What's the difference in the way they work?
Am I right in thinking that a lower spoiler reduces the amount of air going under the car (moves the separation point down) while a splitter moves the separation point forward?
Alpinewhite,
I'm afraid I'm terrible at explaining things via text, but I'll try! You aren't far away with your thinking.
The first thing we need to establish is the difference between a "wing" and a "spoiler".
A spoiler, as fitted to pretty much all production cars, works by disrupting the airflow. To be literal, it "spoils" the airflow, hence the name name spolier. The disruption in the airflow will create drag, and that drag (say over a bootlid) will cause disruption over the affected area, and give an amount of downforce.
A wing works as you'd imagine, splitting the airflow over the two halves of it's surfaces, and the resulting negative pressure underneath the wing causes what is commonly referred to as "downforce".
A production car with a massive spoiler (think mitsi evo etc) is still called a spoiler for that very reason, it spoils the airflow over the rear of the car. The angle of attack of the spoiler is not enough to create meaningfull negative lift, if it could the spoiler would just flutter away because it wouldn't be strong enough to transfer the force.
A proper aftermarket wing will have the materials (read cost) to transfer this force through the car, and can be classed as downforce (as long as the wing mounting points are strong enough to transfer the force).
Getting back to your question, a front spoiler works in the way as described. A front splitter, if large enough and with the correct profile, will work as a wing. The lower pressure created underneath the wing will
help "suck" the car into the ground, whereas a front spoiler will just
help prevent positive lift, not actually create negative.
I hope this makes some sort of sense!
