2006 BMW X3 vs Acura RDX

markcincinnati

The new X5 to my eye looks like a bigger X3.

bodble2

The new X5 (as does the current X5) has a brawnier look. More SUVish proportions whereas the X3 and the RDX have a more tall-wagon look. Not as much as, say, the Subaru Forester, but in that general theme.

bodble2

I agree about the size issue. Aside from the cost, the other factor that would be a strike against the new X5 (for me) would be the bulkier exterior dimensions. But it does look sweet!

driver100

I just saw photos of the new X5 on BMW site. Both the RDX and '07 X3 are ugly ducklings next to the new X5.

Looks very nice and seems to be a nice improvement on the old model. But I like the idea of a smaller SUV so I am happy to keep mine. Hope the redesign looks like the new X5 though.

markcincinnati

Eye of beholder -- the X5 looks porky, bloated and out of proportion. Tall wagon? Wow!

Like I said: eye of beholder. :shades:

corvette

Let me preface this by saying that I have a case of advanced BMW nuttiness. I believe that all German cars have a unique feel to them that has not been duplicated by any other manufacturer, but BMWs best exemplify this feel.

I'll also add that I can't get that excited about the Acura TSX or TL. Maybe I drove them on the wrong roads, or didn't give them enough of a chance, but to me the TL felt like an Accord with Bluetooth and a slightly nicer interior, and the TSX handled well but was lacking in torque.

I own a 2006 3-series (the E90 model). I spent a little over a week with a loaner X3 this summer. I haven't compared dimensions, but the X3 felt like it was within spitting distance of the E90 in length and width. The X3 felt very narrow to be so tall, and the tall greenhouse only amplifies this look. I find it to be a little odd-looking. The interior had some cheap touches, and it used BMW's last-generation flexible-service system (like the E46). It also didn't have the triple turn signal activation of other recent BMW models. My loaner was a non-sport-package model with an automatic. The ride and handling were good, even without the sport package. The engine was not as smooth and quiet as the one in the E90. The leatherette and other interior plastics appeared to be a lower grade than in the E90. I haven't yet seen or driven a 2007 X3, but it is supposed to have the next-generation inline-six engine (with its NVH refinements), as well as an upgraded interior. These improvements should address my main concerns about the X3, but I haven't seen or experienced them yet.

I drove a 2007 Acura RDX a couple of days ago. I will try to attack the issues in a similar order as I did with the X3. The RDX was a comfortable size--I understand its dimensions are nearly identical to the 2007 Honda CRV, which is about the same as the X3. The RDX did not share the same tall-but-narrow feeling I got in the X3. I think I prefer its styling to the X3. There are a ton of steering wheel controls, but if you get the technology package, you only need one or two of them (the voice command buttons). (The X3 I drove was a base model, so I can't really compare the integration of technology between the two.) The instrumentation is similarly busy, with a boost gauge and a graphic that shows you the torque split between the four wheels. I pretty much ignored the satnav, boost gauge, and other gizmos and concentrated on driving the car. I was impressed. Ride and handling were good, as was acceleration. The engine was smooth, although you can tell from its note that it is a four-cylinder, and there is little turbo lag. I did notice transmission lag when I went from about 30% throttle to about 80% throttle--it took a moment before the transmission kicked down two gears. Perhaps this trait would improve as the car's brain adapted to one's driving style. I couldn't believe it, but the RDX was truly fun to drive. Then there is the interior. As I noted, the steering wheel controls (as well as the center stack) are busy, but the dash materials are of a higher quality than the X3. The RDX lacks a memory driver's seat, power passenger seat, and rear A/C ducts. These are things that I could live without, but other buyers may not be so forgiving.

I prefer the 2007 RDX to the 2006 X3. I may prefer the 2007 X3 to the 2007 RDX, but probably not at the price difference it will command.

robertsmx

Very good writeup.

driver100

Great review, much appreciated.

My X3 is exactly 2 years old and I love it. If it is a little high and narrow I don't care...I love the big windows and command seating position. I also like the styling and although the interior could be a little jazzier, it is extremely comfortable and well thought out.

The RDX is certainly worth a test drive though. I wouldn't hesitate to try one out when the time comes, based on reviews and comments like yours.

