Say hello to Motor Mamma. To get to this charming woman we need to head to the Southern Raceway, an area hidden down a little nook to the south side of the map.
Upon entering the area swing a right. There's a gate dead aead, but we need to open it via a switch to the right. There a small encampment (the switch is up above, on the inner building) and you will be set upon by Momma's men. Deal with them however you see fit.
With the gate open, head back to area entrance and take the road forwards. Follow the road all the way round, taking momma's goons out along the way, and you'll eventually find yourself in a huge arena...
Boss â€“ Motor Mamma
Motor Mamma sits atop a large motorcycle and will try to ram you, as well as shoot you with various guns and explosives. Minions will also appear on bikes, too, to cause trouble.
The best way to deal with Momma is to use the large structures around the arena as both cover, and a large 'thing' to lead Momma around, shooting her the entire way. This denies her any ability to ram you, and is just generally an excellent strategy.
With Motor Mamma dead, head back to Moxxi for your reward.