Honda real time AWD

[Hokiefyd]

Originally Posted By: IndyIan
I can't see why having different length drive shafts on the front and back axle would matter at all for torque steer?


I think the influence of drive shaft length symmetry on torque steer is highly overstated myself. I've owned two FWD vehicles with decidedly high performance engines for driving the front wheels, two Northstar Cadillacs. The half shafts were exactly the same length...this is because the 4T80-E transmission lays transversely behind the engine, rather than all crunched together on one side like most TFWD transmissions.

Put the pedal to the mat, though, and those Sevilles would weave and bob all over the place. Now, they may have been even worse if the half shafts were of unequal length, but there's more to it than shaft length alone...ahem.

You have a lot going on in the front suspension, and when toe is essentially variable (due to the movable toe control links...also known as tie rods), you can get pulls to one side or the other. You also have a lot of suspension compliance bushings, engine cradles that might shift or flex under load (to which suspension links are attached), etc. Lots of stuff going on up front.

Our MDX doesn't have equal length half shafts in the front, and it has minimal torque steer...probably because it's helped by the rear wheels. But if you turn the steering wheel a little bit during slower speed WOT, it will get a little jukey on you.

 

[Kiwi_ME]

Originally Posted By: Thermo1223
Actually if you ever drove a FWD car with a bit of power you'd realize that equal length driveshafts do help a lot with trq steer.
Why would you assume I haven't?

The image you have posted shows equal length left and right driveshaft lengths. To make a point about torque steer you might want to post one that has unequal lengths, probably not as easy to find as Subaru would have you believe.
Even in that case, torque steer is not going to affect the AWD ability.

 

[Thermo1223]

Originally Posted By: Kiwi_ME
Originally Posted By: Thermo1223
Actually if you ever drove a FWD car with a bit of power you'd realize that equal length driveshafts do help a lot with trq steer.
Why would you assume I haven't?

The image you have posted shows equal length left and right driveshaft lengths. To make a point about torque steer you might want to post one that has unequal lengths, probably not as easy to find as Subaru would have you believe.
Even in that case, torque steer is not going to affect the AWD ability.


Actually look at it they just encased in a rigid housing. Now it may minimize it but it is not going to eliminate it. It is still an unequal length driveshaft.

If the trq is getting shifted due to unequal mass on a opposite wheel it sure as heck would throw it off. Will it make or break the AWD capability? Most likely not but it isn't helping that is for sure.

Suspension is another thing, most FWD/AWD setups don't change the suspension much which further complicates it by adding more weight to the front of the car. Therefore throwing the distribution off.

A lot of cars use unequal length for FWD and then later on decide to throw on AWD to gain market share. I just know Subaru works, without question where others fall short, even in some cases Audi.

Edit: Sevilles were really front heavy no? Didn't they literally as you seem to describe but the engine on top of the transmission to get it to fit? Not bashing them in any sense(rode in one a few times)just an observation. I still can't get it around my head WHY I wasn't even offered my grandmother's Sedan Deville

I would have paid her too.

 

[Kiwi_ME]

Originally Posted By: Thermo1223

Actually look at it they just encased in a rigid housing. Now it may minimize it but it is not going to eliminate it. It is still an unequal length driveshaft.

Yeah, umm, sorry, it might be a good idea to learn more about the subject, and certainly not from Subaru's marketing department (or anybody elses!) Try a book on mechanical engineering forces and moments.

As you note, the diff outputs are rigid, irrespective of the gear casing being offset. How the power is delivered to the inner CV joints is not relevant to torque steer, only the geometric symmetry of the CV joints, suspension pivots and steering axis. Even minute variations (due to normal tolerances) of those positions from side to side will produce barely-perceptible torque steer, common on every FWD car, especially while turning.

I can see you like Subaru but trust me, Audi's engineers are no fools.

 

[Hokiefyd]

Originally Posted By: Thermo1223
Edit: Sevilles were really front heavy no? Didn't they literally as you seem to describe but the engine on top of the transmission to get it to fit?


No; the transmission sits transversely behind the engine. In this picture, the engine is missing, but you can see where it would sit in the cradle. The bulk of the transmission tucks in behind the engine, under the rear of the "V". It's tight, but it's pretty smart packaging.

 

[Hokiefyd]

Originally Posted By: Kiwi_ME
Even minute variations (due to normal tolerances) of those positions from side to side will produce barely-perceptible torque steer, common on every FWD car, especially while turning.


And in the case of something like a FWD GM vehicle with a full front suspension cradle, if that cradle has any flex or any movement within the large rubber cradle bushings, you'll get a "misalignment" there as well...because the suspension points move with the cradle.

 

[Thermo1223]

Originally Posted By: Hokiefyd
Originally Posted By: Thermo1223
Edit: Sevilles were really front heavy no? Didn't they literally as you seem to describe but the engine on top of the transmission to get it to fit?


No; the transmission sits transversely behind the engine. In this picture, the engine is missing, but you can see where it would sit in the cradle. The bulk of the transmission tucks in behind the engine, under the rear of the "V". It's tight, but it's pretty smart packaging.

