Some Explanation Needed Of Synchromesh Action.

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With all the manual tranny vehicles I have used I notice that if you are shifting into a gear and you are part way thru and hesitate and then try to complete the shift, the dog teeth clash. You have to go back into neutral and shift again in one smooth motion. It looks like the synchro clutch looses its effectiveness once you hesitate. Why is that?
 
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IMHO what you experience is normal in synchromesh'ed M/T box. The way synchro'ed gear works (to eliminate gear clash) is when you perform the shift action (shifing into gear), you dis-engage the clutch while pushing into gear; the shifting motion will force the bronze synchro rings to mate to it's mating surface on the other side, which, by force of the shifting operator, the ring has enough friction on the contact surface to eventually stop slipping and rotate at the same speed before your it mates into gear. That's why I insisted not to play scientist RE: adding molyslip gear additives into Honda M/T box that calls for Honda type MTF fluid, or go against the understanding that certain M/T box like Ford Festiva(Mazda 121) must go with proper 70W80 M/T gear oil instead of ATF, etc. Q.
 

George7941

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I agree with everything you said but it still does not explain why if you do not shift in a continous motion, the synchromesh doesn't seem to function.
 
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that's because you partially dis-engage your synchro rings, which starts to turn at a different RPM, thus the clash. You are not suppose to do that (either all the way "in-gear", or all the way "out of gear", not suppose to fumble or hesitate when shifting. *simple* Q.
 

George7941

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If I have partially disengaged the synchro rings, would they not reengage when I resume the shifting and complete the shift? What is the problem that exists when I go from 50% complete to 100% complete that does not exist when I go from 0 to 100%?
 
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If you cannot engage 100% but somewhat stuck 1/2 way before you manage to force it in (either with a bit of brute force, or manage to slide it in somewhat with a "notchy" or even gear "clashing" then you may have worn out syncho(s). normally, a syncho, when sync' up with the gear to be mated (shifting into gear), should be smooth and one seamless action (from 0 to 100%). Anything that stops you from properly siding (engaging) the gear means your gear lube maybe at fault, or worn synchro, or both. Q. p.s. I'd prefer to go from 0 to 100% (from neutral straight into gear) in one single, continuous motion...and never hesitate or stop somewhere in the middle. Also: once engaged into gear, your shift stick should stay in position. If you have a poor habit of laying your hand on the shifter stick, you may (over time) wear out the gear detention spring or checkball, which will cause the stick to jump out of gear.
 

George7941

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Quest, the gearbox is fine, nothing wrong with it. The discontinous shift motion is by choice, it is not that the shift cannot be done smoothly. It is just that if I deliberately stop halfway thru a shift and then resume I notice the teeth clash and was wondering why. Just idle curiosity and nothing else.
 
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Originally Posted By: George7941
Quest, the gearbox is fine, nothing wrong with it. The discontinous shift motion is by choice, it is not that the shift cannot be done smoothly. It is just that if I deliberately stop halfway thru a shift and then resume I notice the teeth clash and was wondering why. Just idle curiosity and nothing else.
When you stop the motion halfway through a shift, the force that is pushing the synchro clutch components against each other is relaxed, therefore the clutch basically disengages and sync is lost. When you start pushing again with the lever mostly through the shift, you're possibly already past the blocking ring and so the dog teeth can gnash. That's why a single smooth motion is best- the blocking ring will stop the motion until sync is achieved, then the teeth immediately engage.
 
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Originally Posted By: 440Magnum
Originally Posted By: George7941
Quest, the gearbox is fine, nothing wrong with it. The discontinous shift motion is by choice, it is not that the shift cannot be done smoothly. It is just that if I deliberately stop halfway thru a shift and then resume I notice the teeth clash and was wondering why. Just idle curiosity and nothing else.
When you stop the motion halfway through a shift, the force that is pushing the synchro clutch components against each other is relaxed, therefore the clutch basically disengages and sync is lost. When you start pushing again with the lever mostly through the shift, you're possibly already past the blocking ring and so the dog teeth can gnash. That's why a single smooth motion is best- the blocking ring will stop the motion until sync is achieved, then the teeth immediately engage.
This is the most reasonable explanation. But it is certainly not true for all manual trannys. Many do just fine with the delayed shift, and can even be better.
 
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