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2006 BMW X3 vs Acura RDX

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Comments

  • bodble2bodble2 Posts: 4,519
    "Do you mean since the RDX is FWD biased why isn't it subject to understearing..."

    Yes, that's what I meant. And thank you for your explanation. :)
  • varmintvarmint Posts: 6,326
    Given that the front brakes must be quite a bit "beefier" than their rear counterparts and it is some generally accepted that the front brakes must account for ~80% of the braking HP I suspect the weight shift for hard acceleration, while certainly not anything even close to 100%, will be substantially more than 10% - wwest

    That 80% figure is typically applied because most vehicles in our market are something like 60% nose-heavy before you even add the weight shift. And, unless we're talking about track cars, most braking systems will apply far more decceleration force than the engine can provide in terms of acceleration force.

    "so, there's no reason to have 100% of the torque sent aft..." - Varmint

    Not unless you're accelerating into a hard/tight turn wherein the front tires' contact patch is best reserved for lateral control. - wwest


    This line of discussion was started when you commented on my remarks about straight line acceleration. My follow-up remarks held to that line. Obviously cornering is a different scenario, requiring different dynamics and I wrote as much in my original post on the topic.
  • varmintvarmint Posts: 6,326
    "And how would you suggest that ESC (VSC, PSM, etc.) prevent engine braking from becoming hazardous, raise the engine RPM to the "sweet spot", or maybe kick the tranny into neutral?"

    I suspect he means that stability control can treat the symptoms (straighten the car out), not cure the disease at its source.
  • corvettecorvette United StatesPosts: 4,085
    My understanding of the way an automatic transmission works is that the torque converter will slip and allow the wheels to turn at the road speed of the car when necessary. With a manual, it's a little more tricky--you have to push in the clutch if the drive wheels are being engine braked, or else they will cause understeer (FWD) or oversteer (RWD). I know that Volkswagen specifically advertises the ability of their ESC system to compensate for this problem--I don't think it is advertised with other manufacturers, but the capability probably exists with other brands.

    With oversteer, you generally have a split second to correct the problem before the car swaps ends and you no longer have any control over it. With understeer, it's more likely that you have a few seconds to correct the problem before you hit anything due to understeering.
  • wwestwwest Posts: 10,706
    Assuming that the engine compression braking on the front wheels is having an adverse effect:

    If understearing is the case then the ESC, VSC, PSM, etc, will likely apply braking at the rear to slow the vehicle so as to regain some of the lost traction at the front. The VSC system description for my 2001 AWD RX300 indicates that it will apply braking to both rear wheels in this instance. I have no doubt that some systems will selectively brake only one rear wheel depending on the driver "assist" direction required.

    If overstearing, most systems will brake the outside front wheel, or in the case of brakes already having being applied "unbrake" the inside wheel, to help prevent the rear from swinging out in the direction outside the desired turn radius. Note that this can only be done momentarily, briefly, should the vehicle "pendulum" continue to swing beyond the centerline of the vehicle's "moment".

    I think it's pretty clear that in the case of engine compression braking at the front the more likely scenario is overstearing. In that case the stability system clearly CANNOT unbrake the inside front wheel (absent raising the engine RPM) and additional braking on the outside wheel would only serve to exacerbate the condition.

    The obvious solution is for the stability system to provide additional braking at the rear to "balance" the overall braking dynamics. The exact reason drag chains are often required on the rearmost axle for tractor-trailer rigs here in WA when our mountain passes are in wintertime conditions.

    The problem with that is that once the stability system activates the rear has already broken traction and has begun to swing around and therefore braking the rear wheel would likely, again, only exacerbate the condition.

    With drag chains most truckers prevent this by going to the brakes on those wheels with drag chains first, initially, when they need to apply braking in wintertime conditions.

    So the only real answer is the one recommended by the AAA, quickly shift the tranny into neutral unless you have clutch.

    Come to think about it the stability control system could be programmed to do that, shift the tranny in neutral, as an automatic response to the circumstance.

