PierreTT

Hi folks,

After scaring myself at the track (lost the back in a 100mph curve...), I would like to make the car more predictable at the limit. What would you do to introduce more Understeer??

- More camber ( I had -1.4 Front and -1.9 Rear)?
- More Toe-In ( I had 0.04 Front and 0.18 Rear)?
- Play with tire pressure (30 front , 33 rear)
- A combination of these?

I found out that the AWD system is not very usefull when in trouble on a track... Rear wheels were spinning when I was drifting and the front ones didnt seems to do anything at all (except pointing in the right direction...)...

Your toughts please!

Cheers
Pierre

ca993twin

The oversteer you are experiancing is quite unusual in a stockish turbo setup (unles you lift, obviously). Have you had your kinematic toe set correctly? What size tires do you use? what brand and condition are they? Are the front swaybar links connected correctly? If everything else checks out, you can always go for a stiffer front sway bar (or lighter rear).

chris walrod

I'm with Steve on this one. This dynamic isnt common with 993 AWD cars, much less 993 RWD cars. This is, of course, without lifting throttle mid corner or other chassis upsetting moves.

Let us know more about your car.

PierreTT

Guys,

I did not lift (I think... ) but was obviously upsetting the chassis on a very tricky corner. The rear tires started to spin when I reapplied the gas after APEX. In this specific situation, this curve is "off-camber" or banking on the wrong side if you see what I mean... I think this was a major factor.

As a novice, I'm just trying to get some advice to setup the car the way I like it. Maybe the only answer is getting more time behind the wheel!!

To answer your question Steeve, stock setup, brand new Yoko A048 R compound tires.

Thanks for your replies by the way
Pierre

ca993twin

Pierre,

Hmmmm.... that corner sounds exactly where the AWD should come in handy and start applying some force to the front tires. This should help you drive out of the corner. So hmmmmm again. Get that kinematic toe checked (by an expert), and look at your front sway bar links. I don't know anything about those tires, but the key pressure measurement is generally HOT, and should be closer to what? 40 psi?

Is it possible that your viscous coupler is not functional? This is not too unusual.

ScottMellor

I believe those tyres work better at lower hot temperatures than 40. It sounds like you added too much gas too soon if the rear tyres were spinning. Definately get your Viscous oupler checked as well as your Kinematic Toe and alignment. What did your instructor have to say? (You say you are a novice, so hopefully you are riding with an instructor. ) Normally power-on oversteer can be ameliorated by feathering the throttle back a bit.

Jean

Pierre, your settings and tire pressures Fr/Rr seem fine, I don't know how your temps are like nowadays in Canada but a bit higher pressure might help as Scott mentioned to get you closer to optimum grip on those tires. In my stock TT I used to put the same pressures front and rear to induce more oversteer!

I have to think that it is a driving error, you might have not been smooth on the throttle coming out of a turn, you need to squeeeeeze it progressively. The 4WD will not save you from oversteer, it is not good enough.

Check out your sway bars to make sure everything is there as it should. If this problem persists, off to a suspension guru, you could have your rear kinematic toe off or front schocks worn out. I am assuming tires are in good condition. Careful out there.

Carrera GT

There's something else going on. It's incredibly difficult to spin the rears on a 993 Turbo and at 100mph, doubly so.

Aside from driver error, here's a check-list in order of likelihood:

Brake, bearing or suspension problem.

Viscous coupling overheated and permanently failed.

Gross alignment problem.

Damper failure.

Broken, collapsed spring.

After-market product (drive-train, suspension, brakes, aero.)


And here's some questions:

* What do you mean by "lost?" -- at 100 mph in a turn, if someone says lost, I imagine the car is totalled or close to it.

* What tires and mileage/condition? -- street tires can't deal with repeated 100 mph sweepers and will get greasy and behave in a way often described as "the back stepped out" where the car recovers itself (mostly) give or take some driver input.

Also, I think it's possible to end up with an 030/HD setup where the rear springs aren't properly seated (because they're a bit short unladen) and one spring might be jacked up.

Have you lowered the car? (looks like it in your avatar) In which case, unless you've corrected the front geometry, you've got excessive bump-steer and that will make the car feel unpredictable through high speed sweepers where the suspension is loaded and bumps will change the track.