Thanks, that was very helpful.

driver100

I see big full page ads for the RDX in the newspapers today. IMO it would look a lot better without the second upper grill - just leave the one grill on the bottom. Then it looks more like a Cayanne!!!!

bodble2

"Then it looks more like a Cayanne!!!!"

Good Lord, hope not!! :surprise:

dmitrys

I did take an extensive test drive of an RDX with the Technology Package ($37K.)
Am I the only one to notice that RDX has a lot less front leg room than X3? I am certainly not a giant (6-1), but there is no way I can fully stretch my legs in an RDX. That's pretty much where my test drive ended before it even began.
No problem at all in X3... Or in my wife's FX35...

markcincinnati

I am x-act-ly 6' -- so, no, I did not notice an issue, but I am glad you posted it for all to see.

The RDX when well equipped, next to an X3 also well equipped with be $10,000 less out of the chute.

The BMW is in many ways a superior vehicle -- even if you are not taken by its styling. The same styling comments apply to the Acura.

The question will be the $10,000.

In my mind, the BMW is easily "worth" at least $5K more.

I mean, I can FIND the differences at that price point.

The remaining $5,000 differences are more subtle, more abstract and for some -- even if they can detect them -- will be difficult to justify at that level.

Things cost what they cost and are priced accordingly. I am not suggesting there is price gouging going on here, not at all.

I am suggesting that the RDX has "merit" when put next to a $10,000 more expensive X3.

I also didn't say I would get an RDX.

I think it is not inaccurate to say, however, that I would at least "pause" now that I have driven one. For some who are less thrilled with the German cars, but who might be candidates for a new X3, this pause may be enough for BMW to lose a convert.

I cannot imagine BMW will radically drop the price of the X3 -- I don't know how much wiggle room there is in terms of content or performance even.

I do think BMW has some real competition in the RDX and that is -- for the market, if not for BMW -- a good thing.

After our X school experience, I think I can find that $5000 without too much effort, tho. :surprise:

ctski1

Good post - from a driver’s perspective. From a car enthusiast however, the comparison lacks the fundamental difference. The X3 is built on a rear-wheel platform and the xDrive only distributed power to the front wheels when rear wheel slip is detected. Once the X3 is moving, the power is applied to the rear. The RDX on the other hand, is the exact apposite. It sends power to the rear when substantial throttle is applied to avoid any wheel spin, buy once the car is going - it acts much like an ordinary front driver.
So fundamentally, the chassis dynamics create a very different driving characteristic.
If you love to drive - go with the BMW. If you prefer the RDX, get the even cheaper Honda CRV.

You are so right about the interior. The materials used in the X3 look like a cheap imitation from Korea. Fit and finish could be better too. The Acura on the other hand is lined with very nice soft leather, the controls are much better placed and feel better to the touch. The RDX is also quieter on the road, although you can hear the turbo whistling moderately.
RDX’s engine definitely feels more powerful off the line, but that should be fixed with the introduction of the updated, 260bhp 3.0L engine on the 2007 X3. At highway speeds I prefer the X3’s smoother engine and a better auto transmission. 90% of the time, the BMW’s auto shifts where you would if you had the manual, although the “Sport” mode will bring more joy.

All in all, two fantastic vehicles! The only shame is that BMW had to wait for some decent competition to finally address some of the basic issues, like the lack of power and interior quality.

ctski1

The 2007 X3 should arrive at your local BMW dealer about the same time as the new X5 - end of October, begining of Novernber.

corvette

Like I said, I didn't pay much attention to the torque split graph on the dash of the RDX, but it seemed to be about 50/50 when accelerating. I didn't push the car that hard. It seems to be a well-designed system. I think there's a much larger difference between FWD and RWD than front-biased AWD and rear-biased AWD, especially with an AWD system that seems to be very predictive in sending power to the opposite wheels.

It's obvious even during sedate driving that the Acura TL or Volvo S80 is FWD, and this is a difference that's more than just academic to me as it makes them less enjoyable to drive. It's less obvious that the RDX or X3 is biased towards the front or rear--this distinction is less important to me so long as the car doesn't exhibit torque steer due to FWD bias.

markcincinnati

I do not want to pick too many nits, but I just returned from the X school in SC and we were told:

The BIAS of the X3 is REAR, it can shift power where it needs to go. It is virtually never NOT an AWD vehicle, which is what I think is an interpretation of your post.

