Moderator note: this thread is a continuation of the astray 4WD portion of the discussion from this thread
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Stability control systems don't beat the laws of physics, but what they do is work within them using a few techniques. You're incorrect regarding 30 mph. The systems work at any speeds and help to correct the vehicle to its intended path. Put it this way, you will be far better of with the system than without. Not all stability control systems are the same though. Some are better than others. The systems on MB and BMW vehicles (made by Bosch or Conti-Teves) are considered the most sophisticated on the market today, probably because MB invented stability control back in the early 1990's.
VTM-4 is quite simple really, in a manner of speaking. It uses the ABS wheel speed sensors to detect how fast the vehicle is going, and other sensors (ex. throttle position sensor) to detect if you're going from a standstill etc. From a dead stop, some power is routed to the rear (the rear differential clutch packs are energised to a certain extent) so that you minimise front wheelspin. Acura has said that the system will keep re-routing some power to the rear wheels if you are accelerating, at least up to a certain speed.
Once you reach this certain speed - Acura has never mentioned this threshold to my knowledge - the VTM-4 differential starts disengages completely leaving you with FWD only. The idea here is to save wear and tear on the clutch packs in the rear diffy (clutch packs do tend to wear out over time) and to also reduce some power transfer losses, as the vehicle is already in motion. Using these same ABS wheel speed sensors, the vehicle can compare the rotations of each individual wheel and if one starts to spin a preset limit faster than the other, this is interpreted as slippage and power is re-routed to either axle; this should work at all speeds.
What is interesting is that the MDX's AWD system does not have a centre differential. This means that it has to at least disengage one wheel for it to work in turns (or you will get binding), so you basically get 2WD or 3WD in turns on surfaces with a high friction co-efficien. On snowy/icy surfaces, there is probably enough slippage for the system to remain active, just as you can leave part-time 4WD systems engaged on slippery surfaces.
There are a few cons with this system, versus a permanent 4WD system with stability control. First of all, you don't get the benefit of permanent AWD since this is only a full-time part-time AWD system. This means that in turns, it is easier to overwhelm the tires' limits (causing slippage and understeer/oversteer) since the power is usually only directed to one pair of wheels.
With a permanent system, you are working far below the tires limits since the same amount of power is being routed to all four wheels, and not just two. This is why AWD vehicles can put down the power a lot quicker (hence accelerating far quicker and earlier) out of the apex than 2WD vehicles can.
VTM-4 does not know when you are sliding because it doesn't have any yaw and or steering angle sensors to tell it if the vehicle is not following the driver's intended path. Therefore it connect help at all. Not only this but even if it could help, because it is a torque-on-demand system, it requires that you are using the throttle (and hence getting the engine to produce enough torque) for it to be active. If you let go of the throttle, it goes back into its freewheeling (probably FWD) mode. There is also so much that reducing power can do. Stability control systems brake specific wheels to control directional stability and intent.
Another thing too...a spinning tire (when a tire spins, it means that it is overwhelmed) actually has <b>less</b> traction than a tire that is near the edge of its tractive limit. This is where traction control comes in. By braking the wheel and keeping it at its optimum limit, you get maximum traction, sort of similar to ABS. Spinning the wheels are useful when you are trying to build up your momentum. This is why most traction control and stability control systems have defeat/off switches.
As far as off-roading goes, traction control is also much more environmentally friendly as it doesn't allow for the same kind of soil erosion that a normal "spinning wheels" 4WD system does. Off-roaders don't like systems that can transfer power back and forth between axles because if you are in a delicate situation and inching your way past/through obstacles, the sudden unpredictable torque transfer to the opposite axle can be disastrous. Permanent or part-time lockable 4WD systems are ideal (as are long travel throttle pedals for much greater throttle control), since they have fixed torque split ratios.
The MDX's VTM-4 system is not unique. The Isuzu Trooper uses a similar system (and with a cool dash display) made by the same company, Borg-Warner. The major difference is that the Isuzu is primarily RWD whereas the MDX is primarily FWD. The Trooper's system can also be locked 50/50 like a traditional part-time 4WD system.
