Why do bikes back wheels still rotate reving not in gear ?

[B10Hazard]

Little example
https://www.youtube.com/watch?v=eqxawtr6slk&feature=fvwrel
I have never understood this and cant picture or work out why. Can
someone explains
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[Pete.]

Gears are still in mesh even when you're in neutral, it's the oil film that causes drag between the gear and the shaft it sits on that makes the wheel turn. You could stop the wheel with one finger usually.
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a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[AlexW]

What the man above said, If you take time to look at how a gearbox works you will see what causes the drag.

Cars do it too.

[weasley]

Exactly what Pete said - it's known as 'viscous coupling'. The same effect acts across your clutch, even if it is open (assuming you have a wet clutch).

This is why a bike is harder to push around if it's in gear with the clutch in, than if it's in neutral (especially when cold as the oil is thicker).
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[0l0dom0l0]

The problem is enhanced when the bikes cold too because the oil is thicker and causes more drag incase you were interested
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CBT Passed: 30/08/2009, Theory Passed: 31/08/2010, Mod 1 Passed: 6/9/2010, Mod 2 Passed: 13/09/2010. Restriction ended 13/09/2012.

Bikes: 2007 Derbi GPR 50, 1998 Yamaha Fazer 600 (written off), 2002 Yamaha Fazer 600, 1994 CBR 600F, 2003 Triumph Daytona 600, Kawasaki ZX6R J1.....Current: 2006 Yamaha FZ6, 1998 Suzuki TL1000R and a Honda VFR 400 NC30.

[1cyl]

So bikes with dry clutches don't?
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[Pete.]

A dry clutch can drag just as a wet one can. Doesn't have to be oil stiction, any kind of friction can cause drag.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[weasley]

A dry clutch bike will do it if the bike is in neutral and the clutch is closed, but theoretically it shouldn't once the clutch is pulled in as there's no coupling from the engine to the wheel (assuming the clutch opens fully).

In related news, it is this viscous coupling across a wet clutch which causes the 'jolt' when you select 1st; you have the clutch in but there is still some drive through the gearbox as you select 1st, hence the jolt (worse when cold).
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Yamaha XJ600 | Yamaha YZF600R Thundercat | KTM 990 SMT | BMW F900XR TE

[Pete.]

[weasley]

[Pete.]

But a fully released clutch doesn't mean the plate isn't dragging - the same as a fully released brake doesn't mean the brakes aren't dragging.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[Walloper]

Engine Fairies and Gearbox Goblins have quite a lot to do with the fenomenum

https://www.gadgetjq.com/transmission.htm

The ratios of gearing between the engine flywheel/crankshaft and rear wheel make it 'easier' for the friction between component drive the rear wheel when in neutral.

The same thing will not happen as easily in the reverse direction (wheel driving the gearbox). There will be too much resistance in the engine. Even with the plug/s out.
It will be possible to detect torque if you have a sensitive meter.
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