Quattro vs. Symmetrical

[fzanetti]

Also:

 

Audi's quattro system is definitely more special than most retro fitted all wheel drive systems out there. By retro-fitted I mean that the all wheel drive system has been engineered into the package as an afterthought rather than a purpose engineered from the outset AWD system (Subaru). Let's look at BMW's AWD system X-Drive for example, we all know that BMW steadfastly pursue a rear-wheel drive layout for a great many important reasons and this imparts their cars with brilliant driving dynamics and driver appeal. In order to pursue this philosophy, BMW ensure a perfect front to back weight distribution by mounting the engine well back in the engine bay. But when you wish to fit all wheel drive, this is where BMWs get ugly. Heavy componentry needs to be fitted to get drive all the way back to the front wheels - it's an exceedingly inelegant form of all wheel drive: it adds weight and compromises balance.

 

The biggest drawback with good all-wheel drive systems in the Audi and Subaru is the mass of the engine ahead of the front axle. This also affects driving dynamics as the nose of the car is less willing to turn in quickly and brings about the un-fun phenomenon of understeer.

 

Subaru use lightweight aluminium boxer engines that are short and sit very low in the engine bay thus improving the centre of gravity to alleviate this problem. Their Symmetrical All-Wheel Drive system is engineered as the whole combination of engine and drivetrain.

 

Another mechanism by which to get around the understeer problem (particularly in ultra sporty cars like the RS4 and STi) is to employ a rear biased all-wheel drive where more power is either automatically directed to the rear axle (RS4) or can selectively be sent aft (DCCD in STi) by the driver. Thumbs up for the STi!!!!

 

 

Cheers,

 

Flavio Zanetti

Boston, MA

 

 

Also look at:

http://www.audiforum.nl/Technique/Quattro/quattro.html

[fzanetti]

The biggest advantage that quattro has over Subaru is the use of the magnificent Torsen centre differential as everyone knows, this differential is unlike the those employed on our Subarus and also on all Subie line including the STi...

 

 

Whilst both cars employ a centre differential to split drive front to rear, it becomes important to have a device that ensures that even if grip is lost at one axle, then the wheels that still have grip get the power directed to them. This is where the limited slip device becomes so important. In the case of Subaru, these devices range from simple viscous coupling type to multi-plate clutch packs that use electronics inputs to determine which axle to send drive to.

 

With Torsen, it's a purely mechanical device that is, somehow, able to sense which axle has the most grip and then send the drive to axle and back, seemlessly and unbeknown to the driver before even traction has been lost. It's just great.

 

Subaru have some sophisticated electronic limited slip differentials that really do the job well, but I feel that Torsen is still king of the hill...

 

 

Ok, enough for now, tired of typing...

 

 

Flavio Zanetti

Boston, MA

[SoldonSubie]

Maybe the considerable difference in torque between a base OB and an OBXT makes the turbo model feel a little tougher to control in bad weather? Just a thought..

 

Of course that has to do with some of it. However, I am a VERY conservative driver in the snow. I'm not a putz but I don't push it. I probaly hardly get into the boost if that.

 

he 92a's on the 05 were dangerous. I did much better with the the upgrade to a better all season tire.

[bdwg22]

I traded an '02 A4 for my LGT and the Audi handled noticeably better, but I think the tires had a lot to do with it.

[fzanetti]

Which tires did you have on your A4????

 

Flavio Zanetti

Boston, MA

[supersport6]

I hear that the stock tires borderline unacceptable. I drive 95% highway miles between Nj and Vt. Are the stock tires really that bad.

[Edvig]

I hear that the stock tires borderline unacceptable. I drive 95% highway miles between Nj and Vt. Are the stock tires really that bad.

 

RE92s aren't THAT bad...but it depends on how you drive. "Normal" people don't have any problems with them. You'll quickly see the tire's limits as you push the car.

[eles1]

Of course that has to do with some of it. However, I am a VERY conservative driver in the snow. I'm not a putz but I don't push it. I probaly hardly get into the boost if that.

 

he 92a's on the 05 were dangerous. I did much better with the the upgrade to a better all season tire.

