Audi A4

[MrH]

Meh.  It's not about just what diffs are open anymore.  When you can individually control brake application at all four corners, that changes the game completely.

[12,000 RPM]

Open diff is the other way around - the wheel with least traction gets the power.

Yep, peg leg. Not a big deal w/4WD, but quattro is pretty much representative of available AWD systems on the market. Its superiority is just marketing and legacy at this point. Still a good system though.

[12,000 RPM]

Meh.  It's not about just what diffs are open anymore.  When you can individually control brake application at all four corners, that changes the game completely.

Not if u drive spiritedly or do track days. Wat happens when ur brakes begin to fade? Back to peg legging. Naw, no substitute for an honest to god LSD.

[MrH]

Not if u drive spiritedly or do track days. Wat happens when ur brakes begin to fade? Back to peg legging. Naw, no substitute for an honest to god LSD.

We're talking about bad weather traction...

[GoCougs]

LSD is limited too - by itself it can't split power between axles on demand.

[Raza]

Open diff is the other way around - the wheel with least traction gets the power.

Really?

[GoCougs]

Yep:  https://www.youtube.com/watch?v=gIGvhvOhLHU

Fast forward to 2:18 to see a visual. Watch the previous ~2 minutes of why that is (not intuitive).

[MrH]

Open diff is the other way around - the wheel with least traction gets the power.

No.  Neither wheel gets power.  Because there is no traction on one wheel, it just spins, and no power is transmitted.


It's simple.

Open diff - Both wheels spin with the same torque, but speed can vary.

Locked diff - Both wheels spin at the same speed, but torque delivered to the ground can vary.

[GoCougs]

No.  Neither wheel gets power.  Because there is no traction on one wheel, it just spins, and no power is transmitted.


It's simple.

Open diff - Both wheels spin with the same torque, but speed can vary.

Locked diff - Both wheels spin at the same speed, but torque delivered to the ground can vary.

If a wheel is spinning, it's getting power, if it's not spinning, it's not getting power . In that vid at 2:18 the spinning non-traction wheel is getting 100% engine power and the non-spinning traction wheel is getting 0% engine power.

Locked diff is the same torque and speed left and right. This is virtually impossible to drive on the street owing to the inability to negotiate turning. These are usually referred to as a spool (typically used in drag racing) which is not even a diff but simply a set of right angle gears or a locker (typically used in serious 4wd rigs) which is a diff with a pneumatic or electronic soleniod that locks the axles together when off road.

Most any LSD on the street is clutch based, which means the L & R axles are not locked but coupled with a degree of slip to negotiate turning.

[12,000 RPM]

Yea LSDs only have a % lock, and vary the amount of lock significantly for acceleration and deceleration.

We're talking about bad weather traction...

By that measure, in combo with EBD (which is pretty much standard on every car now), there is def no material difference among the various AWD systems in the luxury market.

[GoCougs]

Uh, there are lots of differences, as already mentioned: active vs. reactive (i.e., reacting to car dynamics or just wheel slip), rear L vs. R torque split or not, rear L vs. R speed split or not, degree of nominal F vs. R torque split, viscous vs. clutch pack t-case, and probably other things I'm missing.

[Cookie Monster]

If a wheel is spinning, it's getting power, if it's not spinning, it's not getting power . In that vid at 2:18 the spinning non-traction wheel is getting 100% engine power and the non-spinning traction wheel is getting 0% engine power.

Locked diff is the same torque and speed left and right. This is virtually impossible to drive on the street owing to the inability to negotiate turning. These are usually referred to as a spool (typically used in drag racing) which is not even a diff but simply a set of right angle gears or a locker (typically used in serious 4wd rigs) which is a diff with a pneumatic or electronic soleniod that locks the axles together when off road.

Most any LSD on the street is clutch based, which means the L & R axles are not locked but coupled with a degree of slip to negotiate turning.

I thought most LSDs were gear based? I know the Torsens used in Miatas are gear based.

