Shark Racer

Followed the advice given by a book Clutchdust is letting me borrow.

Anyhow, the test involves you holding the wheel at 1/2 turn in whichever direction, I chose a right hand turn. First you run the turn @ 5 mph, then at 20/30 mph.

Results: 5 mph: 4" in from starting position.

20 mph: ~4 feet in from starting position.

Seems like maybe there is moderate oversteer? Am I wrong?

I think I'm running 460 lb springs in front, and 300(7-leaf) in back. I definitely need to upgrade to 550s.

-Steve

flynhi

This sounds more like a test of steering ratio.

Fevre

He is basically making 2 passes starting with the wheel turned 1/2 turn and without turning the wheel any, he is measuring how far off from his original starting point he ends up when he comes back around. I haven't got into the over/under stear thing yet, just trying to go fast in a staight line first. :)

ddecart

Hey Steve. Can you clarify what exactly you're doing. Sounds interesting. You're basically running a constant steer angle test.

Where are you running, how far are you going? What exactly are you measuring?

And what's the book you're reading?

:seeya
Dave

Shark Racer

It's a book called "Handling! What It Is And How To Get It" by Roland de Marcelluss, and it was published by Addco. (Same people who make the swaybars.)

If it were a test of steering angle, I suppose only one pass would have sufficed. But the increased speed showed a tendency to oversteer, because the increased speed caused the tires to grip more. (A constant 20 mph turn at half a steering turn feels FAST.)

"In the middle of an empty parking lot, crank in a one-half turn on the steering hweel. Put the car in gear, and drive it five miles and hour and when car completes the circle, you will have returned to your original position. Have someone measure the approximate diameter of the circle. Try it again with the same amount of steering turn at 20 mph and 30 mph, being sure to use a steady gentle throttle. If your car describes a larger circle as you increase the speed, it understeers; if it describes a smaller circle, and you end up closer to the center than where you started, it is oversteering. If it is set up to steer neutral, you will end up slightly further out than you started because the car still drifted, but it drifted evenly frunt and rear, thus the difference was not magnified by the car being pointed inward or outward by the drift. If it is set up to oversteer slightly -- just enough to offset the drift -- you will end up where you started. This slight oversteer condition is what most drivers seek; it means that the car goes exactly where it is steered."

-Steve

VegasJen

hey! i never even read that part! :lol:
you know the way i determine understeer or oversteer? i go into a turn at about mach 2.5, when i'm about four feet from the apex i nail the brakes and turn the wheel. if i go in the dirt front end first, then i understeer; back end first is oversteer! :lol: :jester :hat :p:


[Modified by clutchdust, 2:46 PM 9/12/2002]

ddecart

That is WEIRD. That method isn't a generally accepted method for determining Understeer levels in any Vehicle Dynamics related area that I'm aware of. And frankly, if your car oversteers (in the technical sense of the word) at 20 or 30 mph, I don't want to be anywhere near you when you're driving. An oversteering vehicle is inherently unstable. It's like throwing a dart backwards. Throw it slow enough and you'll be able to get it to go backwards. Throw it fast enough and there's a speed where it will swap ends. Same goes for a car that oversteers. At the critical speed, it will swap ends.

Measuring understeer is generally done in one of 2 ways. Constant Speed, or Constant Radius. And generally, understeer levels at 20 mph aren't all that interesting. You'd be more interested in the levels at 60mph or so. I'll try to post more detail about them in the morning.

Shark Racer

You'd be surprised.

This test does seem to work. My friend's Corolla ended up a about 5 feet OUT, indicating a moderate amount of understeer. (As expected.)

The Corvette does feel tail happy approaching the limit, but I have never suffered from snap oversteer...

-Steve

VegasJen

: the more i think about it, that does sound weird. my last post was kind of a pun but i think there is some reality to it. from a strictly high-speed track standpoint, you have to figure that a "neutral" handling car is a deathwish in actual application. figure if your driving right, i mean really driving right, when you go into a turn you have just scrubbed off a whole bunch of speed which means the weight transfer the car feels makes the back end light. that would turn a car with a neutral handling car at constant speed real light in the back really, really fast. this is especially true with a c-3, i've spent a fair amount of time on the track and i promise you the rear end of the c-3 as it was designed is just there to make you wet your pants! :eek: the way i set my car up is for the back end to be very plyable, as forgiving as i can get it. that may change when i install smart struts but for now i want forgiveness in the rear.
i want an inherent mild 'push' in the front because when you decelerate coming into a corner, you load transfer to the front, increasing front grip and decreasing rear. in an ideal set up the load transfer would create that elusive balance.
you know the idea of trail brake? brake hard straight to just faster than the appropriate turn in speed. turn in slightly early and modulate the brakes to scrub off the speed, drift out and balance the car. if your really fast, your decelerating right up to the apex of the corner and then your accelerating from the apex out.
that may be a little much for a regular street driver but i guarantee you that a car that is neutral at a constant speed will kill you in an emergency manuver, like simultanious brake and lane change. D.O.T. will be scraping you and your corvette off a wall. :nonod:

Chuck Harmon

Steve,

You have entered the world of road racing! This intro is why I enjoy it so much more than the straight line stuff. From a realistic point, evaluating over steer at really low speeds is not practical. As I think DeCart said, if you have oversteer at 20mph, you will be in real trouble at racing speeds. Different corners will also behave differently, oversteering with one, a bit of understeer in another and maybe neautral in a couple of others. Uphill, downhill, banked, flat, etc, all makes a difference.

The C3's can also be setup for 4 wheel steering. The rear wheels will angle to the outside if the half shafts start parallel to the ground before the spring is compressed in a curve. This would also further any tendency to over steer.

If the half shafts angle down from the differential at rest, it will make the car understeer more. i.e. In a hard left hand turn, the outside wheel will move up in the wheel well bringing the half shaft level with the differential. This will angle the outside tire "in" in relation to the car. The inside wheel will be unweighted and will move lower in the wheel well increasing the downward angle of the half shaft and pointing the wheel "out" in relation to the car. This means both the right wheel and the left wheel are pointing slightly left in relation to the car.

This is why setting up the rear ride height is so important with the C3. You really change the handling characteristics when you mess with it ignorantly.

As for determining understeer oversteer on a car, I have found nothing better than a wide deserted road with a fresh layer of snow on it. You get the same driving feel at 30mph as you do at a 100mph. A wet road is the next best; but you must travel proportionally faster to get the same feeling. Haven't found a good snowy road in San Jose yet.

It really takes some ***** to gently add a bit more power when you get into trouble in a curve to get better bite at the rear through weight transfer. It does work, but you are walking a very thin line. That's why guys that are very, very good lap after lap make the big bucks. If you panic and "lift" you will spin out in a neutral car.

My track time is regretably very low, but I have found that working on one corner at a time, progressing steadily, sneaking up on the eventual best speed the car is capable of doing through that corner, is the best way. Hitting the apex late also buys you much more room to recover in case you overjudge your abilities in a curve.

A good driver in a 4 cylinder Miata can seriously embarass an over confident ZO6, 911, or Ferrari owner on a twisty track.

Chuck

gkull

Other than what Chuck had to say. It really comes down to tires. In the mid 80's i went to 550's and and 340-360 or so TRW mono fiber rear with a 1 1/8th front and 3/4 rear. I was using 8.5/15 centerline wheels with 245/50 and 265/50 VRrated. What it really comes down to is the tires and alignment settings.

You have to go get your settings likes VB&P book I found that nearly 4 caster and Zero toe with .75 front and rear kept a nearly even tire temp across the face of my tires.

Then at some point i determined that your never going to do it wit 15 inch wheels because you can't get Z rated tires in the size you need. So i went 17/9.5 and 17/11 wheels.

Then like th stupid person I am. Tonight I needed to do a U turn at stop light on a two lane each way. So when i was well into the turn I floored it. Of course This was in front of many cars watching the rear didn't whip around because of my rear 315/35 tires and square out the turn. I had to stomp on the brakes to keep from hitting the curb in the direction I wanted to go.

Blue smoke and a little reverse and smoke them down the street and i was fine.

The only way to really figure out what your doing is a 100 foot skid pan with a clock. Like they have at early in the year SCCA auto-x races. do as fast as you can CW & CCW Then use a thermal across the face of your tires. The keep adjusting your camber and toe until you have the same temp across the face of what ever tire your using.

Then it's just how much power and road surface you have to whether you have over or under steer.. :) I can come into a turn way to fast and saw the front end back and forth. The front tires are loosing it. Or if i brake to early and have to get on the gas you can determine how far you want the rear to come around with the gas on. Like on a freeway fast on ramp.

gkull

I should add that i don't agree with Clucthdust. No real race machine can ever brake while turning the front tires. All real heavy braking is done before you ever turn in.

Shark Racer

When I saw you, I forgot to ask to go for a ride. :eek:

Hell, I'd pay just to get a couple nice video tapes of you driving. :)

I'm in way over my head. I hope what you said in an earlier post is right! I'd love to be able to control drifts like that.