Whew...'nuff said. This has got to be one of the lengthiest post that I've seen on this board ;-)
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Grip has to do with the tires. That's why those are crucial and you don't cheap out on tires. Gravity pulls you downwards, you wouldn't want to overcome it anyway ;-).cardingtr said:
Stability control systems don't beat the laws of physics, but what they do is work within them using a few techniques. You're incorrect regarding 30 mph. The systems work at any speeds and help to correct the vehicle to its intended path. Put it this way, you will be far better of with the system than without. Not all stability control systems are the same though. Some are better than others. The systems on MB and BMW vehicles (made by Bosch or Conti-Teves) are considered the most sophisticated on the market today, probably because MB invented stability control back in the early 1990's.
VTM-4 is quite simple really, in a manner of speaking. It uses the ABS wheel speed sensors to detect how fast the vehicle is going, and other sensors (ex. throttle position sensor) to detect if you're going from a standstill etc. From a dead stop, some power is routed to the rear (the rear differential clutch packs are energised to a certain extent) so that you minimise front wheelspin. Acura has said that the system will keep re-routing some power to the rear wheels if you are accelerating, at least up to a certain speed.
Once you reach this certain speed - Acura has never mentioned this threshold to my knowledge - the VTM-4 differential starts disengages completely leaving you with FWD only. The idea here is to save wear and tear on the clutch packs in the rear diffy (clutch packs do tend to wear out over time) and to also reduce some power transfer losses, as the vehicle is already in motion. Using these same ABS wheel speed sensors, the vehicle can compare the rotations of each individual wheel and if one starts to spin a preset limit faster than the other, this is interpreted as slippage and power is re-routed to either axle; this should work at all speeds.
What is interesting is that the MDX's AWD system does not have a centre differential. This means that it has to at least disengage one wheel for it to work in turns (or you will get binding), so you basically get 2WD or 3WD in turns on surfaces with a high friction co-efficien. On snowy/icy surfaces, there is probably enough slippage for the system to remain active, just as you can leave part-time 4WD systems engaged on slippery surfaces.
There are a few cons with this system, versus a permanent 4WD system with stability control. First of all, you don't get the benefit of permanent AWD since this is only a full-time part-time AWD system. This means that in turns, it is easier to overwhelm the tires' limits (causing slippage and understeer/oversteer) since the power is usually only directed to one pair of wheels.
With a permanent system, you are working far below the tires limits since the same amount of power is being routed to all four wheels, and not just two. This is why AWD vehicles can put down the power a lot quicker (hence accelerating far quicker and earlier) out of the apex than 2WD vehicles can.
VTM-4 does not know when you are sliding because it doesn't have any yaw and or steering angle sensors to tell it if the vehicle is not following the driver's intended path. Therefore it connect help at all. Not only this but even if it could help, because it is a torque-on-demand system, it requires that you are using the throttle (and hence getting the engine to produce enough torque) for it to be active. If you let go of the throttle, it goes back into its freewheeling (probably FWD) mode. There is also so much that reducing power can do. Stability control systems brake specific wheels to control directional stability and intent.
Another thing too...a spinning tire (when a tire spins, it means that it is overwhelmed) actually has <b>less</b> traction than a tire that is near the edge of its tractive limit. This is where traction control comes in. By braking the wheel and keeping it at its optimum limit, you get maximum traction, sort of similar to ABS. Spinning the wheels are useful when you are trying to build up your momentum. This is why most traction control and stability control systems have defeat/off switches.
As far as off-roading goes, traction control is also much more environmentally friendly as it doesn't allow for the same kind of soil erosion that a normal "spinning wheels" 4WD system does. Off-roaders don't like systems that can transfer power back and forth between axles because if you are in a delicate situation and inching your way past/through obstacles, the sudden unpredictable torque transfer to the opposite axle can be disastrous. Permanent or part-time lockable 4WD systems are ideal (as are long travel throttle pedals for much greater throttle control), since they have fixed torque split ratios.
The MDX's VTM-4 system is not unique. The Isuzu Trooper uses a similar system (and with a cool dash display) made by the same company, Borg-Warner. The major difference is that the Isuzu is primarily RWD whereas the MDX is primarily FWD. The Trooper's system can also be locked 50/50 like a traditional part-time 4WD system.
Whew...'nuff said. This has got to be one of the lengthiest post that I've seen on this board ;-)
Nice try troll, but the VTM-4 system is not a "normal 'spinning wheels' 4WD system". When you are in a "delicate situation and inching your way past/through obstacles" as you put it, there is no "sudden unpredictable torque transfer to the opposite axle" cuz VTM-4 has a Lock feature. I know this from personal experience, not from what I've read on the ML forum 
