markcincinnati

Some folks shopping for a new S4 look at the bundling of the Sport Differential, Adjustable "Shock Absorbers" and Red Brake calipers.

I regretted my 2013 decision to NOT get the Sport Differential -- for $1,100 -- when it was a stand alone option on my 2014 S4.

I thought "next time" I'm going to get it.

Then Audi ticked me off by bundling -- three things and calling it the sport package. I was ready to check the Sport Diff option box.

So, I went ahead a ordered a Prestige version with Premium Paint, Napa Leather and Driver Assistance (I specified, by doing nothing, summer only tires.)

I had read about the so-called Brake Based Torque Vectoring and wondered how such a thing (Vectoring) was possible via an application of the brakes. As far as I can figure out, putting on a brake on one or both wheels on the side of the car that corresponds to the direction you want to turn, doesn't really vector torque. Maybe someone can explain how this would work.

But, onto the point -- the S4 comes standard with what Audi calls Selective Wheel Torque Control. It is described on page 146 of the owner's manual.

"The selective wheel torque control operates while driving through curves. The front wheel on the inside of the curve or both wheels on the inside of the curve are braked selectively as needed. This minimizes sliding in the front wheels (over driving the steering wheels, aka under-steer) and allows more precise driving through curves."

I believe you may be familiar with the practice of entering into a sharp left curve, at speed too much for the front wheels to comply with, and at the moment immediately before the onset of under-steer you pull the emergency brake lever and turn the wheel to the left, which causes the car to want to pivot left.

As they say, "don't try this at home."

Besides, I doubt it is even possible with an electric emergency/parking brake as comes on a new S4. The point is, Selective Wheel Torque Control works like a minor miracle by increasing the speed that you can take corners.

I have argued, but I really mean it to be more of a consideration of an alternative, that SWTC for use on the public highways won't engage until a speed that most other cars on the road can't even dream of taking a corner at -- then SWTC allows that speed to be higher still.

I cannot prove that the full-on torque vectoring rear-differential called "Sport Differential" would allow even higher speeds than SWTC, but I take it as a tenet of faith that the sport diff does go SWTC one better (maybe one and half better).

My personal "disappointment" in Audi taking the Sport Diff stand alone option away aside, I must say it is unlikely that it would be better than SWTC for ME. You may live in urban or suburban or even rural circumstances that will both encourage and allow you to appreciate Audi's Super-Handling AWD gem called Sport Differential.

If you were either unaware of SWTC, what it is, what is does and its effectiveness, hopefully the preceding will prove of some value.

The new RS 3 uses SWTC -- and doesn't even offer the mechanical Sport Differential. But, in case you missed it, or thought it was a typo, the RS 3 does something in addition to really enhance handling. The RS 3 uses wider FRONT tires than it has in the rear (by 20mm).

I have often advocated inflating your S4's front tires +2lbs over the rear tires as a kind of budget way of emulating the RS 3's tire approach. In my case 40 front 38 rear. I also use at least UHP designated tires (W, Y or Z rated) with traction characteristics AA and load characteristic XL. Audi uses such tires when you do NOT select all season tires. If you do choose Audi's A/S option your tires will be "H" rated -- which simplly means "performance," but NOT high performance.

If you want to crank up the performance without cranking up the price of your new S4, it is possible to do so "at no extra cost" if you determine your driving circumstances will be more than adequately addressed by SWTC.

In no way does this mean I wouldn't have gone with the Sport Diff had it not been bundled.

Submitted for your edification and information.

James Cole

I have experience on the MQB platform, with VCDS you could set how much "brake vectoring" you could get from Weak-Normal-Strong setting options

All cars came with the "Normal" setting, I guess its just a matter of time when someone finds this same setting in the AWD module which I have not been able for access with my Hex-Net for the S4.

The problem with the "Strong" setting is that if you track you car it will sacrifice the brakes for handling but it did make the car more "RWD" like on my 17 S3 on street driving.

On the RS3, I have seen it in person and it does indeed have skinnier tires on the rear which is not very noticeable and doesn't look "off", the RS3 requires this because its haldex with no center differential, that makes 50:50 torque distribution at best but its much more FWD biased than Torsen 60:40 full time units.

As for tire pressures I thought lower pressure = more traction to a point... like if you want more RWD feel you will inflate your front tires to spec "40psi" and the rears to "45psi"

superswiss

Let me try to give you some info here. The reason this does indeed torque vector is because applying the brakes on one side of the axle has the effect of the torque to the axle being redirected to the other wheel through normal open diff action. That's how an open differential works. The effect this has is comparable to riding a bicycle fast around the corner while dragging your foot on the side to which you are turning. You are effectively sling-shooting yourself around the corner. That's essentially what's happening here.

There are several downsides to this system. The most obvious is that it wears your brakes, so if you do a lot of cornering the reduced mileage you get out of a set of brake pads could be significant. The other downside is that it doesn't work in all conditions. The system has to make sure that if you are driving on a slick surface, that applying the brakes doesn't lock up the wheels, so you could find yourself in a situation where the selective wheel torque control doesn't engage and you end up understeering when you are already in a precarious situation, for example you are driving on ice and the front of the car starts wiping out. Another downside is heat. The continuous brake action will contribute to heating up the brakes and potentially overheat them at which point the system will also disengage.

The sport differential doesn't have these limitations as it does active torque vectoring. It can send more torque to one side of the axle w/o having to slow down the other side, so it's much more effective in slippery situations. For example it can keep your car on course during aquaplaning, something I've experienced first hand. There is a limit to this, but the sport diff can steer the car to some extent by sending more or less torque to the left or right rear wheel. So while the front wheels are floating on water, the sport diff can continue to push the car around a turn by sending more torque to the outer wheel. A similar effect comes to aid if you are driving on snow and start to slip around a corner. The sport diff will help rotating the car. I've said this before, it's often overlooked that the sport diff is not all about driving as fast as possible around a corner.

markcincinnati

Superswiss,

Thank you for that comprehensive explanation. I do not track my S4, I assume that is the case with the vast majority of S4 buyers.

