Last post on Mar 24, 2008 at 9:20 AM
You are in the Sedans
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Acura RL, Audi A6, BMW 5 Series, Cadillac STS, Cadillac CTS, Infiniti M35, Infiniti M45, Lexus GS 300, Lexus IS 350, Sedan
#18 of 102 FWD, RWD, AWD in older cars
Jun 19, 2006 (6:40 pm)
I have a related question, but with respect to older cars.
Context: I already have a nice 'summer' car, a RWD convertible. However, I live in a hilly part of NJ where we do have ~15 days/year with snowy/icy conditions in the winter. I need a car to drive ~6k miles/year including through the winter, and am considering a used car from mid-1990's.
Many cars of the vintage I am considering have ABS but are RWD. My question is, how much of a difference is exists between:
(1) FWD sedan, e.g. 1996-97 Audi A4
(2) RWD sedan (or coupe) WITH SNOW TIRES, e.g. MB 320E from 1994-95, BMW 6 series from 1985-1989
(3) AWD sedan, e.g. 1996-97 Audi A4 Quatro, BMW 325 ix from 1988-91, MB 320E 4Matic from 1994-1995
I'm hoping that the consensus is that while (3) and (1) are better than (2), the difference isn't large, and a careful driver of a RWD car with snow driver should be fine - but I'll wait for your expertise! Thanks,
#19 of 102 Re: FWD, RWD, AWD in older cars [carfast]
Jun 19, 2006 (8:48 pm)
3a: FWD "based" AWD are often front torque biased making then just as dangerous in/on adverse roadbed conditions as an actual FWD.
3b: RWD "based" AWD which are often torque biased to the rear leaving the majority of the front tires' roadbed adhesion factor available for life-saving directional control.
Look at the new Acura RDX SUV and RL sedan for a virtually perfect implementation of a modern day AWD system.
Or the GS AWD that runs a close second.
Subaru keeps advertising "Safety of AWD?...??
I ask you ALL, what or when is AWD safer or a safety factor.
Does FWD or even a 4X4 do anything more than get you up and going, moving, on a slippery surface, resulting in a false sense of security for many owners??
Isn't stearing, the ability to maintain directional control of the vehicle and/or quickly coming to a stop a lot more important..??
Jun 27, 2006 (6:03 am)
"Does FWD or even a 4X4 do anything more than get you up and going, moving, on a slippery surface, resulting in a false sense of security for many owners??"
I am sorry, but you seem to be stating your opinion rather than facts. The fact is that an AWD has engine power connected to all its wheels. Hence, it allows engineers to design additional features like anti-spin, anti-rollover etc. The BMW X_Drive is a perfect example. there is even a video on it on their website. In the context of a BMW, it is capable of rotating a single wheel more than the others when it encounters unequal spin in order to ensure proper steering. This has been designed to a point that the car does this until you are driving in the direction pointed by your steering column. The Acura RL has a similar patented system, as I am sure the GS-300 would too. These are FACTS !!!! I'm sorry, but I'd rather put my faith in the world's finest engineers than your opinion.
#21 of 102 Re: "adverse roadbed conditions" questions [wwest]
Jun 27, 2006 (8:15 am)
"3a: FWD "based" AWD are often front torque biased making then just as dangerous in/on adverse roadbed conditions as an actual FWD."
The language suggests that this is a quote from somewhere. Is it? Or are you an automotive engineer? I'm not being sarcastic, just wondering where the strong sense of conviction is coming from.
What do you (or what does the author of the quote) mean by "adverse roadbed conditions"?
Is the Audi Quattro in this category?
#22 of 102 Re: "adverse roadbed conditions" questions [sfcharlie]
Jun 27, 2006 (9:38 am)
I am most knowledgeable about the Toyota/lexus AWD systems derived from a FWD series vehicle, and somewhat knowledgeable of the Chrysler T&C minivan AWD.
Both of these are front torque biased ~95/5 but use different methods to get there.
The T&C had direct drive to the rear driveline and uses a VC, viscous clutch/coupling, in line, "in series", with that rear driveline. As long as the rear tires are turning at an equal rate with the front the VC remains "flaccid" and very little engine torque is coupled to the rear tires.
If the front, primary drive wheels, develop wheelspin/slip then the VC fluid "stiffens up" due to internal heating and begins to couple more of the engine torque to the rear.
The RX300 also uses a VC but in a slightly different way. The RX300 uses a standard "open" center differential but with differing final drive ratios front and rear so the front receives the clear majority of engine torque "natively".
