All wheel drive, four-wheel drive, is it really necessary?
#256
Lexus Test Driver
Thread Starter
(not my footage, sorry for the crappyness)
i eventually got it out but in the process just about every single warning light came on (transmission problem, adaptive cruise control problem, SH-AWD problem, braking system problem, FCW problem)
the car actually wouldn't let me put it into P, have really any power at all... i had to turn it off and back on again before it would even let me select a gear! and i was literally just defending the capable AWD system of the MDX to my friend who had just scoffed at it...
#257
Lead Lap
#259
Lead Lap
I think this video was what I was referring to as part of the issue with AWD systems. Its basically a system that experiments in low friction environments.Loss of traction takes a bit of time to register to the system. Whereas an full time 4wd system, forward momentum is always available
funny you bring this up, i too remember seeing this clip back when the MDX really was (in my view) king of the midsize luxury 3 row suv market, which is why i was quite surprised when i got my mom's 2016 MDX stuck in some mild snow the other night
https://www.youtube.com/watch?v=mUs9urbaMFE
(not my footage, sorry for the crappyness)
i eventually got it out but in the process just about every single warning light came on (transmission problem, adaptive cruise control problem, SH-AWD problem, braking system problem, FCW problem)
the car actually wouldn't let me put it into P, have really any power at all... i had to turn it off and back on again before it would even let me select a gear! and i was literally just defending the capable AWD system of the MDX to my friend who had just scoffed at it...
https://www.youtube.com/watch?v=mUs9urbaMFE
(not my footage, sorry for the crappyness)
i eventually got it out but in the process just about every single warning light came on (transmission problem, adaptive cruise control problem, SH-AWD problem, braking system problem, FCW problem)
the car actually wouldn't let me put it into P, have really any power at all... i had to turn it off and back on again before it would even let me select a gear! and i was literally just defending the capable AWD system of the MDX to my friend who had just scoffed at it...
#261
Lexus Champion
I think this video was what I was referring to as part of the issue with AWD systems. Its basically a system that experiments in low friction environments.Loss of traction takes a bit of time to register to the system. Whereas an full time 4wd system, forward momentum is always available
#262
Lexus Champion
iTrader: (3)
I think this video was what I was referring to as part of the issue with AWD systems. Its basically a system that experiments in low friction environments.Loss of traction takes a bit of time to register to the system. Whereas an full time 4wd system, forward momentum is always available
#263
Super Moderator
#265
Super Moderator
Elements of EM
Both EM clutches and brakes share basic structural components: a coil in a shell, also referred to as a field; a hub; and an armature. A clutch also has a rotor, which connects to the moving part of the machine, such as a driveshaft.
The coil shell is usually carbon steel, which combines strength with magnetic properties. Copper wire forms the coil, although sometimes aluminum is used. A bobbin or epoxy adhesive holds the coil in the shell.
Activating the unit’s electric circuit energizes the coil. The current running through the coil generates a magnetic field. When magnetic flux overcomes the air gap between the armature and field, magnetic attraction pulls the armature — which connects to the hub — into contact with the rotor.
Magnetic and friction forces accelerate the armature and hub to match rotor speed. The rotor and armature slip past each other for the first 0.02 to 1.0 sec until the input and output speeds are the same. The matching of speeds is sometimes called 100% lockup.
Both EM clutches and brakes share basic structural components: a coil in a shell, also referred to as a field; a hub; and an armature. A clutch also has a rotor, which connects to the moving part of the machine, such as a driveshaft.
The coil shell is usually carbon steel, which combines strength with magnetic properties. Copper wire forms the coil, although sometimes aluminum is used. A bobbin or epoxy adhesive holds the coil in the shell.
Activating the unit’s electric circuit energizes the coil. The current running through the coil generates a magnetic field. When magnetic flux overcomes the air gap between the armature and field, magnetic attraction pulls the armature — which connects to the hub — into contact with the rotor.
Magnetic and friction forces accelerate the armature and hub to match rotor speed. The rotor and armature slip past each other for the first 0.02 to 1.0 sec until the input and output speeds are the same. The matching of speeds is sometimes called 100% lockup.
To speed responses, some EM clutches and brakes use overexcitation. The unit’s power supply gives the coil a burst of voltage significantly higher than its nominal rating for a few milliseconds. Higher voltage lets the coil generate a more-powerful magnetic field more quickly, starting the process of attracting the armature and accelerating or decelerating the load.
Three times the rated voltage typically gives around one-third faster response. Overexcitation of 15 times the normal coil voltage produces responses three times faster. For instance, a clutch coil rated for 6 V should be overexcited to 90 V to cut response time to one-third of the original.
Once overexcitation is no longer needed, the power supply returns to its normal operating voltage. Overexcitation can be repeated as needed, but the high-voltage bursts must be short enough that they do not overheat the coil.
Three times the rated voltage typically gives around one-third faster response. Overexcitation of 15 times the normal coil voltage produces responses three times faster. For instance, a clutch coil rated for 6 V should be overexcited to 90 V to cut response time to one-third of the original.
Once overexcitation is no longer needed, the power supply returns to its normal operating voltage. Overexcitation can be repeated as needed, but the high-voltage bursts must be short enough that they do not overheat the coil.
The bottom line is, anything that involves changing the speed or torque delivery of something with significant inertia does not happen in nanoseconds. Force can certainly be applied that quickly, as in the case of EM clutches. But overcoming inertia still takes at least milliseconds.
Last edited by geko29; 01-13-18 at 06:44 AM.
#266
Lexus Fanatic
#267
Lexus Fanatic
Yes and no. It depends on how the center-differential and torque-management is set up....and whether power flows to all four wheels, all of the time, or whether it goes to the two primary drive-wheels, and shifts to the other wheels only when the primary wheels spin.
#268
Lexus Fanatic
mmarshall, it does not depend, they are not the same. Do not spread false information. A full time 4WD and part time 4WD have a transfer case that can lock in addition to a LO gear. These Subaru's do not. And for the record, full time 4WD is determined because it has a LO range gear and a center lock diff. None of these AWD set ups have it. The Escalade once had AWD with no LO gear, it was always referred to as AWD, typically GM cost cutting. No center lock either.
Last edited by Toys4RJill; 01-13-18 at 08:49 AM.
#269
Lexus Fanatic
mmarshall, it does not depend, they are not the same. A full time 4WD and part time 4WD have a transfer case that can lock. These Subaru's do not. And for the record, full time 4WD is determined because it has a LO range gear and a center lock diff. None of these AWD set ups have it. The Escalade once had AWD with no LO gear, it was always referred to as AWD, typically GM cost cutting.
#270
Lexus Test Driver
Thread Starter
https://re.reddit.co/r/explainlikeim...drive_and_all/
i'll leave this here, he does a pretty good job of explaining the difference. didn't think there was still this much of a debate between the two systems
i'll leave this here, he does a pretty good job of explaining the difference. didn't think there was still this much of a debate between the two systems