Ahh I see now, I don't know why I thought it sat on top.

 

[Thermo1223]

Originally Posted By: Kiwi_ME
Originally Posted By: Thermo1223

Actually look at it they just encased in a rigid housing. Now it may minimize it but it is not going to eliminate it. It is still an unequal length driveshaft.

Yeah, umm, sorry, it might be a good idea to learn more about the subject, and certainly not from Subaru's marketing department (or anybody elses!) Try a book on mechanical engineering forces and moments.

As you note, the diff outputs are rigid, irrespective of the gear casing being offset. How the power is delivered to the inner CV joints is not relevant to torque steer, only the geometric symmetry of the CV joints, suspension pivots and steering axis. Even minute variations (due to normal tolerances) of those positions from side to side will produce barely-perceptible torque steer, common on every FWD car, especially while turning.

I can see you like Subaru but trust me, Audi's engineers are no fools.


Umm try not assuming I haven't (by your own words)

Audi makes a very decent AWD for cars/wagons but their foray into the SUV market all they did was slap on the same center diff but in a transfer case style. Granted there are only so many ways to do things but it's nothing special. They just copy & pasted what was already done for the VW Touareg & Porsche Cayenne.

The Q5 is actually much better in this regard they stuck to their roots but that is because the Q5 is technically built up from a car, the A5.

Not to say it's bad or non-functional but it is easy to see why it was wandering around on that test, that is all.

 

[SteveSRT8]

Originally Posted By: IndyIan
I can't see why having different length drive shafts on the front and back axle would matter at all for torque steer? Both sides of each end are effected the same, by their drive shafts?
I think almost all higher hp cars with streetable bushings in the suspension will do some odd things under power, especially a luxury type car like an Audi. I've watched a WRX sti with all sorts of mods on sticky autocross rubber, start to get almost wheel hop torqueing itself around in a tight section. Symmetrical drive shafts or not, the driver was getting all sorts of "torque steer" through the steering wheel.


The different length half shafts are usually due to packaging concerns, and unfortunately they do nothing about torque steer. TS is exactly what you think it is, pure force moving suspension components in such a way that it affects the steering wheel, while different wheel speeds and traction tug hard on the wheels as well.

Unequal length half shafts and even different diameters are used to combat wheel HOP, completely different issue...

 

[rjundi]

I occasionally notice the torque steer in my 2007 MDX with SH AWD for a really brief moment if you press on gas.

It does have 300HP/273 ft-lbs peak but not sure how much is being put down when I notice it.

 

[SteveSRT8]

Originally Posted By: rjundi
I occasionally notice the torque steer in my 2007 MDX with SH AWD for a really brief moment if you press on gas.

It does have 300HP/273 ft-lbs peak but not sure how much is being put down when I notice it.


More than likely their extremely sophisticated engine and chassis management systems detect it before you do and reduce timing or close the throttle, etc., to reduce it. Modern autos often have a great deal of software dedicated to protecting the driveline...

 

[gregoron]

Originally Posted By: Thermo1223
Originally Posted By: Kiwi_ME
The "symmetric" thing is equally of little practical advantage other than giving the marketing department something to do. In the end, the number of 90-degree changes in drivetrain re-direction are the same, no matter how they spin it.


Actually if you ever drove a FWD car with a bit of power you'd realize that equal length driveshafts do help a lot with trq steer. That is what symmetrical is referring too. Equal length shafts at the back & front(or close to it).

In the above video you can see the Audi Q7 fighting trq steer.

This pic you can see why.

Actually, this picture shows equal length CV axles for both front and rear axles. There's a differential in the front which I believe is limited slip. That differential allows both front CV axles to be the same length so there shouldn't be a torque steer issue.

 

[SteveSRT8]

Originally Posted By: gregoron
Actually, this picture shows equal length CV axles for both front and rear axles. There's a differential in the front which I believe is limited slip. That differential allows both front CV axles to be the same length so there shouldn't be a torque steer issue.


Torque steer can be mitigated by a limited slip diff but it is not eliminated, as it has little to do with the length of the halfshafts and everything to do with suspension compliance...

 

[gregoron]

Originally Posted By: SteveSRT8
Originally Posted By: gregoron
Actually, this picture shows equal length CV axles for both front and rear axles. There's a differential in the front which I believe is limited slip. That differential allows both front CV axles to be the same length so there shouldn't be a torque steer issue.


Torque steer can be mitigated by a limited slip diff but it is not eliminated, as it has little to do with the length of the halfshafts and everything to do with suspension compliance...


Yes, I understand. A lot of factors contribute to it ( I.e. axle flex, suspension angle, tire flex, etc.). But, having intermediary axles to equalize the length of the drive shaft can mitigate it.

What I'm really trying to say is that symetrical AWD is a good thing. But, there are other AWD and 4WD systems that are asymmetrical and are quite capable off-road. Here are a few:

1. Land Rovers
2. Land Cruisers
3. Jeeps
4. MB G Wagens
5. Unimogs

If they have transfer cases in the middle of the vehicle, their front axles will most likely be unequal lengths. In some cases even the rear axles are unequal also.