Auto Tech: Acura’s Super Handling All wheel Drive how stuff works auto tech
Acura SH-AWD; photo by Paul Williams. Click image to enlarge

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By Jim Kerr

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Acura SH-AWD

Acura takes a different approach to vehicle stability control than other automobile manufacturers: instead of cutting power or braking wheels, they’ve added performance while controlling the vehicle direction. It’s called Super Handling All Wheel Drive (SH-AWD).

First introduced on the 2005 RL luxury sedan, SH-AWD has now been put into production for all Acura’s all-wheel drive models, which include the RL and TL sedans and the RDX, MDX and ZDX utility models. To identify vehicles equipped with SH-AWD, look for the SH-AWD emblem on the rear of the vehicle. That’s about the only way you can tell that the car has SH-AWD: it is so unobtrusive when driving that you hardly notice it working its magic.

Auto Tech: Acura’s Super Handling All wheel Drive how stuff works auto tech
Acura SH-AWD; photo courtesy Honda/Acura. Click image to enlarge

The safety of All Wheel Drive on slippery or loose traction surfaces is well noted by driving experts. Many vehicles use wheel brakes and engine power management as their stability control when the vehicle is in an oversteer or understeer situation. If the front of the vehicle is pushing out in a corner and won’t turn, the electronics decrease engine power and brake the wheel on the inside of the corner to pull it around. Similarly, if the rear of the vehicle is sliding out of control the electronics will decrease power and brake one wheel to stabilize the vehicle in a straight line, and then allow it to turn if the driver desires.

This brake intervention and decrease in engine power is very noticeable in many vehicles. Get the vehicle a little out of line on a slippery road and it sometimes feels like the engine has died.

Acura uses the operation of the rear axle to accomplish much of the vehicle’s stability control. Instead of braking a wheel on one side of the vehicle to make it turn, the SH-AWD system accelerates the opposite rear wheel. This extra wheel speed converts into torque to one wheel, which is used to push the car back into line, all without a decrease in vehicle speed or performance.

Auto Tech: Acura’s Super Handling All wheel Drive how stuff works auto tech
Auto Tech: Acura’s Super Handling All wheel Drive how stuff works auto tech
Acura SH-AWD; photo courtesy Honda/Acura. Click image to enlarge

To achieve the directional stability, the rear axle must do two things: it must accelerate the rear wheels faster than the front wheels. Secondly, it must direct this speed to each axle independently. Let’s look at how the speed is increased first. A carbon-fibre driveshaft, used to decrease rotational inertia, transmits the front driveline speed into a planetary gearset located in the front of the rear axle. Two disc clutches and a one-way roller clutch are used to lock or hold components of this planetary gearset. In forward gears, the unit drives at a 1:1 ratio through the one-way roller clutch. If the vehicle starts to oversteer or understeer, the computer will use oil pressure from an internal rear axle pump to apply one of the disc clutches, causing the planetary gearset to operate in overdrive. The rear axle can turn up to five per cent faster than the front axle.

In reverse or during coast-down, the other disc clutch can apply so that the one-way clutch is bypassed, allowing for engine braking of the rear wheels.

The second unique part of the rear axle is actually two planetary gear units, one for each rear wheel. Each planetary gear unit is connected between the differential ring gear and one axle. An electro-magnetic clutch for each axle controls the rotation of the planetary gearset sun gear. By applying the clutch progressively, torque output to each axle shaft can be controlled. Using both the front to rear and side to side torque splitting ability of the rear axle, up to 70 per cent of the total engine torque can be applied to one rear wheel.

The SH-AWD system monitors throttle position, wheel speed, lateral acceleration and yaw rate to control the optimum driveline torque split front to rear and side to side. The acceleration feature of the rear axle works between 35 km/h and 120 km/h while the side to side torque splitting feature works at all speeds. On the road, the system definitely feels different than what we are used to. Get the vehicle out of line on a slippery road or corner and the car maintains the direction the driver intended with no performance loss or vehicle braking. That keeps the handling optimised for all driving conditions.

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