When you notice your clutch slipping, or not engaging properly before a shift, you have to ask yourself what's wrong - is it my clutch disc, clutch release bearing, or my clutch pressure plate? The truth is that it could be any of those parts. There's no way to know without taking apart your clutch kit and looking at each component. But once you perform a visual inspection it will be obvious. If you see that some of the diaphragm springs (or "teeth") on the clutch pressure plate are broken off, then you have your answer. This part is responsible for providing the means for your friction disc to pivot back and forth, and if its integrity is compromised then the clutch will not engage or disengage fully when it is supposed to. This leads to poor shifts and rough driving, and can ultimately render your transmission useless until your clutch is replaced. No one wants to have their car out of commission for too long, so make sure that when simple issues like a noisy or poorly functioning clutch present themselves, you also make sure to get them attended to right away.
For many drivers, the way their clutch functions is a mystery. Sure, we know how to use it... but how exactly does it work? There are three main components: the clutch disc, the clutch pressure plate, and the clutch release bearing. When your car is in motion, the friction disc is mated to the flywheel so that the engine's rotations transfer to the transmission. When you want to make a shift, the clutch pedal is depressed and the release bearing pulls the disc away from the flywheel so that it is separated from the transmission input shaft for a short time. The device that allows this disengagement to happen is the clutch pressure plate. Tooth-like springs in the middle of it allow the release bearing to remove the pressure on the disc so it will temporarily float; allowing a shift. When the release bearing snaps back, the plate exerts pressure on the disc and it then mates back with the flywheel.
Replacing a worn out or broken clutch pressure plate is usually done when performing regular clutch service. That means the other parts in the kit are replaced as well. However, sometimes just the diaphragm springs get broken and only the plate needs to be swapped out. This is how it's done: