2BWise

I have been tracking and autocrossing my 85.5 944 for the last two years now. It is basically stock with the only major modification being my set of Hoosier R3s. Up unitl now I haven't had much problem on the track with the open diff and only having limited issues with the inside tire spinning. Recently I've been doing more autocrosses and am finally reaching the limits of the car, but I am now struggling with major wheelspin on corner exit. The last event I did last year was on street tires and I had a couple near spins on corner exits. Yesterday I did my first autox of the year (on the Hoosiers) and had a horrible time getting out of the corners. The worst part is that I can not figure how to drive around the problem of the inside tire spinning wildly.

* I can't increase throttle or I'll get more wheelspin.
* I can't lift because I already have decreased rear grip and lifting will just lower my grip more.
* I can hold throttle constant, and wait until the tire hooks up once I've got the car pointed straight and get back on the power. (This is what I did)

At the autox yesterday it was probably costing me nearly a second on a 40 second course and is making it tough to be competitive. I know the simple answer is to buy an LSD, but right now its not in the budget Secondly, I want to learn how to drive with the open diff to make myself better.

Please enlighten me with your driving know how

Larry Herman

It's probably because you are taking an earlier apex in most turns. This will cause you to have to turn the car more in the second half of the turn, exactly when you are trying to accelerate. The combination of heavy lean and lots of power unloads and spins the inside rear tire. Try going a little deeper and turning the car more in the initial part of the turn, so that you can take a straighter line with more power coming out of the turn. Yes you will give up a little on entry, but you should gain more on exit.

One car setup change that you can make is to reduce the rear roll stiffness (soften the sway bar). It will give you more understeer which you will have to drive around (by using the technique above), and compensate for with tire pressures, but it will allow you to put more power down.

2BWise

Out of the three corners inwhich I was struggling with one was decreasing radius, one a hairpin, and the other a 270 deg traffic circle. I was definitley taking an earlier apex for the decreasing radius as I was trailbraking in, the hairpin I could have used a later turnin now that I think about it, and the traffic circle thing I was pretty much screwed due to how tight the darn thing was.

I had dropped the rear tire pressures after the second run to induce understeer as that is my only possible adjustment (no adjustable rear sway).

Now when I'm at the track I rarely encounter any major issues with the open diff. It is a function of me just not pushing hard enough or is the effect small enough that I'm not consciously noticing it. Or since the car tends towards understeer, I'm thinking that its not appearing as that when I have traction the equal speed of both rear tires is one of the causes of this understeer.

M758

I have been racing with an open diff for 7 years now in my 944. Larry is correct. You will need to adjust your line and also reduce the rear roll stiffness mostly with a small sway bar.

I don't autocross the car much any more so I can't give you tips there, but on race track it is easier to deal with. Reason is that you can spend time tweaking the car to minimize it's impact. For me the open diff is problem in one corner and some times in a 2nd corner. So over the years I have learned to adjust the suspension and tweak my lines. Adjusting the suspension is key and right now I have found the balance between minimizing wheel spin and minimizing understeer. I actually allow a little wheelspin as I can mangage that better vs tweaking the chassis that much. Results in lap times within the typical variation of LSD shod cars. So in the end it is not hurting me that much.

One thing you will need learn however is to feel the wheelspin and don't over do it. Anywheel spin is bad and at best results in a overheater prematurly worn tire. You need to simply backoff a bit or use a higher. Of note also is to minimize the camber as lots of negative really hurts that inside of the tire.

Z-man

I AX and DE my open-diff 944S2. With the added power of my motor, inside rear wheelspin used to happen quite often.

The key to getting down the power effectively, IMHO, is to have patience with your car and throttle. On a less-than grippy surface for AX, that typically means:
1. You have to wait until the car is pointing almost in a straight line.
2. You may have to take a wider arc through a corner to get the car straight as possible.
3. You need to feather the throttle, and back off a little if you start to feel the wheelspin happen. It's kinda like the same concept of throttle steering around a skidpad, except instead of lateral grip/no grip, you're dealing with forward momentum grip/no grip.

To me, it sounds like you're mashing down on the throttle a bit too quickly. If you allow the car to take a set before you go WOT, you won't be fighting wheelspin as much.

If you are concerned about losing exit speed in a corner because you're not getting to WOT as soon as before, consider the fact that with wheelspin, you're tranferring less than 100% of the power to the ground -- and that will cost you through the whole straight section. (The old saying is true -- sometimes you have to go slow in order to go fast...)

Also - if you carry more momentum into the corner, your exit speed will be greater, and you can get the car pointed straight-ish sooner, and you can get the power down more effectively.

For the track, there are only three corners where wheelspin was an issue for me: T8 - the turn before Bridge Straight at Shenandoah, the Keyhole at Pocono South, and Oak Tree Turn at VIR. With a little patience at all three of those turns, wheelspin is no longer an issue.

