Understeer / Oversteer

[sacook]

I know this is a really stupid question, but, Can anyone explain what over/under steer means and what each feels like? Would like to be able to talk intellegently about this topic.

Thanks

[VARam1500]

Imagine a left hand turn. Oversteer kicks out the rear wheels to the right leaving the car pointed too far left of the target. Result: rear end spins out. Understeer allows the front wheels to drift to the right leaving the car pointed too far right of the target. Result: couldn't make the turn.

Understand that the results can be highly variable; from correctable in the turn with steering and/or throttle all the way to complete loss of control.

If you watch nascar, they call this loose (oversteer) and tight (understeer).

What it feels like......well, sometimes "Yeeehaahhh".....sometimes "Oh, crap!"



--Case...

[rhaas]

I heard oversteer/understeer explained once and never had a problem after that, here it goes:

understeer: The front of your car hits the wall

oversteer: The back of your car hits the wall

[Dave at Pelican Parts]

Another way to think about it--

Understeer == you turn the steering wheel and the car goes straight. Or at least, straighter than you are pointing the wheels.

Oversteer == you turn the steering wheel and the back end starts sliding.

Understeer can be a little tough to detect sometimes. Sometimes the steering will get a little lighter when this happens, and sometimes you'll feel "something funny" up at the front of the car. And, of course, the car won't be pointing in the direction that you think it should.

Oversteer is usually a lot more obvious. Your seat can usually feel when the back end starts to step out. To me, it's one of those sickening/thrilling/fun feelings. Plus you'll notice that the car is rotating more than you are steering it...

Would it be worthwhile to talk about recovery techniques??

--DD

[sacook]

Originally posted by Dave at Pelican Parts


Would it be worthwhile to talk about recovery techniques??

--DD

Thanks that helps a lot. some recovery techniques would be helpful to me...

Also, what do you adjust on the 914 suspension to correct these problems. I do not have any sway bars

[rfoulds]

Before sway bars, be sure you have good shocks struts and springs. I mean, be sure they are in good condition. Opinions vary on what brand and stiffness. DO your own research and go from there. If the car is a daily driver and sometime autocrosser, 100 or 140 pound springs are enough.

Next, the best tires you can afford. Go to tirerack.com and do your research, and then buy the tires you want locally for the same price. Get them from a reputable tire shop that knows how to mount and balance competition tires, even if you bought street tires. My tire guy can balance without weights by rotating tire on rim until he finds the right spot.

Then, a good alignment and corner balance from a shop that knows how to do it on a Porsche, preferably the same place that mounted the tires. Do NOT go to the corner tire store or Muffler shop!!

It sounds like a lot of work, but the difference is great!!! Next up, adjustable drop link sway bars.

And, my Nascar buddy calls "Loosey" and "pushy", pronounced Lewsy and Pewshy. I get a kick outa that.

recovery technique: GO to Skip Barber Driving School. You will know it inside and out after that!!

[Zeke]

What do you want to do with the car? Autocross or fast street driving (not reccomended)? Tell us more about the car, i.e., tires, wheels and springs and shocks. Temporary tire pressure adjustments can change the way your car corners, but you won't want to leave them that way all the time or you might get bad tire wear patterns. An example: higher press in back and lower in front will lessen understeer (assuming you started with tire manufacturer's reccomended pressures and the tires match the rims in size).

[sacook]

Originally posted by Zeke
What do you want to do with the car? Autocross or fast street driving (not reccomended)? Tell us more about the car, i.e., tires, wheels and springs and shocks.

Okay here is waht I have: New Koni adjustable shocks front and rear; New 100 lb's springs; New ball joints and front bearings; New four lug repoduction Fuch's; 205x60x15 Fulda tires.

Don't plan to race at the moment and getting to old to stree race ; just want to have lot's of fun on those twisty-windey back country roads.

Got most all my parts from Pelican's (thanks Dave)... and

I will be installing a Jake Raby 2270 next week..

[rfoulds]

You've got everything but the sway bars. Alignment and balance AFTER the new engine is in, and you will be in great shape.

[sacook]

Originally posted by rfoulds
You've got everything but the sway bars. Alignment and balance AFTER the new engine is in, and you will be in great shape.

Good Thanks, now I just have to find an alignment shop that understands the 914

[Brad Roberts]

Tell us where you are located and we can point you in the right direction for alignments. Or.. I can give you some agressive street specs and you can hand them to your local alignment shop and have them set the car the way you want it.

I can give you 914 body measurements to get the corner weighting almost dead on after you give me your weight. (90% of the cars come out almost identical in the cross weights.

B

[914timo]

B, I am here in Finland. I know the place where I can do my alignments, but I don´t have the numbers. Can you suggest something for me too?? The place where I can do the alignment job has very accurate digital measurement instruments.

Now I think my rear tires have too little toe in. If I first accelerate fast in the long turn, but then loosen a little, my car tends to spin. It didn´t used to do that before I swaped the rear trailing arms. What do you think, am I right ??

[sacook]

Thanks Brad

That would be helpful, I live in Columbia, MO -- halfway between St. Louis & Kansas City.

If you could pass along the numbers that would be great also.

Steve

[Dave at Pelican Parts]

This has to be short, because I've gotta run in a few minutes.

Recovery--more or less, undo whatever it was that got you into trouble.

Understeer recovery: Back off the throttle, unwind the wheel a bit. Maybe most of the way. Back off the throttle the whole way if that doesn't work.

Oversteer recovery: Most 914s do not have enough power to kick the rear end out using power, except in the wet. So in general, when the tail starts sliding add more power and unwind the wheel. You may wind up having to go past neutral and steer the other way. ("Opposite lock.") If you got into trouble by feeding in too much power, then you need to back off the throttle and unwind the wheel.


