are-dee-ex

I have had my RDX for about 7 months and it has yet to see snow but will very soon(New England). I have a good understanding of how the SH-AWD works in that it can vector torque to the outside wheel in corners, but I was wondering if it functions as a form of traction control in slippery conditions. Will it apply torque to a certain tire if you start to get sideways while on throttle, like in a drift, in an attempt to correct the cars direction? Will the rear differential also act as a locking diff or limited slip diff based on wheel spin?

Thanks

MMike1981

rear diff doesnt lock, when slippage is detected, someone correct me if im wrong, i think its biased to the front and if the front slips power will be sent to the rear (or the tire that the comp believes has contact)....its behaves a bit different for me in certain circumstances, best bet is to take your RDX to a parking lot with snow and feel out the system for yourself, that is the only way you will know how it behaves and how it reacts, you can do it safely in a lot to understand the car...

the tq vectoring is basically done to my understanding not on slippage but on throttle & like it says "vectoring" to the outside wheel that compensates for understeer and reacts to the angle/lateral gs.

When slippage is detected usually power is routed to the wheels that the computer detects as having traction...but in my experience the RDX will tend to plow ahead in snow with FWD bias only reverting to the rears when the front loses grip....

are-dee-ex

Yea, I just didnt know if that was how the system was programmed by acura or something like that. Like if you were to powerslide the car, and stayed on the throttle through the slide, will it attempt to correct with torque vectoring.

Now that I think about it, it would probably spin the wheel needed to correct the slide anyway assuming you were counter steering since that would be the "outside" wheel according to the steering wheel.

Im interested cause I have a CRV with awd(fwd primary until slippage) and its horrible, if u are on the gas at all and turn, it goes straight. If u do by some chance get it sideways it does not want to come back around. Multiple times i have had troubles making corners in it and wanted to see if the RDX would be a lot better. It has good all season tires on it that are rated well in snow too. I also have a Lincoln LS which is a rear drive V8 without an open diff and I have an easier time getting around in that than the CRV.

thanks

Carbon2008RDX

The SH-AWD is completely separate from the VSA (Vehicle Stability Assist). Keeping the VSA on significantly improves control of the vehicle in almost all driving conditions. The only time to turn it off is if the RDX gets stuck in serious snow, etc) where the VSA prohibits full torque. Once the RDX is moving again turn the VSA back on ASAP.

I tested my RDX last winter in a big open parking lot with a few inches of fresh snow. With the VSA on it was hard to get the RDX to mis-behave. Turn it off and holy crap!!! Donuts galore! (with little control of the vehicle... once its sliding, its sliding). Keep the VSA "ON".

big1128z

Keep the VSA on at all times while in snow unless like Carbon2008RDX said you want to do donuts down the freeway. The SH-AWD ECU receives commands from the ABS VSA control unit to perform the traction control function as well. SHAWD and VSA work in unison and when both are workin in unison you will have no problems, unless you hit a patch of ice, then it doesnt matter if you even had tracks on the thing you would still loose control.

MMike1981

i turn my VSA off when im really hammering the RDX

Carbon2008RDX

Mike – Please elaborate. What exactly are you doing when you say “hammering the RDX?”

MMike1981

VSA is all about limiting or shall i say restraining power to achieve what the computer may deem as more safe or efficient take offs, wide open throttle etc.

So, with it off, im not sure if the VSA is a non-defeat system in the RDX or not, but it gives you more control of the car in a sense, powers comes on harder, the computer wont limit wheel spin or dampen power delivery in situations where you really want it.

on straight line accels and throttling thru turns, vsa off with the car in sport mode is pretty good

black label

Fixed

The ZDX and MDX have "cooperative VSA" which works in conjunction with the SH-AWD, the RDX does not.

To the person who said their CRV wasn't great in the snow, there is a reason. Real Time 4WD that the CRV has is front drive until the front wheels slip, then it can send 20% of engine power to the rear wheels. The problem is that by the time the rear wheels get power, you have already dug the front wheels down in the snow. SHAWD (in the RDX) always powers all 4 wheels iat all times. It can send as much as 90% to the front with the remaining 10% going to the rear or as much as 70% to the rear with the remaining 30% going to the front. It makes these adjustments on the fly based on what you are telling the car to do as well as the available traction.
100% of the power that is going to the rear wheels can be sent to the outside wheel in when cornering and it can spin that wheel 1.7% faster than the inside wheel.