I could be wrong, but I thought our instructor told us that the X3 moving on down the road was about 30% FWD and 70% RWD. This RWD Bias is also the rule for Mercedes and Infiniti's AWD systems (I don't know if this is true for Lexus cars, tho.)

Audi has been 50 50 across the board with a system that "binds" therefore transferring power without wheel slip, but in part to shut the naysayers up, Audi, too has made a "statement of direction" that all future Torsen models will be nominally 40 60 split.

Acura, Honda, Volvo and some to many Toyotas are FWD with RWD transfer possible. Volvo is 95% FWD with the Haldex system able to quickly ("almost" instantly) shift the power where it is needed based on wheel slip.

Torsen since it is mechanical does "bind" in real-time, i.e., a difference in wheel speed evidenced by slip is not a prerequisite.

In the real world, this is probably not a show stopper -- but either equal power distribution or, more to the world's liking -- rear biased power distribution is certainly part of the requirement for a vehicle to not be labled a "poser."

The Germans, to their credit do understand this. Acura, on the other hand, has a pretty nifty technology, SH-AWD, but the cars are, fundamentally, FWD biased and NOSE HEAVY.

The combination is, er, less than optimal.

X-drive and the BMW "balanced" design is a better approach if performance driving is even a tiny goal for you.

corvette

I thought the RDX had close to a 50/50 weight distribution. Mom's S80 is one of the most nose heavy cars I've ever experienced--it has the 2.9 liter inline six, transversely mounted. The car feels like it is being propelled by a giant boat anchor somewhere way, way out front, and don't even think about trying to turn out into traffic quickly from a dead stop--you'll spin the tires. Add to that the torque steer from FWD and the overboosted steering, and the car takes quite a bit of getting used to.

Also, there was a video on Google video or Youtube a while back that demoed Subaru's AWD system showing up a Volvo XC70 (which was pretty much stuck). If it was accurate, it's an impressive demonstration.

ctski1

You're absolutely correct! My point (even if I did not make a good case before) is the chassis. Acura RDX is built on a "Small Truck Platform" aka. front drive, capable of being AWD.
BMW inherently designs platforms to be RWD with AWD capabilities.
That in itself translates to different driving dynamics....corner approach, hitting the apex, and corner exit will "feel" different regardless of front-rear bias.

I’m just beyond ecstatic that BMW had finally “fixed” the X3 for 2007. The F1 style paddle shifter should be great fun too. I'm ordering my as soon as I tase it.

robertsmx

I'm going to ask a quick question to you, doesn't relate directly to RDX, but it uses a similar system, and concerns F/R bias perceived necessary in an AWD system to "perform".

Acura RL uses SH-AWD, that has a "normal" F-R distribution of 70-30. Straight line acceleration results in 60-40 split, and going around a corner under throttle, the bias is 30-70 (rear biased).

Why do you think Honda engineers opted to maintain a 70-30 cruising split when they could have simply made it rear biased since SH-AWD is capable of being rear-biased (it is in RDX too, but only when cornering abilities necessitates it).

robertsmx

RDX has 52-48 weight distribution, so it is quite close to 50-50 (thats actually better "static" weight distribution than many RWD cars).

wwest

Just as long as there are no lateral, high engine torque or engine braking forces acting on the front tires' roadbed contact patch there is no reason not to have them apply more than 50% of the engine torque to the roadbed, or even 95% (except for tire wear) for that matter.

The SH-AWD seems to very nicely shift engine torque, leading or lagging, to the rear in all circumstances wherein it would be detrimental to handling dynamics, safety, were it left at the front.

The nice thing about SH-AWD is that the position feedback for the left and right rear drive clutches could likely be easily modified, offset, by an aftermarket device such that it is always rear torque biased.

I suspect, that while Acura says snowchains should only be used on the front wheels, using then only at the rear would be dramatically more safe, and would not in any way damage the driveline. Snowchains only on the rear would provide additional engine torque biasing to the rear due to "increased" rear tire/chain circumference, a very definite plus on an adverse, slippery, roadbed surface.

markcincinnati

Whenever these kind of questions arise (and I often ask them) and no obvious answer, no technical or theoretical answer comes to mind, I generally assume the reasons for "whatever" are financial.

Acura, one would imagine, has the engineering talent and manufacturing capability to build a rear biased, weight balanced, AWD vehicle WITH SH-AWD.