Alignment suggestions:
Front caster either stock (6 deg) or max you can get and have both wheels even.

Street tires, stock suspension, reasonable driving:
-0.5 deg camber front
-1.0 deg camber rear
~1/16 inch toe-in front and rear.

Street tires, stock suspension, fairly aggressive driving or some autoX:
-1.0 deg camber front
-1.5 deg camber rear
~1/16 inch toe-in front and rear; possibly ~1/32" toe-OUT in front for a car biased more for autoX.

I wouldn't go much past this setup for any car with street tires.

Sticky tires, 140# springs/stock torsions/19mm front sway bar, autoX with some street:
-1.5 deg camber F
-2.0 deg camber R
~1/16 inch toe-OUT front
~1/16 inch toe-IN rear

Sticky tires, 220# springs/21mm torsions/22mm front sway bar, track w/some autoX:
-2.0 deg camber F
-3.0 deg camber R
~1/16 inch toe-OUT front
~1/16 inch toe-IN rear


Note that these are all very much "general recommendations", and are based pretty heavily on PCA Zone 7 class rules. Everything from exact choice of suspension components, to exact tires, to pavement, to driving style all affects the setup!! So these might be a half-decent place to start from, but to find the "right" place you really must test and tune.

The lesser-modified cars are set up for more benign behavior and less uneven tire wear. The more-modified ones will be less kind to tires and will require more attention when you drive them.

The advatage B has is that he's done enough of these to refine "general recommendations" into more specifics for individual use and so on. And he knows the questions to ask about what things will affect what setup you need. Gee, I guess that "experience" stuff really comes in handy, don't it?

--DD

[SummerSledSix]

Brad, my car is at 43:57 frt:rr with the batt and spare in the front trunk (3/4 gas, 200lb in dr seat). With the 3.2 / 901, can I get any closer to 50:50 without sandbags?

[Andyrew]

Understeer= not steering unough
Oversteer= steering to much,

You guys have really complex deffinitons for easy things


-- Now that i think about it your guys's defnitions are better, dang i hate it when your right --

[sacook]

Dave,

Mucho Thanks for the tech info on alignment. I think I found a guy in St. Louis who can align my 914.... Although he was pretty "crusty" on the phone..

Hey if your good, your good!!

[Brad Roberts]

Hum.. didnt get the responses from this post until after 3-4 people responded.. anyway.. Dave gave some good numbers.

Summer... I normally shoot for 45/55 Your real close.

Now the reason for 45/55.

I want the car to be balanced under braking (this is when you turn in "correct") With weight transfer... 45/55 works nicely for 4cyl car. Your 6 cyl car could probably stand 40/60 depending on what size torsion bars and what you have your shocks set on (assuming they are adjustable) Soft on the dampen will transfer weight forward a tad faster.

The key to setup is to think past the static settings and compensate for actual running settings. Look closely at a fast AutoX car. The guy's who win Nationals normally have some really jacked up alignment settings and cornerweights.

You can get 50/50 by raising the rear (this is why I use threaded collar spring perches) or dropping the front. I have set cars up with a great balance and watched the lap times go UP and not down. What sometimes happens is: you lower the front so much that the shocks lose their amount of travel and the car begins to "bounce" around the track and make the driver uncomfortable.

Slowly over the years.. I have figured out: setup is 90% what the driver wants and not what the car needs. To go fast, the driver needs to be comfortable driving fast. So.. there is no one setup for the 914's. Its different per driver per car.

B

[SummerSledSix]

Brad, thanks for the reply - makes a lot of sense the way you described it. What about *after* you turn in? I'm assuming front/rear bias shifts back to the rear wheels, good for traction, but encouraging oversteer? I have stock 911 torsions up front, 200 lb'ers on adjustable perches in back, slight forward rake, Bilstein sports all around, front sway bar of unknown parentage one notch up from "loose". Sound like a reasonable combo to you?

[Dave at Pelican Parts]

B, I didn't think you could affect the fore-and-aft weight distribution by more than a couple of percent by raising one end of the car and lowering the other. Corner weights, yes, you can affect them a whole lot. But I am pretty sure (well, was pretty sure, I'm doubting now) that ride height didn't have much effect on total front vs. total rear... The only effect I can see changing the fore-and-aft weight distribution in that case is the very small amount that the CG will move forward as you lower the front end of the car.

First, are you sure that just changing the ride height will change fore-and-aft balance? And if so, can you 'splain why it does? I'd like to know...

OK, on to SummerSled. Sounds like a not-unreasonable setup, but knowing exactly what size "stock 911" torsion bars you have and exactly what size front sway bar you have would help even more. And of course, you can fiddle the balance by messing with the alignment.

But I think the bottom line is, "How does it feel to you?" If it seems balanced when you're driving it, then that's good enough for most use. If you're trying to get that last tenth of a second out on the track/autoX course, then buy yourself a pyrometer and get educated on how to use it... And be prepared to spend money on suspension bits, tires, etc!!

To quote a friend of mine (who I think was quoting a pro driver), "You want the car to understeer on turn-in, and oversteer on track-out." That way, you can trail-brake into the turn (wish that was a skill I had!) to get the car to rotate, which may also make your braking zone just a little bit longer/later, which is generally A Good Thing. And you have to stay on the power while exiting the turn, which is the whole point of "setting up" the turn correctly anyway. With your 3.2 (I think it was, right?) you will have to be somewhat careful with the throttle so you don't overpower the traction of the drive wheels and go to oversteer that way... But still, getting on the throttle as you exit the turn is the way to make speed down the straightaway.

--DD