I do feel, after your post, even better about SWTC. The effect of a slight inflation difference (F v R) air pressure is in consideration of the additional front end weight and was told to me as a minor under-steer compensation technique. The person who told me was one of the instructors at one of my four driver training courses taken in Austria.

If this advice is wrong, I am unaware of any negative effect -- but I also wonder if the ends were reversed, if I could tell the difference. I keep telling myself this set up makes the car turn-in better.

mplsbrian

You have it correct. I think Mark has it backwards but maybe he just made a typo.

That's possible, even probable.

To the point above, you would want to set a "baseline" of neutral handling pressures, and then increase rear pressure (or lower front) to encourage oversteer aka. RWD-like handling. Not the other way around as that would encourage understeer and cause the car to "push" more.

Edit: it's also possible to do this with suspension tuning: stiffer shocks in rear + softer in front = more oversteer. Tighter rear sway bar = more oversteer.
Edit2: this is a key benefit of adjustable coilovers at the race track. Not only can you adjust the suspension as a whole to meet the unique characteristics of each circuit (some tracks have wayyyyyyyy better pavement than others), but you can also tune the car to negotiate those circuits very differently depending on need. Totally different suspension (and tire pressure) requirements at Nurburgring than at Circuit of the Americas if your goal is fastest possible lap in the "same" car.

markcincinnati

The Audi driving instructor in Seefeld, Austria was a long time Audi race driver and a long time Audi driving school teacher. His advice was to slightly (2 pounds) overinflate the FRONT tires to mitigate some of the sidewall's tendency to roll over on itself, thus providing a bit more control when entering a curve, braking, waiting for the load to shift and then using the accelerator to help you get through the corner.

At the time, I had a 2003 allroad 2.7T -- my instructor said, "all Audi's are nose heavy, putting a couple of pound of air in the front tires more than in the rear will reduce under-steer."

I tried it -- maybe it was just a psychological effect. Ever since then, I have practiced this and I have been pleased.

Are you suggesting to increase the rear pressure +2 over the front instead?

I just followed the "advice" provided by some folks who could do amazing things with cars "as if" they knew what they were talking about.

Often wrong, but never uncertain, I stand.

superswiss

The recommended tire pressure for pretty much all Audi models are always higher pressure in the front than the rear for this very reason. For example my RS5 is 39 front and 35 rear for normal load or 44 front and 41 rear for full load, so 3-4 pounds higher in the front. The difference varies by model. A models generally have only slightly higher recommended pressure in the front.

markcincinnati

You know, too, that the new RS3 uses wider (by 20mm) front tires than rears, I would suspect for the same reason.

superswiss

They are optional and yes, they are to mitigate understeer, but for different reasons. Wider tires increase grip. Not only is the RS3 nose heavy like every Audi, but also the Haldex AWD system is FWD-biased, so the front wheels do more of the work, than the rear wheels. For the same reason RWD cars generally have wider rear tires to have more rubber on the axle that puts the power on the pavement. The standard quattro system found in the A4 to the RS7, excluding the new Quattro Ultra system are rear-biased. 60% of the engine torque is going to the rear wheels under normal driving and as much as 85% can be forced to the rear wheels without ESC intervention. The RS3 can't do that. The R8 is the most rear biased and also most weight balanced Audi and as such has wider rear tires.

mplsbrian

No, that will be way too extreme for this car.

Rather, what I am saying is simply take your starting pressures -- aka whatever pressures you consider "neutral" or "normal" -- and adjust as I have described from that reference point. For the sake of illustration let's take "36 PSI front, 33 PSI rear" which as per page 346 of the owner's manual is Audi's recommended pressures for your 18" OEM summer tires as neutral -- and adjust as I've described from there. Let's call 36 front and 33 rear "neutral" since that's what Audi wants you to run (and they probably want that for the exact reasons you stated -- heavier front end etc).

If those pressures feel like understeer or push to much, you can try balancing it out by adding:

+2 rear (36 front, 35 rear), or
+ 1 rear -1 front (35 front, 34 rear), or
-2 front. (34 front 33 rear)

All will have slightly different characteristics but all *should* induce oversteer more than your starting reference pressure of 36 front 33 rear.

VERY IMPORTANT TO NOTE that just because you neutralize the handling through tire pressure adjustments doesn't necessarily mean you are adding lateral grip...although it could. It is possible to neutralize handling or give a car a rear/oversteer bias while also reducing total available grip -- e.g., you induce oversteer by keeping front grip the same and then jacking the pressure up in the rear so that it actually sort of balloons the contact patch and reduces its total size, thus removing rear grip. So the car - which has a natural tendancy to understeer, now has just as much grip up front but less in back, mitigating the car's natural tendancy to push and so it now "feels" like neutral handling but actually it's just slipping the rear sooner and will actually turn slower laps on the track.

A lot of rear sway bar upgrades have this same effect. It "feels" faster through corners because body roll is gone and it feels more neutral, but really it is just giving up rear grip sooner. Common on Hondas.

To really dial a car in for not only best feel but also maximum balance of grip + handling characteristics takes lots of time at the local track (though be advised track pressures != street pressures). But a quick play with pressures can really balance a car's feel out if you don't care about max grip at the limit and just want to give a little bit more goose in back. And you can of course reverse this to make a car push the front more and swing around less.

Don't go overboard though. +/- 2 or 3 PSI in either direction is about all I'd mess with before you start to really affect other **** that matters like braking performance / tire avg. operating temperature / treadwear.