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- Feb 23, 2007
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I found this thread on another site, something we should all be aware of...
I wanted to share with you an experience of mine with an off-road vehicle recently. I have not heard of this happening before, nor was I aware of this potential situation. But this is worth being aware of, I believe. I was wheeling my 84 CJ-7 a few weeks ago. I had just descended a rather steep downgrade and was in second gear, low range. The terrain flattened out to a wide sandy wash, with just a very slight downgrade. I pushed in my clutch (but left the transmission in gear) and let the vehicle gradually accelerate down the wash. This not something I normally do, but I knew that at the end of this wash, (which was about 1/4 mile long), that I would have to climb a steep upgrade and would have to shift back in second gear away. When I originally pushed in the clutch the vehicle was traveling around 2 mph. As I kept the clutch in, the vehicle speed gradually increased till about a minute later, towards the end of the wash, the vehicle speed was approx. 20 mph. Just as I was getting ready to apply the brakes to slow the vehicle down, I heard a loud explosion.
This was the sound of my clutch disc exploding. The clutch disc basically flew apart due to excessive RPM. The clutch was disengaged, (pedal on the floor) when this occurred. Engine RPM was at idle. (800 RPM) Because the clutch disc is splined to the input shaft of the transmission, it turns at the input shaft speed of the tranny. Because the transmission was in gear, the input shaft (and clutch) were being driven by the rear wheels of the vehicle, thru the rear drivetrain, in coast mode. My second gear tranny ratio is 2.37. My Dana 300 low range reduction is 4.0 (terra-low gears) My rear end reduction is 4.1 This gives my total gearing ratio at the time of approx. 39 to 1. At 20 mph, my 33" tires are turning at approx. 211 RPM. (20/60x5280=1760 FPM) 100" is static loaded circumference of 33". (So 100/12=8.33 and 1760/8.33=211) So through the gear reduction, (in this case gear amplification because the tranny is being driven by the rear wheels, 211 x 39 = 8229 RPM. The clutch disc was spinning at over 8000 RPM when it exploded. Stock clutch discs are usually only rated to 6000 or 7000 RPM. Even racing clutch discs are usually only rated to 10,000 RPM.
Worth thinking about....
I wanted to share with you an experience of mine with an off-road vehicle recently. I have not heard of this happening before, nor was I aware of this potential situation. But this is worth being aware of, I believe. I was wheeling my 84 CJ-7 a few weeks ago. I had just descended a rather steep downgrade and was in second gear, low range. The terrain flattened out to a wide sandy wash, with just a very slight downgrade. I pushed in my clutch (but left the transmission in gear) and let the vehicle gradually accelerate down the wash. This not something I normally do, but I knew that at the end of this wash, (which was about 1/4 mile long), that I would have to climb a steep upgrade and would have to shift back in second gear away. When I originally pushed in the clutch the vehicle was traveling around 2 mph. As I kept the clutch in, the vehicle speed gradually increased till about a minute later, towards the end of the wash, the vehicle speed was approx. 20 mph. Just as I was getting ready to apply the brakes to slow the vehicle down, I heard a loud explosion.
This was the sound of my clutch disc exploding. The clutch disc basically flew apart due to excessive RPM. The clutch was disengaged, (pedal on the floor) when this occurred. Engine RPM was at idle. (800 RPM) Because the clutch disc is splined to the input shaft of the transmission, it turns at the input shaft speed of the tranny. Because the transmission was in gear, the input shaft (and clutch) were being driven by the rear wheels of the vehicle, thru the rear drivetrain, in coast mode. My second gear tranny ratio is 2.37. My Dana 300 low range reduction is 4.0 (terra-low gears) My rear end reduction is 4.1 This gives my total gearing ratio at the time of approx. 39 to 1. At 20 mph, my 33" tires are turning at approx. 211 RPM. (20/60x5280=1760 FPM) 100" is static loaded circumference of 33". (So 100/12=8.33 and 1760/8.33=211) So through the gear reduction, (in this case gear amplification because the tranny is being driven by the rear wheels, 211 x 39 = 8229 RPM. The clutch disc was spinning at over 8000 RPM when it exploded. Stock clutch discs are usually only rated to 6000 or 7000 RPM. Even racing clutch discs are usually only rated to 10,000 RPM.
Worth thinking about....