The alignment and tire pressures don't look hazardous. Do you have adjustable anti-roll bars? Any other modifications?

And the very last thing you want is more understeer -- especially in a powerful 911 like a Turbo, this just accentuates the transition to oversteer and give you less time to react and less capacity to correct momentary oversteer with brief counter-steering.

In terms of diagnosis and remedies, I'd go through the simple stuff and fix any mechanical problems. Chances are you have a blown centre diff (aka viscous coupling) and that's a quick but expensive fix. If no other problems arise, I'd recheck the alignment to see if the car still has those settings. I'll address the bump-steer problem (un-slam the ride height or get a bump-steer kit factory or after-market) and install adjustable sway bars and run full-soft front and full-stiff rear (it should be pointy in this configuration) then dial out one hole at a time until it feels good for you. I'd probably also inspect the tires (which might well be useless after too many heat cycles regardless of wear) and consider Sport Cups or some DOT R rubber of choice.

You might also get the car serviced then dyno'd to be sure you're getting smooth power and torque curves which is generally just a fun thing to do with the car and a good way to diagnose tired old K16s or other boost/lag issues.

All that said, once you solve this little problem, rest assured the AWD in a 993 Turbo will save you from yourself during very big, very fast, very wrong driving errors. This much I know from first-hand experience.

PierreTT

Thanks folks, I took lots of notes. I will definitly have the car realigned and have the center diff checked. The car is 28,500 miles with Bilstein HD and ROW spring installed last year. No big damage on the car by the way (ouf... it is my garage queen...).

I'm not blaming the car, it was definitly a driver error (too much speed entering the curve, too wide, too much gas exiting, the most tricky curve of the track...)

I'll keep you posted.

Thanks, I love this board!!
Pierre

Carrera GT

You can check your diff yourself in a jiffy -- just jack up one front wheel, if you can turn it by easily hand, the diff is dead. It should feel like the brake is dragging. You can turn it, but not easily.

If you want to be thorough, repeat the test by lowering that front side and raising the other front side. They should feel the same (nothing dragging in the front) with equal resistance from the viscous coupling. Do this hot or cold, it doesn't seem to matter much for this simple test.

You can test your LSD by raising both rear wheels and leave both fronts on the ground -- in neutral, if the wheels turn together in the same direction (both forwards or both backwards) the LSD is okay. If they turn opposite, the diff isn't limiting slip, so it's worn out or damaged.

As you say, this incident could just be an over-zealous driver. It's easy to put a rear wheel on a painted surface, especially near a burm, and that can be enough, perhaps combined with some dirt or grease, to cause the car to rotate more than desired. The fact that your car is in one piece suggests something like this may have happened and once you were back on the racing surface, the car regained traction and the fronts had a chance to save you ... : )

Tom 993tt

I could use some help understanding the comments about tire pressure. I think my owners manual suggests 42F and 46R. The initial post in this message talks about 30R and 33R?

ca993twin

Tom,

Be careful with deciphering this thread... Pierre is using "R" compund tires... street legal race tires. They are very sticky and tend to generate more heat and hence, they change pressure more as they heat up. For regular old street tires, most of us find that the Porsche recommendations are way too hard... I tend to use 33 front, 37 rear, and find the ride and grip are improved. Try lowering the pressure a bit. However, you'll have to be aware that your rims are a bit more at risk of bending on a pothole with the reduced pressure.

JJayB

Pierre,
Get back on that horse. I've lost the rearend on several occasions, mostly when I missed the turn in. All wheel drive is a saviour but not when you defy the laws of physics. Lets see, off camber turn with 63% of the weight behind the rear wheels going over 100 mph most likely slightly off the racing line. If it was a video game you would have wrecked. Our cars are sensitive to turn in points and are hard to correct when your through a corner unlike a 944 turbo. More seat time, and if you haven't corner balenced that steed, it would be well worth it as well as having someone who knows how to set up the kinimatic and check the sway bars (especially the bushing).

Now a question, how was the wear on your A048 with your camber settings?

Jimmy the fugitive (currently on bail)