In this case the VC is mounted across, between the two output shafts of the center differential. Here again a differing rotation rate at the front vs the rear results in the VC fluid stiffening up and increasingly "locking" the center differential depending on the duration and/or level of the disparate rotation.
As of 2004 the VC aspect was dropped from the system leaving the RX330, Highlander, and Sienna using only the MB ML's brake proportioning concept to re-allocate engine torque to the rear if front wheelspin/slip developed.
With the production of the RX350 the VC concept was re-introduced and I expect it will also be back in the Highlander and Sienna this fall.
So be aware, all AWD systems are NOT created equal.
FWD systems and front biased AWD can quickly become unsafe and even hazardous on an icy or snow packed, slippery, roadbed.
#23 of 102 Re: "adverse roadbed conditions" questions [wwest]
Jun 27, 2006 (10:00 am)
Thanks for that information. I love cars and am always eager to udnerstand better what makes one or another function in ways I have liked or not.
I ask about Audi because I imagine it (perhaps incorrectly) to be a FWD-based AWD system, but one which has seemed to me (a close friend had one when I lived in sn area with lots of snow and ice) amazingly able to keep traction, and also one which is described as able to keep traction in adverse conditions.
Is Audi, so far as you understand it, doing somethign different than what you are critical/cautious about?
Much has been made about the Infiniti M35's RWD-biased AWD system, but I've not read any comparative review suggest one should expect it to do better in ice/snow/rain than Audi.
#24 of 102 Re: "adverse roadbed conditions" questions [sfcharlie]
Jun 27, 2006 (1:49 pm)
Other than "blind" agreement with you about the Audi being derived from FWD I can't add anything. I certainly wouldn't go out and buy one until confirming that the AWD version is rear torque biased.
If the Infiniti weren't so horribly ugly IMMHO (Bull-nosed...?) I would seriously consider it along with the RDX or X3.
But I think, truly believe, that Honda/Acura's SH-AWD system is going to set the future standard for AWD.
#25 of 102 Re: "adverse roadbed conditions" questions [sfcharlie]
Jun 27, 2006 (2:32 pm)
The AUdi QUatro all-wheel drive system is different than the others out there in that it is entirely mechanical. In normal mode, the drive is distributed (until this year) equally to each wheel (they've modified it slightly for this year to bias it to 40/60 F/R).
It is a mechanical system thatuses a central TORSEN (torque sensing) differential - all mechanical, instantaeous reaction - not computer trying to catch up. It is capable of putting nearly all power to an individual wheel or axle.
If it says Quatro, it is currently mechanical. If it says all-wheel, it may not be mechanical.
If you do a search, there is a video clip done where they had 5 (I think) different all-wheel drive cars try to climb a ski slope. Only one car made it to the top - Audi. Neither the BMW, MB, nor the others tested made it all that way up. Quatro DOES work - Audi has "owned" the Mt Washington race and Pike's Peak race for most of the years for awhile.
I replaced my last car, an Audi A6 Quatro with an M35x. It is basically a rear-wheel drive car that can (nearly) instantly apply up to about 50% of the power to the front wheels as well. So far (we didn't get much snow last year, nor was I home much to drive it) it has done a decent job, but I think the Quatro system was/is better. Still, the Infiniti seems pretty close.
#26 of 102 Re: "adverse roadbed conditions" questions [james27]
Jul 25, 2006 (6:51 pm)
Not too much current participation on this forum but your explanation is almost 100% correct.
The Audi TT, alone, uses the Haldex AWD mechanicals -- it is NOT TorqueSensing.
The TorSen system is entirely mechanical and it is able to act "instantaneously" -- it does not require slippage of any wheel to cause a shift of power. X-drive is "nearly instantaneous" and while I personally would not be troubled by X-drive's reactionary nature, it is NOT literally instantaneous. Some would argue but it is in the larger scheme of things little more than a nit. Given the choice, however, I'd take TorSen.
TorSen as employed in almost all Audis remains to this day nominally a method of splitting torque 50 50 f/r. More and more of the Audi family is offering the newest iteration of quattro -- and it is RWD biased (40 60 f/r.) Frankly, I find this change, given the truly instantaneous capability to shift torque of the TorSen system, to be the result of Audi giving in to "marketing" pressure.
It would be far more impressive if Audis were better balanced in terms of weight than RWD torque biased -- but it makes for less engaging ad copy.
Audi has either "among" the best if not tied for best AWD implementation with Rear biased TorSen AWD these days.
Yet, for years, Audi has offered AWD that was/is 50 50 biased and has time and again demonstrated its leadership in AWD systems.
