Self-Study Program

Glossary

Ratio spread In the context of gearboxes, the "spread" of a gearbox is its "range of ratios". The ratio

spread is the ratio of the lowest (ﬁrst) and highest (sixth) gear ratios. The ratio spread

value is obtained by dividing the ﬁrst gear ratio by the highest gear ratio (in this case,

6th gear).

Example (using the 09G gearbox):

i First gear 4.148

i Sixth gear 0.686 4.148 : 0.686 = 6.05 (value rounded up)

The advantages of a wide ratio spread are:

In addition to a high starting torque ratio (for high tractive power), a low end torque

multiplication ratio is achieved. The latter provides a reduction in engine speed, which,

in turn, curbs noise levels and improves fuel economy.

A wide ratio spread requires a certain number of gears in order to avoid overly large

speed differentials during gearshifts (ratio steps).

When changing gear, engine speed must not be allowed to enter low-torque RPM ranges

which will adversely affect or prevent acceleration.

The best solutions are multiple gears or, better still, a continuously variable transmission

ratio, as used in the multitronic gearbox.

Gearbox adaption A gearbox type is, depending on torque and engine type, adapted to different engine

variants through:

– the number of plate pairs used for the clutches and brakes

– the adaptation of the ATF pressure to the clutches and brakes

– the conﬁguration of the gear pairs, planetary gear sets (e.g. 4 planetary gears instead

of 3), shafts and mountings

– gearbox housing reinforcement

– the transmission ratios of the ﬁnal drive and idlers

– the size of the torque converter

– the torque converter characteristic (torque conversion factor or torque converter mul-

tiplication).

The ratios of the individual gears are generally constant.