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Synchromesh Gear Box- How’s That Work?

Robert Emslie April 29, 2014 All Things Hoon 22 Comments


Have your ever driven a car with a non-synchromesh transmission? Or perhaps an old Alfa Romeo whose synchs have gone the way of Elvis as so many of them do? If so, then you’ll likely genuflect before the name Earl Thompson. He was the engineer who invented the  Synchro-mesh transmission while working for Cadillac in the ’20s. It took Thompson 12 years to perfect the mechanism which was introduced in Cadillac’s 1928 models, but even today few folks ever give much thought to how a synchromesh transmission works.

Fortunately these days we have the Internet and that’s a resource of almost unlimited bounds. This video by Paul Cangialosi from 5speeds.com explains how synchros work, and makes me want to go out and have a cannoli after watching it. Check it out right after the jump.

Source: YouTube

  • Irishzombieman☆

    Never learned to double clutch properly until I lost a 2nd gear synchro. Made driving more fun, and made my car undrivable to anyone but me.

    Cool video.

    • wunno sev

      my MR2 wouldn't go into second without grinding when i bought it.

      the owner warned me about it before my test drive, and apparently after twelve years of ownership he'd never thought of double-clutching to upshift.

      changing the trans fluid fixed it for upshifts, but i still double-clutch the 3-2 on any car i drive now. i love when the lever seems to suck itself into second with a perfect double-clutched downshift.

      • Irishzombieman☆

        YES! That feeling!

  • Driving a 31 Model A with the three speed crash box made me learn to appreciate the synchro.

  • GTXcellent

    I've driven a couple of non-synchro grain/dump trucks before – Earl Thompson, a tip of my cap to you.

    <img src="https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcSs8qZ7zAcXsOkvGgF4IELlGc_b7uPUPZY6pqQHoixPxD78yzwPqA"&gt;

  • nanoop

    I don't know why, but the term "worn synchro ring 2nd gear" is tightly connected with "Alfa" to me.
    I'm quite fascinated by gearboxes, the only thing that prevents me from rebuilding one for my project car is the smell of gear oil (and having stinky metal bits lying open in the garage for weeks, where Mrs. nanoop stores her gardening tools…)

    • Alff

      I know why. It's not a myth.

  • R.K.

    Crash ! Crunch ! Profanity ! Cries of anguish and tears of frustrated desperation ! One word : Moss !

  • <img src="http://clunkbucket.com/wp-content/gallery/1980-kv-mini-1/img_6618.jpg&quot; width="450">

    A box with gears in it? Yes, I suppose in theory that would also work. Sounds like a Rube Goldberg lash-up, to be honest.

  • Drzhivago138

    This is a little embarrassing to ask given my age and background, but…A non-synchronized trans means you have to stop (or at least, slow down) between shifts, right? So, a tractor?

    The first tractor I learned on was a Quad-Range JD 7210 at the age of 8. You only had to clutch between ranges, not speeds, so that was easy enough. The first "real" tractor was a 3020 (and its siblings, the 2510, 2520, and 4020), and that's when I learned speed-shifting (although I didn't know it at the time).

    A tractor is the easiest way to learn manual, since it has so much torque that it won't stall. The problem is if you get out of control, _it_won't_stall_. But once you get the hang of it (as I did), a synchromesh is second nature. I didn't learn syncromesh manual until I was 17, in our like-new '08 F-350 regular cab XLT V10 5-speed farm truck. That thing is the most bad*ss vehicle on the face of the earth, bar none.

    It wasn't until shortly after that that I finally got to (maybe had to) drive the '77 L700 grain truck. Same basic principle, except I had to find out for myself how the 2-speed rear axle worked. Then I was really moving–65 mph empty down a one-lane gravel road, eyes burning with pure insanity.

    Similarly, it was at the same time that I finally drove the old two-cylinder 530…with a HAND CLUTCH. And in the same manner as the farm truck, I was left to my own devices as to how to actually operate it going down the road. Apparently, every farmer before 1960 had 5 arms, to operate the hand clutch, throttle lever, steering wheel, gearshift and hydraulic or other implement controls, possibly all at the same time.

