So I decided to visit the dealer. My question was simple - "can you make the clutch pedal easier to press". It seemed like an easy fix. They might open the hood, look for the knob that says "adjust clutch pressure" and turn it 8 degrees clockwise. Imagine my surprise when they said "yes, we can improve the clutch press - give us a small pile of cash and we will replace your clutch". Hmmm.
Now I consider myself engineering savvy and a bit of a mechanic - why it was only 30 years ago that I rebuilt a 1967 Mustang from scratch. The suggestion that "replacing your clutch" would be a viable countermeasure to excess clutch pressure sounds all wrong to me.
Trusting the dealer, and verifying by testing the clutch pressure in a new Mustang, I asked them to make the change. It should be finished in a few days so I will report back then.
Not wanting to be in-the-dark on why a new clutch would help, I turned to the web. A few keyword searches including "clutch clamping pressure" and "belleville spring" turned up what I think is a reasonable answer.
As many people know, the job of the clutch is to apply or remove pressure from the pressure plate which connects rotational force from the engine flywheel to the transmission. What I didn't know is how the spring that applies that pressure works. This article describes a diaphragm clutch as a Belleville spring. This article describes Belleville spring characteristics (look at Force Adjusting Bellevilles). This article describes what I'm sharing below. Dig in if you want more.
The 2007 Mustang uses a diaphragm clutch. Given the discoveries mentioned above, it appears that as the pressure plate gets thinner, the force required to disengage the clutch goes up before it goes down. Once the pressure plate is too thin, the clutch will start to slip and it's game over.
This graph tells a good story but it needs some explanation.
When the pressure plate is "normal" thickness (e.g. brand new) the "pressure plate travel" in the above graph is zero. Reading from the above graph, 4,500 Newtons of pressure are required to release the clutch. As the pressure plate ages and wears thinner, the spring expands and the clamp load goes up. Spring expansion is shown in the above graph as a negative number for "pressure plate travel". The increased clamp load (i.e. pressure) is good because it delays the clutch slippage. Unfortunately, this also translates to a significant increase in force required on the clutch pedal. That's why my knee hurts.
So, given the above findings, it seems reasonable that a new clutch with a thicker pressure plate will improve my driving fun and reduce knee pain. Stay tuned for actual results. May the running continue!
