# Gauge (automotive engineering)

In automotive engineering, the distance between the two wheel contact points of an axle when viewed from the rear is referred to as the track width ( track width or axle track). This corresponds to a projection onto the y - z plane of the vehicle-fixed coordinate system. In the case of double tires , measurements are taken from the middle of the twin tires . The track width is given in the construction position . The track width is a constructive measure that can only be measured approximately. You can z. B. measure flush on the outside (on the ground) and subtract the tire width.

Track width (measured between the wheel contact points)

The track width in connection with the height of the center of gravity has an influence on the tipping limit . The possible gauge is already essentially the vehicle concept defined by the width of the vehicle.

For optical or other reasons, e.g. B. the aerodynamics, the wheels should not protrude too far from the vehicle contour. In the case of vehicle variants with wide tires, the track width is then often reduced via the offset in order to maintain the external dimensions.

## Change in track width, instantaneous center height

With independent wheel suspensions , the track width changes depending on the spring travel . The wheel contact point moves in the view from behind at the angle to the vertical. By definition, this direction of movement is perpendicular to the pole beam from the wheel contact point to the roll center . The current center height (roll center height) of the axis indicates the height of the current center above the roadway. It results in: ${\ displaystyle b}$${\ displaystyle \ tau}$${\ displaystyle h _ {\ text {MZ}}}$

${\ displaystyle h _ {\ text {MZ}} = {\ frac {b} {2}} \ cdot \ tan \ tau}$

With there is a direct relationship between the change in track width and the change in spring travel , which is proportional to the current center height: ${\ displaystyle \ tan \ tau = {\ frac {1} {2}} \ cdot {\ frac {\ Delta b} {\ Delta f}}}$${\ displaystyle \ Delta b}$${\ displaystyle \ Delta f}$

${\ displaystyle \ Delta b = 2 \ cdot {\ frac {h _ {\ text {MZ}}} {b / 2}} \ cdot \ Delta f}$

The instantaneous center height changes with the spring travel and should decrease with increasing compression. The proportion of the rolling moment that is not supported by the suspension is determined by a suitable choice of the instantaneous center height. A moment center lying above the roadway thus reduces the roll angle . If the instantaneous center is too high, as in the case of a rear oscillating axle , however, excessive wheel load differences occur when cornering, which leads to a tendency to oversteer in the limit area in rear-wheel drive vehicles.

## literature

• Jörnsen Reimpell, Jürgen Betzler: Chassis technology: Basics. Vogel Business Media, Würzburg 2000, ISBN 3-8343-3031-0 .