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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.

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Only one question: Who at Honda is releasing the information? I suspect it is the marketing divison, in which case nothing has to make sense, just sell cars. Sort of like the Great Horspower Race of the 50's - 60's.

Bill

Correct. But you know they are asking the engineers questions and getting answers. Help me people

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I think the mistaken assumption might be that the clutches are fully locked at maximum rearward torque transfer. As I interpret it, the only time it might be fully locking is under certain turning conditions on the outboard side, with the inboard side unlocked, where the tire needs to spin faster anyways and/or any over driven tire slip is helping rotate the car. The rest of the time, even under maximum torque transfer, the clutches are slipping and absorbing much of the constant slip rate between the front and rear.

The 885 ft-lb torque vectoring had me a little stumped, as I actually expected it to be higher in 1st gear after the final drive multiplication. It's all in the wording of the marketeers. Saying "up to 100%" of the "up to 70%" in no way implies that it does so under the most extreme circumstances. So what they're saying is anything that exceeds 885 ft-lb through the axle is just going to result in more clutch slippage.

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-Justin

"Orville Wright did not have a pilots license." - Gordon MacKenzie

The TL states that up to 70% of the torque can be transfered to the rear under hard straight line acceleration events and under heavy cornering.

I agree at first I thought they were just doing it during cornering until I drove an TL and when you step on it hard the AWD indicator shows 4 bars of power going to the rear and 3 to the front so it is putting more power to the rear under heavy acceleration I just can't understand how when fullly locked up would be a 50/50 split.

Here is my only theory. What if the rear diff gearing is different than the front would that work? Like lets say the front is 4.10 gear and the rear is 4.80? That would make driving the rear wheels easier and more power would then be driven to the rear.

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In a straight line, the clutches are still always slipping to accommodate the over driven ratio on the rear axles. Using the ability for a slipping clutch to convert a rotational speed difference into applied torque is the principle they are utilizing.

Think of it this way: With all 4 tires turning at the same speed, the input (driven) side of each rear axle clutch is spinning some amount (1.7% or 2.7%) faster than the output (wheel) side of the clutch. With very minimal pressure on the clutches when cruising in a straight line, they would slip quite easily and not much torque would be transferred to the rear. Increasing pressure on the clutches to near-lockup, but not fully locked, they would be able to transfer even more torque (up to 70%) to the rear axle than the 50% typical of a fully locked system that does not over drive the rear axles.

When traveling in a straight line, locking one or both of the clutches on the over driven rear axles would result in some less-than-desirable forces being reacted through the driveline and tires, and would significantly reduce the life of at least the tires.

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-Justin

"Orville Wright did not have a pilots license." - Gordon MacKenzie

I agree with you Justin. Now I need to measure the amount of amperage being driven to those clutchpacks in a straight line situation to see how much they are really squeezing them down.

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Attention TURBO LOVERS, Great book -----> How to turbocharge and Tune your Engine
Sold: GT30/71R My00 S2k
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