Made by an outside vendor (Often Laycock-DeNormanville, if memory serves), these units were frequently installed at the factory as an optional upgrade. This "independent gearbox" had two ratios available, either 1:1 direct drive, or the "overdrive" ratio, where the output shaft speed would be faster than the input speed. It would receive the power from the conventional transmission, and then pass it (either directly, or over-driven) to the rear wheels (by way of a necessarily shorter driveshaft compared to the one fitted to a non O/D equipped Triumph).
Since the two gearboxes transmit power in series, a few things are worth noting. In theory, you now have eight forward drive ratios/ speeds available to you; Any of the standard four ratios present in the Triumph gearbox, could be used with either of the two ratios provided by the overdrive unit; Either 1 thru 4 "direct" drive, or 1 thru 4 "overdrive". In actual practice, there are only five real ratios available for use. The "default setting" of the overdrive unit is the 1:1, "direct" drive state, so it is "invisible" to the functioning of the car most of the time. The "overdrive" ratio is only sometimes enabled (an electric solenoid opens a hydraulic passage within the unit, engaging a clutch pack). When the conventional transmission is in 4th/ top gear, a switch is closed, which allows electricity to flow to the actuating solenoid on the overdrive. The electricity flows to that solenoid by way of the dash mounted "O/D" control switch or button, where the driver selects overdrive on or off.
Both switches must be in their closed state to engage the overdrive ratio. This effectively gives a 5 speed transmission; The four standard Triumph gearbox ratios while the overdrive unit is disengaged or "invisible", plus the fourth/ top gear, in a modified "over-driven" state.
It is a very simple "hack" to bypass or permanently close the "overdrive enable / inhibit" switch on the gearbox, and give the driver access with the dash mounted switch to all eight of the available combinations, but in actual practice it isn't done. The overdrive gearboxes are not intended to deliver their higher drive ratio while receiving the high torque loads available from first or second gears of the standard transmission. So for their own health, overdrive function is inhibited until 4th gear is selected. (Later EDIT: For another more practical reason, overdrive is not generally available to be used in the lower gears......The hydraulic pressure which locks the clutch packs in order to engage the overdrive is supplied by a pump on the input shaft; At the lower road speeds provided by the lower gears, the pump is not capable of generating sufficient oil pressure to maintain the "overdrive" state. The unit reverts to its "direct" ratio, regardless of switch positions.
One other slightly odd condition set is also worth noting; Pretend you are traveling along at relatively high speed, with overdrive engaged; Now you wish to "downshift" in order to overtake another car, or put on a burst of speed. Turning the dash mounted switch off and disengaging the overdrive would return you to the conventional "top / 4th" gear of the transmission. It is very easy, however, to forget you are in 4th gear
with overdrive, imagine you are in "ordinary" fourth. You may carelessly de-clutch and shift the manual transmission from fourth to third, and doing so also has the effect of simultaneously opening the "inhibit" switch, disengaging the overdrive; Effectively bypassing fourth gear, changing directly from 5th to 3rd; Not usually tragic, but it may get your attention.
There came a time, when the vast majority of conventional transmissions "sprouted" fifth (and sometimes sixth) "overdrive" ratios within the stock gearbox, directly selected with the shift lever, and eliminating the need for heavy, secondary "overdrive" units bolted to the rear of the transmission. These are in fact their own gear ratio, selected with the shift stick, (rather than a button); Like all of the other ratios within the box, the overdrive ratio is selected to the
exclusion of all the others, not as an "addition to" or "modification of" any other gear ratio in the box, as with the Triumph. Also worth noting about the Triumph O/D: The internal workings of this style of unit (and the many variants which exist) are different from the gears in the standard gearbox. As previously mentioned, they rely on hydraulically actuated clutch packs (much like automatic transmissions), so you can switch back and forth between O/D on and off, at the press of a button, no need to declutch or even lift off of the throttle; A practice not advised when going through the first four speeds!
A final bit of "old think"; Fifth gear, or "overdrive" is traditionally thought of as being the highway cruising gear, the gear ratio above the traditional "top gear" of 4th......(Or 3rd, depending on how old you are). Whether it was 3rd or 4th, "top" gear was traditionally a 1:1, direct ratio, the crankshaft speed and gearbox output speed being equal. This convention was largely because in the days of "traditional" gearbox design, the input and the output shafts were directly in line with one another, and the 1:1 ratio was almost free, a "no brainer" for the engineers; Simply couple the input to the output, and call it done.....a "ratio" achieved without any expensive gears. With the increasing use of transaxles, whether for front wheel drive applications, or other drive line layouts, the input and output shafts are no longer in line with one another, so the quick and easy 1:1 direct ratio no longer presents its self to the engineer. If now obliged to use a separate, dedicated gear pair for
all ratios within the gearbox, why bother with a 1:1 ratio at all, if it doesn't serve some other engineering purpose? It is very common to see (for instance) five speed transaxles with conventional numerical gear ratios, up to a .96 "fourth", and a 1.15 "fifth", bypassing the 1.00 "top" step altogether. And as Bjacks said above, sometimes followed by a 1.22 "sixth", or more.
A final thought, and could stand to be corrected on any of this, working from distant memory.
While "Top Fuel" drag racing is perhaps "the greatest show on earth", I always used to enjoy the "Pro-Stock" division....They were very fast, but were still something a mere mortal could relate to, as a "real car". Big horsepower, but retaining the silhouette of a stock road car, with opening doors, and "traditional" drivelines; A clutch pedal and (typically) four speeds the driver had to manipulate. The "gearboxes" employed by many of them (back when I was paying attention), were interesting bits: In practice, they were actually a "chain" of four individual gearboxes, (remarkably similar to the O/D units we are discussing) each with two available ratios, bolted together in a series. The individual boxes were identical modular units, manufactured by a company called "Lenco", each as robust as could be, simple, and reliable as a rock. Each gearbox in the chain had its own shift lever sticking out, allowing the operator to select either a slight "under-drive" ratio, or the bullet proof "direct" ratio, straight through drive. In service, all of the individual shift levers would be pushed forward, selecting the slight under-drive ratio in all of the boxes, when leaving the starting line. At the appropriate moment during the pass, the driver would use each of the individual shift levers a single time only, to change one after another of the boxes in the chain to its "direct" ratio, until by the end of the pass, the whole chain of gearboxes was in direct, offering a single coupling from engine to final drive.
riveshaft rotation). It might just say 5 on the shifter, but it's an overdrive gear.
