Questions about synchronizers.

Discussion in 'OT Driven' started by deusexaethera, Jan 26, 2010.

  1. deusexaethera

    deusexaethera OT Supporter

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    Believe it or not, despite all the crap I get from you guys, I do respect your expertise, hence why I'm posting this question here instead of in Eastern Imports.

    So, the second and third gear synchronizers in my RX-8 have been giving me some grief, especially when it's cold out, but also a little bit when it's warmer; there's sort of a clinking sensation just before they engage, like the synchronizer splines are trying to engage before the cones have finished doing their job getting all the parts moving at the same speed as each other. I'm concerned that this is going to cause damage to the spline teeth before the cones get broken-in enough to do their job properly.

    Now, if my understanding of how synchronizers work is correct, there's more going on than just the cones rubbing together and then the splines sliding into place. The way I understand it is, when the cones first contact each other, because they're moving at substantially different speeds, this causes the inner part of the synchronizer to twist inside the outer part a little bit, and this engages a locking mechanism that keeps the splines from being able to slide into place until all the parts are moving the same speed.

    I think what's happening inside my gearbox is the cones on the second and third gear synchronizers aren't meshing very well, because they aren't broken in, so there isn't enough torque being applied as the parts try to match speeds. This is allowing the locking mechanism to disengage prematurely so the splines can try to slide into place before everything is actually moving the same speed. Again, this is based on my assumption that there actually is a locking mechanism inside each synchronizer, though the way it behaves certainly makes it seem like there is one.

    So, I had this idea the other day, while sitting at a stoplight on a side street that takes several minutes to turn green. I thought, "why don't I shift into Neutral, close the clutch, and then try to shift into second and third gears so the synchro cones can rub together and break-in faster?" So I tried it, and as I expected, the spline teeth never tried to engage because of the torque being applied to the cones. So I did it some more, trying to shift into second and third gears for about 3 seconds apiece, pushing just hard enough to see the tachometer drop a tiny bit, then dropping back into Neutral.

    I've done this about five times now, sitting at the same stoplight, just rubbing the synchro cones together for a couple of seconds and then dropping back into Neutral. I've noticed a distinct improvement in gear engagement in second and third gears since I started doing this, and I've also noticed it's getting easier to make the engine bog down a little -- I don't have to push as hard on the shifter to make it happen. So it does seem like the synchro cones are breaking-in and fitting together better as a result of my experiment.

    What I want to know is, is there something I'm not considering that makes this a phenomenally stupid idea? Is it something anyone here has ever done? Let's go ahead and assume that I'm not an idiot (though perhaps misinformed), and I'm not going to keep doing it forever, and once the gears start shifting the way I like I'll stop breaking in the synchro cones this way. Is this a reasonable thing to do to accelerate the break-in process on these two gears that have been causing trouble, or should I stop right now and never do it again?
     
  2. murderer

    murderer ****** by ***** *********s

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    I'm by far no expert on the subject, but i think it won't get better.


    I have had my share of transmission rebuilds and "freshening ups". But more than anything its always been a bad synchro and nothing else.
     
  3. murderer

    murderer ****** by ***** *********s

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    maybe your blocker rings are bad?
     
  4. deusexaethera

    deusexaethera OT Supporter

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    My what rings? I gotta be honest, pretty much everything I know about stickshifts I learned from a 30-year-old "How Things Work" book and my deductive reasoning skills. If you know of a good 3D exploded diagram of all the various parts in a synchronizer, I'd love to see it.
     
  5. ultimatt

    ultimatt OT Supporter

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    i think you need to spend more time explaining the problem and less time saying what you think it is and how you already fixed it
     
  6. deusexaethera

    deusexaethera OT Supporter

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    I already explained the problem. When the transmission is cold, a clinking sensation comes through the shifter just before the 2nd and 3rd gear synchros lock in-place. It doesn't happen every single time, but it happens pretty often, with decreasing frequency as the transmission warms up.

    As for the rest, yes, it's speculation, but I like speculating. :dunno:
     
  7. deusexaethera

    deusexaethera OT Supporter

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    Okay, I've done some more reading, and it looks like I was right about the existence of a locking mechanism -- I just didn't understand how it worked. So, the inner part of the synchronizer, the part that contains the cone, is called the blocker ring that murderer referred to before, and it is spring-loaded so it twists a bit when it presses against a gear that isn't yet synchronized. The locking mechanism that keeps the lock-ring from engaging actually consists of another set of splines around the edge of the blocker ring, and when the blocker ring twists during synchronization those teeth get in the way of the lock-ring so it can't slide far enough to lock onto the gear being selected.