    In the meantime it appears that Toyota/Lexus is doing everything they can, using ASL, within the firmware controlling their FWD and front biased AWD transaxles to prevent incidents of loss of direction control due to engine compression braking.
  • corvettecorvette United StatesPosts: 4,085
    I think it's pretty clear that in the case of engine compression braking at the front the more likely scenario is overstearing.

    I would have guessed understeer for engine braking via the front wheels--at least that's what I've experienced. I would think it would be like having a car without ABS and locking the front wheels up--you can't steer, but the car tends to keep going in a straight line.
  • wwestwwest Posts: 10,706
    First, the statement addresses engine compression braking therefore a "coastdown" circumstance pretty exclusively.

    But no, that's why rear ABS become available long before full function ABS. Note that with over-stearing your rear wheels always "point" in the direction you wish the vehicle to travel, so if they can be kept rolling....

    That's also one of the more serious problems with snow chains on only the front of a FWD vehicle, and level of braking will be "servere" as a result of the inordinate level of traction at the front vs the rear, oftentimes resulting in the relatively "unbraked" rear desiring to lead the way forward.

    Even your owners manual will advise you that it is extemely undesireable to have higher traction on the front versus the rear, often resulting in loss of directional control.

    That's why you are always advised to install newer, higher traction, tires on the rear, not the front. It's also the reason no tire shop will install studs on only the front tires and not the rear also.
  • I am new at this but the basic dif btwn the VTM-4 and SH-AWD is the diffs between the rear right and left wheels eh?

    How do you reconcile the GM Versa Trac technology when compared to the SH-AWD and or the VTM-4?

    Is there a cross compare type article available somewhere?
  • robertsmxrobertsmx Posts: 5,525
    I have seen that "driver included" weight distribution published by Honda before... S2000. The car without driver is said to have a weight distribution of 48/52 (or was it 49/51?) but a perfect 50-50 with an "average sized" driver (whatever that means. Actually, Honda claimed, it was 25-25-25-25 (if you consider the "corners").
  • robertsmxrobertsmx Posts: 5,525
    VTM-4 is pro-active as well as reactive. Under cruising conditions, only the front wheels are driven, until slippage is detected. Use of throttle will engage the rear wheels too even if there is no slippage (so in a start from standstill, all wheels are powered). The torque distribution is from front to rear and continuously variable.

    SH-AWD is a permanent version of VTM-4 with a few more features. All wheels are powered at all times. So, the vehicle is never FWD (or RWD) only at any time. Like VTM-4, torque transfer between front and rear axles is continuously variable. In addition to that, SH-AWD also incorporates side-to-side torque variance on rear axle depending on situation (cornering). And it is also equipped with an "acceleration device" (as in RL, which can "speed up" the outer wheel during cornering under power by as much as 5% compared to the inside wheel).

    GM's Versatrek (I think it is spelled that way) is similar to Honda's Real Time AWD. It is FWD until a slip is detected when the rear wheels are engaged. So, it is a 100% reactive system.
  • corvettecorvette United StatesPosts: 4,085
    So, it is a 100% reactive system.

    This sounds like a poor design... If the wheels slip, my instincts will cause me to let off the gas anyway, which would put it back into FWD-only mode, since the slipping would cease at that point.

    This sounds like something you would have to pore through all of the manufacturers' advertising materials to figure out for each vehicle.
  • Thanks Robertsmx,

    Now it makes more sense to me.

    I imagine differential transfer to the front L and or R wheels is already incorporated in all three systems yes no?
  • Note: the following is broad; the following is "in spirit" an accurate representation of the subject. It hopefully does not contain enough detail information to spawn disagreements that do not advance/answer the point/question: what is reactive vs realtime?

    Most of the AWD systems used in the cars and SUV's we buy for personal use are reactive systems. Many of them are put on vehicles that were FWD and indeed remain "mostly" FWD until wheel slippage is detected.

    These types of systems react very quickly. They react so quickly that the advertising copy is not misrepresenting the situation when the characteristics are said to be "instantaneous."

    They are, however, reactive. They are not realtime.

    Systems that electronically redirect power (perhaps by applying the brake to a spinning wheel, for instance) are very good, are very fast and are not technically instantaneous. They can't be -- they have to wait for slippage before acting.

