Quote:
Originally Posted by svxistentialist
In my layman's opinion it is not safe to suggest whether a car will understeer or oversteer based on the percentage drive to the front wheels in an AWD configuration.
It is usual for designers of FWD vehicles to set up the cars to understeer when driven beyond tyre adhesion primarily to curb over-enthusiastic driving in a safe fashion. It scrubs off speed and limits what a driver can do without the risk of spinning the car.
What I'm saying is the inherent tendency of most cars to understeer is a function of the chassis and suspension, not primarily the split of the power to the wheels.
In the case of the SVX the VTD gearbox by giving a higher percentage of power to the rear does tend to offset the natural tendency of the chassis to understeer.
Joe
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This is true because any car is going to be balanced so as to not loose the tail end, all things equal, if for no other reason than most cars have a heavy forward weight distribuition(well ok there are exceptions, think 1980s 911) . However, I think what he was trying to communicate was that the us SVX, as are all subarus that use the ACT-4 setup, handle very predictably at the limit and will not swing the tail around like in a RWD car, due to the fact that under engine power, the rear wheels will never
slip ahead of the front wheels In a car with a center differential, it is theoretically possible to direct all of the power to one axle or another, assuming there is no device to prevent such an occurance (obviously there is in the VTD). This can easily cause rear wheel speed that is greater than 100% of total front wheel speed, meaning that the speed of the propeller shaft is higher than that of the pinion shaft. Now let us all recall the big rule:
In a ACT-4 car, you will never have a propeller shaft that is greater than the speed of the front pinion gear. This is not mechanically possible. Therefore, to get the rear wheels to slip, the front wheels must also be slipping. In my 10 years driving subarus with this setup, I can tell you that this almost never happens unless you are trying. Quite honestly, I don't even think the SVX has the horsepower necessary to break the rear end loose on dry pavement, and even if it did, the transfer clutch in a stock setup is no where near strong enough to make this a possibility. However, with a valve body mod and some better friction discs in the transfer clutch, this thing can actually lock the front output to the rear, which makes it so the car is less stable. I can easily swing the rear of my car around nowadays if I get on a low friction surface because the open front diff will allow the inside front wheel to turn much ahead of the outer one, which makes the front end stay planted, but also causes the pinion speed to be equal to the propeller shaft speed in a conditon when the rear wheels have broken traction. Because of the LSD action at the rear I get both of them spinning and out comes the tail end in a classic oversteer fashion.
The SVX was always intended to debut the VTD system to the world, but at the time the US would not allow a car to be full time 4wd with no means to provide 2wd for emissions testing on a dyno, which made the VTD not even an option for a vehicle on US roads. Once this requirement was abolished, VTD came over to the US.
ANYWAY, back to the subject of this thread, I am really not sure that you can simply put in a solenloid and have it work correctly. It may work, but I doubt it will work just as good as it was intended, because the torque split duty ratio can not be the same between the two, or I cannot imagine them to be so. However, It can be assumed with a fair amount of certainty that both control units will behave in an identical manner when there is a difference in wheel speed between front pinion and rear propeller shaft. As a matter of fact, I am not even sure that there exists a complex logic for the vairable apply in the VTD unit as this would not seem to be necessary. And if there is, I would be surprised if it was as agressive. This is because in a
VTD unit you have
mechanical torque split that is fairly close to neutral. As a matter of fact, the slight rear bias would seem to be ideal for a tarmac setup, because of the weight shift to the rear. If I were to guess as to the programming in a vtd setup I would say that it probably has a map that tells the clutch to be at rest much more often than ours, but to apply under conditons of large throttle opening and low wheel speed, such as a full throttle launch, but to let the gears roam freely about their business under any conditon that is what you would see in a normal, casual drive.
In summary, I think that a TCU meant to operate a VTD setup, assuming that the solenoid is the proper one for that type would make the front wheel bias even more pronounced than it is with the US tcu.