 

I wasn't implying that your driving was at fault. Simply pointing out that the extra torque of the turbo engine can "sneak up on you" under adverse road conditions.

 

My Z28 won't get out of its own way when the roads are the least bit bad. Granted, it's RWD, but there are a lot of other RWD cars out there much better under those conditions. Torque/weight ratio is very important for traction, as are the tires (which you've pointed out above).

[eles1]

I hear that the stock tires borderline unacceptable. I drive 95% highway miles between Nj and Vt. Are the stock tires really that bad.

 

Mine seem ok. I have no plans to replace them until they've worn out.

[The B4]

RE92s aren't THAT bad...but it depends on how you drive. "Normal" people don't have any problems with them. You'll quickly see the tire's limits as you push the car.

 

RE92s ARE that bad...i've driven both a wrx and a legacy gt with less expensive Kuhmo ASX's that made a noticeable difference in grip. I have no idea if they last as long as the RE92s though.

 

I have a few spiraling on ramps i like to enjoy...i got a lot more squeal and sliding on the RE92s than on the Kuhmos. I noticed that breaking is also affected...when your tires wear out...opt for some good ones...you'll be like HOLY CRAP!!!

[gfxdave99]

I have RE92's and a 5mt LGT and I think it tracks quite well in the rain.. Straight-line acceleration is smooth with no wheelspin and it corners well. The only places ive had problems with in the rain are a few turns that I have had problems with every car I've ever had that tend to just seem extra slippery when it rains.

 

But i make turns hit the apex, punch it and 98% of the time it pulls straight and true.. Once in awhile it gets wiggly but thats just me pushing the envelope. I think once I ditch the 92s things will hook up better. Esp since im going to wider tires / 18s

[supersport6]

Thanks. I'm coming from a front wheel drive car. Even in huge snowstorms I made it to my destination but cautious speeds.

[RobY]

The biggest advantage that quattro has over Subaru is the use of the magnificent Torsen centre differential as everyone knows, this differential is unlike the those employed on our Subarus and also on all Subie line including the STi...

 

 

Whilst both cars employ a centre differential to split drive front to rear, it becomes important to have a device that ensures that even if grip is lost at one axle, then the wheels that still have grip get the power directed to them. This is where the limited slip device becomes so important. In the case of Subaru, these devices range from simple viscous coupling type to multi-plate clutch packs that use electronics inputs to determine which axle to send drive to.

 

With Torsen, it's a purely mechanical device that is, somehow, able to sense which axle has the most grip and then send the drive to axle and back, seemlessly and unbeknown to the driver before even traction has been lost. It's just great.

 

Subaru have some sophisticated electronic limited slip differentials that really do the job well, but I feel that Torsen is still king of the hill...

 

 

Ok, enough for now, tired of typing...

 

 

Flavio Zanetti

Boston, MA

 

 

Audi Torsens work on a multiplier ratio which I think is 2.3:1 if I recall correctly. If one axle gets stuck it can only send 2.3 times more power to the rear meaning that if it takes 10 lb/ft for the front wheels to spin up in the snow you better be dammed sure that 23 lb/ft is enough to pull you out. If 23/lb ft is not enough to pull you out then you will continue to spin up the stuck wheels.

 

The STI electronic clutch system is WAY better as it has locking capability. It also has ways to sense which axles need the most power.

 

A bias ratio refers to the maximum amount of power that the torsens differential can send to the one set of wheels as a function of the traction on the other set of wheels.

 

If one wheel becomes unloaded or suffers limited traction the other set wheels will share the same limited traction based on the bias ratio. This underlies the main problem in using torsens differentals in limited tractive conditions. That is if one wheel completely looses traction or becomes unloaded the torque transfer to the other set of wheels (wheel) will be ZERO. Any bias ratio times ZERO is ZERO.

 

For example: Suppose in your torsens equipped car you get your front wheels stuck in a snow bank and your rears are on perfectly dry pavement on a sunny day. It takes 10 lb/ft to turn the front set of wheels before they spin up in the snow bank. Your rear wheels will get a maximum of 23 lb/ft probably not enough to move your car.

