Breakthrough: The synchromesh gearbox

It’s another game with a true crash gearbox – get it wrong, especially at high engine speeds, and you will be hard-pressed to decide what’s worse – the wince-inducing grinding assault on your eardrums, or the horrendous vibration through the gearstick as you feel the gears failing to mesh all the way from the palm of your hand, through your arm and right to the amygdala, or whatever bit of the brain that deals with nails scraping on blackboards and other nerve-jangling sensations. It’s truly horrible.

Now, Andrew Frankel – who’s driven more crash gearboxes than I have had cups of tea, including truly terrifying ones like those of 1920s Le Mans racing Bentleys – has rightly observed that drivers of that era were actually no more skilled than are we modern drivers. That means that crash ’boxes are built strongly and can take the occasional ham-fisted shift failure. But it’s still horrible when you miss one.

Our man Earl Thomson agreed. Not only that, he considered that the effort – mental and physical – required to change gear in the cars of his day was actually dangerous, and determined to think of a way to make it easier and, hence, safer. So in 1918 he filed US Patent 1435430, which described the basic operation of what would become known as the synchromesh or ‘synchro’.

Thompson’s invention is essentially unchanged to the present day. Describing how synchromesh works is one of those things that’s like riding a bicycle – hard to describe but fairly easy to demonstrate, if I only had the innards of a gearbox here in front of us on the desk. So imagine that we do…

This here splined shaft is the gearbox main or input shaft – the driven shaft, carrying torque from the engine via the clutch. Assume it’s spinning like hell, as for some reason we’re roaring up a hill in second gear, engine about to nut itself into the rev limiter. Now, when I grab third, that input shaft needs to mesh with this here third gear cog, which is just idling around minding its own business, at a much lower rotational speed.

If this were a pre-war Benters, and I had the nimble feet of Mr Frankel (they call me twinkletoes – af), I’d have to do the fancy footwork required to double-declutch and match the gear speeds by a perfect dose of revs at just the right time. But it’s not – so the gearlever selector fork pulls this here synchro hub along the input shaft to make it press up against third gear. As you can see, the synchro hub has a bit going on. It’s got a sliding inner part, and a set of internal teeth known as ‘dog’ teeth. These dog teeth mesh, when properly aligned, with the external dog teeth on this here bronze bit known as the baulk ring, which sits between the synchro hub and the cog we’re shooting for. Now, look closely at the face of the baulk ring that presses up against the soon-to-be-pressed into service third gear. You can see that it’s machined in a conical shape. Look at the mating face of the third cog – y’see here? – and it’s got a suspiciously matching conical face. So what’s going on?

Well, as you push the lever forward toward third, you will push the synchro hub hard up against the third gear – but with the baulk ring sandwiched in between them. And what makes first contact are the conical faces between the baulk ring and the gear to be engaged. As you push further and harder, the friction between these conical faces will speed up the third gear hub, until eventually it’s spinning at the same speed as the input shaft. At this stage – lo! – the dog teeth will line up and the baulk ring will slide inside the synchro hub, allowing the cogs to mesh smoothly. No grinding and gnashing of teeth, as in the Old Testament, just sweetness and mechanical light.

(If you are still lost, I recommend a bit of sneaky YouTubing – several kind souls have posted slow-mo videos of synchros actually operating. It makes a lot more sense when you see the magic in action.)

All this happens – purely mechanically – in the time it takes for you to pull, or push, the gearlever as fast and as hard as you can. Unless it’s attached to my old 911’s notoriously lazy 915 gearbox, which appreciates a little pause for thought when you drop from third to second; it feels downright rude to rush the old gal.

It took Thompson two years of hard graft – and several trips clean across the US from his home in Oregon on the West Coast to what was already Motor City in Detroit – before he could persuade the car company executives that gearshifting not only could, but should be improved. Predictably, his first customer was a nascent luxury car maker – Cadillac – which was looking for every possible mod-con to establish its reputation as the American Rolls-Royce. So 1928 Cadillacs and LaSalles were duly equipped with Thompson’s gadget, and the trademark they gave it – Synchro-Mesh® – remains with us to this day as the generic term. The now-familiar technology trickle-down started quickly, and as early as 1932 some relatively modest GM models were boasting ‘synchro’ gearboxes.

But for many years to come, synchros were considered unnecessary on first gear (the assumption being that the driver could come to a stop and engage first) or on ‘top’ gears – usually third or fourth. There is a little bit of doubt about who was the first car-maker to apply synchromesh on all forward gears. Some claim that it was not until 1952, on the Porsche 356. But it was well before that – Alvis offered an all-synchro gearbox on its Speed 20 models as early as 1933. I think the confusion is probably due to a patent that Porsche deposed in 1947 – a significant improvement on Thompson’s design that made the synchro rings and cones more compact and thus able to be crammed into the small confines of a 356’s transmission. In any case, Alvis’ achievement has been somewhat forgotten – possibly because Porsche went on to become, well, Porsche, while Alvis, at least as a car maker, sank in the mire that became British Leyland. What was left of the company went on to make Challenger tanks, but that, as they say, is another story.

Maybe because I’m an electrical and not a mechanical engineer, Thomson’s invention seems almost miraculously ingenious. If you think about it, it really is a fiendishly clever idea, both in principle and in execution. And no microprocessors, GPS, GSM, OS, AIs or other acronyms involved – just a set of cleverly machined cogs. Sheer bloody genius, if you ask me. And we all take it for granted, hidden as it is in the bowels of gearboxes we will hopefully never have to expose to the light.

So, please think about Earl A. Thompson the next time you slot fourth in your Civic Type R, or push that stick across the open gate of your Italian classic, if you are lucky enough to own such a thing. Your pleasure is largely due to his determination to get his invention into production, so many years ago.

But – there is always a but… Our man Thompson – who subsequently had a glittering career at General Motors (legend has it that they paid him a million dollars for his patents, long before a million bucks was debased by Dr Evil) before founding his own company – should really have a secure berth in the Pantheon of saints of the enthusiast driver. But, like me with the A110 in the eyes of the manual diehards – yes, I’m talking about you lot down the back – the lad blotted his copybook: badly.

The clue to his sin lies in the title of the patent that made his name. For US Patent 1435430 is titled ‘Automatic gear shifting mechanism for sliding gear transmission’. Yes folks, Thompson was not satisfied with just making manual shifts easier – he wanted (horror of horrors) to get rid of the chore altogether by building an automatic transmission. And y’know what? He succeeded. Working for years at the head of a small team within Cadillac and then GM, Thompson developed a hydraulic torque-convertor gearbox that was launched in 1940 as the ‘Hydra-Matic’. It was and still is the granddaddy of all subsequent slushboxes.

I can only imagine the amount of stick the poor bloke got.