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john vanlandingham
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Probably irrelevant, but now I'm kind of curious.
Not at all irrelevant, its a good question cause it brings the discussion down to basics, or fundamentals, and that's always good---cause its silly talking about "theoreticals" or effectively un-obtainable stuff like $6000 gear sets.
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Is there any advantage to using longer gears in the box with a shorter final drive vs. using shorter gears in the box with a relatively longer final drive to hit the same overall gearing?
For less stress on the box itself, yeas its better to have "taller' gears---or pointedly less speed reduction and therefore less toque multipication in power that comes into the box. let's see if these piccies help;
or
And here's a link to T5 since we can look at the effect of differeing torque capacities on otherwise basically same boxes:
http://www.therangerstation.com/tech_library/T5ID.htm
Notice the horrible 4.03 first gearset with a measly 235 ft/lbs rating, the disgusting 3.97 at 240 ft/lbs, the crappy 3.50 at 250 ft/lbs, the sucky virtually identical to Xratty 3.35 at 260 early and a bit more later and note well the NWC 2.95 gearset at 305 ft/lbs, later 2.95 gearsets at 325 ft/lbs.
It all goes down to how much toque is multiplied from input shaft down to cluster gear....like you see in the power flow piccie.
Whatever torque the engine makes gets multipled by whatever the ratio is from input shaft down to cluster gear and the box "sees' that increased torque. Reduce it a whole bunch so first is 4.08, the box is stressed X% more than if you reduce only a bit to make first 2.95-everything else --specifically the centerline distance between the 2 shafts.
That distance is always the first thing you look at
Example the most commonly used rally box in the world for decades is Ford Type9, a 5 speed version of the 4 speed used way back in the Escort MkIIs
It's 69mm between shaft centers. The gear company Tran-X makes a version that is 72mm between centers--evidently worth it. Ford wanted something a bit stronger in-house than the Borg Warner T5 so they made MT75, MT75 4x4 and much later the fwd MTX75.
Quaiffe makes a direct replacement box for the Type 9 that is 80mm c-c and Fords own Group A 4x4 box was called MS90, 90mm between centers.For comparison the box in my diesel van is a Muncie M89, curiously 89mm between centers for the 2 shafts--so a purpsoe built GpA box is sized like what is suitable for a full sized van with a diesel with who da fawk know what I can tow with it..
But after than the ratio from clutch inputshaft down to cluster is the next prime determinant of what torque the box will tolerate.
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I'd think there would be reduced shaft/gearset speeds but have no idea if that has value or relevance. Since you are moving rotating mass, is there a gain in not having to accelerate the mass to the same speed? in a trans-axle car the half-shafts are past the final drive so moving that mass is irrelevant, but in a conventional RWD using a driveshaft between the gears and the final drive is there an advantage to be had in keeping that spinning slower?
Not following the question too well but the kardan or propshaft speed is changing every gear, (in a typical car in first it will be running 1/3.63th crank speed, 1/1.8th crank speed in second etc) the critical speed is what speed is it when in 4th (cause then its running at crankshft speed in a rwd box) or overdrive cause it is spining pretty damn fast
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This is all theoretical, of course, since the availability of gearsets and final drives is probably the real deciding factor.
Well for some boxes such as Type 9 the choices are mind numbing, but for some cars its no problem at all cause nothing is made or made in such short runs it is prohibitively expensive...
Now on the rear end torque is multiplied one more time with another redcuction gear set and again no or limited choice means we can't get things like ratios with tooth counts or materials for competition like some sizes of US axles, notably Ford 9". There you get not only different alloy than the ubiquitous 8620, but different heat treatments for toughness, not 200,000 miles service.
Real competiton stuff may have same ratios as some streety thing but do the ratios with less teeth so the section or base of the tooth is fatter. There's an interesting note in Fords Sierra book in the list of Homologated rear final drives for the stock 7.5" diff which were in 87:
3.64 (51:14) stock<----toothies count, most of the world talks tooth count
4.44 (40:9)
4.63 (51:11)
]4.66 (42:9)
4.88 (44:9)
5.1 (46:9)[/b]
The say "Note there are two closely similar "4.6" ratios for the 7 1/2" rear axle which are in fact, completely different designs...The 4.63 ratio (with 51 crown wheel teeth and 11 pinion teeth) is not reccomended for use in the fully tuned Group A SIERRA COSWORTH RS/RS500 Cosworth"
The highlighted gears were the common gears used , the shortest earlier with 40mm restrictor "up" to the 4,44 in the 34mm restrictor cars. Engines didn't rev so the made them into toque monsters, and didn't
NEED
shorter gears.
Notice in the normally used gears they go down in tooth count. They were making the things FAT for strength. i think I have a piccie, its shocking.
(clop clop clop clop clop clop clop clop clop clop clop clop clop, oh yeah here it is, clop clop clop clop clop clop clop clop )
Serious, fundamentally different design, for different usage.
