Quattro Handling Characteristics

[Patrick]

Having owned my first Audi and AWD car (S4 Avant) for just over a year now, I'm not sure what to make of the AWD drive experience. All I've ever owned is front wheel drive cars. I've always found a front wheel drive car reasonable predictable to drive hard. The majority of the time they simply understeer, turn in more, or play with the throttle and you can generally keep the car on the road.

My S4 Avant is a different ball game all together. I have never had a car with the rear end step out so many times!

If I had to categorise my car between a front or a rear wheel drive. I would definitely put it in the latter category. I'd go as far as to say I'm very cautious when applying the power as the preference is always for the rear end to move. Where a front wheel drive car will spin, and at worst plough straight on, where removal of the power generally brings the car back into line. Not so in the S4, which like I say feels like the rear of the car is just about to step out or does!

This was confirmed in the snow recently where yet again the rear end was always the first to break loose during straight-line acceleration. I thought with the engine in the front, more weight over the front wheels they would receive the majority of the power and thus pull the car like a front wheel drive. Not so, mind you to be fair, the conditions were very slippery.

I'm so uneasy about the handling of my A4 that I took it to a track day at a disused air strip (Crail) where there is loads of run out to deliberately find the breaking point and characteristics of the car. The experience was very reassuring for the dry conditions on the day. The car just understeered the removal or power or addition of power made very little difference in combination with steering. With hindsight I should try the same on a wet track. However, this is only half the story. From day one of ownership my car was well down on power 220 instead of 265bhp I had that rectified and subsequently chipped to 320+bhp. With this increase in power it is quite easy to get the rear end to step out under hard acceleration on just a dampish road! I thought it maybe the tyres so I fitted a complete set of Bridgestone SO3's. They have improved the handle marginally. Further evidence to support the rear wheel drive theory is that the rears are wearing much more rapidly than the fronts. Worryingly so, as the outer edges of both tyres are wearing. This could be expected on a car subjected to a lot of heavy cornering, but 99% of my driving is on dual carriageway.

Having said all this, I'm not convinced the rear suspension of my car is Ok. It exhibits a strange sideways skipping action over certain potholes. Given the preferential wear on the outside edge of the tyre I'm convinced the rear camber angles are not correct despite having a 4-wheel alignment check. What I am not so sure about is whether the camber was checked or can even be checked with the 4-wheel alignment equipment advertised by Star Performance and AMD?

Anyone know a real suspension specialist not just someone who can check or fit suspension but really know how to rectify a car's handling characteristics?

I've decided to take some tuition from a track instructor to see if I can establish the best from my car.
/ubbthreads/images/graemlins/idea.gif

 

[mramage]

Patrick, does the S4 not have a 40:60 Front:Rear torque distribution as standard? i.e. there is always a rear bias?

Anyone ? /ubbthreads/images/graemlins/confused.gif

BTW, if your from Scotland area, why not come to the meet a week on Saturday. See the meets forum for more info. Also, I can definitely recommend Star Performance if your from the area, they did the dampers on my A4. Very happy.

 

[Mutley]

Contrary to popular belief, all Quattro A4,6 and 8's do not have a fixed torque split front to rear. This is because instead of a cheaper viscous coupling (which achieves bias by gearing one final drive ratio very slightly higher than the other thereby making the coupling try to slip all the time) they have a Torsen diff.

The Torsen diff constantly adjusts the torque according to the reaction from the axles, therefore more grip = more torque sent to that axle. If an axle should lose grip altogether (both wheels), then almost all the torque will go to the other axle, while the axle with no grip will rotate at the same speed.

This makes the system much more effective than a viscous coupling, because wheelspin is not required to bring it into action - the Torsen is proactive instead of reactive.

In extreme cornering, but without wheelspin, the Torsen cars are more effective than viscous ones, since the Torsen 'feels' high reaction from both axles, and therefore tries to turn them at the same speed. This has a profound effect on the centre of rotation of the car, and the famous 'Quattro squat' occurs in corners. Sorry to anyone who drives an S3 or TT - you won't get this.

If the back end of the car is stepping out, it is almost certainly due to the weight transfer on take-off causing high reaction, thus high torque going to the back wheels. Eventually the static friction of the tyres is exceeded and they break loose.

If this happens, don't back off the accelerator - let the Torsen do its job (practice on an empty road first please!) When the rear steps out, the lack of reaction will be detected and power sent to the front. I suspect you are backing off as soon as the rear starts to step out. This transfers weight to the front and removes torque from the back wheels (due to the reduced engine torque). The wheels grip again and the Torsen keeps sending remaining torque in their direction . . . Vicious circle.

Try APR for tracking. Does the back end always step out in the same direction ...??? A small tracking adjustment on the Torsen cars makes a huge difference.

Cheers
Jon /ubbthreads/images/graemlins/smile_smoking.gif

 

[mramage]

Sorry, don't think I was clear. I didn't mean a fixed ratio, but a natural ratio before any slip occurs. IIRC a salesman said the "normal" Torsen split is 52 front, 48 rear, but the S4 is different. I/he/we could be completely wrong though!!!

What's "Quattro Squat" and why wouldn't an S3/TT get it???

Got me interested now /ubbthreads/images/graemlins/grin.gif

 

[DavidR]

Patrick,

Star have the full VAG approved 4 wheel alignment tool which can check all the adjustable parameters (and many non adjustable ones) on the S4. I would recommend them highly as they know what they are talking about and are much better equipped and informed than [censored] fit and there "quasi" 4 wheel alignment tools.

