Make the Shift

The Canadian Lorne Elliott observed that the first man who ate lobster must have been “some kind” of hungry. I’m sure the first person to depress the clutch pedal and slide a rotating mainshaft gear toward a rotating countershaft gear did so with only slightly less enthusiasm and without a necessity born of imminent starvation. At least in the latter case the worst that could happen was noise, broken teeth and, perhaps an embarrassing walk home.

As we now know, it worked! Through repetition, improved technique, and sliding gear transmissions (we call them ‘crash boxes’ today), manual transmissions took hold and were commonplace by 1900. Not everyone could manage them, however, and the gears, whose teeth were quickly burred, chipped or broken, required routine attention. There was obviously room for improvement.

Reason suggested that, rather than sliding the entire mainshaft gear roughly the gear’s face width, along a splined or square-sectioned shaft, if the mainshaft gear could rotate relative to its supporting shaft it could be fixed axially and, therefore, kept in constant mesh with its countershaft partner. This would, in theory, greatly extend the life of the gear teeth, the engaging and load-bearing teeth now being separate. If the engaging teeth could be made shorter axially than the face width it was also possible to shorten the gearbox, making it stronger and lighter in the bargain. Of course, there were extra and more complex parts to be made, the gearbox cost increased, and those with the skill to manage the old sliding gear design saw no need for the constant-mesh improvement. To shift smoothly still required the driver to synchronize the rotation of the sliding element, — the ‘sliding clutch’, with the rotation of the destination gear. speeding up or slowing down each until they rotated at the same speed. It was better, but not enough.

Before proceeding, some theory. The sliding clutch is typically splined to the mainshaft which, in turn, is connected to the tires and ultimately the world through an assortment of shafts, joints, and gears all rotating at known fixed ratios to the sliding clutch speed and therefore remain rotating at an arguably constant speed during any acceptably fast shift cycle. It follows that the necessary change in rotating speed is made by the constant mesh destination gear and all those elements connected to it, including the other mainshaft constant-mesh gears, countershaft cluster, maindrive and clutch disk. Considering each of these several elements as a little flywheel, shifting speed can be increased by reducing the inertia of one or more.

This is why it is commonly beneficial to replace a large diameter single-plate clutch with a multi-plate clutch of smaller diameter. It can, therefore, easily be understood that without introducing a braking device, a ‘synchronizer’, not yet invented a century ago, shifting remained an acquired skill, although advancing clutch technology reduced disk inertia and sped things somewhat.

To review, by the mid-1920’s a technologically current North American automotive gearbox incorporated three forward speeds, with second and third constant mesh selected by a sliding clutch. Low and reverse were still engaged by sliding gear, there being no good reason to increase cost and complexity to achieve a debatable benefit as low and reverse were intended for shifting with the automobile at rest.

While the automotive world waited for the synchronizer, easier shifting without one was possible. Remember that the mainshaft and the ‘downstream’ hardware connected to it were connected to the world and, therefore, did not change speed during shifting? It is not true, however, that they are necessarily so connected. If the driveshaft could be disconnected from the mainshaft during shifting only the mainshaft, the sliding clutch and any sliding gears supported by the mainshaft would need to change speed; the inertia of these items being much less than the ‘upstream’ inertia shifting speed and ease could thereby be greatly improved. Of course, the declutching of the driveshaft needed to be transparent to the driver; there were already challenges enough at the driver’s seat. A way was found to do this. It was called ‘freewheeling’ and consisted of a ‘one-way clutch’ installed between the mainshaft and driveshaft. When torque was transmitted from the engine, the freewheel sent the torque transparently to the driveshaft. When shifting, however, the freewheel interrupted the torque stream, permitting the ‘upstream’ and mainshaft hardware to change speed independent of the driveshaft hardware. Shift complete, engine torque found its way earthward again, all without driver intervention and mostly transparently if he was willing to overlook one tiny problem, the freewheel eliminated engine braking. In the years before hydraulic brakes perhaps this problem wasn’t really so tiny. So, despite freewheeling being widely applied to ‘status’ marques it disappeared as quickly as it appeared once synchronizers became available.

Which brings us to the late 1920’s and another installment.


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