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View Full Version : AWD cars and automatic vs. stick Q?


kanicbird
12-12-2005, 08:44 AM
Some AWD cars (all I have looked at except 1*) have a different AWD system depending on what type of transmission. The stick normally gets a reactive system with a viscous coupling, while the a/t gets a active computer controled system. Why is this?

Does the a/t talk to the drive system somehow on actually work together with the drive system in a way a stick shift would not be able?

* The one that I remember not having a different system I think is the Vibe, which just uses the viscous coupling system in both.

butler1850
12-12-2005, 11:02 AM
<WAG.>

I'd bet it's an issue of dollars. Those purchasing an automatic transmission are paying extra money for it. Along with the automatic transmission you get more computer power, which they leverage for the AWD system.

{from the Subaru Baja brochure}
Continuous All-Wheel Drive Baja Sport and Baja Turbo, equipped with 5-speed manual transmission, utilize a viscous-type locking center differential with torque
distribution normally configured at a 50/50-split front-to-rear. If wheel speed differs between front and rear axles, the system helps
distribute power to the wheels with the most traction.
Active All-Wheel Drive Baja Sport, equipped with 4-speed automatic transmission, utilizes an electronically controlled variable multiplate transfer clutch and viscoustype
limited-slip rear differential to distribute power to where traction is needed. Sensors monitor parameters such as wheel slippage, throttle
position and braking to help determine torque distribution to the wheels with optimum traction.
Variable Torque Distribution (VTD) Baja Turbo, equipped with 4-speed automatic transmission, utilizes an electronically controlled variable transfer clutch in conjunction with
a planetary-type center differential. Torque distribution is normally configured at a performance-oriented rear-wheel-biased 45/55-split
front-to-rear. Sensors monitor parameters such as wheel slippage, throttle position and braking to help determine torque distribution to
the wheels with optimum traction.

As you move up the ladder on these choices, each is more expensive.

</WAG>

Rick
12-12-2005, 11:33 AM
Haven't looked at Volvos have you? :D
At Volvo we use exactly the same system stick or auto on our cars. Allways have. We used to use a viscous coupling, but there are some issues with VCs. Like the system is very sentive to wheel sizes. So much so that the 4 tires had to be perfectly matched or you ran the risk of burning something out. With the electronicaly controlled system the syatem can sense the difference in tire size and it will accomondate it. Different tires present no problems.
I don't understand why a car maker would use both since now they will have to stock twice as many parts for repair. The only thing I can think of, is the VC is probably cheaper, and that allows them to hit a lower price point with the stick version.

asterion
12-12-2005, 12:08 PM
Yeah, the VC requires some thought on the part of the owner and better routine maintainence. Subaru claims, though I haven't verified this, that as little as a 1/4" difference between wheel sizes can cause problems with the VC--we're talking uneven wear here. All I know is that I'm very careful to get my tires rotated and to keep them properly inflated at all times to the same PSI, where I might have ignored a couple PSI difference between tires on a different type of car.