Tom Devoe

rear diff ratio's in 1931

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Were different rear differential ratios available for Studebakers in the late 20's, early 30's, or was it 'you'll get what we give you' back then?  I'm wondering if parts are available to lower my ratio, as my car is geared for stump pulling.  With today's better roads and higher speeds, I'd like the car to move a little faster on secondary roads. 

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I do know there were two different ratios for the late GE; 4.66 and 4.3.  I have one of each. We fitted the 4.3 to the car but it is quite worn unfortunately - maybe one day it will get new gears.

 

For 1931 The Standard catalog only lists one ratio for each series. It is possible there were more, as in some makes the roadster had a different ratio to the sedan.

 

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OK, you have a 1931 Stude. The following comes from the Republic Gear Catalog "S".

 

Dictator 8-61 1930-31 had either 46-9 or 52-11. OD of ring gear was 9.25". Same sets used in 1931 6-54, 6-53.

 

Commander 8-70 had either 48-11 or 52-11. OD of ring gear was 9.75" but the bore was the same as for the 8-61. The pinion was a different length with different diameter rear bearings. The same ring and pinion sets were used in 1932 Dictator 71, 62 and 55.

 

1930-31 President 8-80, 90, and some 1932 91 and 1933 92,  had 51-11 or 56-13. Both front and rear pinion bearings are different diameters to the Dictator and Commander. 

1933 6-56, 8-73, 8-82 (some of each) used another two sets at 48-11 and 52-11 with larger rear bearing diameter than previously.-

 

 

 

Edited by Spinneyhill (see edit history)
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post-47871-143142620111_thumb.jpgThe 1928 Commander GB and EW cars had 3.31 ratio as standard, 3.09 available as an option.  These cars has the slow-turning old 6 cylinder engine, but the Victoria coupes and small sedans could handle the lower ratio for decent speed.  The Studebaker Indy cars of 1931-33 used these axles with 3.09 ratio to go 120-140 mph on 18" or 19" tires while turning 4000-4400 rpm.  Now all you have to do is find one of these axles and fit it to your car.  A more-modern Dana 44 axle might also fit and provide a choice of ratios.

 

On the other hand, an overdrive transmission might be all you need to cruise at 55-65 mph.  Jerry Kurtz might be able to convert your transmission.  The Warner overdrives had a 0.7 ratio, so even a high numerical ratio axle delivers decent speed.  My 1948 M5 truck with 4.82 rear end cruises nicely at 60-65 mph with overdrive.  

 

Edited by Gary_Ash (see edit history)

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9 hours ago, Gary_Ash said:

post-47871-143142620111_thumb.jpgThe 1928 Commander GB and EW cars had 3.31 ratio as standard, 3.09 available as an option.  These cars has the slow-turning old 6 cylinder engine, but the Victoria coupes and small sedans could handle the lower ratio for decent speed.  The Studebaker Indy cars of 1931-33 used these axles with 3.09 ratio to go 120-140 mph on 18" or 19" tires while turning 4000-4400 rpm.  Now all you have to do is find one of these axles and fit it to your car.  A more-modern Dana 44 axle might also fit and provide a choice of ratios.

 

On the other hand, an overdrive transmission might be all you need to cruise at 55-65 mph.  Jerry Kurtz might be able to convert your transmission.  The Warner overdrives had a 0.7 ratio, so even a high numerical ratio axle delivers decent speed.  My 1948 M5 truck with 4.82 rear end cruises nicely at 60-65 mph with overdrive.  

 

Where did you find that 3.09?  Hens teeth.

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Rex:  No, I don't have a 3.09 gear set, only the 3.31 gears.  I think that may limit the top speed of my Indy car replica to about 110-120 mph.  😁

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If memory serves me, I believe  there were some after market gear sets made in the 3.5 ratio range. I believe that they were made for the bigger 30's cars including the Pierce Arrow. I  remember the 3.31 ratio that Gary has used, was standard in the last of the Big 6 Commanders in 1928. That setup with that engine and trans combination was simply amazing. Too bad that it was the last of it's kind.