 

With the subaru viscous couping system if you were in the same situation spinning the viscous couping up enough will induce a center differental (quote) "lock" and your engine power would mostly be diverted to the tractive wheels. This is because a viscous couping does not depend on a bias ratio. It depends on a speed diffrence.

 

Torsens are good but they aint perfect thats why pro rally cars excluseively use electronic diffs. Even most group N cars use viscous diffs. A torsens does not have locking cpability... If you exceed the torsen's bias ratio you will spin up the wheels with least traction.

 

As for the 50mph disengagement... I read it on an audi forum. Its a necessary saftey feature becuse if you are to apply brakeforce to one axle at a high speed you are going to have alot of suehappy americans making VAG broke.

 

The current generation Quattro does not use real diffs to bias side to side power. It uses an open diff that is limited by braking action. (Marketing likes to call it EDL or electronic differential lock)

 

At high speeds clamping onto a rear right wheel while turning left will loop you so fast you will think you are on a merry go around.

[RobY]

Offical Audi Press release....

 

http://media.audiusa.com/article_display.cfm?article_id=9207

 

"EDL reduces tire wear and works up to about 50 mph in the quattro application."

 

 

about halfway down

[rao]

I seem to recall at the time that EDL stopped working at 25. Either way the system was annoying.....

[pdw]

Does anyone have a link explaining exactly how the AWD system in the LGT 5MT works?

[fzanetti]

I love this alias...

You can jump in with any info you know, and someone will always complete you with some more...

 

Great, thanks for the wide explanation man...

 

As I said earlier, QUATTRO is better on some circumstances, and Subie Symmetrical AWD is on others....

 

Flavio Zanetti

Boston, MA

[Deer Killer]

The brakes overheat, that's the real reason why. Hell it will even disable itself after a set time if you keep getting on it, i.e. having some fun in the snow. Viscous = tighter and tigher. I'd say that's more fun Not to mention EDL and torsen are lossy compared to our viscous lockers. We're getting more efficiency than regular open diffs because you're recovering what loss you'd have if you started having a speed differential.

[GetAwayDriver]

Now what about Lancia?

 

Great infomation, especially from Zanetti. I was thinking this could be a thesis or disertation.

[Scotty]

The one advantage of a electronic center diff is it can be set up to optimize the handling of the car. Something a mechanical diff can't do. Take Volvo's lastest Haldex AWD system. While under normal driving, 95% is still up front. Let's face it, under normal driving, your tires aren't slipping much, it really doesn't matter which wheels you drive. When you nail the throttle hard, the previous system wait till the front wheel starts losing traction before it sends power to the rear. The current system immediately sends 70% to the rear where you need the power. In addition, combined with the computer control suspension and stability control system, it is programmed so that to transfer power to the rear to PROVOKE OVERSTEER on turn in, equal power to all wheels to keep the car neutral in the middle and then more power to the front for understeer on the exit. While a skilled driver can do the same thing with a mechanical diff, a computer can do it better and you don't have to be skilled at it. The computer can't defy physics, but will stop you from pushing the car too far.

[IwannaSportSedan]

...Take Volvo's lastest Haldex AWD system. While under normal driving, 95% is still up front. Let's face it, under normal driving, your tires aren't slipping much, it really doesn't matter which wheels you drive. When you nail the throttle hard, the previous system wait till the front wheel starts losing traction before it sends power to the rear.

 

...While a skilled driver can do the same thing with a mechanical diff, a computer can do it better and you don't have to be skilled at it. The computer can't defy physics, but will stop you from pushing the car too far.

 

Front drive is still front drive. Even WITHOUT wheel spin, torque steer can pull the steering away from center. FWD is acceptable to most drivers because of the illusion of more traction for the driven wheels, since the engine is leveraged directly over them.

 

Mostly it is used due to ease of manufacturing. One drivetrain sub-assembly gets mounted under the front of the car, and bam, you're done. Cheap & Easy, Japan-esy (Since Japanese cars mastered this cost saver first, and have almost exclusively used it since, and American and European cars jumped on the bandwagon.)

 

I say the illusion of increased traction, because as long as you are talking about economy cars, with low grip, low weight, and low power, the finite ammount of traction in the front tires can be split between turn, stop, and accelerate (usually one or two of those at a time) without exceeding the traction limit.