Carbon2008RDX

I don't quite understand what you're trying to say here. Yes, I know the ablities of the SH-AWD in the RDX. Are you saying the SH-AWD in the MDX and ZDX are different (inferior or better) to the RDX?

black label

Yes, maybe, sort of. The SH-AWD in all of them is very similar. I believe the maximum potential torque splits front to rear for all of them is 90:10 / 30:70 (I know it is on the RDX, I'm fairly confident it is on the MDX/ZDX).

The VSA on the MDX and ZDX is superior (or at least it can do more stuff) as the VSA on a MDX/ZDX is "cooperative VSA" which works in conjunction with the SH-AWD. The VSA on a RDX does not communicate with the SH-AWD, it just works with the braking system and the fuel delivery.

The only SH-AWD system that is inherently better than the others is that of the RL which uses a carbon fiber reinforced propeller shaft and has a more complex unit running the over drive of the outer rear wheel. While the MDX/RDX/ZDX can accelerate the outside wheel 1.7% faster than the inside wheel (no more no less), the RL can accelerate it up to 5% faster (key words being "up" and "to") due to the planetary gear set in the rear.

black label

Carbon2008RDX, the only reason I quoted you originally was to add that you should turn off the VSA if using the spare tire. Because it is narrower than the rest of your wheels it tends to confuse the VSA system and can lead to unnecessary wear of the brake pads on the corner that got the spare. The rest of what I was saying was really directed towards are-dee-ex as he is the one had had issues with the CRV in the snow, the RDX will be a completely different animal.

MMike1981

wait a sec.

the RDX rear wheels ARE overdriven by 1.7%, not that they can be, they always are. The RL system determines when to drive the rear faster and can do it at a greater rate.

"The rear drive unit of the RL sedan has a clutch and planetary gear set, called the "acceleration device," that can increase the speed of the rear axle up to 5.7-percent. In contrast, the RDX's rear driveshaft is constantly overdriven by 1.7-percent and the resulting overdrive effect is regulated by the left and right rear clutch packs."

also, the differences in the RDX to the MDX is the ability of SHAWD to route power to the rear in hill climbing, also, the MDX has a greater limitation on power heading to the rear in cornering. http://www.hondanews.com/categories/714/releases/4670

MDX operates differently than the RDX, and the RL is the most unique unit in the brand, altho all go under SHAWD. the systems are tweaked in each respective vehicle.

For the RDX, the specs can be found here: http://www.hondanews.com/categories/764/releases/5109

best bet is to turn to acuras literature when in doubt, you will find the answers to basically all of the questions about how the systems in our cars work, including the VSA. "The logic and control of SH-AWD™ is integrated with the RDX engine/transmission Electronic Control Unit (ECU), and Vehicle Stability Assist™ (VSA®) ECU."

black label

The rear wheels aren't always overdriven by 1.7%, if that was the case you would be burning rubber with the rear wheels whenever you drove in a straight line. This 1.7% increase only happens on the outside rear wheel when cornering and hitting the gas.

In the case of the RL it does the same thing at the same time however it can do it at a varying rate from 0%-5.7% depending on the situation (amount of throttle application as well as speed and steering position).

MMike1981

did you read the specs on the vehicles? you arent arguing my opinion but words from acura.

they are constantly over driven but controlled by the clutch packs

RL - no the RL has an acceleration device that increases the rear axel speed when needed; this contrasts with the RDX. The RDX is always 1.7% faster in the rear but modulated by the clutch packs.

taken from ACURA: The rear drive unit of the RL sedan has a clutch and planetary gear set, called the "acceleration device," that can increase the speed of the rear axle up to 5.7-percent. In contrast, the RDX's rear driveshaft is constantly overdriven by 1.7-percent and the resulting overdrive effect is regulated by the left and right rear clutch packs. The difference in the chosen rear axle speed increase is due to the differing handling dynamics of the two vehicles. The RDX achieves the desired SH-AWD™ cornering characteristics with less outside rear wheel speed increase. A matched pair of direct electromagnetic clutch systems, one on each side, control the power delivered to each rear wheel.

acura1972

quick question then: so does this affect durability then given that it's being constantly overdriven? wouldnt the clutch packs eventually "burn out"? [am no engineer here but i saw other posts -not here- that asked this before and i couldnt find an answer so thought about asking it here now given the topic] thanks!

black label

Mike, I'm not arguing your opinion, just your extremely literal translation of the litrerature.

