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Old 08-12-2009, 11:36 AM   #83 (permalink)
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Originally Posted by Bearleener View Post
Engine-off coasting with a manual transmission:
Shouldn't it be better to leave the transmission in gear and hold the clutch until the bump start?
Seems as if synchronizer wear would be more of a problem than clutch throwout bearing / pressure plate wear. Of course, by now my left leg is a bit shorter than the right one from all the clutch holding.

For synchronizer see PaleMelanesian's comments above. When the car is rolling with the engine off and you put it into gear, the tranny has to synchronize the engine's 0 RPM with the tranny's whatever RPM; not easy. If instead you simply disengage the clutch and turn the engine off without going into neutral, then when you want to restart the gears are already meshed and the synchronizer isn't needed.

For the clutch have a look at the illustration on HowStuffWorks "Fly Wheels, Clutch Plates and Friction"
When the engine is off, everything in the clutch is stationary except the clutch plate and output shaft to the tranny. That is, the throwout bearing and pressure plate are not grinding against each other (no wear), in contrast to when the engine is on.

Furthermore, if you leave the car in gear during EOC that's one less clutch press-and-release cycle (unless you pop it out of gear without the clutch), i.e. less usage of the throwout bearing.

Then of course there's the safety issue of being able to quickly restart the engine even when both hands are full (e.g. swerving).
When you're neutral coasting, the only thing the synchronizer has to speed match is the input shaft and clutch disc... about 10 lbs, maybe slightly more.

When you're holding the clutch pedal in, the throwout bearing is fighting against the pressure of the spring action in the pressure plate, which could be 500# or more, depending on the application. You're also putting pressure on the thrust washer/bearing which holds the crankshaft in place laterally, causing wear to it as well.

Reference "crank walk" to see about thrust bearing issues.

Frankly, I'd rather not have to worry about crank walk.. I'd rather deal with a minute amount of wear in my well lubricated transmission, versus the potential of a throwout bearing exploding (again).
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