For a well-developed set of wheels, you'll want to be working the one area of your quads that'll show in and out of your shorts: that little teardrop muscle just above your knee.
Most would use the leg extension machine to develop the VMO (vastus medialis oblique). And there's nothing wrong with this. Research shows that the top portion of the leg extension is one of the most effective exercises for activating your VMO.
But what happens if the leg extension machine is taken and you still want to work on that teardrop? Just grab a foam roller and do a few sets of plank leg extensions.
Plank Leg Extension with Foam Roller
They work the exact same portion of your quads as the leg extensions, and you can get a good isolated pump while doing them. Bonus: You can also do these at home.