Took my 97 328 for a nice drive in the foothills the other day. While
thoroughly enjoying playing games with how well I could set up the
suspension for each turn and then TIGHTENING it by adding throttle to
power through it, I started wondering just why it works this way.
First off, I'm a moderate driver - maybe 5/10ths, so we're talking
entering 45 mph turns at maybe 50 mph and powering out with moderate
throttle at 55 mph, while enjoying the scenery - I often pull over to
let faster drivers pass. When I drive this way, my car TIGHTENS the
turn significantly as I add throttle when entering the turn - lifting at
this point will produce a significant push to the outside, so that you
have to quickly correct to stay in the lane.
I would think adding throttle would produce a push to the outside, not a
'dive' (exaggerating) to the inside of the turn. Can anyone explain to
me how this works in terms of traction circles, slip angles, front to
back weight transfer, etc. I cannot figure it out, and would really
like to understand this.
I am not familiar with all the terms you use so you probably know more
about this than me, but I would say when you accelerate you put more
bias to the rear and therefore more traction for the drive wheels. You
would not start to push to the outside until you had so much lateral
momentum and overcame the grip of the tires. I'm no expert, just my opinion.
I believe this is because you are rear wheel drive and the rear tires
try to overcome the front during acceleration and, therefore, push the
front into the corner as the rear tries to go wide.
This is especially evident in a motorcycle, though a little different.
"If in doubt, hit the throttle!"
Most cars "understeer" in a corner. That is, the front seems to push to the
outside of the curve a little bit, not quite going where you want it to.
It's a momentum/physics thing. Front wheel drive cars tend to do this more
than RWD, because they tend to be heavier in the front, and the front wheels
are both driving and steering the car. To correct with FWD, you either pull
a little harder on the steering wheel, or ease off the gas a little. And
what fun is that?
FWD's also end to torque steer. When accelerating, the car will pull a
little bit to the side that is powering the car (left/right). This doesn't
help in the curves either, as it can exaggerate understeer. This is why
it's harder to throttle up out of a curve with FWD than with RWD. FWD
corrects understeer slower than RWD, and acceleration\torque steering makes
it worse. A limited slip differential can minimize torque steer (which is
almost non-existent on RWD).
Well-balanced RWD cars, like yours, have a very nice, built in solution to
understeer ... give it more gas! This will push the rear of the car more
into the curve, and provide a little oversteer to correct the initial
understeer. In your words, the rear will push, and the front will "dive"
(they are not mutually exclusive). The results are your car is lined up
properly in the curve more quickly, and you can throttle up out of a curve.
RWD corrects understeer faster than FWD, and acceleration helps this (plus,
the gas pedal is going in the right direction). It's also a big reason
BMW's are so much fun to drive!!
As for weight ratios, anything close to 50/50 front/rear is ideal. Not
exactly sure what your car's is, but I've seen other BMW's with something
like 52/48, which is excellent. FWD cars are more in the 58/42 range. The
heavier front end makes the steering work harder, and with the drive wheels
up front, you get understeer\torque steer. Big old American Iron RWD cars
from the 60's and 70's were sometimes around 63/37. Even though they had a
heavy front end like FWD, they were RWD, and tended to have the tail swing
out on turns, thus oversteering. It was the poor weight ratios that made
them bad in weather and bad in curves.
It was also that Big Iron that made people think FWD was better in bad
weather than RWD. Better weight ratios, with more weight over the drive
wheels. Old RWD's would just spin out on wet/snowy surfaces, because they
had no traction. But, it was all relative. A well-balanced RWD car, with
good tires, is outstanding in bad conditions, better than FWD or Big Iron.
It has weight over the drive wheels, like FWD, and the drive wheels are
separate from the steering wheels, like Big Iron. FWD in general will
understeer in a curve, and this is exaggerated in bad conditions, because
the fronts are trying to both drive and steer. A good RWD will slide
through a curve too, but a little gas will straighten the car out, and since
the fronts only have to steer the car, they can grip the road better.
I used to have a Porsche when I lived in Cleveland, and it snowed there. I
had good snow tires, on a RWD car, with a good weight ratio. It was the
best snow car I've ever driven. And it was fun to drive in slippery
conditions, because I knew the car could handle it. Coming up to turn onto
my street, I'd turn the wheels a bit, give it some gas, making the back end
slide out a little, until I was lined up just right, then take my foot off
the gas to stop the slide, and the wheels would grip and I'd be going
straight again. A very fun way to make a turn.
Hope this helps.
'91 Nissan Maxima SE
(yeah, I know, FWD! But I chose grad school over a new Bavarian)
The technical term for what you have experienced is "oversteer",
as opposed to "understeer". To be precise, it is power-on oversteer.
If you google that term, you will find many explanations better and more
technically correct than have been already posted.
You need to take a performance driving course to get practical experience
in this and other facets of controlling a car.
Oversteer is when you go through the fence - backwards.
Understeer is when you go through the fence - forwards.
Neutral handling is when you go through the fence - *just* right.
Ask me how I know this.
Forgive me if I'm making a false assumption about your reply, but I assume
that you're talking about applying enough power to break the rear wheels free,
or at least bring them to the limits of traction. This I understand quite
well - used to do it all the time on the snow covered streets of Mpls Minn as
a teenager. Still practice "swinging the rear end" a bit with my Pathfinder
on crushed gravel roads up in the forest - truck has a nice front/rear weight
balance and does this quite nicely.
With my 328, I'm talking about being way way below these limits - just Grandpa
driving while looking at the scenery - don't understand how I get power-on
oversteer in this situation where the rear wheels are way inside their limits
Again, forgive me if I misinterpreted your reply. A Google search seems to
turn up the 'limits of adhesion' situation, (discussions of racing and
autocrossing) so I couldn't find anything that seemed appropriate for my
Don't forget that power-on also removes weight from the front wheels, which
can eliminate the built-in understeer, which then manifests in the
you experience. Over/Under-steer is a far more general concept than going
through the fence frontward or backward, and describes a wide range of
A couple of the commentators for NASCAR races actually explain it fairly
well during the course of a race, since most of the tire-pressure, track-bar
wing adjustments they make in the pits are designed to change the
steer aspects of the car's handling (they call it "tight" and "loose",
Under these circumstances, I doubt that you do. You are correct in
assuming that power oversteer is a marginal-traction situation. I
suspect that you're incorrectly identifying the effect you're feeling
in your 'Butt-O-Meter'. As a BMW CCA instructor, I have had the fun of
getting to stand in the middle of our skidpad circles as students
accelerate gradually until one end or the other of their cars started
to slide. With stock BMWs, this is almost invariably the front end
(understeer). Almost without exception, even those who should know
better identify it as oversteer. Don't ask me why. That's just the
Your car is also an inherent understeerer unless you do something
drastic (brutal power application or sudden lifting off throttle in the
middle of a turn). I suspect you are being overly sensitive to your
car's behavior and you may *believe* it is tightening when you apply
throttle when it is actually doing nothing at all other than neutral
steering. Either that or something's broken ... =8^O
(Been there; done that)
Your reply just showed up on my computer today - strange beasties these things.
Anyway, I definitely have oversteer - after being set in the turn, if I apply
throttle, car will cross the centerline in a left hand turn, or run off road in a
right hand turn if no steering correction is applied. I went to that autozine
handling site someone recommended, and it seemed that the discussion about adding
throttle increasing the slip angle for the rear tires explained it. Seemed
plausible, anyway, and also seems to fit with the fact that I can change the
somewhat by adjusting front to rear tire pressures. Usually, I enter a turn
'coasting' (just enough throttle to avoid engine braking) and accelerate once
the turn. With this effect, though, it's fun to get in a groove where I enter
throttle on and then set for the turn - car will hold the line nicely, unless I
Ever since I joined BMWCCA, I've been trying to get myself to one of the driving
schools - maybe I'll make it yet.
isn't this a consequence of the rear suspension (semi-trailing links
or whatever the E36 has) I remember reading long ago that for such cars
the best way to control understeer is to accelerate in the curve.
The acceleration makes the rear end go down on one side and this gives
the rear wheels an extra sort of "steering"
I suggest you check out the Autozine Technical School at
http://www.autozine.org/technical_school/tech_index.htm , especially the
http://www.autozine.org/technical_school/tech_index.htm#Handling It has
all the information you need.
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