 

However, Increase the vehicle's weight (more inertia), increase the engine's power (more torque), Increase the brakes (to cover the inertia increase), and even with larger tires, the somewhat higher, but still finite traction limit can be exceeded much more easily. Plus, with the inherent asymmetrical layout of one driveshaft being longer than the other, and the rarity of devices like LSD units on front wheel drive, there are serious detriments to any kind of performance being delivered by a front wheel drive system.

 

Basically, drive systems like Haldex offer a coupler, and a rear drive system. As you mentioned, under most conditions, the rear section gets 5% of the power. And usually never more than 50-70% of the engine's torque. It is basically rear wheel assist, to help remedy the inherent issues with FWD, and add some of the rear tire's traction to the equation, which would otherwise only be used for lateral stability.

 

I agree with you that computers tend to make things easier on the driver, not having to be constantly involved in getting the most out of their car. That is nice. I am afraid some cars go too far. Like a recent off-topic thread about a member here using a mercedes while their subie gets fixed, and when the electronics are turned off (which I am amazed is available...) the car became much less civilized.

 

I wonder if computer technology, with all of it's amazing, and sometimes necessary benefits, is also giving engineers a pass on bad engineering, by simply covering it up with computer control. That seems like a bad system with a coverup fix. At best, it seems like an cheaper electronic alternative to the proper mechanical systems, and not necessarily the better choice.

 

I REALLY feel this way when people talk about Audi's (and lots of others, lately) use of EDL and EBD, where the brake system is used to cover the driveline's duties. Brakes are used for decelleration. Audi may claim that they have lower tire wear with their system, but what about brake pad wear? What about brake system stress, and overheating? What happens in a catastrophic brake failure? does the drivetrain become more difficult do handle, also? (as if a car with failing brakes needs another problem simultaneously...)

 

It just seems to me that computers have their place. Managing the engine's systems, and an electronic center differential seems like fairly good ones.

It also seems to me that simpler, mechanical systems are better in some situations, like braking (even with ABS control) being separate from driveline functions.

 

It seems to me that a locking variable center differential is a very good choice, due to the multiplier explanation given earlier, which was very informative, BTW.

 

I also agree with fzanetti, and I still think that mechancal (helical gear torque sensing LSDs, like torsen and quaiffe) LSDs are a good, even superior role to play at the axle differentials, where more constant differential actions are taken, and smoothness of operation is more important. There is more often a greater speed differential from side to side, than front to back. and if one tire slips, it can usually be managed by transferring to the other wheel on that axle, slightly before modifying the center torque split.

 

A Subaru VTD AWD setup, with quaiffe front and rear helical LSDs sounds like a damn fine street-based AWD setup to me. Certainly better than a Gen-IV longitudinal Quattro, with only a center diff, and EDL. Just my opinion/prediction.

 

This is a fascinating thread. Thanks for all the informative posts... Please continue...

[TSiWRX]

Since there's a sub-vein in this thread about the RE92s.

 

I honestly don't feel that they're all THAT bad.

 

I, too, am among those who feel that they do track well, straight-line, in the wet. Even through rather torrential downpours on the highway, I have never had any problems with them hydroplaning at speeds <70 mph. On washed-out local roads, with appx. 1-inch of standing water, hydroplaning and pull is minimal even at 45 mph., and I just experienced this yesterday.

 

Certainly, they're not the best tires for going around corners, even at moderately elevated speeds, in the wet, but they get the job done fairly enough for anyone who is NOT driving the car "as an enthusiast."

 

Similarly, in the snow, straight-line tracking and off-line traction is admirable. Through metro-Cleveland's unexpected late-spring snowstorm just this past year, the wife's WRX had no problems at all, given that I again demanded only reasonable levels of performance from them. Certainly, I wasn't about to drive them as a wanna-be snow/ice-Rally driver.

 

Once again, I think that driver expectations as well as driving habits will dictate heavily whether or not one feels that our stock RE92s are "acceptable" tires.

 

-A

[smoss]

I had a 2001 A4 1.8T and now I have a LGT limited. The Audi felt great in snow. Never slipped. Of course I needed new brakes at 20k miles and the Audi manager said that was normal. Every other car I have owned has had brakes last until at least 40k. I know brakes are made differently these days, but still. I am glad the Audi came with free service for 4 years and I am glad I no longer have the car.

