Saw an odd thing while changing the front pads and discs at the front this morning.
With the car jacked up and both road-wheels off, I started the engine. The front driveshafts, discs and hubs all spun around slowly forward for about 5 seconds before stopping! And it wasn't in gear.
Anyone have any ideas why this would happen?
thanks-
Edited by Dynamic Dave on 11/11/2009 at 16:59
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It does. I'm surprised they stopped of their own accord.
The gearbox internals are still turning and with cold oil some energy will be transferred.
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Gearbox oil is quite thick and viscous. After a car has been standing it lies in a pool at the bottom of the gearbox. On starting the engine, the input shaft and those gears that are on it, which rotate with the engine when the clutch is engaged, start to swirl the oil around the gearbox turning the output shaft initially. When after a moment the oil in the box reaches its normal 'shape' for that engine speed, evidently, the dragging effect on the output shaft is reduced to a level that won't overcome the friction losses in the wheel and other driveline bearings, and the driveline stops rotating. It isn't anything to do with the oil warming up and becoming less viscous, because that takes some time. It's just that big initial surge of cold oil in the gearbox.
By the way, this illustrates the reason why many drivers depress the clutch when starting their cars. It gives the battery an easier time, not having to move all that oil around.
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Thats very interesting, thanks to you both. I normally press the clutch in, so have never seen this.
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