-Steve

ddecart

One of the differences in conversing here is the definition of understeer. What Steve was trying to measure is honest to goodness understeer in the technical sense of the word. It's defined under STEADY STATE conditions as the difference between the required steer input (by the wheels) rleative tot he reference steer angle (ackerman). So if a vehicle that has 3 degrees per g of understeer was navigating a corner at steady state (constant steer angle, constant speed) at 0.5 g then the front tires would need to be turned 1.5 degrees (3 deg/g*0.5g) MORE than the ackerman angle to navigate that curve.

What does that mean for racing around the track? Either a lot or nothing, depending on how you look at it.

If you have that 'neutral steer' car that everyone desires (and you brake in a turn or apply power in a turn, you're throwing it out of balance. You've essentially taken the fins off the dart and it no longer cares which way it's going. With some level of understeer in the vehicle you're able to keep the fins on the dart and where you put the throttle and brake will in a sense, make them bigger or smaller. Does that analogy make sense?

Years ago (read: before my time) we had a car with something between 0 and 1 deg/g of understeer. Either Road &Track or Car&Driver came out and drove the thing. Now remember that these are the people who are constantly complaining that cars have too much understeer. They came out of the car looking like :eek: The car was almost undriveable on an autocross-type course.

Now the other thing I shoudl mention is that understeer is evaluated at a particluar speed AND a particular g-level. I can have a car that has low understeer at 0.2g, higher at 0.4g and it might actually go oversteer at 0.7g. Or it might have limit understeer and have huge levels of understeer at the limit, or it might not have too much at the limit. So defining understeer requires a specific point like 60mph at 0.25g.

Incidentally, you can get into a vehicle and drive along at 60mph, you can do lane changes, corners, whatever. You won't be able to tell me how much understeer the vehicle has. You can tell me how sensitive the steering is, how quickly the vehicle responds. Those are directly related to understeer. But you won't be able to percieve understeer.

Now on a track, you can definitely feel understeer in the TRANSIENT sense. Does the car push or not is the main question. That's a matter of transient effects and what the tires are doing, how the suspension is reaction, how mcuh camber you're getting under braking or accelerating, how the sta-bars are reacting, how they're balanced front and rear and how that works out with the rest of the suspension. So the steady state understeer is a factor in that all, but it isn't the same.

*rrrrrrrrrriiiiiiinnnnnnnggggggggg*

Whew. Ok kids, there's the bell. Class is over and I'll shutup now. :D

VegasJen

gkull, when i re-read my post, it didn't sound really clear to me too. maybe you'll agree with this: when you are truely at the limit, 95% of the braking is done in a straight line before turn in. there is still a slight amount of deceleration (read engine brake, pedal modulation or the natural speed loss associated with steering input) right up to the apex of your corner at which point you begin your acceleration (if you've done everything else right).
OOPS! let me clarify, again. there will still be a slight amount of deceleration up to the point of maximum steering input. how's that? we know that after total braking effort we don't whip the wheel to full lock, it's a gradual progression from total forward acceleration to maximum lateral acceleration. hence trail braking, maintaining as much speed as possible until maximum steering input is acheived (<--trying to sound smarter than i am :rolleyes: )
obviously, this is going to depend highly on the type of turn we're talking about. in this particular instance, what i'm talking about is a generally two straights connected by a single turn. multiple turns will affect entry speed, angle and theoretical apex versus physical apex, yadda, yadda, yadda.

VegasJen

sorry steve, didn't mean to hijack your post! :cheers:

Shark Racer

All I can say right now is that you guys are about ten times as good as I am.

I can only manage the occasional drift...

Don't feel bad at all. :) I'm learning.

-Steve

Chuck Harmon

Thought some of you guys might enjoy this article. It explains what goes on in a turn in a very down to earth way.

Chuck

ddecart

Hey Chuck, where's that from? I see it's written by Guldstrand, but I don't recognise "The Leading Edge".


I'll concur with what he said about GM. GM has some of the most talented people working in Vehicle Dynamics. Open a VD text book and you'll find their names in the Bibliography. :yesnod: Which reminds me, I need to calculate what rear sta-bar size I need in a particular vehicle.

Chuck Harmon

ddecart,

This description was printed in Guldstrand's catalog. He and his staff are some of the most helpful guys in the business. They also take a real enthusiast's interest in your car and performance cars in general.

Occasionally I think I might be able to get a certain part for a few bucks less elsewhere, but I don't because the questions I can ask and the extremely valuable guidance I get are worth a ton. I have replaced new and expensive stuff bought through other companies with Guldstrand's products. I have never had to replace any of his!

Chuck