flexer

I'm so confused on all the information that honda is releasing on how its SH-AWD distributes the ratio of power to the front and rear wheels. Lets first state the obvious:

If the rear diff locked both of its electromagnetic clutches the system would then be 50:50. Meaning 50% torque to front wheels 50% to rear. This is because the car has no center diff so all locked up would mean every wheel gets equal power assuming all have traction.

Now lets talk about what acura says: They say that on the 2014 TL up to 70% of the power can be directed to the rear of the vehicle if need be. I was SO CONFUSED on how this is possible till I read that the rear is constantly overdriven by 1.7%. AAAAHHHHH ok. So the rear diff is geared 1.7% faster than the front. NOW HOLD ON. So a 1.7% over gearing causing 70% of the torque to then want to go to the rear wheels? I guess. I 'm sure you can't just add 1.7% to the torque split because the over drive would make those wheels drive slightly easier which would naturally send power more towards those wheels which could account for the 30:70 possible torque split acura is talking about if its rear clutches in the diff were FULLY LOCKED.

BUT WAIT! THE STORY GETS CONFUSSING. Acura in its release of the first SH awd in 2005 on the acura rl said that in straight line acceleration the max amount of power available to the rear wheels was 40%. OK I buy that. The RL does have the 1.7% gear increase but they are just not locking the rear diff all the way to get a full 50/50 during acceleration. Now the RL has a front "acceleration device" that actually can increase the rear over drive up to 5.8% in cornering situations, BUT LETS NOT BRING THAT INTO THE CONVERSATION AT THIS POINT. Instead lets go back to the TL.

Ok so the Tl in 2014 is said to allow up to 70% of available torque to go to the rear wheels which we assume is from the 1.7% overdrive of the rear with a FULL LOCK of the diff.

WELL HERE COMES THE CRASHING DOWN ON MY PARTY.

For the 2015 TLX acura has released that with the new design of the SH AWD rear diff it is ditching the electromagnetic clutch packs for hydraulically controlled clutches that work the exact same but are able to shed 30% of the weight of the rear diff. GREAT. Also the rear over drive is being increased from 1.7% to 2.7% to allow even more control and power to be directed to the rear wheels. They state that up to 70% of available engine torque can now be sent to the rear wheels.

WAIT WAIT WAIT. WHAT THE HELL? with a 1.7% you said up to 70 % could be driven to the rear wheels and now you say even more can be driven to the rear wheels with a 2.7% overdrive yet you state that as 70%?

And then you go and quote that the new TLX can supply up to 885 ftlbs of torque to one wheel in a cornering situation. WHAT THE HELL?!?!?! The car is rated at like 270 lbs of torque and your saying a 2.7% overdrive gets me to 885 lbs of torque to a wheel? You must be using some

Help me understand this people. Where am i going wrong in my thought process?

Let me throw some quotes in:

2010 acura TL:
SH-AWD® operating parameters include:

Up to 90-percent of available torque can be transferred to the front wheels during normal cruising.
In hard cornering and under acceleration, up to 70-percent of available torque can be directed to the rear wheels to enhance vehicle dynamics.
Up to 100-percent of the torque sent to the rear axle can be applied to either rear wheel, depending on conditions.

The TL SH-AWD® rear drive unit is constantly overdriven by 1.7-percent and the resulting overdrive effect is regulated by left and right electromagnetic clutch packs which independently control the power delivered to each rear wheel.

2015 TLX :
Up to 90-percent of available torque can be transferred to the front wheels during normal cruising.
In hard cornering and under acceleration, up to 70-percent of available torque can be directed to the rear wheels to enhance vehicle dynamics.
Up to 100-percent of the torque sent to the rear axle can be applied to either the left or right rear wheel, depending on conditions.

The TLX's new, more lightweight (25 percent reduction) SH-AWD rear drive unit is constantly overdriven by 2.7-percent (the previous generation TL system was overdriven by 1.7-percent). The resulting overdrive effect is regulated by left- and right-side clutch packs (located in the rear differential) that independently control the power delivered to each rear wheel. The increased overdrive percentage in the new TLX means that the torque vectoring effect is more pronounced and effective even in corners with a radius of as little as 49.2 feet (15 meters). This means that torque vectoring can be felt more often, such as when accelerating aggressively through a standard corner. Up to 885 lb.-ft. of torque can be delivered to either rear wheel, which gives the system the unique ability to yaw the TLX into turns for superior handling.



2009 Acura RL:

In full-throttle straight line acceleration, up to 40-percent of the power is sent to the rear axle.

In hard cornering, up to 70-percent of available torque goes to the rear wheels for enhanced chassis balance. Up to 100-percent of this torque can be applied to the outside rear wheel that can also be overdriven up to 5.7-percent by a built-in acceleration device.

echodigital

Maybe this will help...

SH-AWD - Wikipedia, the free encyclopedia

I just drive the car and it is better than FWD. Especially in the winter.

flexer

I wish that would help. That only gives basically the same information I gave above, but never talks about how the bias is achieved.

Tonyware

If I recall correctly there was another thread in the TLX section that discussed the issue extensively. I don't remember much, except perhaps talking about something referenced as "torque multiplier"? ... search forum and shall find.

Electromagnetic clutches were replaced by hydraulics because lots of 4G TL ownes reported operating powefull CB and Ham Radios in their back seats (.... Just kidding!!!... or read between the lines!)

