Mad Max

In the not-too-distant future in which society is teetering upon the brink of collapse, berserk motorbike gang member Crawford "Nightrider" Montazano (Vincent Gil) steals a Pursuit Special, which he uses to escape from police custody after killing a rookie officer of an Australian highway patrol called the Main Force Patrol (MFP). Even though he manages to elude other MFP officers, the MFP's top-pursuit man Max Rockatansky (Mel Gibson) then engages the less-skilled Nightrider in a high-speed chase. He breaks off first, but then is unable to recover his concentration before he and his girlfriend (Lulu Pinkus) are killed in a fiery crash.
Nightrider's motorbike gang, led by Toecutter (Hugh Keays-Byrne) and Bubba Zanetti (Geoff Parry), run roughshod over a town, vandalizing property, stealing fuel, and terrorizing the population. Max and fellow officer Jim Goose (Steve Bisley) subsequently arrest Toecutter's young protÃ©gÃ© Johnny the Boy (Tim Burns), who was too high to leave the scene of the gang's rape of a young couple. When neither the rape victims nor any of the townspeople show for Johnny's trial, the federal courts close the case, releasing Johnny into Bubba's custody over Goose's furious objections.
While Goose visits a nightclub in the city the next day, Johnny sabotages his police motorbike in the parking lot. After being thrown into a field at high speed while taking a ride uninjured, Goose borrows a ute to haul his damaged bike back to the MFP. However, Johnny throws a brake drum at the windscreen, causing Goose to crash the ute. Toecutter forces Johnny to throw a match into the petrol leaking from the wreck, triggering an inferno and severely burning the inverted Goose. After seeing Goose's charred body in a hospital intensive-care unit, Max becomes disillusioned with the MFP and informs his superior Fifi Macaffee (Roger Ward) that he will resign. Fifi convinces Max to take a vacation first before he can make his final decision about leaving.
While on vacation, Max's wife Jessie (Joanne Samuel) and their infant son Sprog (Brendan Heath) encounter Toecutter and his gang, who attempt to molest Jessie. Max and his family then flee to a remote farm owned by an elderly friend named May Swaisey (Sheila Florance), but the gang manages to follow them. Jessie and Sprog escape from the gang, only to be run over by them.
With Sprog having been killed instantly and Jessie near death in a hospital ICU, an enraged Max dons his police leathers and takes the supercharged black Pursuit Special from the MFP garage to pursue the gang. He forces several gang members off a bridge at high speed, kills Bubba during an ambush, and chases Toecutter to death by a speeding semi-trailer truck. Max eventually locates Johnny, who is looting a deceased car crash victim for a pair of boots. In a suppressed rage, Max handcuffs Johnny's ankle to a wrecked vehicle and sets a crude time-delay fuse involving a slow petrol leak and Johnny's lighter. Max throws Johnny a hacksaw, leaving him the choice of sawing through either the handcuffs or his ankle within the given time limit. As Max drives away from the bridge, the vehicle explodes, apparently killing Johnny. Now a shell of his former self, Max drives on to points unknown, pushing deep into the Outback.

Question: What did Johnny throw at the windscreen?
Answer: Brake Drum