Synchromesh

Take two spur gears on separate shafts. Spin one, hold the other still, and try to slide them into mesh.

They clash and refuse to enter cleanly.

Now spin both to roughly the same speed by hand and try again. They mesh easily.

That is the whole problem stated: teeth will only engage cleanly at matched speed, and a driver had to produce that match by ear and timing.

Fit a brass cone ring to one gear and a matching tapered seat to the other.

Spin them at different speeds and press the cones together lightly before attempting to mesh.

Time how long they take to reach the same speed. Expect a fraction of a second.

Brass is used because it beds in against steel and carries the oil film that stops it grabbing — the material is a deliberate choice, not an economy.

Fit the blocking ring so its dog teeth are offset by half a tooth while there is any speed difference.

Now try to force engagement while the shafts differ in speed. You cannot. The blocker sits across the path.

Once the cones equalise the speeds, the blocker rotates into alignment and the sleeve slides through.

Friction alone would only soften the clash. The blocker makes a bad shift mechanically impossible, and that is the difference between a synchroniser and merely a friction damper.

Measure how long a full engagement takes from first movement to teeth home.

Expect 0.2–0.5 seconds, most of it spent in the cone phase, not the tooth phase.

That time is set by cone area, cone angle and how hard you push. It is why a heavy truck gearbox still shifts slowly and why a fast shift in a worn box crunches — you are outrunning the cone.

Deliberately force fast shifts repeatedly and watch the brass cone surface.

Expect it to polish, then glaze. A glazed cone takes longer to synchronise, so the shift crunches — and the natural response, shifting harder and faster, makes it worse.

This is the whole life story of a synchro. "Second gear crunches when cold" is a worn cone plus thick oil, not a broken gear.

The dates, separated. Earl A. Thompson conceived it in 1918, received US 1,435,430 in 1922, sold his synchromesh patents to General Motors in 1924 and 1930, and the first production cars were the 1928 Cadillac and LaSalle. Sources routinely give 1928 as the invention date. It is the production date, six years after the patent and ten after the idea.

Thompson was an outsider. An engineer from Oregon, not a Detroit transmission man, who developed it independently and then sold it in. He went on to lead the team that produced the Hydra-Matic — so the same person is behind both the manual gearbox that stopped crunching and the automatic that removed the shift entirely.

What it actually changed. Not performance — a skilled driver double-declutching a crash box shifts perfectly well. It changed who could drive. Gear-changing stopped being a learned skill requiring timing and hearing, which mattered enormously for mass-market motoring.

What is still not synchronised, and why. Reverse, on most gearboxes. It engages only at rest, so there is nothing to synchronise, and the saving in cost and space is worth it. The crunch when you select reverse while still creeping forward is that omission, working as designed.

Where it went next. Multi-cone synchronisers for the gears that need the most work, and then a genuine sidestep: the dual-clutch gearbox pre-selects the next gear on a second clutch, so there is no shift to synchronise at all. Thompson's problem was eventually solved by arranging never to have it.