mrg

For the longest time now I still confuse the terms "push"or "pull", "oversteer" or "understeer", to describe what's happening to a car's handling as it enters or leaves a curve. I was wondering what the cause and effects are when these terms are used to describe how a car is handling.
Anybody .. ? ..

SWCDuke

The NASCAR guys call understeer "push" and oversteer "loose". "Pull" means the car does not track straight. Circle track racers are set up to pull in the direction of their turns - usually left. Road cars and road racing cars are best set up with no pull. They should track straight on a straight and level road and have similar handling response right or left.

Whenever a tire is generating a cornering force it is sliding. At normal cornering loads the slide is so slight that most people don't notice it, but you might notice that the faster you go through a given corner, the more steering wheel angle is required. This is understeer and it is safe and stable. If you get to the point where the rear begins to slide around and you have to back off the steering angle to keep the car on the proper arc, you have experienced oversteer.

The best setup is neutral steer where the amount of steering wheel angle to negotiate a given corner is independent of speed, but this is a knife edge that is very difficult to balance, and falling into oversteer is bad news. Thus, most cars are set up to understeer. Front engine cars with near equal weight distribution can be set up with minimal understeer to achieve near neutral steer behavior. The small block C2 is a good example and one reason why these cars have excellent and fun driving characteristics, but they are so close to neutral that they can transistion into oversteer at the limit of adhesion, so they demand respect and well honed driving skills if you want to push the limit of their handling envelope.

There is a concept in tire engineering called "slip angle". It is the difference between the direction the tire is pointed and the direction the tire is actually traveling - that slight slide I referred to earlier. In normal driving slip angles rarely exceed one to two degrees. At the limit of adhesion a radial tire will probably be operating at about a six degree slip angle.

Chassis engineers refer to "degrees of understeer". This is simply the rear slip angle subtracted from the front slip angle. If that number is negative you have just spun into the weeds! ;)

Duke

Milehigh66

Thanks Duke, I have wanted an easy explanation for a long time, yours was excellent. :yesnod:

mrg

Ditto .. Thanks Duke, for providing a great explantion .. I'll be sure to copy this thread for reference. Taking the car out for a spin is gonna have some new meaning .. :lol:

Milehigh66 - Did you guys finish up the sanding on your car . .? ..

mrg

pittsaj

Perfect as always Duke :cheers:

I guess,
I just need you to take my car for a spin... :eek:
Well, you know what I mean :lol:

And tell me what's causing my handling problem...

I sure haven't been able to completely pin it down.. :banghead:

Tony

JohnZ

Darrell Waltrip's concise explanation - "when the car's "tight", the front end hits the wall first - when it's "loose", the rear end hits the wall first" :jester

0X-AutoProducts

duke, great reply. thanks.
what about 4 wheel drift? is it good or bad?

Viet Nam Vett

Quote*

Darrell Waltrip's concise explanation - "when the car's "tight", the front end hits the wall first - when it's "loose", the rear end hits the wall first"
________________________________________ ________________________

:lol: :lol: :lol: :lol: :lol: :lolg: :lolg: :lolg:

JohnZ

Actually, "4-wheel drift" is a misnomer - what's really happening there is "controlled oversteer"; the front tires are still within their slip angle limits and are steering the car, and the rears are beyond slip angle limits and are sliding, with the degree of "slide" being controlled by power application. The ultimate "ragged edge", one small step from an "agricultural excursion" :D

SWCDuke

John - I think your description is actually controllable power oversteer exiting a corner. The most obvious example of this is dirt track racers. My definition of a four-wheel drift is more narrow.

To use my Cosworth Vega as an example, it is a near neutral car with mild understeer on turn-in and neutral on exit under power. (It's dynamic response is relatively insensitive to throttle, which makes it very controllable at the limit.) I recall once taking friend who is very familiar with the idiosyncratic handling of Porsche 911s out for some hot laps at Riverside. Exiting the relatively slow Turn 7A he cried out: "This car is dead neutral!" Indeed, you can see in the in-car video that as I apply WOT (third gear at about 50 MPH) at the apex I make no steering input, but slowly back the steering wheel off to center as I continually widen the arc exiting the turn, and you can feel the car drifting. The CV exhibited similar behavior in Turn 9, which was an 80 MPH sweeper that I ran in fourth gear.

A true four wheel drift means the front and rear slip angles are equal, a most desireable handling response, but very difficult to achieve because you are on that knife edge between understeer and oversteer. In the case of the Cosworth Vega, it will assume a four wheel drift exiting turns, because it's just short of the amount of power required to break rear traction, which would establish an oversteer condition.

A Corvette is much more difficult to set up dead neutral and achieve this narrow definition of a four wheel drift. It has a lot more power, which is capable of breaking rear traction and they can be trickly on a bumpy surface as the track, toe, and camber change in the rear suspension makes it difficult to establish a constant rear tire slip angle, so they can be a little dicey at the ragged edge.

A well set up solid axle design like the Cosworth Vega actually has better limit handling behavior than most IRS designs except for the most modern five-link or SLA with toe link designs, such as the C5. The old Corvette three link design and semi-trailing arm IRS on vintage BMWs places a premium on driver control skills.

