In the near future I will replace the clutch friction plates for my motorcycle (1984 Yamaha FJ1100) and most likely also the steel plates (the bike's done 80k km so I think they are due for replacement, will open it up and measure thickness). On this model of bike the spring used is a diaphragm spring (see image above), I was wondering how should I check if this spring has enough spring tension left/should be replaced with a new one? On 'regular' coil springs I've heard you can simply measure the length of the springs to check if they are due for replacement.
If the diaphragm spring indeed does need replacement, are there any other parts I should check/possibly replace?
Any info would be great, thank you in advance!