I read the article about the DSM. Certain things stood out to me.
[quote=“DSM article”]For example, to reduce understeer on a FWD car on corner exit, one approach is to increase the amount of rear weight transfer distribution by softening the front bar, stiffening the rear bar, or both. One would expect this to increase the amount of load on the outside rear tire (at the expense of the load on the inside rear tire) and thus reducing the amount of grip on the rear of the car, promoting oversteer. Often overlooked is that the increased outside rear load will increase the load on the opposite diagonal (on the inside front) which, all else being equal, should increase the amount of front grip, which in turn promotes oversteer.
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I read an article by 034 Motorsport a long time ago and they only recommended changing out the rear sway bar on a B5/6/7 car to get more oversteer. If both the front and rear sway bars are changed then the relationship stays the same the car will still understeer. With my B6 S4 I installed the Hotchkis RS4 big rear sway bar and it made a noticeable difference with kicking the rear out but it wasn’t an extreme change. It seems like a reoccurring theme throughout the article that AWD decreases the effect between changes in front or rear end grip and I agree.
[quote=“DSM article”]The net result of trying to create power-on oversteer on an AWD with a 50/50 diff is a car that likes to occasionally snap-spin on corner entry or midphase. It makes for a very difficult and unforgiving car, especially in transitions or the rain. The car will also be slower than a car set up to maximize grip. And you still probably won’t get the car to exhibit power-on oversteer anyway
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I have noticed this with my car at times. Especially on a skid pad while trying to modulate the throttle. I was at an education day and the instructor was trying to teach me how to modulate the throttle while going around a circular skid pad to have the rear end come out then go back. I was at a point where if I let off the throttle then it was snap oversteer and I was spinning in circles with both feet in (clutch/brake). If I gave it more throttle then the back end would slide out and I had to apply counter steer, then I was drifting pretty controllably. This has happened to me on certain road courses also. There was noticeable oversteer midphase through a corner if I got on the gas too early/hard into the corner and I had to apply counter steer to keep the car going where I wanted. It is actually nice for really sharp hair pin corners but longer more gradual corners require a more delicate touch. Good examples of this is at VIR. Turn 12/Oak Tree at VIR I have to roll on the throttle because if I jump on it then the back end will come out enough that I have hit the gaters on the far left side of the straight with the left rear tire. I really struggle with Turn 1 at VIR because it is such a long sweeping right hand turn and I can mess it up pretty easily. One of the instructors that I had at VIR drove a B5 S4 and he would understeer through turn 1 really badly but I didn’t. Instead I would end up cutting the turn too tight or spin out. The only time that I get understeer is when I am spinning all four tires at once, which does happen.
To give you some background with my B6 S4. My S4 has the JHM 4:1 center diff and Wavetrac LSDs in the front and rear. It also has most all of the other power mods that JHM sells like SC, Headers, FI 2.5" catback, modified intake manifold + spacers, and a bunch of other stuff.
Also there is a right hand uphill turn in my neighborhood that can make my car’s ESP flip out if I leave traction control on. I can feel the front end lose grip then the power transfers to the back and it loses grip and then the car can’t decide which end is going to lose grip and finally it straightens out and goes up the hill. Very frustrating turn for like two seconds with traction control on and a heavy right foot though.
This part is about the Torsen LSDs:
[quote=“DSM article”]It does, however, have a couple of quirks. The first is that it is only availible in a 50/50 torque split (and that may or may not be a consequence of the design, I don’t know). The second is that it requires a little bit of resistance from the inside wheel to “push against” before it can start locking. The net result is that unless there is a secondary locking mechanism installed in parallel (like a clutchpack or viscous) if a wheel comes off the ground the Quaife in that slot becomes an open diff.
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- We know that Torsens can be F40:R60 or even better still with the Crown gears so that part is no applicable any more.
- That is true but the Wavetrac LSDs use a cammed piece so that if a wheel is lifted then it creates internal load to keep the LSD function.
To give even more background on my car. I got the JHM 4:1 center diff mod first and it made a slight difference making the rear end more tail happy but I wasn’t happy with the change because I wanted more. Then I got the rear Wavetrac LSD and that made the car slightly more tail happy but it felt weird to me. The front end did lose grip easy too. So then I kept bugging Wavetrac to make me the front LSD and I had to ship them my front factory open diff for them to make their LSD version. There is more to this story in the AZ thread that you posted a link to. The Wavetrac front LSD made the biggest difference of the three diff mods. It made the steering easier (especially at low speeds) and more controllable when the rear end was going tail happy.
EDIT: Since I got Wavetrac to make a front LSD for the 0A3 transmission that is used in both the B6/7 S4s and RS4s (the RS4s use different gear ratios but they are coded as the 0A3 manual transmission) then the B6/7 S4 front LSD fits the B7 RS4. I think that BenSTI also got the Wavetrac front LSD. Kind of pricey for a LSD though. In comparison for the rear LSDs, JHM’s is much more reasonable price wise than Wavetrac.
EDIT #2: I am about to pass out but I thought that I should mention that wider tires helps get rid of both understeer and oversteer. Both understeer and oversteer are caused by a loss of grip and wider tires allow more grip because of the wider contact patch. For example my S4 from the factory runs 235/40/18 tires and currently I am running 265/35/18 tires. I know that they have made a big difference as far as the handling through turns goes. I bring this up because a lot of RS4 guys go from the stock 255/35/19 tires to 275/30/19 tires and that does seem to make a noticeable difference for them as well. Later this year I will be trying out wider tires still than my current 265/35/18 tires to see if they make a bigger improvement but that will be a bit before I get to that point. There is a downside though to running wider tires and most likely wider wheels. That downside is that a wider tire and most likely wider wheel will weigh more and cause more drag. That additional drag will slow down straight line performance and it puts more stress on other driveline components like axles, differentials, etc. I have broken a shit ton of left front inner CV joints because of my wider tires/wheels and my aggressive launches while at the dragstrip.