The wheels cannot possibly be constantly overdriven. If the rear wheels always spun 1.7% faster, every mile you drove in a straight line the rear tires would have spun 1.017 miles for every mile the front tires travelled which would mean the rear wheels would constantly be squeeling .

It only accelerates it 1.7% when cornering and and giving the car gas. I could give you a list of my credentials with Acura and Honda as well as listing the awards I have received from the manufacter if that will help convince you but I feel you are a smart guy and can just understand that if the rear wheels spun 1.017 miles for every mile the front tires spin that you would be burning rubber constantly and rear tires on an RDX would be bald in about 7000 miles. You are reading into the literature too much, they are only talking about outer wheel overdrive when the SH-AWD is actively performing torque vectoring of the rear wheel power.

MMike1981

I understand they obviously cant be running at 1.7% faster. that woldnt make any sense.

my point is this, which you should also understand. in the RL, the rear tires are not over driven UNLESS the car determines to power them, which happens thru the RLs acceleration device.

the RDX, the rear is continuously powered at 1.7%, but this is kept in constant check by the clutch packs. Thus, the RL, the power isnt in the rear TILL ITS NEEDED, the RDX, the power is always AVAIL, but regulated thru clutch packs, same result, but 2 totally different methods in achieving that result.

i should have cleared that up before, but thought itd be completely obvious that you cannot have rear tires moving faster than fronts at all times errr

MMike1981

"The clutch packs and friction material are specially designed to withstand the small amount of continuous slip between front and rear axles (created by the 1.7-percent speed differential), while ensuring the expected level of Acura durability. An oil-temperature sensor allows the ECU to estimate the clutch plate coefficient of friction (which changes with heat) in each clutch pack and then adjusts voltage sent to the electromagnetic coil that controls each clutch. To ensure that the amount of torque transmitted remains optimized as miles accumulate a coil provides a feedback loop that the ECU uses to adjust voltage to the electromagnetic clutches to compensate for any clutch wear."

DJ Iceman

Fascinating. So on the RDX, to save a little money (I'm guessing) they used a simpler, more brute-force design which in turn forced them to overdesign the clutch packs. They're essentially using up their useful life with every mile you drive, much like tires themselves.

I have to wonder if it wouldn't have just been cheaper to use the same SH-AWD system across the entire vehicle line-up rather than customizing the solution on a vehicle-by-vehicle basis. I guess not; I would expect that the engineers and bean-counters have done very thorough analyses of exactly scenarios like that.

cwepruk

Bean counters aren't always right though.

Good info in this thread. Though I feel zero engine response difference with VSA off. It's only an issue once slip is detected and then it cuts back power/applies the brakes.

MMike1981

the same setup (over driven rear) is in the MDX as well, so maybe $$ wasnt the ONLY motivation behind it, who knows

but when i first read about the RDX back when it was released, the first thoughts i had was shit, does this system just burn itself out over time....my guess is that maybe not, but hard to imagine it being totally reliable in such a set up over an extended amount of miles

MMike1981

i feel the throttle applied more aggressively, and dont sense any brief delay, i can chirp the tires much more consistently (not like thats a benefit of having vsa off) but it tells me power is being sent when i want it & aggressively

i thought i had read somewhere about the change in attitude with vsa off, if i can come up with something ill post it....for some reason i thought power was applied differently - not sure tho will have to check on it...

MMike1981

i couldnt edit ran out of time, but vsa has always been a power drainer/limiter in my book. taking the computer out of the equation in my experience usually unlocks some power, especially on straight lines and accels thru turns

cwepruk

I've never seen it unless slippage is detected. I'd like to see some info that it does anything for performance. Those systems are typically reactive an only do anything once slip is detected.

MMike1981

if i hammer the gas in a corner, the VSA will retard power due to what it thinks may be an unsafe angle (relative to yaw rate) etc so i think there are other factors that come into play other than slippage...if it thinks the car is going out of control or will go out of control, thats when i thought the system kicked in as well.....but ill be in the hunt to find some answers

cwepruk

I can definitely see it in a hard corner or with some light slippage. But in day to day driving, even from a dig, throttle response and power seems the same (provided there is no slippage).

If you want to have some fun, it is better to turn it off otherwise it does intervene on any wheel slip.

corduroygt

So doesn't this mean that 1.7% of engine output is being wasted when you're not cornering aggressively? Another reason for poor mileage...