Guys,
Prolly been explained before here on this forum, but have a few questions about simple transmission internal layouts.
So we don’t get sidetracked like we usually do and go off on a tangent, I’ll try and keep it as straightforward as I can.
Looking at a basic 3 speed transmission (forget the reverse function for the time being) , say for the purpose of the exercise, be it either car or truck, one can assume that the two lower gears (1st and 2nd) the “flow” through the gear train comes in via the input shaft, and gears, through the cluster (or countershaft) and then back into the mainshaft. All the top gear does, in this scenario at least, is lock the mainshaft to the input shaft,i.e. 1:1 ratio. In reality, the countershaft although rotating, is carrying no load and other than splash feeding the actual shafts, is simply “along for the ride”.
So far so good. The only way to change these actual lower gear ratios is to either vary the tooth count(s) on the cluster shaft and corresponding mainshaft.
However, by altering the actual input teeth counts, but leaving the lower gear sets alone, the OUTPUT ratios will have been altered. NOT in the case of the DIRECT gear where the input has been locked to the output, it would remain unaltered, but the “lower” ratios which utilise the cluster gear would now be different to original.
I have done this myself in an NP435 transmission I have in the shed,
What in actuality happened is that with an altered input tooth count, the original 3rd gear (NP435 being a 4 speed box) has now become an overdrive (in my case a 0.71 OD), the first two gears also unchanged in tooth count, are now given different output ratios on the mainshaft.
Chrysler did this with a 4 speed light truck gearbox when the Yank oil crisis hit, although they changed the actual tooth counts on the mainshaft/cluster to attain the same result.
They maintained the same “4 speed H shift pattern” at the remote control linkages by simply turning one set of linkages upside down. Had they left it in the original position, 3rd and 4th would have been “back to front” i.e. the same position as many older overdrive transmissions, Spicer, Fuller, Eaton to name a few.
I recently came across a bit of a curiosity (to me at least) where in a 5 speed light truck box, where the overdrive position is in a normal (extended) “H” pattern. This box is NOT like a lot of others where the overdrive set of gears have been housed in a separate section and simply bolted to the rear of a conventional 4 speed.
Going by the published ratios, it is definitely an overdrive box, but rather than having the overdrive added to the rear of the main casing, internal gearing sets the ratios.
My understanding of it is that the 5th gear (or overdrive) would have to have the countershaft under constant load, much the same effect as driving a vehicle around in 2nd gear in the case of a 3 speed, or 3rd gear in the case of a 4 speed. Obviously not an issue as those common 3 speed Spicer auxiliary were built like this from scratch (under/direct/over) and did millions of miles with trouble free operation.
Actual position of overdrive gear at the gearknob and/or the transmission itself is dictated by the way the linkages are set.
Where it gets a trifle confusing is perhaps in the case of something like those earlier Fuller Roadrangers with the “double overdrives” say in the case of the older 13 speeds, 10th and 11th gear would be direct one to one input to mainshaft lockup, 12th and 13th would be where the “double overdrive” comes in.
In reality, a 13 speed was only a conventional 9 speed with an addition splitter section added to give you the full spread of gears.
OR, do I have this arse backwards as usual?
Seems these days a lot of car (and truck) transmissions are actually overdrive in the “top gear” position.
In the case of a lot of 5 speeds, 4th gear is actually the 1:1 mainshaft to cluster gear lockup.
Thoughts?
Dave_64
