"When a target is struck by a Concussive weapon, he must take a Toughness Test (...). If he fails, the target is Stunned for 1 Round per Degree of Failure. Additionally, if the target takes Damage greater than his Strength Bonus, he is knocked Prone."

The "additionally" part is unclear to me. Is the target knocked Prone whenever he takes Damage greater than his Strength Bonus, regardless of the Toughness Test? Or only if he failed the Toughness Test?

Thanks!

They're independent effects. The prone bit happens regardless of how the toughness test goes.

I assume the damage calculation used is after toughness and armor?
Related question: What happens in a bike duel, when one rider is hit by a concussive weapon, in each of the following circumstances:
1. He fails the toughness test, but takes less damage than his strength bonus.
2. He passes the toughness test, but takes more damage than his strength bonus.
3. He fails the toughness test and takes more damage than his strength bonus.

Obviously, these results will be stunned, knocked prone, and stunned and knocked prone, but the book is unclear on the exact mechanical effect of each of those in a vehicle (and specifically, in a bike) Does the bike go Out Of Control? Is the rider thrown clear? What happens?

I assume the damage calculation used is after toughness and armor?

Related question: What happens in a bike duel, when one rider is hit by a concussive weapon, in each of the following circumstances:

1. He fails the toughness test, but takes less damage than his strength bonus.

2. He passes the toughness test, but takes more damage than his strength bonus.

3. He fails the toughness test and takes more damage than his strength bonus.

Obviously, these results will be stunned, knocked prone, and stunned and knocked prone, but the book is unclear on the exact mechanical effect of each of those in a vehicle (and specifically, in a bike) Does the bike go Out Of Control? Is the rider thrown clear? What happens?

The damage used to determining Prone is definitely after Toughness and Armor.

1. Stunned, not Prone.

2. Prone.

3. Stunned and Prone. (hello, +40 to hit with melee!)

As for vehicles... isn't there something in Only War about Concussive attacks Stunning the occupants?

I'd say that a concussive attack has little effect on large vehicles, but perhaps on a bike it would do something. Force an Operate test with penalty equal to the Concussive Quality x 10 or crash?

Yeah, the rules are clear on what happens to the person re: stunned/prone/both, but in the case of the bike, I'm talking about the attack hitting the rider, not the vehicle itself. What happens to a vehicle whose operator can't take actions?

If a vehicle's operator cannot take actions, I would put it out of control, determining it's direction accordingly (speed tactical +1d10). I would determine randomly if the operator lead-footed or slipped off the pedal. If leadfoot, rerolling each turn until eventually the vehicle is no longer getting acceleration. After that, cut the speed by about 1/3rd per round. Dragging the operator out of the seat may take time depending on the nature of the vehicle, but grabbing the wheel from him should be a quicker option for some course correction.

I would consider the rider not on the vehicle anymore (like in a joust) making the vehicle independant, with all the consequences this should provoque.