Motorcycle pressure plate springs are much weaker because motorcycle clutches typically consist of multiple clutch disks and steel plates that are all stacked together, so a little pressure results in a lot of friction from multiple surfaces.
I went to college with a guy that could barely use his legs. he could walk with special crutches if he needed to. he usually used a wheel chair. his arms were huge (makes sense). he had both the brake and the gas on a stick that you squeezed (their were two separate sticks) they went down to the pedal so I assume they were still attached to the pedals. he never used his feet to drive. it was an automatic but it may work. I wish I had cared more back then to find out how it all worked but I was worried that I might offend him.
he drove an extended cab smallish truck (maybe a mazda) but he kept the wheel chair right behind his seat. it worked very well for him.
maybe this will help, I don't know what they are called.
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Hmm- I didn't know those were multi layer- Do you know if switching to one of these will result in an easier pedal?
I suspect that they come with pretty stiff springs. However, they're designed to handle 800hp or whatever; it might be possible to modify one with a light spring for use with your stock engine.
The big problem with them, as I understand, is that you don't really ever get to slip the clutch. You have to rev up to launch hard and dump the clutch for every launch. I'm not sure if it's because of the multi-disc design or the heavy spring.