced in late 2008 that the 2010 Acura TL SH-AWD will be implemented with a six-speed manual transmission. Previous implementations of SH-AWD have all been with five-speed automatic transmissions.
On July 20, 2009, Acura announced the Acura ZDX four-door sports coupe, equipped with a new six-speed automatic transmission and SH-AWD. This would be the first time a six-speed automatic transmission would be mated to SH-AWD, as well as being the first six-speed automatic transmission introduced in either the Acura or Honda line up.
One week later, Acura announced a similar six-speed automatic transmission and SH-AWD with the 2010 Mid Model Change (MMC) refresh for the second generation Acura MDX.
In December 2010, the 2011 Acura RL was announced with a second Mid Model Change, featuring, among other things, a six-speed automatic transmission.
On November 30, 2011, as part of the publicity surrounding the 2011 Tokyo Auto Show, Honda announced a new implementation of SH-AWD as part of its "Earth Dreams Technology". The new hybrid electric SH-AWD would be based on an all new 3.5 L V-6 and three electric motors and lithium-ion battery technology. The V-6 would be mated to a 30 kW electric motor which could operate independently of the V-6, unlike earlier mild hybrid Honda IMA configurations. In addition, each rear wheel would have a 20 kW electric motor that would operate independently, varying torque as conditions demand. On a turn, the electric motor would apply torque to the outside rear wheel, while the inside rear wheel motor would drag, amplifying the SH-AWD effect. At the same time, the dragging motor would act as a generator, feeding power to the outside wheel motor. If the lithium ion pack became depleted, the V-6 would power the front electric motor as generator to feed electricity to rear wheels while also charging the battery. A prototype of this system was shown in a test mule using an 8th generation North American version of the Honda Accord.
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