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Hi, All;   Commander Dave here. I have a 1927 Commander sport roadster currently with a 3.31 diff. gear ratio. Not sure if that was stock for the roadster or the 3.07. However at one time in the sdc forum there was a formula to determine the rpm at a certain speed by using the tire height if the final drive ratio is 1 to 1.  IF-- I figured it right   not to sure about that though    with my 3.31 axel the rpm at 50 mph would be 1,737      at 60   2,079    at 70  2,432 . Book says max h.p. is at 2,400,  most of us know about power curves. Therefore if these calculations are right this car should be good for close to 75 m.p.h. The old big six has a stroke of 5 in. If any of this is wrong please correct me, that is if this is even worth responding to.       Rest assured this car with worn engine will go faster than I'll ever take it. Happy motoring, Commander Dave

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7 hours ago, commander Dave said:

with my 3.31 axel the rpm at 50 mph would be 1,737

That equates to a 32" diameter tire?

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When I did my similar calculation, I measured the radius from the centre of a hub dust cover to the floor, because the loaded height is less than the unloaded radius of the inflated tire. I can't remember the difference, perhaps an inch.

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Spineyhill   Yes. I measured from floor to top of tire taking into consideration the hub sticking out a bit .This was taken with the tire on the car with the weight of the car on tire of course, and even looked like the tire needed a little air. There is room for a little error here, but I think it is still fairly realistic.   This is all considering I did my math right; my weakest subject in school.  Commander Dave

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For what it may be worth:  The top speed numbers are theoretical without calculating wind resistance.  Fairly low hp (by modern standards) engines will use a LOT of power overcoming wind resistance. 

 

For me, a more practical calculation is that of safe and comfortable (for the engine) cruising speed.  For my Babbitt-bearing long-stroke cars, it seems that a safe and comfortable cruising speed is about 70% of published redline rpm for each engine, and that is substantially below the peak horsepower rpm.

 

Google the McCullough formula for converting rpm to road speed, which was in a regional CCCA online pub some years ago (so look for that).  I have the actual formula on another computer and can post it if you can't find it.

Edited by Grimy
correct typo (see edit history)

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It is pretty simple mathematics to convert. Let's work out our own formula.

 

Engine RPM x 60 = RPH (per hour).

 

Drive axle rotations = RPH/(diff. ratio x top gear ratio) noting that the top gear ratio is usually 1:1 but may be less if an OD is fitted.

 

Distance travelled per axle rotation = pi x running diameter of tire (D). Pi = 3.1412

 

So distance travelled in one hour = Drive axle rotations x distance per rotation = RPM x 60 x pi x D /(Diff. Ratio x gear ratio).

 

If we measure tire diameter in inches, we need to correct to miles. Noting that the distance travelled in one hour is the same as the speed,

 

speed (mph) = RPM x 60 x pi x D                             

                            Diff. Ratio x gear ratio x 12 x 5280

 

                        =  RPM x 5 x pi x D                     

                           Diff. Ratio x gear ratio x 5280

Now cancel 5 into 5280 and we get

 

speed MPH = RPM x pi x D                            

                         Diff. Ratio x gear ratio x 1056

We can even remove pi because we know what it is. Thus

 

speed MPH = RPM x D (inches)                 

                         Diff. Ratio x gear ratio x 336

 

And rearrange if we want to know RPM at a given speed.

 

RPM = speed MPH x diff. ratio x gear ratio x 336

                                 D (inches)

 

This is dead easy to feed into a spreadsheet. Note that the horrible constant 336 has units; it converts from minutes to hours and from inches to miles. A metric one would be different!

 

 

 

Edited by Spinneyhill (see edit history)

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To be accurate, you should really measure the distance traveled by one revolution of the tire. Just mark a radial line on the tire, push the car until the line is perpendicular to the ground and mark your starting point. Then push until you get one full rotation (when the mark is perpendicular again). Then measure distance traveled.

 

Measuring the tire radius from the ground to the center of the axle is not as accurate as the circumference of the tire doesn't change. However just using the circumference is not completely accurate as there is some very minor skidding that occurs during rotation and will slightly change the rolling circumference.

Scott

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