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rear wheel spinning while bike is on stand... why? how?

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I've noticed a few two stroke bikes on Youtube, that when the bike is running on the stand, the rear wheel spins slowly on its own. I got my cr125 out to see if it did the same and it didn't move when the bike was cold or warm, or when I revved it up a little. I'm just curious to know why some bikes do that and how. Does it have to do with the clutch or the transmission? SHOULD the bike do that? should it not? Just curious

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Not really sure what causes it since the bike is in neutral so its not the clutch and it doesn't matter if it does it or not, some bikes like my 4t 250 do it and some don't

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well if its in gear its gonna spin. But I assume your talking about when its in neutral. I have seen my bike do this before I'm going to take an educated guess and say its just physics at play. The piston is moving and the crank is revolving and maybe some of the vibration causes the wheel to slowly get started turning and then its own mass helps it to keep rotating slowly.

Edited by dan2581

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Even though the transmission is in neutral, the transmission internal gears are all still fully engnaged.   There is a pair of sliding gears in there, perhaps three, depending on transmission, which engages the secondary shaft (output) to your countershaft sprocket.   When you are in neutral, the sliding gears just do not completely engage the driving gears. Although the floating gears are not "driving", there is oil in there, which is causing some drag among the gears.  

 

This is most likely what is actually causing the countershaft sprocket to turn, and why when you put a load on the wheel, it stops.   You are actually turning the rear wheel in a way similar to an automatic transmission.   Simply hyrdraulic pressure causing the rotation.  

Edited by DethWshBkr_
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+1

Viscosity of the oil causing drag inside the tranny, thinner oil = less drag. 

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Like others have said, spinning parts move oil around, which causes other parts to rotate.  It's actually a good sign, in that you don't have anything binding or causing drag.  Don't worry about it, and don't try to do anything about it.  

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