Note that the players can Concede at any time before they get Taken Out, so you shouldn't worry about killing them, but you should figure out what the bad guys want (presumably just to subdue them and leave?).
The biker gang, too, can Concede if the fight doesn't go their way, or maybe even if they want to keep from taking any Consequences. How invested are the enemies?
Devonapple's right on the money here, except I need to add that my group at least doesn't think of Conceding until it's too late. So with that in mind, just don't have the bikers KILL the PCs if they Take them Out.
-They'll want to make an example of the poor schmucks who crossed them. Time for a deathtrap! Or something more cruel but realistic, like roadhauling or something like that.
- If the PCs held their own, maybe they might want to keep them alive for recruitment or turn them into weregators (I don't know if yours transmit by bite or anything)?
- If some PCs are Taken Out and some of the others escape, they could do a ransom demand. Or prisoner exchange, esp. if the PCs have some of the bikers captured too.
All in all, it's a lot more fun to keep your PCs alive, and unless it is a blood-sworn vendetta or something truly obvious that would end in one party's death (maybe this is so for their NPC friends but you indicate the PCs are just "going along"), it's not hard to come up with a reason to keep someone alive.
Another way to pull your punches is to focus on the NPC allies first and through doing so show the PCs how the bikers work. Maybe the PCs see a weakness in their strategy and use it to their advantage or maybe they see that fighting was a mistake and without the NPCs to egg them on, they'll seek escape or negotiation.
Or they'll come back later with explosives.