Duke

stngry63

Duke,
When my '63 had 0 deg. rear camber, the rear end would swing out and then stop; with the car stable, but going through the corner with the rear hanging out. This is by seat of the pants. Using a carpenter's square to keep both sides equal, I corrected by adjusting in negative rear camber until it stopped doing it on my favorite "safe" 60 mph curve. A gentleman explained that the car had been rolling faster than the suspension could react with negative camber when it was set at zero. Is this so? By the way when getting the car aligned professionally, the rear turned out to be right on -1.5 deg.
Brian

SWCDuke

Tires will generally develop maximum cornering force when they have a bit of negative camber relative to the road - about one to two degrees for a "conventional" high profile tire - say 60 series or above. Very low aspect ratio tires are a lot more camber sensitive and should be held near zero to just a slight negative camber bias. The static rear camber spec at curb weight for '63 is minus one-third degree, and at this setting the camber at "design" ride height (full gross weight with two pax and a full load of cargo) is minus 1.5 degrees. I believe later C2s have the same or a very similar rear camber spec. As you can see the negative camber increases with suspension jounce and is referred to as "negative camber gain."

Both the front and rear suspensions have negative camber gain with jounce. Thus, in a corner this negative gain will tend to compensate for body roll to maintain the wheel near vertical to the road, and the slight static negative rear camber, plus the higher negative camber gain coefficient at the rear than at the front will help keep the car out of significant oversteer, but bushing compliance reduces the theoretical negative camber gain computed without allowance for bushing defletion.

As you found, if you don't have a little static negative camber at the rear the car will have a tendency to oversteer. Suspension tuning is critical to achieving good handing response from a Corvette, and because they have near neutral handling characteristics, they are quite responsive to changes in static camber settings . That's why I have both a "touring" recommendation and a "high performance" recommendation with a little more negative camber bias at both ends for the latter.

The gentleman's explanation was basically correct.

If you have negative 1.5 degrees rear camber at curb weight, it's a bit over spec, but if you are happy with the handling performance, and aren't experiencing excess inside tire wear, then it's okay. Corvette dynamic response is also very sensitive to rear toe. With radial tires about 1/16" total toe-in at the rear works well, and it's very important that it be equally divided between both ends. The total could be measured at 1/16", but if it's divided 1/8" toe-in on one side and 1/16" toe out on the other the car is not going to handle very well, especially turning in the direction opposite the side that has the toe out. The car will tend to oversteer on turn-in and when powering out of a corner. Toe-in has a big effect on transient handling response.

By contrast the C5 has much less camber change with suspension movement as this minimizes track and toe-in changes, which is very important with wide low profile tires. Camber change relative to the road are controlled primarily by having lots of roll stiffness to limit body roll, most of which is provided by the anti-roll bars so that the springs are not overly stiff. The modern Z06 OEM camber recommendation is slightly negative, front and rear, with more negative camber bias for track work, and their substantial roll stiffness limits roll to about three degrees per g, versus probably about four to five degrees per g for base suspension C2s.

There is a big difference in the design philosophy of the C2 and C5 suspensions, which is due to the significantly different characteristics of the narrow, high profile tires on C2s relative to the wide, short sidewall tires on C5s, and this is also the reason why I don't recommend short sidewall 17" or 18" tires on C2s. The suspension design is just not suitable to modern short sidewall tires, but is very well suited to capitalize on the qualities of the highest performance seventy series 15" tires.

Duke

stngry63

Thanks. The explanation is easy to read and understand. I laughed though. Before I got to the last paragragh, I thought that's why he doesn't recommend low profile tires on a C2 in other posts. And then, there it was.
Brian

JohnZ

Great explanation, Duke, as usual :yesnod: I may have mentioned this before, but we've built every Viper since '96 with removable "sport shims" on the upper control arm shaft attaching studs; Viper camber is set with cam bolts on the lower arms, but we designed-in one fairly thick shim on each upper stud - even bent the tops to provide a "handle". For weekend track work or autocross events, owners can loosen the nuts, pull all four shims (two on each side), re-torque the nuts, and it provides one degree of additional negative camber. Weekend over, loosen nuts and re-insert the "sport shims" and it's back to the normal street geometry, without disturbing the critical primary adjustment on the lower arm pivots (due to the Viper's 13"-wide 30-section tires, our alignment machine in the plant sets front and rear camber within two-hundredths of a degree). It's also the only car on the planet (besides Ferrari) where the steering rack attachments are shimmed in the alignment machine to achieve zero bump-steer geometry. Viper owners DRIVE 'em - hard! :steering:

Milehigh66

Not even close, and I am already really sick and tired of sanding. We are going to do the fiberglass work this weekend, a couple of cracking issues to be dealt with.

Kurt

mrg

I know what you mean. I kinda took the "easy" way out and used paint stripper on mine. Well, the label said it was for fiberglass! .. This was done before the "Paint Tech" file I received from Lars, in which he says not to use the stripper on fiberglass. It would be great to hear how the CCC progresses on your project. Thanks, Kurt ..

mrg