Calculate rear end ratio

mrtexas

mrtexas

Well-known member
I jacked up one wheel and rotated it 10 times. The driveshaft rotated 19 times. What is my rear end ratio? 36 ford woodie.
 
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You do not say what you are working on. If it is a 33-48 Ford car or PU, check the bottom rib on the banjo differential housing, the number of pinion teeth and ring gear should be stamped there. You could also drain the lube, and pull the fill plug, turn one of the wheels with a rag stuck in the hole against the ring gear to wipe as much grease off it as possible, mark a tooth with a piece of chaulk and count the ring gear teeth while having someone turn a wheel. Post the ring gear count and we can tell you what you have.
 
Turn the wheel twice, count the driveshaft turns, that is the axle ratio.

Turn the wheel twenty times, divide the driveshaft turns by ten and that is your ratio.

In this case, he turned the wheel ten times and got 19 driveshaft revs.

Double both numbers, twenty wheel turns and 38 shaft turns.

Divide by ten and you get 3.8.

Knowing the available ratio's, 3.78 is the nearest fit.

I have a video on youtube that will make it as clear as mud.

http://youtu.be/bkvp3Fq21ZM

Mart.
 
all you have to do is keep one wheel on the ground ant turn the other wheel 2 complete turns while a friend or you count how many turns the drive line makes, then guess how far it turns after the full turn, if it turns 3 1/2 turns its a 3:50 to 1 if it turns 3 3/4 its a 3:75 to one you can get real close, this only works on open drive lines
 
Tricky devil...
Your ratio computes out to a tad less than 2 to 1...but having one wheel left touching the ground halves the ratio. Double that, and closest ratio below 4 that was available in 1936 was 3.78. If you had gotten a bit over 20 instead of 19, the likely suspect would have been 4.11.
Common (there were others)stock early Ford ratios with various applications '32-48 were 3.54, 3.78, 4.11, and 4.54. There were MANY others out there, stock, foreign, and aftermarket, but those are the likely suspects.
 
Bruce Lancaster said:
Tricky devil...
Your ratio computes out to a tad less than 2 to 1...but having one wheel left touching the ground halves the ratio. Double that, and closest ratio below 4 that was available in 1936 was 3.78. If you had gotten a bit over 20 instead of 19, the likely suspect would have been 4.11.
Common (there were others)stock early Ford ratios with various applications '32-48 were 3.54, 3.78, 4.11, and 4.54. There were MANY others out there, stock, foreign, and aftermarket, but those are the likely suspects.
Click to expand...

When I lifted both wheels off the ground and repeated I got the same number of turns of the driveshaft. ???
 
Mart said:
3.78:1

Should I say any more??

Mart.
Click to expand...

That is nice. I expected a 4:11 ratio as station wagons usually have high ratios. I've had a couple wagons over the years and the rpms were always higher than the comparable sedans that I also owned.

I looked but couldn't find the stamped rear end ratio number supposed to be on the bottom of the rear end. I wanted to check as well as over 75+ years no telling how many times the rear end might have been changed.
 
mrtexas said:
I looked but couldn't find the stamped rear end ratio number supposed to be on the bottom of the rear end. I wanted to check as well as over 75+ years no telling how many times the rear end might have been changed.
Click to expand...

Mr Texas, Not just the bottom, but the bottom rib. This will be found just forward of the banjo, at the bottom of the pinion bearing location. Still, if not there, some banjos reportedly were not stamped. At best, this will tell you what the original ring & pinion was within the banjo, which may have been changed out at some time. You already have the answer you need, anyway.
 
And...I do believe I got the wheel situation backward: Both wheels turning (one will contrarotate to further astound you) gives 1/2, one wheel on ground gives correct. Tired, not enough coffee, so I better totally disassemble the thing and count teeth...
 
Bruce, it's much easier to count the teeth if you take out your plate and hold it in your hand. Mirrors and fingers don't work that good.:) If all else fails, ask your dentist.
 
Here you go, you can see the numbers stamped in the center rib. 9 37
H36
P1020877.jpg


This photo turned 90 degrees.
P1020879.jpg


 
32phil, good point! Made me think, hurt my head. Mr Texas has another thread here showing his rolling frame.;)
 
Bruce, drink the coffee, my friend.

If both wheels are free and contrarotating (at equal speed) you would have zero driveshaft revs. Any movement of the other wheel spoils the calculation.

Guys, you can use a variation of this technique even in a fully built car, but a starting handle would be a big advantage.

Jack up one rear wheel. Put the trans in top. Turn the starting handle to a convenient position and hold it there. mark the rear tire and the ground, or some other convenient place.
Turn the crank until the raised wheel has done two full turns. Count the crank rotations and the number of turns will equal the axle ratio.

I described this method via pm to a barner and he reported back that it worked well.

Mart.
 
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