Beware of the EXPLODING clutch!
No, sadly this isn’t another one of those over-used quotes from a Monty Python movie…this is an actual occurrence!
A few weeks ago I got a call from my good buddy Adrian. He had blown his clutch in the middle of the desert and wanted to give me a heads up as to what happened, so I could warn/ remind others of a potential hazard of low gearing. Adrian is no dummy; he was the crew chief of one of the most technologically advanced racecars in the world. And understands exactly what caused his premature clutch failure, but a fraction of a second laps caused it all.
The scenario
You’re driving down a steep hill. You “wisely” shift in low gear to allow the engine to do the work. The hill gets steeper, and the engine RPM’s start to increase to redline speeds. Quickly you dump the clutch to reduce engine speed and prevent damage. Still in 1st gear with the clutch in, the engine RPM’s begin to drop and so does the hill. The Jeep picks up speed…
Do you see a problem here?
1. Engine RPM: It’s good, the clutch is pushed in, it’s idling.
2. Steep Hill: Ya, but it flattens out in a few yards…you’re only going 15 mph anyway, no problem
3. You’re too tired to put the transmission in neutral, and besides, it’s only a little way to flat ground, all you need to do is coast a few more feet, let the clutch out, and go. What could go wrong?
This is exactly what Adrian thought while driving his 513-geared JK down a steep sand wash. Here’s the problem-
Without working through all the intense math equations, here’s what happened. The rear wheels are turning one revolution to every 5 revolutions of the driveline, assuming a 513-gear ration. The driveline now spins the tcase output shaft. Because you’re rockin a 4:1 low, the shaft speed on the input side of the transfer case is now 20x faster than the wheel speed. The transfer case drives the transmission, which in this case is still in 1st gear. Using the gear ratio of the transmission, the input is being spun at a staggering or should I say grenadine rate! The clutch disc is riding on the transmission input shaft, and because of the clutches diameter, centrifugal force comes into play and the disc, designed for about a 6,000 rpm max, reaches speeds of 10,000 rpm or more. Not Good.
This all happens in about 2 seconds, there is a sound, much like a formula one car just before it shifts. The clutch hits warp speed and the term “big bang”, takes on a whole new meaning. The clutch completely disintegrated, and he was walking. It took about .5 seconds for Adrian to realize his injudicious actions, (the word of the day on my phone app, I was dying to use it) and start berating himself. It’s a lot less painful, and not to mention less costly, to learn from the experience’s of others. Thanks for sharing your pain with us, in an effort to too help others avoid the same fate.
So what do we do to avoid the above from happening to our Jeeps?
Don’t coast in a low gear with the clutch depressed.
If you are dying to know what your clutch rpm would be, >>this is a good web site I visit<<
Wood out