Mark Hooper asks:
> Silly question, but I always thought the primary reason for castor was to
> make the wheels pull back straight when moving and not just running all over
> the road....Any angling of the handlebars ends up raising the bike,
I'm not familiar with motorcycles (though my neighbor just bought a new
Triumph Bonneville! - obligatory LBC content) but I can speak about
bicycles. The fork rake of a bike moves the wheel forward so that the
contact patch is in front of the steering axis ever so slightly. If you
project the steering axis downward you can see that without rake it would
hit the ground *in front of* the contact patch. (The exact amount would be
the wheel radius divided by the cosine of the head tube angle. And the
steering radius would be the wheel radius times the sine of the head tube
angle. I'll send you a picture if you want.) Since the contact patch would
be behind the steering axis, the contact patch would move up w.r.t. the bike
with any steering input, causing the bike;s nose to drop down! By adding
rake you move the contact patch to the "other side", i.e. in front of the
steering axis, which makes the contact patch go down with steering input.
The whole point is decided by the steering radius, that imaginary arc
whereby the contact patch (actually its centroid of forces) is rotated about
the steering axis. Castor actually makes it bad from this standpoint but
the rake fixes it.
With cars, things are complicated a bit by the fact that you have no rake
per se but the orientation of the steering axis can be arbitrary. Normally
it is tilted inward at the top for two reasons. This moves it toward the
contact patch center, making the steering radius small. Equally as
important, the lateral inclination is typically much higher than castor,
perhaps 10 to even 20 degrees. With no lateral inclination, castor would
make the wheel move up with any steering, ruining straight-line stability.
But with no castor, lateral inclination would make the wheel move down with
any steering input. The actual movement is the "sum" of both effects, and
since lateral inclination is much larger, it wins out. So the car's mass
really does fall to its lowest point when no steering is dialed in and it
really does want to go straight. But castor isn't the reason! The steering
radius rotating about the axis does it.
In fact, a suspension designer can make the steering axis intersect the
contact patch anywhere, typically in front of and slightly inside the patch
center. (By adding the equivalent of rake, you could make the axis fall in
front of the patch even without castor.) On some cars, for example the Audi
Fox and VW Dasher if I remember right, the steering axis intersected the
road to the outside of the patch. This was referred to as "negative radius
steering" and was supposed to be an aid to stability through snow. It
worked like this:
Imagine that you are braking and you hit ice with the left front wheel.
Suddenly only the right wheel has traction. The result is uneven braking on
the vehicle, rotating it to the right. But it gets worse. Steering
kickback is the result of forces from the wheel torquing the steering axis.
You normally feel very little because the two sides balance each other. But
with the left wheel on ice, the right wheel is the only one providing
steering axis torque. When the steering axis falls inside the contact
patch, a force backwards on the wheel will torque the steering to that side,
yanking the steering wheel in your hands and actually steering the vehicle
with its "good wheel" to the same side as it is being pulled by the uneven
braking. So by moving the axis to the outside of the patch, they made
braking forces on the right wheel torque the steering wheel to the left,
helping you steer through the trouble. The same effect happened when you
ran over ice while accelerating with FWD. The car might jerk to one side
but the steering wheel jerked to the other!
Just another two cents (plus change :-),
Jim Muller
jimmuller@pop.rcn.com
'80 Spitfire (Percy)
'70 GT6+ (Nigel)
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