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Double Lucky! New AWD Transmission also fixes front-end vibration issues

6 messages,  Last post on Nov 02, 2010 at 10:34 AM

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#4 of 6 Re: Double Lucky! New AWD Transmission also fixes front-end vibration issues [yatesjo] by wwest

Nov 02, 2010 (9:17 am)

Replying to: yatesjo (Nov 01, 2010 6:49 pm)
My guess is that NO ONE would question the UNSAFE aspect of RWD when too much engine torque is used for current roadbed conditions, spin-out, skidding, over-steering, etc.
 
So what happens with FWD or F/awd (FRONT torque biased awd system) when too much engine torque is used for current roadbed conditions...??
 
TOTAL loss of directional control, that's what.
 
ALL modern day FWD and/or F/awd systems have hyper-active Traction Control systems that INSTANTLY applies braking to most quickly return to front traction, and also INSTANTLY dethrottles the engine, FULLY dethrottles the engine, upon detection of even the slightest level of front wheelspin/slip.
 
Many FWD and/or F/awd (Cadillac SRX) with high output engines also pre-emptively detune/derate the engine using DBW "gain" in the lower gear ratios. Some of these even PREVENT a downshift if it would/could/might result in the driver's ability to apply too much drive torque.

#5 of 6 Re: Double Lucky! New AWD Transmission also fixes front-end vibration issues [yatesjo] by wwest

Nov 02, 2010 (9:43 am)

Replying to: yatesjo (Nov 01, 2010 6:49 pm)
"..I still prefer AWD if I have to drive in the stuff.."
 
It is now no longer satisfactory to use the simplistic AWD term.
 
Even if you say/use F/awd there are a myriad of designs, with widely differing functionality, in the marketplace today.
 
Reactive...
 
A) TC only. Basically a ONE-Wheel drive system, three open differentials, with TC activation to ENFORCE, as a LAST resort, at least some level of AWD functionality. With these systems (Highlander, RX330, etc.) braking, INSTANT braking, is used to virtaully instantly return traction for directional control to the front wheels while sustaining engine torque to a fairly high level and simultaneously reapportion engine torque to the rear wheels.
 
All well and good..
 
And that would be THAT if the driver could be relied upon to either release or "feather" the gas pedal upon the onset of TC activity. The driver doing so would thereby alleviate the possibility of brake component overheating and subsequent damage, rotor warping, etc.
 
But the driver CANNOT be trusted...
 
So, simultaneously with braking from TC activation the engine is dethrottled, FULLY dethrottled, if not all the way down to idle then very closely thereby.
 
Basically while the TC type F/awd system has now been made somewhat functional it must now be CASTRATED.
 
So nowadays many of these reactive TC enforced F/awd systems have a TC "off" capability for use in those situations wherein a "true" AWD system would undoubted suffice.
 
B) VC, Viscous Clutch (99,00 RX300, F/awd Chrysler T&C, etc.) With sustained wheelspin/slip, front vs rear, the VC liquid heats up and since it cannot expand (hermetically sealed fixed volume case) its effective density increases thereby increasing the coupling between the two interleaved clutch disk sets.

#6 of 6 Pre-emptive F/awd. by wwest

Nov 02, 2010 (10:34 am)

Pre-emptive....??
 
So, ask yourself, when is my FWD or F/AWD most likely to become unsafe via loss of traction on the front, DRIVEN, wheels, driving on a slippery, low traction, surface...
 
The answers are...
 
A) Low speed acceleration, moreso with increased, increasing, engine HP/torque availability/use...
 
Example: Taurus SHO with a TwinForce 350 HP twin turbo V6, available as F/awd ONLY.
 
B) Turning, turning tightly.
 
HIGH lateral force, centrifical force, uses up the majority, if not all, or even beyond, front tire traction coefficient.
 
C) Accelerating while turning, turning tightly, accelerating rapidly into a turn.
 
Most new F/awd vehicles (even as far back as my '01 F/awd RX300), and ALL modern FWD vehicles, will not allow you to accelerate rapidly while turning tightly, engine dethrottling INSTANTLY. Often requiring a COMPLETE release of the gas pedal to first overcome the dethrottling effect.
 
Now, we ALL know about the caution that must be used while driving a TRUE 4WD vehicle on a reasonably tractive surface...
 
DAMAGE to the drive train may result from use of the 4WD system on a tractive surface.
 
So what is a design engineer to do when stuck by the beancounters with the manufacturing and production efficiencies of a base FWD and therefore the need to improve the safety factor of FWD via converting it to F/awd..?
 
The reactive F/awd systems have now quite clearly proven themselves to be inadequate to the "AWD" task.
 
There is, today, no easy nor inexpensive system for implementing a method for "forecasting" the onset of wheelspin/slip. Other than Ford using the OAT to modulate, moderate, the level of regen braking to be used on their hybrid series I know of no one.
 
So, what is that engineer, engineering team, to do other than ignore the 4WD edict against drive train stress, OVER-stressing, issues but only under the circumstances wherein loss of traction, and the resulting loss of directional control, is most likely.
 
The Honda/Acura SH-AWD system, for instance.
 
A) Automatically engage the rear drive, distributing at least a fractional portion of the high engine torque to the rear during low speed acceleration and thereby avoid, maybe avoid, the need to automatically detune the engine using DBW throttle "gain" in the lower gear ranges.
 
B) Automatically engage the rear drive when turning, increasing the rear coupling coefficient incrementally as the turn tightly or with higher and higher throttle openings.
 
Note that engaging the rear drive while turning, turning tightly, and/or while accelerating is the WORSE possible time insofar as driveline stress, driveline windup, tire scrubbing, on a tractive surface. But what other choices does an engineer have when stuck with the basic flaws of a FWD vehicle..?
 
So after ten years of experimentation, PUBLIC use experimentation, Ford is now using the Escape's Pre-emptive F/awd across the FWD product line. Just ignore those PTO/PTU failures.
 
And Acura..? Just ignore those transaxle failures.
 
At least Ford has now learned enough to add cooling to/for the 2011 Ford Explorer's F/awd PTO/PTU.
 
FWD(????) Ford Explorer, F/awd optional. End of the line for the Ford Explorer, like the Taurus X before it and the FreeStyle before that.

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