 

I havent' really driven the Subie in the snow yet. I have snow tires for it, and I hope it is acceptable in the snow this year. I love the car.

 

No problems in heavy rain with the RE92's so far. I drove through major major rain last week. I couldn't see past 40 feet in front of my car. Traffic was going 20 with hazard lights on. Of course a jeep and Audi had to pass us all at 50 mph. Just because you have good traction doesn't mean you can see any better.......

 

 

As I side note, the A4 leaked oil from the head gasket at 18k. Serivce guy said that is not abnormal for audi. He also told me he drives a toyota.......

[pdw]

Front drive is still front drive. Even WITHOUT wheel spin, torque steer can pull the steering away from center. FWD is acceptable to most drivers because of the illusion of more traction for the driven wheels, since the engine is leveraged directly over them.

 

Mostly it is used due to ease of manufacturing. One drivetrain sub-assembly gets mounted under the front of the car, and bam, you're done. Cheap & Easy, Japan-esy (Since Japanese cars mastered this cost saver first, and have almost exclusively used it since, and American and European cars jumped on the bandwagon.)

...

 

I REALLY feel this way when people talk about Audi's (and lots of others, lately) use of EDL and EBD, where the brake system is used to cover the driveline's duties. Brakes are used for decelleration. Audi may claim that they have lower tire wear with their system, but what about brake pad wear? What about brake system stress, and overheating? What happens in a catastrophic brake failure? does the drivetrain become more difficult do handle, also? (as if a car with failing brakes needs another problem simultaneously...)

 

It just seems to me that computers have their place. Managing the engine's systems, and an electronic center differential seems like fairly good ones.

It also seems to me that simpler, mechanical systems are better in some situations, like braking (even with ABS control) being separate from driveline functions.

 

...

 

FWD has one other advantage, at least over RWD and that is that it tends to be more stable (less likely to wander) on the highway. Think about the basic physics of pulling a four wheeled vehicle vs. pushing it. The difference isn't huge and I'd rather drive a good RWD car, but it's probably appreciated by people in Florida driving Buicks even if they don't totally realize what's going on.

 

You're right about manufacturers liking FWD for costs reasons but it wasn't the Japanese who started it. It was Europeans. The first car to use the layout was the original Mini and the next car to really popularize it was the VW Rabbit (Golf I). Back in the 70's, most Japanese cars were still RWD except for Honda who's always been fully FWD until quite recently.

 

As for EDL, I think people here are getting a bit worked up about it. It does work and works reasonably well without the non-linear activation that mechanical solutions can give. Mercedes 4-matic is, essentially, all EDL since it has 3 open diffs and uses braking to direct all torque both sideways and fore and aft. In practice, it doesn't eat brakes. You also mention "simpler" mechanical solutions. The mechanical solutions are not simpler. They're more expensive. The hardware for EDL is already in place if you have ABS. It's just a few lines of code.

[pdw]

I had a 2001 A4 1.8T and now I have a LGT limited. The Audi felt great in snow. Never slipped. Of course I needed new brakes at 20k miles and the Audi manager said that was normal. Every other car I have owned has had brakes last until at least 40k. I know brakes are made differently these days, but still. I am glad the Audi came with free service for 4 years and I am glad I no longer have the car.....

 

I don't think new brakes at 20k had anything to do with EDL. I had a '98.5 A4 1.8TQ and consistently got 40k or more out of a set of brakes and I wasn't exactly gentle on the thing. My wife has a '99 Passat V6 which has EDL on the front and we get similar mileage on a set of pads.

 

One reason Audis (and VW's) eat brakes is because the pads wear unevenly. For some reason, the inside right pad would hang up in teh tracks of the caliper and wear at an angle. I buzzed off the tracks with a dremel and never seized them and still had problems. Even then I still got 40k on a set of pads so I wasn't too worried about it. It's also due to the type of pads they stick in at the factory. Mercedes are famous for the same thing.

 

EDL simply doesn't put that much force on the brakes compared to stops from 50mph or so.