    • Without synchros it is not inherently necessary to slow the vehicle but it is necessary to allow the engine to slow during upshifts so that the speed of the input matches the speed of the output before engagement. Of course, in practice this means the vehicle is temporarily coasting and it will, under ordinary circumstances, decelerate. Downshifting requires one to speed up the engine between deselecting the higher gear and selecting the lower gear, again so as to match speeds. The vehicle is again coasting during this period and, again, probably slowing down. The trickiest bit is developing a sense of when the engine speed is correct for the conditions. I'm terrible at it, but I've had little reason to practice.

      • Drzhivago138

        See, I started double-clutching/speed shifting before I even knew what it was, and now that I do know, I have a harder time doing it–the centipede effect.

    • ptschett

      Well, there were some more-progressive farmers prior to 1960 whose tractors had foot clutches and therefore only required 4 arms. (The tractors I'm thinking of had 4- or 6-cylinder longitudinal inline engines with vertical cylinders rather than a transverse engine with two horizontal cylinders, and were generally painted red…) 🙂

  • wunno sev

    as an aside: anyone who liked this video should look up AgentJayZ on youtube and thank me later

    • JayP2112


  • Once again, proof that absolutely nothing about motorcycle mechanics translates to cars.

    <img src="http://tanshanomi.com/temp/motorcycle-transmission.jpg"&gt;
    Let's face it, car designers do everything the goofy way.

    • Drzhivago138

      I learned on our '95 Yamaha Timberwolf before I ever understood the principles behind "regular" shifting. Up equals up, down equals down. Nothing hard about that, to me at least. Oh, and you had to let off the throttle right when you shifted, otherwise the engine would go WAAAAAAAHHH and the trans would go GRRRRKKKK and it sounded bad. Sounds bad=is bad. Easy enough logic for a 10-year-old.

  • smalleyxb122

    It's a good video, and it does a good job of explaining what synchros do, but it misses a few key points of the how. He glossed over the struts and ignored entirely their real purpose. He also was a little bit off on what the blocker ring is actually doing (and at no point are you necessarily turning any component backward).

    I'll see if I can fill in the gaps without the aid of audio visuals:

    You saw in the video how the components stack together. The strut (or rather 3 struts) have a detent that engages a groove in the sleeve (referred to as the slider in the video). Just like any spring detent, there is a certain force required to disengage it. As the strut is engaged with the groove, it moves into contact with the blocker ring when the sleeve begins to move. The strut actually makes first contact with the blocker ring (referred to as the sychronizer ring in the video). The force required to disengage the strut determines the force imparted to the blocker ring. This imparts the initial contact with between the blocker ring and the gear cone, the drag of which ensures that the blocker ring and the sleeve do not line up (he touched on this a bit, but could have explained it better).

    As long as there is relative motion between the gear and the sleeve, the blocker ring remains clocked to one side (preventing the sleeve from moving). The torque holding the blocker ring in misalignment is determined by the angle, coefficient of friction, and axial force applied to the cone clutch, but requires relative motion. When that relative motion disappears, the torque acting on the blocker ring goes away, and the lead in angle of the blocker and the sleeve teeth pulls them into alignment (and subsequently into alignment with the dog ring on the gear).

  • Synchomesh

    Unbelievable! My nick is finally explained on Hooniverse! 🙂

  • buzzboy7

    I've owned a car with non-syncro 1st gear. Not too bad really. I've driven cars without 1st or 2nd gear syncros due to wear. Not too bad. Even with syncros the shifts are so much smoother with a proper double-clutch. I've gotten to a point of double clutching every down shift and when I skip shift up.

  • ptschett

    My Challenger has a TR-6060 and I've been trying to work out how its triple and dual cone synchros would work. After watching this, I think I know. [youtube pele5vptVgc http://www.youtube.com/watch?v=pele5vptVgc youtube]

  • Dave

    I found fivespeeds.com when I wanted to upgrade the non world class T5 in my Astro van to a world class unit. It is filled will G-force gears and has Paul's proprietary counter gear support. It is a neat transmission in what would appear to be a derelict vehicle.