Ps. The 4g shop repair manual has an excellent description of the entire SHAWD system starting at chapter 15... Seek, download, read. Hope the TLX shop manual will "leak" soon to read up about the new diffs!!!

Riboflavin

From everything that I've been reading (wikis, various articles, and Honda's releases), my understanding is that the torque distribution isn't caused by the overdrive, it's regulated with the electromagnetic clutch packs in the rear diff.

The front axles are powered though the transaxle, as normal with a FWD/transverse engine setup, and the rear axles are powered through the torque transfer unit (attached to the transaxle, where the propeller shaft is sent to the rear diff). The torque transfer unit isn't computer controlled, and is just the mechanical connection from the transaxle to the rear diff.

Then, due to the electromagnetic clutch packs in the rear diff, this is where the front/rear torque vectoring magic happens. With both rear clutches fully disengaged in the TL, only 10% of the available torque is transferred through the rear axles, and the remaining 90% stays with the front wheels. When both rear clutches are fully engaged, 70% of the available torque then goes through the rear axles, and the remaining 30% stays with the front wheels. Since both clutch packs can be independently actuated, that's where the 100% right/left vectoring comes from. Since all 4 wheels aren't connected to the same diff, they don't have a default 50/50 distribution.

Now regarding the 5.7% acceleration device in the RL, or the default 1.7% overdrive from the torque transfer unit in all other SH-AWD implementations, this doesn't have anything to do with the amount of torque being distributed, it just controls the actual speed of the rear wheels after torque has already been transferred via the clutch packs. Higher overdrive percentage means the activated wheel will spin faster, and that inward yaw moment (SH-AWD rotation) becomes more pronounced. Without the overdrive, you could still do the rear-outside wheel torque transfer via the rear clutch packs, but the activated rear wheel won't be turning any faster than the front wheels, and you won't get that inward yaw moment.

This is why even though the RL has the acceleration device overdriving the rear wheels at a higher rate than the TL, it doesn't have a greater torque distribution since that's caused by the rear diff clutch packs. The 2.7% rear overdrive in the TLX means that the activated outside wheel will be able to spin 1% faster than the TL's wheel would, producing a higher inward yaw moment even if the clutch packs are still distributing the same amount of torque.

Oh, and I don't think that 885lb-ft number was an actual number for the TLX output, I think it was just referring to the amount of power that its clutch pack system can physically cope with.

Below are all the random ass articles I've been going through, in no particular order.
Honda all wheel drive explained | awd cars, 4x4 vehicles, 4wd trucks, 4motion, quattro, xDrive, SH-AWD, Haldex, Torsen, wiki - How it works
SH-AWD - Super Handling All Wheel Drive from Honda/Acura
SH-AWD | David Sigalingging
http://www.atraonline.com/gears/2010.../2010_5_52.pdf
The Temple of VTEC - Honda and Acura Enthusiasts Online - Articles - "and/or": SH-AWD
SH-AWD - Wikipedia, the free encyclopedia

liquidneon

I don't understand how it works, but it feels good, and I like it.

flexer

Thanks for the response. One problem is that while the wheels are no connected to a common diff, they are connected. But you may have found the answer. When looking at the transmission the rear output shaft is taken off the drive to the front wheels, so they are locked all together and the 90f/10r distribution is obtained by just not locking the clutches in the rear diff. But these wheels are naturally all locked together, so a full lock up in the rear would resort to a 50/50 split. UNLESS What if the transfer case is not a 1:1 ratio? Lets say the rear diff was geared down so that it is easier to drive the rear wheels than to drive the front wheels. This would make it so that when you lock up the rear diff its not 50/50 because the rear is geared down. This would make it easer for the motor to turn the rear wheels and power would go there first. This must be it. The gearing in the transfer case or they could have done the gearing in the rear diff to be different.

Example, the front could be a 4.10 gear, but the rear could be a 4.80 gear. A normal car couldn't do this as the difference in gearing would rip the car up, but since the SH awd allows that slip to occur without causing problems.

What do you guys think about my theory?

Reorge

Some things are just meant to be misunderstood; like women. They can be fun to drive but difficult to understand. Don't question their logic or you could be "living in a van down by the river".

dtrayers

Here are two more resources:

Honda Worldwide | Technology

Really scientific stuff, free registration required: https://www.hondarandd.jp/index.php

lift3d_

thanks for the laugh, summarized my feelings exactly on this thread

Riboflavin

That's what I figured too. Since the TL doesn't have an acceleration device between the torque transfer unit and the rear diff, one of those two has to be generating the rear overdrive. Now I think it's the rear diff that changes the gearing, since all the documentation for the TL and TLX specifically refer to the "rear-drive unit" being overdriven by 1.7% (or 2.7% in the TLX) and not the torque transfer unit transferring overdriven torque through the prop shaft and into the diff.

2009 Acura TL | car review @ Top Speed

That could account for that default 10% coming from the rear diff when the clutch packs are disengaged rather than completely free wheeling.

LOL we're ruining the magic. At least I'd get to live in a TL down by the river. Uncomfortably trying to sleep on that hump on the rear seat.

Reorge

Confucius say: One who tries to understand SH-AWD approaching a slippery slope.

Riboflavin

"Man who stand on toilet gets high on pot."

Tonyware

hahahaha!!!

...and to paraphrase a famous comedian: I spent the first half of my life trying to understand SH-AWD. I'll spend the rest of my life trying to forget what I learned