Also, isn't what MrH is saying that open diffs provide the same torque to both wheels, the diff can only send as much torque to both wheels as the wheel with the lesser traction can handle? So if you have one wheel on ice and one on the road, the wheel on ice can handle (effectively) 0 torque, so the diff can only send that much torque to the wheel on the road with the traction.

Or is that wrong?

[MrH]

If a wheel is spinning, it's getting power, if it's not spinning, it's not getting power . In that vid at 2:18 the spinning non-traction wheel is getting 100% engine power and the non-spinning traction wheel is getting 0% engine power.

Locked diff is the same torque and speed left and right. This is virtually impossible to drive on the street owing to the inability to negotiate turning. These are usually referred to as a spool (typically used in drag racing) which is not even a diff but simply a set of right angle gears or a locker (typically used in serious 4wd rigs) which is a diff with a pneumatic or electronic soleniod that locks the axles together when off road.

Most any LSD on the street is clutch based, which means the L & R axles are not locked but coupled with a degree of slip to negotiate turning.

It's not delivering any power to the ground.  Spinning freely but the car not moving = zero torque being applied.  RPM x Zero torque = zero power.

-1 internetery by Cougs

[MrH]

I thought most LSDs were gear based? I know the Torsens used in Miatas are gear based.

Also, isn't what MrH is saying that open diffs provide the same torque to both wheels, the diff can only send as much torque to both wheels as the wheel with the lesser traction can handle? So if you have one wheel on ice and one on the road, the wheel on ice can handle (effectively) 0 torque, so the diff can only send that much torque to the wheel on the road with the traction.

Or is that wrong?

Si.  If one wheel has zero traction and an open diff, zero torque will be applied to the ground for both wheels.

[GoCougs]

Well, there are variations on the LSD, viscous, gear and clutch. I don't know what the market share is of each and perhaps the clutch based aren't the most popular (they used to be). 

Yes, that is basically right, however he said neither wheel is getting power which isn't correct - if a wheel is spinning it's getting power, if it's not spinning it's not getting power.

[GoCougs]

It's not delivering any power to the ground.  Spinning freely but the car not moving = zero torque being applied.  RPM x Zero torque = zero power.

-1 internetery by Cougs

Unless the surface's coefficient of friction is 0 power is most definitely getting put to the ground. Chronic spinning on ice or sand or mud or w/e will make noise and melt/heat/burn the tire and/or surface even if the vehicle doesn't move (and heat and sound = power).

[MrH]

Totally insignificant amounts and worthless to the driver.

[GoCougs]

The point is with an open diff in Catch-22 fashion the wheel with the least traction gets the most/all power, and there are various ways to combat that.

[Madman]

And we all used to wash clothes in a bucket and churn our own butter. It's a bad argument.

Hey, quit making fun of my wash bucket and butter churn! 

[Jon?]

I find the most annoying trend is the near identical shape of models across a manufacturer's range.  I can't tell an A3 from an A4 without spotting the badge on the trunk.  They're identical if you're not a hard-core Audi-phile.

[Gotta-Qik-C7]

I find the most annoying trend is the near identical shape of models across a manufacturer's range.  I can't tell an A3 from an A4 without spotting the badge on the trunk.  They're identical if you're not a hard-core Audi-phile.

[Char]

Shit.

[giant_mtb]

Piss.

[Jon?]

Poop

[giant_mtb]

Boogers

[12,000 RPM]

Jizz!

This car does not excite me at all

[Tave]

It looks like my Jetta with a different grill. Unacceptable for what they charge.

[giant_mtb]

Glad I got outta the A4 game when I did.  Every time I see a newer Jetta, it looks just like my car, especially since mine was a base model without the fancy LED/HID headlights...the halogen headlights on the newer base Jettas are essentially identical to the ones that were on my A4.





I mean, they're basically the same car as it is, but the new Jettas are so similar to the looks of my 2010.

[MX793]

Despite the styling, the Jetta is on a completely different platform from the A4.  One has a transverse engine, the other longitudinal.  The A3 sedan, on the other hand...

[giant_mtb]

Oh, yeah. I was speaking more in terms of styling, styling cues, interior design, etc.