Craig Blome wrote:
> If you're thinking about posting on this blasted
> rollover topic, don't. Go and read
> <http://www.rqriley.com/suspensn.html> instead,
> specifically the "Rollover Threshold" section. You
> might learn something, and 850+ mailboxes will thank
> you.
This article, in my mind, makes some grievous errors in some places,
and some relevant omissions in others. What do we know about the
author?
I'm no physicist, but based on my understanding, here are some errors:
> The weight bias of the vehicle determines its inherent oversteer/
> understeer characteristics. A vehicle that is heavier at the front
> will tend to understeer and one that is heavier at the rear will
> oversteer. A vehicle in which the weight is equally distributed
> between the front and rear axles tends to exhibit neutral steer
> characteristics.
I have a hard time believing that weight distribution is the major
factor here, unless we are assuming the same tire sizes and dynamic
alignment at both ends, and only under static cornering (e.g. on a
level skidpad at constant speed). It seems to me that if a car was
ever designed to understeer, it's the rear-heavy Porsche 911.
Certainly once you get the tail of a 911 moving laterally, it'll keep
going that way -- but the skinny little tires up front with no weight
on 'em, and poor camber control from the front strut suspension,
don't do much for front traction.
> Understeer results when the slip angle of the front tires is greater
> than the slip angle of the rear tires. A greater steering angle is
> then required in order to maintain the turn. When the steering angle
> reaches full lock and the turn cannot be maintained, the vehicle
> drifts to the outside.
Wrong again in that second and third sentence. When understeering,
more steering angle won't help, since the driver is already asking the
front tires to do more than they are capable of doing -- so asking
them for even more certainly isn't going to help. Also, drift occurs
long before the steering angle reaches full lock.
> Also a lower inflation pressure will cause the tire to reach its
> limit of adhesion at lower g-loads. Consequently, increase the
> inflation pressure at the end of the vehicle requiring greater
> cornering forces (greater adhesion). Reduce the inflation pressure
> for reduced adhesion and cornering forces.
This is only true if the starting pressure is below the optimal
pressure. The author ignores the possibility that there's such a thing
as too much pressure.
> Larger diameter tires tend to ride more smoothly, and they also
> develop greater cornering forces.
I'll agree with the first point, but is there anything to the second?
The author offers no proof.
On the Rollover Threshold point, the author ignores the concept of
roll center, and the fact that when the suspension and bump stops are
fully compressed, the roll center moves to the outside contact patch,
thus making the incipient roll even more likely.
I'm sure there's more, but I haven't read the whole article in detail.
Mark
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