1-Make sure you are using a mod program that lets you change the game files without
breaking the game. LIKE (MAGIC GTA IV)
2-replace (handling.dat) and (vehicles.ide) in
X:\Program Files\Rockstar Games\Grand Theft Auto IV\common\data
3-Download (sparkiv) tool if you dont have.
4.Open (vehicles.img) (in pc\models\cdimages) with sparkiv tool and import
(fbi.wft + fbi.wtd + merit.wft + merit.wtd + noose.wft + noose.wtd +
police2.wft + police2.wtd + police.wft + police.wtd + polpatriot.wft + polpatriot.wtd).
after that click on rebuild and after click on the save icon and exit to finish.
Modified by Polodave
Theres a couple issues though: You can't get the sound of the bikes (they get the same sound as the copcar they replace) and you can't lean back (wheelie) or forward (stopie) on the bike either;
But I don't think there is something you can do about it... Despite that, I loved the mod as now cops are more accurate when they shoot me from their bikes. I just replaced one car (police2) for the Sanchez though to make it realistically "balanced".