Multi-group transmissions
In order to cope with the widely varying freight and loading conditions, a tighter transmission layout and thus a higher number of gears is required in commercial vehicles when compared to passenger cars. There are generally two possibilities in order to raise the number of gears:
An additional gear wheel pair is intended for each additional gear in the main transmission,
Front-mounted or rear-mounted groups are coupled to the main transmission.
The second possibility results in multi-group transmissions whose advantage lies in multiplying the number of gears without a corresponding increasing in the number of gear wheel pairs and shifting elements. This implies a saving in construction space, weight, and costs.
The figure above shows a two-group transmission with a two-speed planetary train as rear-mounted group.