    Systems that are comprised of gears (e.g., worm gears) "bind" in real time, no wheel slippage is required for their effects (and benefits) to manifest themselves.

    In a practical sense is a Haldex system, such as is employed on some Volvo cars, "inferior" or dangerous? The answers are "probably not" and "no." The practical answers are "no" and "no."

    Yet, Torque Sensing (TorSen) equipped vehicles -- especially the latest design where the "at rest" torque split has been shifted from 50:50 to 40:60 -- do have some important advantages. They ACT in realtime, they do not react after a wheel has slipped. The argument goes something like this: no matter how rapidly the reaction is, it is still a reaction. Control has to be lost, even if for a nano second, for a non realtime system to "kick in."

    Some further argue that the FWD biased reactive AWD systems still "feel" mostly like their FWD only cousins more than true AWD vehicles or RWD (biased) vehicles.

    RWD or rear-biased AWD is "the holy grail" don't you know?

    For me, I say give me better f/r weight distribution and a 50:50 power split with realtime AWD, a powerful engine and a great 6 or 7 speed transmission (preferably a DSG) and I have found automotive nirvana.

    Today, the quest to be able to claim RWD biased AWD seems to have overshadowed f/r weight distribution and realtime AWD's advantages.

    I believe the reasons for this are entirely cost based. TorSen differentials are expensive. Making a perfectly fine (but nose heavy) chassis better balanced, likewise, is expensive.

    The FWD biased AWD (95 5 in the case of some Haldex equipped vehicles) put in "nose heavy" 60 / 40 f/r weight biased vehicles are quite good from a practical and cost standpoint.

    They are not great, however.

    Maybe they don't need to be.

    The disadvantages to my nirvana, as noted above, CO$T! Big time CO$T and some porkiness. TorSen differentials add a bunch of weight (this has improved somewhat, but such means to an end still weigh more and cost more than many or most of the reactive systems.)

    Of the two cars under consideration in this thread, the BMW X3's X-drive (RWD biased and reactive, not realtime) coupled with the BMW's "nearly ideal" weight distribution make this a completely lop-sided contest. The RDX doesn't stand a chance.

    Yet, I have driven both vehicles extensively now and the main difference I can see is the stack of 100 $100 dollar bills that the RDX has in its glove box when the two cars are comparably equipped.

    Were the RDX to offer both a 6 speed manual and a 6 speed automatic and retain the pricing scheme, the X3, then, doesn't stand a chance.

    Are you willing to pay the extra $10K for the X3? It is or at least it can be demonstrated to be "superior" -- technically, logically, mechanically, engineering-wise, empirically, etc. (styling and "option content" is a separate issue, however.)

    My wife and I will probably be "serial" X3 owners -- but damn, if that $10K difference doesn't give a person a good reason to pause and ponder (your navel.)

    Good is the enemy of great, methinks.

    The RDX is good, damn good. The X3 is better -- but I am hard pressed to find it $10,000 better.

    We are walking contradictions. Often wrong, but never uncertain. :surprise:
  • wwestwwest Posts: 10,706
    "...Systems that are comprised of gears (e.g., worm gears) "bind" in real time, no wheel slippage is required..."

    Really.

    If I have a "gear" based system and both tires driven by opposite ends of the "gear" based system have equal traction and turn at equal speeds what action does the "gear" based system take? (answer..NONE) On the other hand if one of those tires has less traction and that creates a differential rotational rate between the two then the resulting "gear" windup will bias torque to the non-slipping wheel.

    If that isn't reactive then you need to define the terms better.
  • wwestwwest Posts: 10,706
    http://www.freepatentsonline.com/6588860.html?highlight=5343970

    Wow...Double that, WOW..!

    I have NEVER seen a more direct admission of the hazards of engine braking on a FWD or front biased AWD in my life, never really expected too, from FORD yet. Good-o.

    You should note that the FEH/MMH regenerative braking is significantly reduced ONLY at, near, or below freezing.

    Regenerative braking is instantly reduced to zero if ABS needs to release the brakes to prevent lockup. But what about the ICE itself, does it raise the ICE RPM simultaneously to prevent actual engine braking?

