Increase max chance of knockdown on legs, decrease on body, remain same on head.
Personally I would also add held attack to matter the most for knockdownchance. Even more than weight, raw damage and end damage. Especially end damage.
Another thing is having "knockdown ability" in it's own weapon stat instead of having it based on weight and raw damage.
About weak hits vs strong hits:
While having the high damaging hits have a higher knockdown % makes sense, isn't that bad in a gameplay perspective? If I hit someone once for half their health, that's already pretty nice to me. Hitting someone once for 10% of their health on the other hand, getting that "free" (aka, free on EU2 cause no roll, not free on EU1 cause you can roll away) hit is more a bonus for choosing a low damage weapon. Ofc weapon speed might make up for it, but meh. This is why I want knockdown to have it's own "stat". Same when I get knockdowned myself. More annoying when it's a high damage weapon because it kills me before I can get up. I'm saying this as a guy who loves the mallet.
Also make roll less retarded. Especially the part where you have to find out if the server even allows you to roll (looking at you EU2).