Why are the sound and shift feel different for some shift steps with an MGU?

This phenomenon is related to what we call "double shift operations". These are shift steps in which the force-fit pawl changes in both units of the gearbox. There are two such shift steps for each MGU on the 12-speed version, these are steps from gears 4 to 5 and 8 to 9 .

During these gear changes, the gear change takes a fraction of a second longer and can, under certain circumstances, generate significantly more noise than the other gear changes. A certain amount of pedal play is also noticeable.

The gearbox components are designed to withstand the loads that arise during these gear changes. This does not damage or increase wear of the MGU. 

Under these conditions, ‘rough/hard’ gear changes occur more frequently:

-Riding at low cadences

-Riding with high pedal load 

-Riding in Auto.Shift (A) or Auto.Shift Pro (A+) mode.

The combination of these factors also results in maximum gear change noise.

Noise and crank backlash during gear changes can be reduced by:

-Cycling at medium to high cadences

-Using manual shift mode when cycling sportily and reserving Auto.Shift (A) or Auto.Shift Pro (A+) modes for more relaxed cycling.

Explanation of the technical context:

The design of our gearboxes and MGUs, consisting of two gear stages, makes it possible to achieve a wide range of gears in precisely uniform steps while still maintaining compact external dimensions.

Here is a brief description of the general principle of a Pinion MGU with 12 gears. (for 9-speed version see below):

The 12 gears are formed by two units consisting of 4 and 3 gear pairs (4x3=12). 

One pawl is always active in each of the two gear units. In first speed, the first gear is selected in both units. In the shift steps 1 to 4, the change always takes place in the first sub-unit from gear 1 to 4. 

When shifting from 4>5, the first sub-unit shifts back from gear 4 to 1 and, at the same time, the second unit shifts from gear 1 to gear 2.

The same happens when shifting from 8>9, with the difference that in the second unit, the shift is from 2nd to 3rd gear.

This means, of the 11 steps between gears 1 and 12, steps 4<>5 and 8<>9 differ from the other gear shifts.

The 9-speed version consists of 3x3 gear pairs and besides that works in the same way, with double shifts occurring after 3 gears: 3<>4 and 6<>7.