    Now I am very sure, certain, sure, I never want to drive a FWD or front torque biased AWD in wintertime conditions.
  • varmintvarmint Posts: 6,326
    I wonder if the whole "rear-baised vs front biased" debate just got a little murkier.

    "An updated xDrive system provides full-time all-wheel-drive with a 60/40 front/rear split."

    http://www.leftlanenews.com/2006/08/08/2007-bmw-x5-revealed/

    If that's not a typo, then I feel for poor Audi. If Mark is correct and they redesigned their AWD system to match the rear-bias of BMW, they must be kicking themselves.
  • The BMW AWD system, unless hell has frozen over, is rear drive biased. I cannot imagine BMW bringing a car that is FWD biased to market.
  • Audi's TorSen quattro cars were, for years, 50:50 f/r biased. Some of the newest TorSen quattro cars (mostly S or RS versions of A4's, 6's and 8's) are 40:60 f/r biased.

    The Haldex equipped quattros: TT and A3, e.g., are FWD biased and until the engineers and accountants figure out how to put out an otherwise neutral or rear biased alternative, I would suspect they will remain that way.
  • Not only is no wheel slippage required, none is possible in the TorSen system. The non-TorSen, non mechanical systems, on the other hand, require wheel slippage to react.

    Perhaps there are some very subtle nuances in the definition of realtime that somehow I overlooked or perhaps they escaped me then and now.

    Here is an additional explanation:

    The mechanical nature of the TorSen-based AWD system (like most quattro cars, but hardly limited to just the Audi brand) helps prevent wheel slippage from occurring by diverting power to the axle that has more grip at the exact moment it is needed, not after a triggering event (i.e., wheel slippage.)

    By comparison, viscous coupling and electronically controlled AWD systems that are widely used in many all-wheel drive systems are reactive since they only redirect power after wheel slippage has occurred.

    The realtime, TorSen in this instance, advantage is felt under hard acceleration during turning as the power transfer between axles is not sudden, virtually eliminating the chance of spinning.

    The TorSen-based AWD system offers another performance advantage that is the opposite function of distributing power to the wheels, i.e., engine-braking. When engine-braking is used to decelerate, the associated loads on both axles are stabilized by the TorSen system just as engine power is diverted -- mechanically. The effect is to spread the engine-braking load among the four wheels and tires.

    The TorSen AWD vehicle is able to execute a highly stable high speed turn under such conditions (slowing down) with low risk of spinning due to loss of grip on either end.

    Moreover, such a configuration substantially reduces so called torque steer. This is because of the design: equal length drive shafts on the front axle.

    All is not automotive nirvana, however, with this system. Audi, perhaps the most prominent (but NOT the only) advocate of this system, uses a longitudnial engine placement. With such placement of the engine/transmission assembly, thus far (changes are a commin' with the B8 generation of the A4, however), the front axle is placed behind the engine. The common criticism of Audi vehicles, of course is that they are nose heavy (ya think?) In other words, the ideal 50/50 weight distribution cherished by many driving enthusiasts has, heretofore, not been possible.

    Changes are coming -- and I am suggesting that Acura will need to follow suit, eventually, at least.

    Recently the TorSen differential has been adapted to a 40/60 f/r at rest torque split (i.e., when the coefficient of friction is "equal on both front and rear axles"), thus providing more RWD-like driving and handling characteristics.

    This 40/60 f/r TorSen system was first introduced in the 2006-model RS4 and shortly thereafter in the Q7 SUV and it is or soon will be implemented across the entire TorSen line. The power split between left and right wheels has progressed in the TorSen system through various combinations of differentials.

    A torque sensing mechanical system -- it weighs a lot, it costs a lot: disadvantages.

    A torque sensing mechanical system -- it has become, as of now, a rear wheel drive biased realtime AWD system that few companies employ, but it certainly seems to be, if not THE absolute best way to go, closer than most in use today.

    Practically speaking, it may not actually make THAT much difference. For, as I noted earlier, good is the enemy of great.

    These AWD systems, overall, are all good, that is. :surprise:
  • wwestwwest Posts: 10,706
    "...helps prevent wheel slippage from occuring by diverting power to the axle that has more grip at the exact moment it is needed..."

