Overstreer & understeer??

[BeEfCaKe]

OK, I need to get this straight.. I have heard many things from people.. some say Understeer is in FWD cars and Oversteer is in RWD cars... but i'm also hearing the opposite from some... so I want to sort this out...

[Grendel]

I'm pretty sure its basically Understeer in FWD, and Oversteer in RWD...

My first Z was RWD, and it oversteered...

My current Z is RWD, and it oversteers...

My parents Accord is FWD, and it understeers...

My Moms old Tercel was FWD, and it understeered...


So basically... It looks like RWD=Oversteer, FWD=Understeer

-Grendel

[swedish]

yeah, thats the general rule. however, there are cases where rwd will understeer (mr2?) and vice versa

[Grendel]

Originally posted by swedish
yeah, thats the general rule. however, there are cases where rwd will understeer (mr2?) and vice versa

Yea, I think it depends on the placement of the engine...

Rear engine/RWD would be different than front engine RWD...

Don't ask me why tho... I have no clue :P

-Grendel

[hermunn123]

Originally posted by Grendel


Yea, I think it depends on the placement of the engine...

Rear engine/RWD would be different than front engine RWD...

Don't ask me why tho... I have no clue :P

-Grendel

well on an MR2, the front tires have little weight on them so they don't have sufficient traction.

[FYRHWK1]

from what i understand, understeer is when the front tires lose traction, oversteer is when the rears do. you could make a FWD car oversteer, just yank the Ebrake, massive oversteer, probably better classed as spinning out but its the same principal. braking hard and jerking hte wheel to full lock i'm sure you could get almost any car to understeer as well, MR and RR cars tend to have some understeer when you accelerate due to weight transferring to the rears, i believe anyway.

[CraigFL]

Most cars are set up to understeer because it is considered safer. Driving hard into a corner with an oversteering car can be disasterous -- a nasty spin. How a car handles normally and in the transition from understeer to oversteer can be affected by things like tires, pressure, shocks, swaybars, etc.

I would rather have it this way too because you can always create the oversteer situation by adding power in a RWD car or a trailing throttle in a FWD car.

The test to see if your car oversteers or understeers is to find a big parking lot. Pick about a 100ft diameter circle and start driving at a constant speed. Gradually increase the speed until either the front washes out (Understeer) or the rear washes out(oversteer)

[Gasoline Fumes]

Originally posted by CraigFL
...The test to see if your car oversteers or understeers is to find a big parking lot. Pick about a 100ft diameter circle and start driving at a constant speed. Gradually increase the speed until either the front washes out (Understeer) or the rear washes out(oversteer)

I would think that most cars would understeer in that situation, unless you break the rear wheels loose with the throttle.

[SaabJohan]

Under- and oversteering depends on the traction like FYRHWK1 says (and the inertial mass of the car).
This means that there is many things that affects the car. How much throttle you gives is one of them, and this causes a FWD car to understeer more when you gives more throttle and less when you lifts the foot of the pedal, and this can even cause the FWD car to oversteer. For the RWD car it is the other way around.
And then there is the 4WD car, here there can be a lot of differance between the cars since 4WD doesn't says anything about the power distribution. Some 4WD cars are 2WD (usually FWD) until the front wheels are spinning, this can do that the car can change from understeer to oversteer very aggressive and make the cars very hard to drive. But these are not the only 4WD cars with that problem, konstant 4WD cars can also have that behavior, this was what Audis rally drivers in the eigties learned.

[CraigFL]

Originally posted by Gasoline Fumes

I would think that most cars would understeer in that situation, unless you break the rear wheels loose with the throttle.

Unless you had an early Porsche 911 that had sudden terminal oversteer.

[sleeperguy]

when going in a circle or turning, when you accelerate, there's a force imbalance and the weight of the car is shifted to the rear meaning a greater normal force on the rear tires (more traction) resulting in understeer. If you brake a little thereby shifting weight to the front, there's left weight on the rears resulting in oversteer.

That's why in a fwd when oversteering, simple get on the gas a little to get more weight shifted to the rear, and vice versa (when understeering, brake a little to shift weight forward).

as people have pointed out, oversteer/understeer is ultimately determined by weight distribution.

[higgimonster]

My 95 saab 900s (Non turbo ) has snow tires in the back and normal tires up front. On rainy days i can slide that car all over the place and when things get hairy all i have to do is mash the gas and the thing straightens right out.
Just thought i would through in my $0.02.

[pimpin4profits]

Weight distrubution as well as drive wheels is something to keep in mind...

