Will It Ever Run Again Posted June 30, 2019 Share Posted June 30, 2019 Please be warned that the following question is probably completely stupid: With both rear wheels off the ground only one of them goes around when engine on and in gear. When engine off, if I turn one wheel the other does go around (in the opposite direction which I assume is correct). I've never delved into the murky world of diffs but am I correct to assume my diff is toast? Thanks Quote Link to comment Share on other sites More sharing options...
mblowing Posted June 30, 2019 Share Posted June 30, 2019 Thanks perfectly normal Quote Link to comment Share on other sites More sharing options...
RobPearce Posted June 30, 2019 Share Posted June 30, 2019 Yep, perfectly normal, and shows you have a standard diff. A limited slip one would behave differently. A broken one would most likely behave differently again. Quote Link to comment Share on other sites More sharing options...
glang Posted June 30, 2019 Share Posted June 30, 2019 yes I think in a perfect world both wheels would turn but as theres always slightly more resistance on one side the power goes to the other. Of course when your driving this small difference is insignificant compare to the load of moving the car so it has no effect..... Quote Link to comment Share on other sites More sharing options...
Will It Ever Run Again Posted July 1, 2019 Author Share Posted July 1, 2019 Oh! I wasn't expecting that. Surely though both wheels should be propelling the car forward? There's no forward propulsion whatsoever with one wheel. With the wheels off the ground, engine on and in gear, I can turn one wheel either way with no resistance just as if the car was out of gear. The other side is spinning. Surely when back on the ground, the car would therefore only be propelled by one wheel? Sorry. I did say it was a stupid question and my knowledge on diffs is obvs limited! The reason I was prompted to ask is that up until recently BOTH wheels were spinning. Only yesterday something changed and one now doesn't. Quote Link to comment Share on other sites More sharing options...
RobPearce Posted July 1, 2019 Share Posted July 1, 2019 Well, you say your knowledge of diffs is limited so let's start at the very beginning. Why do you need a diff at all? To go round corners. In a straight line, both wheels turn at the same speed because the car is moving in a straight line. When you go round a corner, the inside wheel travels less far than the outside wheel, so you need to let them rotate by different amounts. BUT... you still need to propel the car forward (or retard it when using engine braking) So what does a diff do? A conventional diff allows the wheels to rotate independently of each other with the specific constraint that the AVERAGE of the two rotations must be what the input shaft enforces. So for every 3.89 rotations of the propshaft (assuming a Mk4 Spitfire standard ratio) the rear wheels must rotate by one full revolution on average. The diff cares not one jot whether that's exactly one rotation each, or two rotations of the right-hand only, or half a rotation of the right and one-and-a-half of the left, or any other combination, so long as the average of the two is one revolution. Then why would only one wheel turn? Something is making it harder for the other one. Remember that the diff itself doesn't care. Each wheel has a bearing and a drum brake. A week ago, when you jacked it up, both brakes released completely and both wheels were free. Yesterday, a fly got caught in one of them. Or the wheel cylinder didn't slide back as much. Or any of a huge number of other little things that you won't notice on the road because, as glang says, moving the car is so much more effort. Remember that you did your test with the engine idling - wheels turning slowly with no resistance. The slightly sticky wheel stays still while the other begins to spin. Once you're in that condition, the already spinning wheel has intertia that keeps it spinning and any stiction is already gone. You can turn the other wheel by hand because the difference in stiction is very small - the diff is quite loose and allows this difference very happily. Why did the opposite wheel rotate backwards when in neutral? Like I said, the diff imposes (Vl+Vr)/2=Vprop/R - the mean rotation of the wheels matches the propshaft divided by ratio. When you rotate one wheel, this constraint can be met either by rotating the propshaft or by the opposite wheel rotating backwards. The propshaft is harder to rotate because of the gearbox load and the ratio, so equal-but-opposite rotation of the wheels is the easier way to meet the constraint. Quote Link to comment Share on other sites More sharing options...
Will It Ever Run Again Posted July 1, 2019 Author Share Posted July 1, 2019 Awesome! Thank you Rob, you're a star. I appreciate the time and effort to explain. That's great news! I'll crack on with the other issues and get this Spit on the road again! 😆 Quote Link to comment Share on other sites More sharing options...
Clive Posted July 1, 2019 Share Posted July 1, 2019 https://youtu.be/yYAw79386WI Old but BRILLIANT Quote Link to comment Share on other sites More sharing options...
Clive Posted July 1, 2019 Share Posted July 1, 2019 https://youtu.be/yYAw79386WI Old but BRILLIANT Quote Link to comment Share on other sites More sharing options...
Will It Ever Run Again Posted July 1, 2019 Author Share Posted July 1, 2019 Quoted from cliftyhanger- Old but BRILLIANT Thought you meant Rob for a moment.... Great video! Thanks muchly. All is now clear! 🙂 Quote Link to comment Share on other sites More sharing options...
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