"Everyone" says this is the holy grail of design of this particular kind of vehicle.

Acura has marketing and engineering people that can and do read what "everyone" thinks.

The reasons for not doing something such as this seem to be, then, cost. If such a change were "free" would they do it? (yes) If such a change were to have a multi-billion dollar price tag with a 9 year payback would they do it? (no)

There is a place at which it is an acceptable cost. For whatever reason, using the RL as an example, that point just below "too much" must not have been able to be obtained. Ditto the RDX.

It may be a "who cares" kind of thing in the overall scheme and being able to put a fully loaded RDX on the showroom floor for $10K less than an X3 may have had top priority.

If the sales show this approach had merit -- and probably some research suggested it does -- the financial folks will in effect have been correct: "we don't need rear biased RWD, we don't need weight balanced vehicles, a la BMW, etc."

I know that Audi "listened" -- Audi does believe in perception is reality. They have already brought several rear biased 40 60 f/r torque split Torsen quattros to market.

Too, Audi is moving the engine back, putting the battery in the trunk, making the front end out of aluminum and striving for better weight balance.

They, at least, drank the Kool Aid.

I believe the marketing pressure made Audi do it, for example. I believe that a 50 50 distribution nominally in a Torsen set up was fine. I believe that were it my money, I would have spent more of it sooner on bettering the weight balance rather than biasing Torsen to the rear.

But, perception is reality. The pundits say "rear biased AWD is king." Well, better talk and walk the talk, apparently.

Acura, or perhaps better said, Acura's intended customer may care little or at least less than a BMW's intended customer about REAR biased AWD nominally.

I suspect a monetary consideration ruled -- for, as I said, Acura certainly has the engineering smarts to design and subsequently manufacture a balanced and rear biased AWD vehicle -- it is just a matter of how high the stack of $1,000 bills have to be to make this a reality.

That's my take, and, I'm stickin' to it. :surprise:

carlitos92

"Acura lists weight distribution as 52 front/48 rear, but with its front-biased power delivery, the RDX behaves like a front-drive vehicle and understeer is its defining characteristic."
-Car and Driver, July 2006

Road and Track's test in October 2006 did not list 52/48 distribution, but rather something more like 56/44 if I recall. Maybe one of them is weighing the car with the driver, and one, without.

robertsmx

I stand corrected. Somehow I remember seeing somewhere that it was 52/48. It is actually 57/43 per hondanews website.

wwest

It seems to me that once the decision is made to incur the expense of coupling the rear wheels into the driveline the decision as to how to bias the engine torque becomes a fairly simple one.

robertsmx

Whenever these kind of questions arise (and I often ask them) and no obvious answer, no technical or theoretical answer comes to mind, I generally assume the reasons for "whatever" are financial.

Acura, one would imagine, has the engineering talent and manufacturing capability to build a rear biased, weight balanced, AWD vehicle WITH SH-AWD.

But, SH-AWD already has the capability to be rear-biased! In fact, it has the capability to keep any torque split upto 70% going to rear wheels. It already does that, in both applications, and likely in the new MDX too.

So, cost as a concern does not make sense to me. Its got to be more about dynamics of the chassis itself than anything itself, and how it is oriented. For instance, one area where SH-AWD differs in RDX compared to RL is in overdriving the rear outer wheel. In RL, the overdrive is upto 5%, in RDX, it is fixed at 1.7%.

markcincinnati

The fact that SH AWD CAN deliver 70% bias under low coefficient of friction conditions does not mean it can be set to a 40/60 torque split with the flick of a switch or slide of a lever.

Witness Torsen -- the torque split, for years, in an at rest situation was 50 50. Torque could be sent to the rear or the front thus creating, temporarily, a bias.

Money and engineering talent did have to be expended to come up with a 40 60 split using Torsen. Were it simple, all new Torsens or all new SH - AWDs would be "dialed in" to the desired (by the market and the pundits) torque split.

But the reality is that these things takes time, engineering and money to create and adopt. With enough money, I do assume Honda could make its SH AWD vehicles be rear biased.

Cost, to me, makes the most sense since "everyone" who writes about this stuff for a living remarks when a vehicle is FWD biased (as an AWD vehicle, i.e.)

The C&D comments are certainly not what Acura/Honda marketing folks want to see printed. And, were there a potentiometer that could be cranked from 90/10 to 40/60, you would think it would be already cranked. Indeed, doing so, were it possible, would seemingly get C&D and others to lavish some praise on the RL and RDX in this area.

