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You'd think this would be awful hard to do; after all, one of the major design criteria for a clutch would seem to be the ability to withstand high rotational speeds, but it turns out that it is possible, in normal operation of some vehicles (mainly 4WDs), to create circumstances where the clutch's design rotational speed is exceeded. (Note: there are probably multiple ways to go about this, I'm only covering one type of clutch explosion)

Picture this: you're taking your new manual Jeep Rubicon for a drive on a particularly steep 4WD road. To negotiate a hill, you use your first gear, along with your low-range four-wheel-drive, which adds an additional 4:1 ratio to your already low first gear. In this configuration, the hill is cake, and you now find yourself looking down a mellow downslope ending in another steep climb.

Being the lazy sort, you decide not to leave low-range, and instead coast down the hill in first gear, low range, with the clutch engaged. As the car accelerates, you hear the transfer case and transmission whir and clink, and finally, you start to think about dropping the clutch as you near the bottom of the hill, and your speed nears 15 MPH.

As you start to lift your left foot, you hear something not unlike the sound of a bucket of chain being dropped several meters, and you notice your clutch is now inoperable. As you sit, miles from the nearest highway or telephone, pondering how you will make the 10 mile walk out to the road, you begin to wonder what happened.

While this may not be a likely scenario for most drivers, it is a concern for those who operate vehicles with manual transmissions and low-range 4WD. It only takes a bit of simple math to figure out why:

In most cars, the line between first and second gears sits below 10 MPH, and pushing first gear to 15 or 20 MPH may bring your RPMs near red-line. By coasting, in first gear using the clutch, the RPMs could easily build to 6 or 7 thousand without warning, as the engine provides no auditory feedback. The low-range 4WD compounds this problem, by multiplying the RPMs experienced by the clutch by 2-4 times, depending on the car. In a rock-crawler, with ratios near or exceeding 4:1, it is not difficult to achieve speeds in excess of 20,000 RPM at the clutch, which is well greater than the 10,000 RPM design speed of many clutches. Well before this 20,000 RPM point is reached, the clutch will likely catastrophically fail, leaving the lazy driver miles from home, sans clutch.

This is certainly not exclusive to cars with low-range transfer cases, but if you're trying to hammer your transmission into first at highway speeds, you probably deserve any transmission or clutch damage you get. Neither are sheer RPMs the only way to cause a clutch to fail in this way: most clutches cannot take too great an RPM differential without some sort of failure.

The moral is: be nice to your clutch; cars, especially those designed for off-roading, are not as automated as some may be led to believe. It is certainly possible to effect this sort of failure from the comfort of your air-conditioning, heated-seats, and open-top, and besides, it's not all that much effort to drop the car into neutral while you coast.

 

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