    Older transmissions had brass blocker rings with a single synchronizer cone apiece, but I know (because the sales rep mentioned it about every 30 seconds when I was test-driving the car) that my transmission has steel blocker rings, lined with carbon-composite, with three synchronizer cones apiece. The carbon-composite stuff apparently wears down a lot more slowly, which obviously has an impact on break-in time as well.

    The big thing, however, was mentioned by one article I read: the blocker ring will only twist during synchronization if the driver pushes hard enough on the shift lever for there to be enough friction between the gear and the synchronizer cone. If the driver shifts really slowly, or if the RPM difference between the synchro and the gear is small enough, or if the torsion spring on the blocker ring is unusually strong (such as, if it's designed for hard shifting in a sports car transmission), the blocker ring won't twist enough and it won't prevent the lock-ring from trying to engage before everything is moving the same speed.

    Considering that I've never had this problem when shifting hard and revving high, but only when I'm cruising around town and "granny shifting" at 4000rpm, it sounds like the synchronizers just aren't being worked hard enough to make the blocker rings operate properly. I'm still not entirely sure if my approach to breaking-in the synchro cones is a really bad idea or not, but it does sound like what I'm imagining when the shifter clinks is accurate, and that and the dealer tech is right, it really isn't a malfunction per se.

    Still, I'd rather not have any clinking at all, which is why I'm still trying to figure out how to make the synchro cones grab harder. But the fact that they might be malfunctioning due to a lack of friction makes me wonder if switching to synthetic gear oil won't just make the problem a lot worse. But then again, synthetic gear oil doesn't thicken as much in cold weather, which is when I notice the problem most often, so maybe the cheap gear oil is actually helping the synchros start doing their job before they even touch because the oil is so thick, and that's keeping the blocker rings from twisting as much as they're supposed to. I dunno; lots of stuff to consider.
     
    Last edited: Jan 28, 2010
  8. victimizati0n

    victimizati0n New Member

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    my car did this when their was air in my hydraulic lines, once everything warmed up and the fluid expanded a bit it would work normally

    the slave cylinder wasnt pushing the pressure plate off of the clutch all of the way and causing it to keep spinning somewhat
     
  9. deusexaethera

    deusexaethera OT Supporter

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    Funny you should mention it, someone on RX8Club.com suggested that too, and I've been suspicious of it ever since. I can't say that isn't what it is, and the engagement point does move enough when the weather changes that it throws off my shifting for about a day, but at the same time the clutch pedal doesn't feel spongy like I'd expect if there were air in the clutch line.
     
  10. murderer

    murderer ****** by ***** *********s

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    you could always just trade the car off and fix the problem? That's my response to flat tires, cracked windshields, and out of date registration....
     
  11. victimizati0n

    victimizati0n New Member

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    my clutch pedal felt perfect when it happened, no sponginess, but that sounds a bit funny if the engagement point is changing, i would try and bleed it and see if it helps the problem.
     
  12. murderer

    murderer ****** by ***** *********s

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    Hope that helps! My blocker ring issue (always was a mustang) was during a low speed or high speed shift (depending on the gear, 3rd and 4th were everywhere) but it was just a grinding issue. Simple fix was learning how to replace them myself. g'luck! :x:
     
  13. victimizati0n

    victimizati0n New Member

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    :bigthumb:

    i started having the problem a while back only when it was cold out, during the summer it rarely did it, and if it did, it would only happen a few times.

    i changed my clutch, and it started doing it again, but really bad. I had a new slave in it, and after some reading i found it is VERY hard to bleed my car, so i bought a mityvac and sucked the everliving hell out of it for a few hours, and my clutch has been fine since.
     
  14. deusexaethera

    deusexaethera OT Supporter

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    Um...I've had this car since October. I kinda still like it.
     
  15. deusexaethera

    deusexaethera OT Supporter

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    See, I was thinking the engagement point shouldn't be changing like that. The whole reason for having a hydraulic clutch on a RWD car (which could just as easily use a solid metal linkage) is so the clutch pedal can auto-adjust for wear and temperature changes like the brakes can, so the engagement point should stay the same no matter what.
     
  16. murderer

    murderer ****** by ***** *********s

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    :mamoru:

    I bought an '05 corvette last may, it left in november.....nothing wrong with swapping cars every so often
     

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