I am, however, assuming the issue is "cost." I hereby state, I am speculating and was when I posted earlier. There are no super expensive chassis dynamic reasons I can come up with, hence I revert to my educated guess that this is, overall, a cost issue.

:shades:

wwest

"But, SH-AWD already has the ability to be rear-biased...."

Yes, but for how long?

Engine torque is shifted, proportioned, to the rear via "partially" engaging the left and right rear clutch packs and the fact that the rear driveline is constantly overdriven, someone said by 1.7%

Absent the rear driveline being "overdriven" the highest ratio even with those clutches fully engaged would be 50/50.

How long do you think the driveline, say those clutch packs, would last if the torque split remained at 50/50 for an extended period?

Pretty rough on tire tread life too.

If the front drive could be somehow "uncoupled" proportioned, as the rear biasing increases, then you could have long term rear biasing.

I suspect that one of the benefits of the planetary gearset in the rear driveline for the original SH-AWD implementation was to allow continuous 50/50 torque biasing by reducing the rear overdrive factor to zero.

robertsmx

The fact that SH AWD CAN deliver 70% bias under low coefficient of friction conditions does not mean it can be set to a 40/60 torque split with the flick of a switch or slide of a lever.

Not a flick of a switch, but by reprogramming, it certainly can be. We already know SH-AWD can send upto 70% to the rear wheels. Now if it couldn't, I wouldn't be making this point.

I suspect chassis dynamics (not just static weight distribution that all of us seem to focus too much on) played a role in how torque distribution is managed. Even in case of straightline acceleration, don't you think Acura could have done a 50-50 or 40-60 split? Instead they kept it at 60-40. Again, that makes sense if you consider the position of the engine (just ahead of the front axle) and the static weight distribution.

wwest

If you look at the actual implementation you will see that there is no way provided to modulate or proportion the level of torque to the front driveline. Think of the SH-AWD as being like two people of unequal weight on a see-saw.

In this case we always have a 100 pound person on one end and at the other either a 50 pound person or a 150 pound person or something in between.

If the rear clutches are completely disengaged then 100% of the engine torque is at the front. With the rear clutches fully engaged but without the "overdrive" of the rear driveline the F/R ratio would be 50/50. But with the 1.7% overdrive to the rear and the clutches engaged the rear wheels will "want" to turn faster than the front and thereby more torque is now delivered to the rear drive wheels.

Absent the F/R "slip, slippage," allowed by the left and right rear clutches you would encounter driveline "windup" and/or tire "scrubbing" just as you would with a part time 4X4 system engaged on a high traction surface.

Many modern day AWD systems such as the RX series use a VC, Viscous Clutch, to provided the required F/R slippage so the driveline isn't under undue stress on high traction surfaces.

The RDX therefore cannot have a significant proportion of the engine torque allocated to the rear wheels during straight-line cruising except or unless the roadbed surface is slippery enough to provide the required "slippage".

As an example I could quite readily convert my 2001 AWD RX300 to a rear biased AWD system by eliminating the functionality of the center differential by solidly coupling the engine drive to the rear driveline, opening the "solid" coupling to the front driveline, and thereby allowing the VC to be used as the ONLY coupling to the front driveline.

Or I might buy an X3.....

robertsmx

The RDX therefore cannot have a significant proportion of the engine torque allocated to the rear wheels during straight-line cruising except or unless the roadbed surface is slippery enough to provide the required "slippage".
SH-AWD doesn't require slippage to provide 40% of the torque (up from 10%) to the rear wheels in straightline acceleration. If it can increase from 10 to 40 to 70, I don't see why it couldn't increase from 10 to 50 or 60 in straightline acceleration. I believe it is all in the programming. The optimization of how much and where probably considers more factors than those you and I can come up with here.

As for proportioning to the front, it does send more to the front during deceleration (or when settling down from acceleration to cruising mode). If you meant side to side proportioning, in the rear, it is handled by ATTS, which was implemented at the front wheels in Prelude SH (and in Honda DualNote Hybrid concept, which coincidentally had a very similar AWD system as SH-AWD albeit using electric motors).

wwest

"....it does send more to the front during...."

No, as I tried to explain, it partially "uncouples" the rear which results in the front driveline "recieving" more of the engine torque.