HTH,
-Z-man.

2BWise

Thanks with the input so far. Now that I think about it again I believe Z-man has pinpointed my particular issue; I was trying to go WOT to soon. I have a tendency when I see the track straighten I'm impatient and try to get WOT as soon as possible. My throttle inputs are not too quick, as a like to slowly roll into the throttle, but that I am rolling onto the power too soon and need to wait slightly longer to do so. I need to look at the rules again, but I think I need to put on a front or rear adjustable sway bar so that I can slowly tune the car away from the issue.

M758...since you need to adjust your line while racing does that effect your passing/blocking on track? I can imagine having to take an early apex to hold off a pass, but that does that ever compromise you on the exit?

M758

The LSD allowance for wheel spin on the way out of the corner has no really impact on my ability to race into the corner. What I mean is potential wheelspin does not make me loose a corner on the way in. Clearly the biggest impact is on the way out of the corner where I may be late getting the power down or simply have less speed down the next straight. One thing to consider is that if you can defend on the way in often times you can slow the chasing car too so you both are slow on the way out. Plus the biggest thing is that where the open diff hurts the most putting the power down it is very easy to defend on the way in and fact my line tends to be very defensive. Even chasing other cars I notice that I only give up a very little on corner exit speed and all they need to do is not get it just right and can still hang. All of this has to do with the particular corner however.

It think really the biggest weakness I have with an open diff is one that I can't fully quantify. That is the impact during heavy braking. I know it helps stablize the car under hard braking and should make trail braking a bit more stable, but I just have no idea how much I am missing out. I have never driven and LSD 944 hard enough to understand how it impacts braking, but I know I am losing time or simply working harder than everyone else. Seriously I can manage the wheelspin both driving wise and mentally, but I have no idea what to do about braking impact. One day I will have guard LSD, but I will just have to make do unit then. Just too many other things (non-racing) spend money on. Family comes first.

Larry Herman

Joe, you are missing out a lot. I just removed the standard LSD from my car (works fine - any buyers?) and installed an early cup car diff. What a change it made! I can stay on the brakes much farther into the corner and the rear stays planted, which means that I can brake later. I'll be better able to evaluate the full impact of it when I get to a track that I know well (like the Glen in May) but suffice to say that on it's initial outing at Road Atlanta I am very pleased.

chrisp

I don't have an LSD in my SC but in researching and asking around the paddock drivers who have driven their car without and then with say it's pretty significant. Some have even gone so far as to say it's "seconds" per lap. The ability to trail brake is greatly enhanced.

I am talking to the air-cooled crowd so the above comments may not translate 100% to front engine cars

Keep in mind tires get their most grip at about 10% wheelspin. So, a little wheel slip shouldn't be detrimental to acceleration....but you are losing any lateral grip that the inside tire may otherwise contribute.

smlporsche

^^^^^^^+ 1

I installed a Gaurds LSD (50/80) in my car over the winter and all I can say is that I can now put down ALL the limited power I want which is at or before the apex. Plus as Larry said it allows much deeper/later braking going into the corner.

I was having problems with rear inside wheel spin at VIR in 1/2 and Oak Tree. Those are but a distant memory now.
Best $$ I ever spent (on the car!)

Sean F

Great...something else to buy

M758

That statement is a major reason I have never picked up a used stock 944 LSD. Stock is good, but I think better to go all out and get the Guard one and be done with it.

Now I know it is supposed to "feel" better, but I don't know how that turns into lap times. Right now all the fastest cars are LSD cars and mine is right there maybe a 1/2 second off with an open diff. With typically lap to lap variation of club level drivers can hang with these guys. So while I am not feel "blown away" and I feel like I have better thing to spend money on. Some folks say and LSD is "seconds" a lap. If I consider "seconds a lap" then I am either WAY in front or they are over estimating the impact.

I think alot has to do with car type, track and driver skill/style as to how much of an impact the LSD really has. The fairly easy to see with wheelspin as it is obvious when it happens, but while braking is better I just can't understand the magnitude of the benefit. $2500 is ALOT to spend on $10k car to get better "feel". One day... One day... Dang 2006 it was HANS & helmet, 2007 it was a fire system, Maybe 2008?

Larry Herman

Yeah, that's why you need a more expensive car!

chrisp

Agreed but once you spend the $$ it becomes a $12,500 car .

Then the $2,500 is only 20% of the value whereas on a $10,000 car it's 25%. It's all how you rationalize it.

Seriously though, if this one upgrade could have you running up front wouldn't you do it?

M758

The issue is I already can take a top 3 spot with the open diff. Won the regional championship in 2006 with an open diff with 4-5 straight 2nd place finisehs. Got 2nd by less than second 3 or 4 of those times too. It was ALOT fun last year and this year has been fun too. Not quite so many 2nd places due to tire issues, but still right there.

That is why it is so tough... I might spend $2500 more, but will I have any more fun racing?