The Stovebolt Site Forums The Shop Area Driveline Rear Axle Ratio of 1.625?

I’m disassembling my truck (1955 2nd 3100) for rebuilding/restoration. It hasn’t run for about 20 years. It’s a modified truck with a hopped up mid 1960s 327, Saginaw 3-spd, and an early ‘60s rear axle. Thought that I’d Jack it up, spin the wheel and see what the axle gear ratio is. The axle has a posi tag on it but upon spinning the wheel, it’s not a posi…bummer.

Here are the numbers on the rear axle. L143, 3725899, GM34
3725899 = casting number, used from ‘55 to ‘64.
L143 = date cast, L - December, 14 - 14th, 3 - 1963
I’ve read that typical ratios for this axle were 3:70 or 3:36.

I was expecting 3:70 or 3:36 but the result was 1 rotation of the wheel/tire equaled 1 & 5/8 rotation of the pinion. A ratio of 1.625 ??? Can that be???

I was certain that I was doing this wrong, so read some tech articles, YouTube videos and then spun the wheel again, 4 more times. Same results. Am I doing this correctly?

-Mike

3.90:1 would be common.

If you've only got one wheel jacked up, the spider gears double the speed of the ring & pinion. One way to get a more accurate guesstimate of the rear end ratio is to roll the vehicle forward with both wheels on the ground one turn of the rear wheel and count the number of turns the driveshaft makes. The best way is to drop the rear cover plate and look for the tooth count stamped on the ring gear- - - -10-39 for instance, would be a 3.90:1 ratio. 9-37 would be a 4.11:1, etc.
Jerry

Actually, you need to turn the wheel TWO revolutions to figure rear end ratio. The pinion speed is halved turning one wheel with the other on the ground.
If you came up with 1.625 turning the wheel once, then your ratio is 2 x 1.625 or 3.25. So given the 3.70:1 or 3.39:1 choices, you have the latter.

Hmmm…. I think I see what’s happening.
I had both wheels off the ground but,….there was some friction/drag on the left side wheel. When I would rotate the left wheel by hand, the right wheel rotates the opposite direction. Ok, that’s expected (the posi tag is obviously a ruse). But when I rotate the right wheel by hand, the left wheel doesn’t rotate. I did the ratio check while rotating the right wheel by hand and because the left wheel wasn’t rotating, it was as if it was still on the ground?
In the past I have only ever done this type of check with both wheels off the ground and didn’t know that with one wheel on the ground, two revolutions were required. Lesson learned.
I’m going to pull the drums today to remove the e-brake cables. Hopefully it’s just the brakes that are causing friction/drag in the left wheel. If the left wheel frees up, I’ll do the gear ratio check again. Then it’s off to the media blaster….to see what remains when I get it back. 😳
Thanks,
-Mike

I think Kevin and I are saying the same thing with slightly different semantics. When we did "on the car" wheel balancing back in the 1960's, one of my first jobs at Dad's shop was "driving" the car while one of his mechanics did the balancing. With one rear wheel off the ground, I'd hold a steady 30 MPH on the speedometer- - - -which ran the wheel at 60 MPH due to the multiplier effect of the differential gears. That's something to remember if you're ever trying to "rock" a vehicle out of a mudhole or a snow bank- - - -If only one wheel is turning, the spinning wheel is traveling twice the speed that's showing on the speedometer!
Jerry

Jerry, very true.

I had to think for a long time to be sure I had it right. Your analogy of the on-car balancing is correct. The wheel that's spinning is indeed spinning at twice the speedo reading.

I've seen people stuck on ice on a flat parking lot where one rear wheel is spinning and they probably have the speedo at 60 MPH. They'll eventually melt thru the ice and start burning rubber, or possibly snap something in the drivetrain. The old trick of gently applying the brakes on an open rear end while easing into the throttle to get moving is beyond their capabilities.

I hear you barking and agree/understand.

I jacked it up again today and removed the brakes & E-brake cable. The left wheel turns freely now. Apparently, as you can see in the photo, the previous owner didn’t believe in re-lining the brake shoes!!!!
Anyway, I’m not sure what’s up except that the axle will need some TLC…rebuilding. When I rotate either wheel, the opposite wheel doesn’t rotate at all. However, when I grab and rotate the pinion, both wheels turn in the same direction for a few revolutions and then one of them will stop. Rotating the pinion in either direction, clockwise or counterclockwise, yields the same result….both wheels spin in the same direction for a little bit, then one of them stops rotating. This is different than what happened yesterday went the left wheel was dragging.
Oh well, it’s off for blasting and metal work and I’ll tear into the axle after it returns.
Thanks,
-Mike

LOL, there is a LITTLE bit of brake lining there, just not connected to the shoes.

The wheel spinning/pinion spinning exercise may not indicate a problem in the differential, unless it's a posi axle with a problem inside. You'll find out when you remove the cover.

A little update.
I pulled the rear axle (a while ago). Turns out that the posi tag was ruse and the ratio is a 3:36. Finally getting around to disassembly, cleaning and rebuilding as necessary. The insides look fine, so off to good start.

Found a couple of kinks in the frame due to a front end accident sometime in the truck's lifetime. :-( Not happy about that and it's at a local truck frame straightening shop. Their specialty is big boy trucks/rigs but he does antique car/truck frames when things are slow. He just finished a '55 truck frame for his buddy and now he's finishing up mine. Not a bad price either, $750 to ensure that it will go down the road straight. :-)

-Mike34

According to the photo you posted, it has a 5 lug bolt pattern which was not available in a truck. 57 through mid 60's cars used a
3.36 rear axle ratio with an automatic transmission. That's likely what you have.
Fred

Fred you are spot on. I mentioned in the 1st post that nothing in the drive train of this truck is original. Decoding the rear axle casting numbers shows that it is from a full size (Belair or other) and the axle was manufactured in Dec 1963, so it would have been installed in a '64 model year. Hopefully will get it back onto the frame in a week or two. I need to get this project back into storage so that I can get my daily driver into the garage for the winter!

Mike