Look at it this way, for how many hundreds of milliseconds, or even seconds, can you have straight line acceleration? So having a F/R torque ratio of 10/90 for that short period of time represents no problem insofar as driveline windup, tire scrubbing, etc.

10/90 F/R torque distribution for an extended period of time (in motion) would undoubtedly result in ~5,000 mile life for the tire treads.

markcincinnati

I see little or no chassis influence on the torque split.

The system has been designed to nominally send the most torque to the front. The car is a FWD biased AWD set up and to change it at this point would cost something.

Were it a low cost matter, it would be done if for only marketing reasons.

As others have stated, the system is NOT designed to keep a rear wheel drive bias going for more than short bursts of time.

The cost to design, manufacture and implement a change to RWD bias is, apparently, too high at this point to justify.

If by dynamics you mean, perhaps, weight distribution, well even Audi has shifted to RWD biased Torsen in cars that remain nose heavy.

They are praised for this.

BMW started with the premise of balance is important. Then they added AWD. Acura, at present, builds cars that to the folks who are the trend setters and pundits, are FWD like in feel. Because they ARE FWD like more than neutral or RWD like.

It doesn't make them bad vehicles, it just makes them NOT neutral or RWD. Perhaps customers or their target market cares little about this.

The fact remains: companies, such as BMW make marketing hay out of being balanced and AWD.

Audi and Infinity have, in their own ways, responded. Acura has marched forward creating FWD cars that can shift power for short periods rearward.

C&D editor's butts and yours may be miles apart in their ability to discern such things -- these magazine editors do, however, suggest "what we all SHOULD like" and what is best for us.

In 80% of the on the real roads world, I suspect it matters little.

There IS a difference in an X3 and an RDX in this regard -- the BMW can perform at a higher level.

In real life does it matter?

That question's answer is up to you.

For me it is a $10,000 question that I have reduced to a $5,000 question. For, I do think the BMW X3 is easily worth at least $5K more for road competence reasons alone.

I do doubt if the general public would agree.

Perhaps BMW is not looking to sell to the general public.

Why, other than cost, would Acura bring a 2007 model to the market and NOT be ever so slightly RWD biased were it not COST prohibitive to do so?

The RDX, in RWD bias mode, could seriously challenge the BMW X3 when MSRP is factored in.

Acura had to make the decision based on the cost vs return issue.

Again this is speculation.

robertsmx

The system has been designed to nominally send the most torque to the front.

What would that nominally imply? What could make 70-30 nominal for RL and 90-10 for RDX?

markcincinnati

By nominal, I meant approximate or the "named" or designation of.

I don't have the "at rest" torque split memorized for very many cars -- I do recall reading that the Volvo system is nominally 95 / 5 split (F/R) but that when needed the torque up to "X" amount can be shifted rearward.

I do believe I also read that Acura's system "at rest" is 90 / 10; if they also offer SH AWD with some variability, such as 90 10 in car "A" and 70 30 in car "B" it doesn't change the remarks made by C&D and others. SH AWD's implementation thus far remains FWD biased AWD, plus the cars are nose heavy and the driving "feel" tends to be more FWD than neutral or RWD and the trait that leaps out at the C&D editors is "understeer."

I have taken several folks on test drives of cars that I know are both nose heavy and FWD biased -- my observation is that most folks on an initial test drive don't notice this, perhaps because they stay at well below 80% of the performance envelope of the car (probably both during the test drive and in their daily driving on the public roads.)

My "marketing" instinct would be to make the advertising copy and the engineering be RWD biased AWD and for those who care it would be a selling feature and for those who don't know or don't care, it would not be a negative for they would probably be oblivious to it.

robertsmx

You didn't address my question. Why (and I will add, how) do you think Acura would tune SH-AWD to a cruising 90-10 split in one, and 70-30 split it another? Why would those engineers tune acceleration split to 60-40 and not 70-30 or 50-50 (although SH-AWD is "continuously variable, 60-40 is the max split in straightline). And why would they make it rear biased with a 30-70 split during aggressive cornering.

As you can see, the system itself is flexible, as to how and how much torque is provided to each axle. But those design decisions must boil down to something. You say cost, I would be curious, how so? How would 60-40 split in a straightline be cheaper than 70-30 or 50-50 split?

wwest

Easy question...