Now, in the real world of highway driving, I submit that -- in order -- the optimum drive layouts are: AWD, FWD and RWD. Now of course this statement is likely stir controversy and, for once, that is not my purpose.
Under today's Urban and Sub-urban conditions and traffic, FWD is able to be driven without the concern of over-steer.
RWD can become tail happy; and, for most folks understanding that the way to bring an understeering auto back under control requires "turning into the skid" is a concept that, these days, is unlikely to have been taught by dad, mom or the school driver's ed instructor.
Understeer, on the other hand, can respond to increasing the turn of the wheel in the direction one wishes to go (which is the opposite of what will right an oversteering car -- and therefore counter intuitive.) Further, applying the brake during the onset of understeer serves the purposes of "shifting the load" to the wheels that are attempting to steer the car (assuming lock up does not occur, which with most cars ABS systems is increasingly unlikely) AND slowing the car enough to reduce the tendency of the car to continue understeering (2, 2, 2 benefits in one.)
If one is driving a RWD car in such a fashion that it begins to oversteer, simply applying the brake may, as it reduces the load from the rear tires, increase the "tail wagging" (oversteering) behavior. Further the inherent reaction to a car that is oversteering left may actually be to turn the wheel to the right, thus exacerbating the skid. If the car is "apparently skidding left" many folks will not "naturally" turn the wheel left as they do not associate turning the wheel left with "making the car turn right" thus countering the apparent direction of the skid.
FWD for the vast majority of drivers on the Interstates, primary and secondary ROADS and Highways is almost benign. RWD can be perceived as more difficult to control.
AWD, even Volvo's and Acura's RL (95/5 f/r torque split) are capable, at least, of keeping the hapless driver from getting into trouble better than an RWD system alone.
Traction control mitigates this, somewhat, of course.
But, oddly, the main advantage that RWD has over FWD is the ability for the manufacturer to put ever higher HP and torque to the driven wheels. This higher HP and torque may tend to facilitate the car's rear end to break traction and induce tail wagging (a form of oversteer, more or less.)
FWD seems to have come to the US in the fuel crisis of the 1980's as a way of packaging. As horsepower and torque increased over the years, the ability to put power through the front wheels alone became more and more problematic -- torque steer is not a problem in a relatively low HP and Torque environment, but with many of today's high output engines, FWD can be limiting.
For years, Audi engineers believed 200HP to be the upper limit for a FWD chassis.
The highest performing Audis and Porsches are AWD. Virtually every Mercedes is offered in AWD and the 7 series BMW will be joining the 3 and 5 series in its next generation due out soon.
Lambos highest performing autos, too, are -- yep -- AWD.
Today, AWD (and another old technology -- diesel) is seen as the high performance choice and several mfgrs, Audi notably, have made a career out of mastering it.
If you must have 2WD, make it FWD -- especially if you live anywhere that has "weather." Otherwise, go for the highest performance and the extra measure of control and fun that is part and parcel of most of the AWD systems on the market today.
Although NOT literally true, for many folks, RWD vehicles can require either skills or electronic assists (ESP for instance) that are NOT yet universal. FWD is both more pragmatic/prudent and safe than an otherwise identical RWD variant.
Nothing of my remarks disputes some of the obvious RWD advantages when driven at over 9/10ths and/or by a skilled driver, used to "power steering" an oversteering (RWD or AWD) car. In most of the driving we mere mortals encounter, FWD is better suited to the job -- not a better drive system.
AWD, on the other hand, is best suited to almost all jobs and is a better drive system.
If you can, get at least a 50 50 system; yet, don't sweat if it is X-drive versus quattro unless you plan to compete and even if you end up with a Volvo S60 type R with its 95% front bias, you are very unlikely to ever suffer any issues due to the nominal torque split.
Instant response is better than split second reaction, however, no matter what.
TorSen is not as widely utilized due to its weight and extra cost -- many folks think it remains almost without peer and from a practical standpoint, without peer -- period.
#27 of 102 Re: "adverse roadbed conditions" questions [markcincinnati]
Jul 30, 2006 (4:52 am)
Odd remarks, yours, about RWD's purported "tail wagging" tendency. I've driven a BMW 330i (sport pkg & 5-speed manual) under all sorts of conditions & have never experienced this. (I do not, though, go out in this car when snow or ice is on the road; I have a 4WD vehicle for that sort of thing.) Moreover, I've been frequenting these boards since 1998 & can't recall anyone describing a "tail wagging" incident. Have you personally experienced this? If so, please post the details: make & model, road conditions, etc.