Most pickup trucks especially smaller trucks with v8's have A LOT of weight in the front and not near as much in back, the result, understeer....

[911GT2]

Originally posted by swedish
yeah, thats the general rule. however, there are cases where rwd will understeer (mr2?) and vice versa

The MR2 tends to understeer because it has so much mass in the back end (compared to a front engined car) that it's hard to break it free, making the front end lighter. Therefore, the front end will plow, creating understeer, seeing that it can't really break the rear wheels free because of its lack of power.

That's why Porsches of the past were considered so hard to drive. If you break the rear wheels free, you're never getting traction back until you slide a long way or hit something. There's so much mass back there that it builds up a large amount of momentum. The momentum combined with the power able to break the wheels free creates awful oversteer. But in skilled and experiences hands, they were some of the best handling cars int he world.

[replicant_008]

You can get oversteer and oversteer regardless of where the wheels are driven - it's about weight transfer and slip angles although granted it is easier to create the necessary slip angles depending on which wheels are driven.

Understeer is effectively a yaw movement where the forward portion of the vehicle is propelled away from the corner axis relative to the intended arc ie what is called "pushing." This is generally when the slip angles on the front wheels exceed those of the arc around the corner.

Oversteer is effectively a yaw movement where the nose of the vehicle is propelled toward the corner axis relative to the intended arc ie it begins to get "taily" alternatively it can be described as the condition where the rear wheels have slip angles exceeding those of the arc around the corner.

Most passenger vehicles usually are intended to provide understeer as it is thought this is more benign and easy to recover for most drivers than oversteer. Why? I'll try to explain...

The first thing to consider is weight transfer - under acceleration, the car tends to "squat" (ie weight is transferred to the rear) and under braking the car tends to "dive" as weight is transferred to the front. This weight transfer has the same effect as lift and downforce - when you have lift your mechanical grip to the road is reduced and when you have downforce mechanical grip is increased.

You usually get understeer (particularly plough-on understeer) where the grip of the steering wheels is exceeded by the sideways cornering forces ie you are cornering too fast for the mechanical grip of the car. This causes the vehicle nose to push wide of the apex of the corner. To alleviate this you can apply more steering lock (ie turn the wheels toward the axis of the corner) but at some point the yaw force (understeer) will be to great to compensate. To alleviate this, you can reduce the throttle gently this reduces the cornering forces and reduces the slip angles bringing the car back to neutral and also subtly transfers weight over the front wheels adding to grip. This is accepted to be easier to control for most drivers.

However, sudden lifting off of the throttle and/or braking when experiencing understeer can cause what is termed "snap-oversteer." This is because weight is sudden transferred to the front of the vehicle (ie pitch) which reduces the weight on the rear of the car. This weight transfer reduces the downward force on the rear which reduced grip. The cornering forces which were held equal at the rear of the car now exceed the grip of the rear tires. This causes the rear of the car to be pushed outside the intended arc (ie oversteer) in a very rapid transition.

This rapid transition is often difficult to control as it must be compensated rapidly by a reduction in steering lock or alternatively to a steering angle OUTSIDE the intended arc ie the steering wheels are pointed AWAY from the apex of the corner! This is called 'opposite lock' and must be applied as rapidly as the transition to oversteer to be corrected successfully. This is generally accepted to be outside the skill levels of most drivers of passenger vehicles.

Front Wheel drive cars are inherently more likely to understeer because the steering wheels are also charged with the job of applying drive so acceleration forces as well as the bulk of braking forces are applied to this end of the vehicle as well as having the job of steering. However, with sudden weight transitions or differences in front-rear grip (eg worn tires on the rear and grippy tyres on the front) oversteer is still possible. Weight distribution front to rear can also affect the tendency to under/oversteer eg unladen forward control front wheel drive vans.

Rear-wheel drive cars are inherently more likely to oversteer than FWD as the rear wheels have acceleration forces to contend with. Again weight transfer/distribution is significant. High powered RWD pickup trucks can produce oversteer due to the unladen rear end having less weight over the driven axle.

Finally, polarity of inertia is also a major factor. Rear-engine Beetles and Porsches inherently have oversteer characteristics as the weight of the vehicle is skewed behind the rear axle which creates inertia towards the oversteer yaw movement. Front Engine and in particular FWD cars tend to understeer for similar reasons as weight is skewed over the front axle.

On the other hand, mid-engine vehicles have a low polarity of inertia as the weight is centred between the axles. While improving weight distribution and reducing the tendency to produce yaw movements, this does have the disadvantage of having a higher propensity to spin on the axis of the engine location.

Phew...