The RDX's drive train is less expensive to produce since it does not include the variable ratio planetary gearset in series with the rear driveline. And given the SUV "nature" of the RDX the planetery gearset would likely be subject to a lot more use/wear and that raises the question of long term reliability. So it would have likely needed to have been "beefed up" for use in the RDX.

It is also that variable ratio gearset that allows the F/R torque split more variability.

robertsmx

I don't agree with your assumption on greater wear and tear due to the bodystyle. That said, I'm more concerned about front-rear torque split than acceleration device (the planetary gearset) which speeds up the outer rear wheel around a corner.

Besides, there is more variability in F/R torque distribution in RDX (from as low as 10% to as high as 70% to the rear axle) compared to RL which has it from 30% to 70%. And it is all continuously variable (not that it would add much value to the ongoing discussion)

pg48477

I think you mistaken the RWD platform with RW torque distribution. It does take a lot of work and money to create a new platform, RWD or FWD; however it does not take any money or effort to switch torque distribution on already existing AWD system, especially if the system is already capable of transferring most of the torque to the rear when needed. In case of SH-AWD, all you have to do is update the program controlling the system.

Look at it this way, fore pure performance and handling standpoint RWD is the way to go. But with addition of AWD, performance decreases on the RWD based platform (for one due to additional weight, which would make 50/50 balanced car, nose heavy). The opposite works for FWD vehicle, addition of AWD will add weight to the rear, therefore better balance the car.

With all that said, torque distribution has nothing to do with the platform, and can easily be changed without additional cost. Sure it would make sense to apply more power to the heaviest part of the car, but just look at Audi, FWD platform, rear biased torque distribution.

wwest

I think you would agree that a higher percentage of SUV "bodystyles" will be put to use in the wintertime on adverse roadbed conditions than would a standard passenger vehicle "bodystyle". Many SUVs are even preferentially purchased for this very reason.

So that inline planetary gearset were it adopted for the RDX would undoubtedly need to beefed up in order to provide reliability equal to the sedan.

And doesn't that planetary gearset also provide F/R differential drive capability vs the "solid" driveline in the RDX?

markcincinnati

My question remains, for if it is "cheap" to bias the torque (in the Acura for example) to be able to claim rear bias, why NOT do it, since the results would be "praise" (and sales?) from the likes of C&D and R&T, etc?

Were it this easy, why not just do it?

It almost always ends up being a use of funds question.

An opportunity cost question.

Perhaps, the customers that Acura is seeking are not as likely to be dissuaded from purchase with the car being, primarily, a FWD car with AWD/RWD capabilities from time to time.

Were it "free" one would imagine just doing it so that C&D would stop ragging on their FWD feeling vehicle(s).

robertsmx

My question remains, for if it is "cheap" to bias the torque (in the Acura for example) to be able to claim rear bias, why NOT do it, since the results would be "praise" (and sales?) from the likes of C&D and R&T, etc?

Again, a temporary rear bias exists, so it is not something that is being avoided. Only provided when it is deemed necessary (the greatest advantage to having a RWD vehicle is in cornering, and to a lesser extent in drag runs. I don't see that advantage during cruising).

This leads me to believe that those engineers look at things far more than what C&D and R&T have to say. Perhaps it is all about optimizing performance/traction/efficiencies under different situations. I would think so.

robertsmx

I just don't see why an AWD car would not see the same adverse conditions as an AWD CUV. I'm willing to bet that much of the SH-AWD components between RL and RDX is shared, except for a few upgrade in the RL like carbon-fiber reinforced propeller shaft.

The planetary gearset in SH-AWD is the acceleration device. It doesn't control the F/R torque distribution.

pg48477

The answer is simple, Acura is applying torque to the wheel s with more traction. Because it’s nose heavy, at stand still front has more grip. With change in traction AWD system changes the torque distribution. In any case, I don’t think torque distribution will help Acura’s sales, they sell as much as the can produce (RL would be an exception). I also think most magazines are complaining about FWD platform and not torque distribution.

Look at Audi for example, they changed torque distribution to 60/40, but still have pitiful sales. I also don’t think FWD platform is the reason Rl’s sales are so poor, I think it more have to do with styling (to conservative), pricing and marketing and lack of V8.

wwest

Once ski season begins maybe I can send you pictures taken at some of the local ski area parking lots dominated by pickups, vans, busses and SUVs, passenger cars in the definite minority.

