Ample Gearbox

My old idea, "Overdriven Driveline" (q.v.) was duly fishboned, but nobody seemed to hit upon the true reason it was fishboneworthy. Oil drag, being a fluid drag, might reasonably be expected to vary with the square of the rotational speed involved. If one considers that for all intents and purposes the power loss associated with an all-roller-bearing gearbox derives entirely from oil drag, we can therefore expect that power loss to vary with the *cube* of the relevant rotational speed.

[kevinthenerd]'s idea, "A New Chevy Nova" (q.v. also) got me to thinking about that. It seems that the requisite upgradability would ideally (i.e. without recourse to swopping gearboxes) require a gearbox equally capable of handling huge torque inputs and offering very little resistance to low-powered engines. Is that possible?

The thesis here is that there is an optimum ratio of engine and gearbox speeds where the effects of oil drag and the necessity of making the gearbox stronger and heavier to accommodate the multiplication of torque are balanced; and that that optimum will probably be found when the gearbox is turning more slowly in relation to the engine than would normally be the case.

Simply scaling a twin-shaft constant-mesh gearbox up by the requisite ratio might produce a gearbox that is far too bulky and heavy for use in anything that is supposed to be light and low-powered, however hardy also. It is, however, well known that epicyclic gear trains as found in automatic transmissions are capable of handling remarkable levels of torque, compared to conventional manual trains of comparable bulk.

The twin considerations of avoiding unnecessary proliferation of gearing and suitability for use with some of my prior suspension ideas suggests a transverse gearbox. So: a drive shaft from the engine and clutch drives a hypoid-bevel primary reduction gear, taking the speed down to, say, half engine speed. A hollow shaft from the ring gear takes the drive to a compound planetary as found in, say, a THM350, which gives three speeds and reverse. The output therefrom is also hollow, and drives a simple two-speed splitter planetary on the other side of the ring gear, through the hollow input. The output from the splitter is directly coupled to a differential, one output of which runs down the middle of the input/output concentric shafts, to the other end of the 'box.

So, we have a transverse barrel-shaped gearbox, with an input in the middle of the length and outputs at the ends. Different combinations of the available ratios allows six forward speeds and (potentially) two reverse. Final drive is by chains, the best of which, also, are capable of handling much more power than is commonly appreciated.

Now, the planetaries are somewhat larger in diameter than those found in the typical automatic. Because clutches and bands and their attendant hydraulic systems are dispensed with, the entire assembly is much more compact than an automatic, especially in length. Instead, the necessary couplings are effected by what are essentially large-diameter synchromesh mechanisms, controlled by multiple, motorbike-style axial cams (each controlling both planetaries: there are several only to obviate the need for shift forks to be heavy in order to handle the large diameters). A result is a motorbike-style quadrant shift pattern, which would require a reverse lock-out and warnings.

Of course, by adding perfidious electronickery and electric servos an efficient automatic may be had for a while. This may be important to those of you who demand gratuitous microchips.