It is also interesting that you can look at these two bikes in the pictures above, and see the shock angles are completely different.
The red bike may also have a different length swingers.
These factors would require some differences in the shocks, most likely different springs, as the leverage is different, possible different damping as well.
On all of the twin shock designs, the leverage on the shocks is far less the most mono shock designs. This allow us to use far lighter spring rates, and even when we are changing the spring rates for different weight riders the damper changes if they are needed are very small. Usually the standard damping can cover 2 to 4 spring rate changes without the damping needing any change.
Most mono shock designs use a linkage, for two reasons, the first is to make the shock travel (stroke) shorter, this reduces the length of the shock, making it easier to package in todays motorcycles.
the second is that with a linkage you can design it to be either digressive, very rare to see this, linear, or progressive.
Progressive is what all sport bikes use. Usually a 3% to 5% progressive rate, works the best for solo riding. As well as track use.
When twin shocks are mounted straight up and down, as they are on the red bike pictured above, they are linear.
When the twin shocks are mounted an angle as on the Black Commando above, they are progressive.
As British bikes advanced, you can see they all started doing this, meaning mounting the shocks at an angle, making them progressive.
There are also Linear and Progressive springs. like most choices, they both have advantages and disadvantages.
But first what is linear, it means as the shock or spring is compressed, it produces the same movement or rate increase for each mm or if you prefer inch of movement.
Example: a 22 Nmm spring, or spring mounted on a shock, that is linear, as is the Stock Norton spring produces 22 nmm of resistance for each mm of compression, so the first 1 mm of shock movement produce 22 Nmm of resistance, and the last 22 mm produce 22mm of resistance, Linear.
This on a graph, appears as a straight flat line, as you can see in the spring chart I posted earlier.
Progressive:
With a progressive design the amount of movement of the shock stroke is changed, as it is moving thru its stroke, the further it is in its stroke, the more the shock movement is increased. the amount of change is based on the angle the shock is mounted at or the design of the shock linkage.
This means the shock is moving more at the end of its travel than it was at the beginning( for a given movement of the rear axle !) so even if the spring on the shock is linear, because the shock is moving more, it is compressing the spring more, so there is more spring resistance. If you compress a 22 nmm spring one mm at the beginning of the shock travel you will get 22 Nmm of resistance, but at the end of the shock travel, if you are compressing it 1 mm PLUS another 5% you will get 22 Nmm of resistance plus another 5% of 22 Nmm of resistance.
on a graph, this looks like a line at an increasing angle.
I will post some graphs.