The planetary gearset is used to control the overdrive ratio to the rear wheels and therefore the proportioning of the level of torque to the rear.

I don't know how it works but I have always understood that a planetary gearset, like a an open differential, will allow the two shafts to have differing rotational rates.

markcincinnati

Audi is evolving their f/r split to 40 60. I assume that is what you meant.

And, Audi's sales are currently at record levels, which is on the heels of record levels and so on.

Acura, at least in the RL (the A6's contemporary) has not gained traction as was hoped and forecast when the new RL was introduced.

To play in this market (lux and near lux performance cars), Acura would seem to benefit from addressing some of the criticism laid on it by the magazines of late.

The RL, as a flag ship, is struggling, apparently, even against Audi -- in relative terms.

Audi looks at its low sales numbers in context -- the best in the company history. Acura in this respect has seen a decline.

Perception is reality is part of the point I am advancing.

When the review of a very good product is ever so slightly marred by comments pertaining to its FWD underpinnings, well that has to be SOME impact on sales.

carlitos92

Acura, at least in the RL (the A6's contemporary) has not gained traction

...I'm sorry, but whether that was intentional or not, that was just hilarious, given the conversation... :P

-c92

markcincinnati

I actually had meant to insert my own wise crack, but I timed out and couldn't edit it, once I got to it.

Thanks for picking up on my slack.

:shades:

varmint

The original press kit for the RDX did list weight the distribution at 52/48. They have since changed it to 57/43 on Hondanews.com.

R&T listed their own specs for the RDX, which included a person in the driver's seat. With the driver on board, the split is 57/43.

Coincidence? Perhaps, but I think it's pretty clear Acura has simply changed the way they publish their info. Now they include the weight of a driver, while earlier press releases did not. That is not inconsistent with the way they publish other specs (like towing).

BMW does not specify whether they include a driver in their published distribution (which is roughly 51/49), but traditionally that is how it's done.

varmint

With the RDX, the rear axle is constantly overdriven by a degree of 1.7%. When cornering, that percentage of overdrive is put to use by accelerating the outside rear wheel.

I doubt that is news to anyone here.

The question which hasn't been asked is, "what happens during straight line driving?" How can the rear wheels be moving faster than the fronts when driving in a straight line?

The answer is simple. It gets wasted. The rear clutch packs absorb a constant slip and scrub it off. This avoids drivetrain binding.

(Interesting side note: The system includes a wear sensor which checks for any loss in friction as the clutch packs wear. The electromagnetic field is then increased to compensate for that loss with more pressure.)

Anyway, this is different than the SH-AWD unit in the RL. The RL uses an acceleration device to vary the overdrive from 0-5.7%. So, it can eliminate any drivetrain binding by reverting to 0% as needed.

Why do I bring this up? Someone (I forget who) wrote that the RDX could not have a significant rear-ward torque bias because of the potential for binding. Aside from the fact that we're talking torque, not speed, that's obviously not the case. Binding cannot happen. It's already built in. Actually, the clutch packs used in the rear diff for the RDX have been beefed up (compared with those in the RL) to accommodate that constant slip.

If 30/70 is possible, why not more? - Robertsmx

In theory, all that is required from a hardware standpoint is reprogramming the SH-AWD ECU.

Why did they chose 55/45 for the RDX and 60/40 for the RL?

Weight distribution. The RL is decidedly more nose heavy than the RDX. When it comes to straight line acceleration, you want the power to go to the tires with the most traction. In most cases, that would be the ones with the most weight on them.

Sending more power to the rear would make sense if the vehicle's is tail-heavy. Otherwise I see no reason for it.

(Time for anudder side note: When Infiniti released the story of developing the FM platform, they specifically mentioned the strive for a "perfect" 52/48 weight distribution. They claimed that under acceleration, weight shifts to the rear making 52/48 into 50/50. If we keep that 2% shift in mind, and assume the 57/43 balance is correct for the RDX, then it becomes roughly 55/45 during acceleration. Coincidentally (?), the same ratio as the torque split under the same conditions.)

Even if there is some theoretical advantage to rear-biased power delivery during the straights, in the real world it appears the RDX is faster. (This based on the 0-60 and 1/4 mile measurements from various magazines.) So, drivetrain output trumps any potential AWD bias in this instance.