    Action: "..diverting power..."

    "...at the exact moment it is needed..."

    When landing an airplane I do my absolute utmost to pull back on the "stick" at the exact moment needed for a "soft" landing. But I have numerous senses that I use to know when the exact moment arrives.

    So, can you please explain to me just how the "gear" type torque apportioning device knows just when the exact moment has arrived to "divert" power.

    "...slippage..."

    "...none is possible in the TorSen...."

    Also keep in mind that just as a standard open differential allows for "slippage" when turning, your "gear" type torque apportioning device MUST, at least to some degree, make the same allowance.
  • varmintvarmint Posts: 6,326
    I strongly suspect the editors just missed a typo. It's probably supposed to be 40/60, rather than 60/40.

    It's just the "what if" scenario that gives me a giggle.
  • "The bias ratio characteristic of the Torsen differential instantly reacts [realtime, by another name] to unequal traction conditions by delivering an increased amount of torque to the drive wheel having better traction before the other drive wheel exceeds the limit of traction available to that wheel (before wheel slippage, i.e.) The bias ratio characteristic also remains substantially constant over a wide range of torque conveyed by the differential, and is not sensitive to changes in ambient temperature or conditions of vehicle use."

    Perhaps this is what you are looking for to quench you thirst for information:

    TorSen Tech Talk
  • varmintvarmint Posts: 6,326
    Matt Davis gives the 2007 X3 a rave review in the lastest issue of Winding Road. (Didn't know this on-line mag existed until a few days ago.)

    With the new engine and 6AT, they clocked 0-60 in 7.1 seconds. Certainly more competitive with the RDX than the earlier engines. Overall, his opinion was something like, "More mature driving dynamics that the original," or something like that.
  • We drove into the BMW dealership yesterday in a 2005 X3 3.0 manual transmission (with the sport package and all other options avail in 2005.) The car was there for a trim piece replacement that had somehow faded.

    While we were there, our salesperson threw us the keys to a new X3 3.0si -- all options, save nav and a 6 speed AUTO.

    We know the routine: go up (and later down) BMW Hill as we call it, go onto I71 and accelerate to a high 2 digit speed or maybe a low 3 digit speed if traffic is light.

    We got OUT of a perfectly fine 225HP X3 and IN to an also perfectly fine 260HP X3.

    The suspension must be recalibrated for less harshness, similar fimrness, similar sticky-ness and the chassis or the insulation or something has been calibrated to further mute the road, engine and wind noise (which was already pretty low in the 2005.)

    The car had 65 miles on it.

    It was "wicked quick." Exceeding expectations quick -- and remember we drove in a 6 speed MANUAL X3 with the sport set up, so we had high expectations.

    The transmission in first and second gear held well into the power curve of the engine. We tried it in both D and S modes -- hell S mode held the gears on acceleration much longer and downshifted crisply "just about the point" where one more second would have been too long to wait.

    This car, like ours, had the Servotronic steering ($250) -- and the beefier "M" steering wheel, yep, slightly thicker and meatier than our 2005 Sport Steering Wheel.

    Even at 65 miles on the OD this was one horse that was limber, ready to romp. At full cry, the engine revved to well above 6,000 RPM. At 5000 miles I can only imagine the strong power pull would become a plus size.

    The chassis, suspension, engine and transmission are now all on the same team, and all receiving their instructions from the same coach -- simultaneously.

    The X3, already car like, is now a car that just happens to have some utility and some off road talent (as witnesses, my wife and me, recent graduates of the two-day BMW X driving school in SC.)

    Now, to the interior and a little bit, the exterior.

    This was, at $47K minus sat nav, but otherwise, it seemed to have had all the option boxes checked. Who orders such a thing? Were you to deck one of these guys out to $47K would you NOT want navigation? I mean, it is not like you ordered a strippie and put ONLY nav on it -- and, in a odd way, even that seems more like a car that would be easier to sell than a fully loaded one without nav.

    Of cours, no one at the dealer responsible for such decisions consulted with me.

    Anyway, the new dash, the new materials are now on par with the other BMW's on the show room floor.

