Undercarriage
A perhaps slightly different perspective than you're used to—that's the purpose of this chapter. This image is usually used to explain that all wheels essentially roll in circular arcs around a single point on the extended rear
axle. And the angle on the right is the toe difference angle, essentially the difference in wheel angle between the two front wheels.
This is achieved by the steering trapezoid, shown here only schematically because such a one-piece tie rod is only available on rigid front axles, and these have become rare, even on buses. The greater the difference
between the length of the tie rod and the distance between the two pivot points on the axle, the greater the toe difference angle, measured at a 20° wheel angle on the inside of the curve.
Here's a not-so-uncommon design with a steering trapezoid in front of the steered axle. For this to work, the track conditions would naturally have to be reversed. In this case, with a vertical axis of rotation, the wheels
wouldn't pivot in place, but would instead rotate in small circular arcs. However, the track difference angle is the same in both cases at approximately 2°, depending on the distance between the wheel centers of an axle
(track width) and the distance between the front and rear axles (wheelbase).
In addition to track width, there's also the toe. This should actually be defined as the deviation from the straight line in degrees, and not as the distance between the front and rear wheels on an axle. This isn't because of
the length rather than the angle dimension, but because the steering wheel must always be in the center position according to a mark on the steering gear, and then each wheel is measured or corrected individually.
| With toe-in (plus) the wheels are closer together at the front and with toe-out (minus) at the rear. |
There was so much talk in the past about toe-in and toe-out. Supposedly, front-wheel-drive vehicles, for example, needed more toe-out because the wheels pulled themselves into a straight-ahead position due to the play
in the suspension and wheel bearings. Firstly, the play is now minimal, and secondly, the drivetrain is now often in gliding or coasting mode.
Toe-in is now close to zero on almost all vehicles. However, one shouldn't assume that a track difference angle is chosen for the smoothest possible driving. Try pushing a car around a tight corner, preferably on a surface
with good grip, and you'll not only feel the inconsistencies through stiffness, but you might also hear them.
An undercarriage is simply complex and a car is not designed for easy pushing around corners. Dynamics play a role here, namely, assigning a little more lateral force to one or the other tire to achieve effects such as
more direct steering. There's also understeer, when the car swerves at the front in a corner, and oversteer, when this happens at the rear, and adjustment options that can mitigate this behavior.
1: Vertical axle (yaw), 2 longitudinal axle (roll),
3: transverse axle (pitch) |
Oh yes, camber is also part of it. This is the angle of the vertical wheel axis to the vertical. But is this wheel axle always the same? Of course not. The car leans outward in the curve. Camber is supposed to keep the wheel
perpendicular to the road surface, despite the increasing roll angle (image above), so that the entire width of the tire covers the road.
As a first approach to the solution, here's a particularly common variant of the double wishbone axle: making the lower wishbone longer than the upper one. During compression (dashed lines), the camber becomes more
negative, and the wheel tilts more inward at the top. This effect becomes more pronounced with increasing compression.
As a first approach to the solution, here's a particularly common variant of the double wishbone axle: making the lower wishbone longer than the upper one. During compression (dashed lines), the camber becomes more
negative, and the wheel tilts more inward at the top. This effect becomes more intensified with increasing compression.
For the steered front axle only, here's a second solution using caster. This is the inclination of the top pivot axles toward the rear. This also creates negative camber. However, this only affects the outer wheel, which is much
more important when cornering because it bears the majority of the load. On the inside of the curve, the camber becomes more positive.
For the steered front axle only, here is a second solution using the caster. This is the inclination of the top pivot axles toward the rear. This also creates negative camber. However, this only affects the outer wheel, which is
much more important when cornering because it bears the majority of the load. On the wheel inside of the curve, then the camber becomes more positive.
Why are we showing the caster on a bicycle here? Because this image dispels a common misconception. It's claimed that the caster promotes the restoring forces of the steering. For this front wheel, just imagine turning
90°. Then the wheel touches the ground at the front piercing point. Is the handlebar higher or lower?
Yes, of course lower. And if you don't believe us, try it on a regular bike. And how do the restoring forces come about? Not through the caster angle, but through the caster offset. We've added that to the image below,
meaning we've shifted the wheel forward while maintaining the same pivot axle.
This is the principle behind the wheels under a shopping cart. They always face backwards in the direction of travel, also a kind of reset. If you were to measure the second front wheel shown, the distance from the
handlebar to the road would remain the same over a much larger swivel angle and would only decrease from perhaps 45°. Until then, the reset forces act through offset, which allows us, for example, to ride hands-free.
Additionally, although not as effective on bicycles, there's what's known as tire caster. This affects all wheels that aren't currently being driven at full speed. The contact patch runs afterwards the tire and thus also has a
stabilizing effect.
On a two-track vehicle, it's also possible to tilt the front wheel pivots slightly inward at the top. This then results in the spread angle γ. To plot it correctly, the pivot axis for this McPherson strut must pass through the
top center of the strut bearing and through the ball head of the wishbone at the bottom.
On the outside wheel, the spread angle reduces the caster angle, while on the inside, it complements it. It is therefore usually smaller than the caster angle. The point at which the swivel axis, shown in the image above,
penetrates the ground is also important. If this occurs off-center, this so-called caster radius is negative; exactly in the middle, it is zero, and inside, it is positive.
Zero is chosen during design to minimize road surface influence on the steering. Negative is necessary when the brake circuits are divided diagonally. If one fails, the intact circuit with the front brake exerts greater effect.
The vehicle tends to swerve. A negative steering roll radius causes the steering to automatically counteract this tendency.
All too easily, important basic vehicle settings are thrown out the window. Manufacturers seem powerless in the face of this activity, or even participate. This refers to wheel spacers with a lower offset (pictured above) or
wheel spacers, which increase the track width of the axles but unfortunately also reduce the effect of the steering roll radius. And unless, for example, a brake circuit fails, you won't notice.
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