Fatalities due to Better Tyres and greater Side-Forces at Front Wheels?

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Cars unstable
with greater side-force
and better tyres
at front wheels

Several people killed in skidding accidents
after mounting new tyres at front wheels only

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December 21, 1996

Avoid accidents (tomorrow)
by understanding (today)
the relevant laws of nature

Tyre side forces (S), their sum or 'resultant' (F) and its application point (P)

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Small sideslip oscillations are always occuring, due to road unevenness, aerodynamic forces, etc.
However, the current sideslip tends to be amplified by the resulting external forces, if their sum, F, applies to the vehicle (at P) in front of the centre of gravity, T (about which the vehicle is rotating).
Therefore, the vehicle may become unstable (like the upper car), when the side forces increase more with the sideslip angle at the front tyres than at the rear.

Hydroplaning at the rear wheels only may be simulated and perceived in this skid training car

Yaw Stability may be lost ...

... and your car may spin around by itself (like the nearmost yellow Combi Coupé in the picture) because:

the stud protrusion is greater at the front wheels of the average front wheel driven car
heavy luggage and rear seat passengers (centre of gravity moves rearward) increase side-wind sensitivity
heavy luggage and rear seat passengers decrease power over-steering tendencies in a rear wheel driven car, but may increase the risk of rear wheel skid in lane change manoeuvres, irrespective of driven wheel position
heavy luggage and rear seat passengers affect yaw stability on icy roads in different directions depending on both tyre studding and axle weight
the rear wheels hydroplane when they come out of the tracks of the front wheels (hydroplaning at front only will increase stability, though steerability is lost)

the rear wheels lock up if the passenger pulls the parking brake (accidents have occured in Sweden when amateur teachers in passenger seat tried to stop their private car with single controls - allowed for driver training on all Swedish roads)
too much of the braking forces are distributed to the rear wheels - due to inadequate brake linings

Spin-out and plow-out cars before collision

too much of the braking forces are distributed to the rear wheels - due to corrosion or inadequate maintenance of the braking system components
too much of the braking forces are distributed to the rear wheels - via the driving shafts in a four wheel driven car without centre differential or with visco-devices limiting the difference in angular velocity between front and rear wheels
too much of the braking forces are distributed to the rear wheels - upon unloading of the rear wheels
In the opposite case (too much braking forces at the front) deceleration capacity and steerability will diminish - such as for the distant brown Sedan in the picture - contributing to fatalities (in Swedish)

Spin-out may also be triggered ...

... if the driver mis-use the antilock (ABS) function
... by emergency braking in a car without ABS
(in braking tests with 66 ordinary drivers, loss-of-control was much more frequent with disconnected ABS)
... by pendulum steering, inducing yaw resonance
... if the driver releases the accelerator in a rear wheel driven car on an icy road
... if the driver reduces the spin of the front driven wheels (by releasing the accelerator pedal e.g. in finishing an over-taking manoeuvre)
... by pressing the clutch pedal in a front wheel driven car before turning the steering wheel from straight-forward position
... if the driver counter-steers too late and too slow in a rear wheel skid (check out in television broadcasts from rally and racing cars seemingly not yawing at all, though the steering wheel motions are very quick and frequent)

Official Statistics of Sweden indicate that instability and rear wheel skids are much greater problems than poor steerability for the average driver when roads become slippery. Accidents with oncoming vehicles "Head-on" and Single vehicle accidents (poor directional stability) contribute to a significantly greater percentage of the accidents when the road has ice or snow on its surface. Compare the left and right columns in the diagram, showing annual averages from the years 1988 to 1995.

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To be completed later with descriptions of relevant accident cases
Under Construction

Webpage opened in February. To VETA website in April 1996.

plasmalinje

Best Wishes from Sweden and Lennart Strandberg
Charter Chairman of the non-profit VETA society
Email: lennart.strandberg@veta.se

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