    Softer plastics, more wood -- every where you look or touch is smoother, more upscale looking, more befitting a nearly $50K BMW.

    BMW may not have all the electronic gizmos the RDX has, for instance, but its new underwear and move from the GAP to Ralph Lauren make it competitive in the looks department. And, with respect to the electronics, the only thing missing, oddly, is voice command of the telephone, sound system and navi controls (were the vehicle so equipped, that is.)

    Otherwise, the BMW does everything the way you want it once the light turns green.

    This new engine, tranmission and new set of clothes goes a long long way to answering the question "where's the additional $10,000?" (which is the difference between the RDX and the X3 similarly equipped.)

    Now, if they put the engine from the new 335 coupe/sedan in this guy, well, "sign me up!"

    This evolution, er, transformation, is one of the most significant changes I have ever had the pleasure to see (and feel) happen.

    You must test drive one of these if an SUV-lite vehicle is anywhere on your radar screen. Somehow, too, this car actually has better rear seat seating than the 3 series (and, btw, the X3 now has heated REAR seats available, too.)

    Were out of the market for about 12 or 13 months, but this -- today -- would be a no brainer for my wife and it actually would be a consideration for me, if you asked me the question NOW. We'll see what happens in 12 months.

    The cool thing is there is no reason the "35" turbo engine cannot be made for the new X3.

    :surprise:
  • I test drove a 2007 RDX in Sept. At the time I had a 2005 x3 2.5 loaner that I drove for 2 weeks. The RDX was wonderful but We are getting a 2007 X3. No question, I liked the ride, handling and ergonomics of the BMW.
  • I'm a big BMW fan. I leased a 2001 BMW 540i for three years, loved the car, zero problems. I've been driving an Acura RDX for three weeks now. All this talk about rear biased vs. front biased AWD: on the Acura SH-AWD system, don't worry about it. The RDX feels as if it's carved from a single piece of aluminum billet, the handling is UNREAL. I personally prefer the Acura interior/ergonomics to BMWs (I have a 2004 TL as well). Very subjective. The handling of the RDX is nothing short of astonishing. Read the magazine road tests, which essentially confirm my observations.

    What's not subjective is the price. I paid $33k for a base RDX. Amazing car. A bargain in my view. The X3? A tad more cargo room, but otherwise similar in interior space. Worth an extra $5-$8K? It's your dough. I have two Acura cars and a Honda motorcycle in my garage. Very happy. This from a guy who drove nothing but European cars (BMW, Mercedes, Porsche, Saab, Volvo) for the better part of 20 years.

    So spend the extra money. You'll get a BMW, but you won't get Honda reliability.
  • bodble2bodble2 Posts: 4,519
    Yes, you've made a wise choice, but the big question is, can you beat an M5 around a racetrack? :P ;)
  • >>but the big question is, can you beat an M5 around a racetrack? <<

    Why would I ever be on a racetrack? I race bicycles not cars. Though racing cars might be a bit safer : ).

    BMW doesn't have to 'consider being afraid' of cars like the RDX. They're BMW already. They sell half their cars on the sheer mojo of the little badge on the hood. Plus they're lovely cars - overpriced or not. And they're not overpriced if car weenies will pay for them.

    I personally think the X3 isn't nearly as nice as the RDX - and costs more. But that's quite subjective. And if your X3 makes you happy that's sort of all that matters.
  • bodble2bodble2 Posts: 4,519
    Don't mind me, my friend. That was just a bit of an "inside joke". There was a bit of a "discussion" earlier on one of these X3 topics (I don't even remember which one. May even have been this one), where someone commented that an X3 can beat an M5 around a track, or something to that effect.

    I wasn't trying to imply anything negative about the RDX. It's a nice rig. I don't have one, nor an X3, yet, although I admit I do have a fondness for anything BMW. I currently drive a TL.

    Happy motoring with the RDX! :)
  • Let me paraphrase something my buddy the bicycle shop owner said about high end racing bikes: 'Once you get over $3000, none of it sucks.'

    Same thing for cars over $30K for the most part, BMW's and Acuras included.
This discussion has been closed.