My S4 has ESP and that comes on fairly rapidly if I open the throttle wide on a wet corner so I haven't really experienced much in the way of tail out antics, except on the recent snow, when I put ESP off.

Interestingly, it is the front tyres on my S4 that wear much more rapidly than the rears and that makes me think that your rear suspension is faulty (especially as the wear I have experienced on the rear is equal across the tread of the tyre)

I would be v. cautious at crail because of the amount of debris on the track pebble dashing the front of your car (it did to mine) and also the horrendous potholes (one of which blew a sidewall on my michelin pilot sport) Do you go to the 1/4 mile runs at all.

Also, as Mark said, we are meeting up in Dundee on 1st March so come along and compare S4's to see whos is handling badly

Cheers

Dave

 

[AL_B]

Talking of backends stepping out...how do you explain this then...

Haldex & Torque transfer thread

ta

AL

 

[amoffat]

[ QUOTE ]
Worryingly so, as the outer edges of both tyres are wearing. This could be expected on a car subjected to a lot of heavy cornering, but 99% of my driving is on dual carriageway.


[/ QUOTE ]
I had exactly the same thing on my S3. I had the camber and a full 4 wheel aligment check by a specialist wheel alignment company and the camber was fine.

This is merely the effect of the Quattro system /ubbthreads/images/graemlins/confused.gif

 

[DavidR]

[ QUOTE ]
S3 Moff said:
This is merely the effect of the Quattro system /ubbthreads/images/graemlins/confused.gif

[/ QUOTE ]

Not convinced, as I said, my rear tyres are wearing very slowly and very evenly...

 

[Patrick]

Mutley,

Interesting comment on the backing off point. When I was younger all my cars had a hard drive. From the outset I determined first of all how good the hand break was, and how the car would respond to pulling it with both the power on and off to generate opposite lock sideways responses. This used to give me some idea about the cars handling characteristics and was a hell of a lot of fun. - granted some may consider reckless but no bods were injured in the fun and it never seemed the damage the Polo, escorts, Orions, 205 and Mx6. However, since moving down from the Highlands where there was plenty of disused runways etc for that nonsense I've not been so keen to do this and particularly with a £18K mint S4! But it's limiting my confidence with what is my first decent performance car. So at the weekend I had a little 'play' with my car on some damp tarmac. I tried a couple of sharp turns in second gear deliberately 'flooring' the car to see which end of the car would break loose. It's a hairy idea, NOT recommended unless you have loads and I mean loads of space to get things horribly wrong! The initial problem is that because you have 4 wheel drive and now, in my case 320bhp, but through turbos that take a small amount of time to spool this results in a smooth initial acceleration then POW you get the full boost and hard acceleration (bear in mind this is maybe just over a second) you've achieved quite a speed before as usual the rear end breaks loose. A lot of opposite lock i.e. steering into the slide, keeping the power on to my surprise had the car in a 4 wheel power slide but at a speed I'm simply not used to, (Bear in mind what I used to get up to!) so I had to back off because I simply didn't have the space to anticipate/judge stopping / spin out of control distances. On this very limited basis I'm still uncertain of the cars handling, but slightly more confidant. I really need more space, and time to go through the learning curve of establishing the cars reactions and more importantly develop my counter reactions. Something like a large damp smooth supermarket car park would be ideal, but of course risky. A lot of the Crail track tarmac sections are far too pot holed and rough. However, until further 'playing' determines otherwise, the Quattro system on MY S4 always breaks the rear wheels first! Which, like I said in my first post is the complete opposite of a front wheel drive car under power and more akin to that of a rear wheel drive car.

Putting all the technical differences between the various 4 wheel drive systems aside. What are the real experiences of you lads with Quattros? Rear, front wheel drive or other?

Of course, all of this hinges on whether the rear of my car is set up correctly. Despite both Star Performance and my local dealership's investigations I'm still convinced there is something odd with the rear end of my car.

Assuming the suspension is ok is there any way to determine if the torson or rear diff is not working correctly apart from an obvious whining grinding clonking noise?

Any recommended handling experts? Do Lotus offer a service? Jim at Star performance suggested letting a professional racing driver have a go as he'd soon discover what if anything is wrong with the car! That's what got me thinking about a personal driving lesson /track session at Knockhill, using my car.

 

[mramage]

Hi Patrick. All I can suggest is what David R has already said. Come along on Saturday and see how your car differs from his. I'll PM you my mobile, check out the Scottish meets thread for more info.

 

[Patrick]

Bushy you have described exactly what I did on a large damp roundabout. It's just the rapid rear end step out that makes me nervous. I need to try this is on a really large smooth damp area that doesn't put the tyres/car under too much stress and allows you to 'feather' the throtle to establish what the car will do.

In the dry you just get understeer, irrespective of power on or off when at the limit, which is just like a front wheel drive car. But when it's wet with the power on my recent experiences have demonstrated rapid rear end step out!

More practice is required. My girlfreind bought me a Driving lesson at Knockhill so I can take my car on the track. Lets see what the instructors make of me?

I may get round to booking a time at the track over the next month or so once I get under the floor my car to check out all the suspension and bushes. I'll let you all know it goes. /ubbthreads/images/graemlins/beerchug.gif