Teach me about differentials.

N

nosoup4u

Senior Member
Got a car. Actually more of s speedster but it has a model A rear axle and brakes. Last time I drove it, I let off the clutch, and nothing happened. Messed with the clutch linkage a bit, though I think it was fine and tried again and nothing. Then for some reason I pumped the brakes and it took off and I drove it around slowly a bit.
Anyway in trying to figure out what happening I realized I wasn't sure how a differential is supposed to act. With one wheel off the ground and the ****** in neutral, I can turn the rear wheel but the drive shaft doesn't turn where you can see it at the speedometer gear hole on the torque tube. It should, shouldn't it?
With the wheel off the ground, and the ****** in gear, if I turn the engine with the crank the rear wheel turns.
What on earth is going on?
 
I suspect you have a broken axle or keyway. The open diff in a Model A sends the power to the wheel with the least traction. A broken axle has zero traction, so that's the side that will spin. When you tapped the brakes, you equalized the traction on the two sides, and the side with the good axle started turning.
 
Checked one keyway this morning before work. I'll check the other later. Are broken axles common on model A's? The engine I am using actually has less power than the original A engine.
 
Breakage is not real common but it can happen. These cars are old and depending on the individual car's use, there could be fatigue in the metal.

The pinion gear is tapered & keyed to the driveshaft as well so there can be problems there too. There could also be a sheared key on an axle.
 
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Neither axle key seems to be sheared. It's supposed to be at a show this weekend, so I guess it is going to be a trailer queen. Then I guess I will be doing exploratory surgery.
 
nosoup4u said:
With one wheel off the ground and the ****** in neutral, I can turn the rear wheel but the drive shaft doesn't turn where you can see it at the speedometer gear hole on the torque tube. It should, shouldn't it?
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Yes, it should.
 
redmodelt said:
RE the broken axle, if broke there is nothing holding the wheel/hub/brake drum to the rear end, and you could pull the broken stub along with everything else out of the housing.
There is more to this story I think and if this is the same rig might have something to do with the duel hi; https://www.fordbarn.com/forum/showthread.php?t=265427
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It's the same rig, however, something is disconnected between the rear wheel and driveshaft. It's not even turning the back of the dual high.
 
Would I be correct in guessing the driveshaft was shortened between the dual high and rear end, if so, wonder if the two ends were just welded and the weld has broke free?
 
Dual high? Is this a model A based speedster? It's starting to sound like a model T. Model A chassis generally use an overdrive of some sort.
 
This past weekend, I came across a similar case where the engine could not be turned by rotating the rear wheels in any gear. It turned out that the clutch was slipping.
 
Bob Bidonde said:
This past weekend, I came across a similar case where the engine could not be turned by rotating the rear wheels in any gear. It turned out that the clutch was slipping.
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I took the speedometer cable attachment off at the front of the torque tube, and I can see that the driveshaft is not turning when you turn the rear wheel. However, jack up either rear wheel and it will turn when you turn the engine.
 
nosoup4u said:
I took the speedometer cable attachment off at the front of the torque tube, and I can see that the driveshaft is not turning when you turn the rear wheel. However, jack up either rear wheel and it will turn when you turn the engine.
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When you spin the one rear wheel and notice that the drive shaft isn't turning, what is the other rear wheel doing? If it is turning in the opposite direction of the wheel you are spinning, that is normal. Think about it, the spider gears can spin on their bearings without the carrier turning. If you turn one axle and hold the carrier still (so that the ring and pinion and driveshaft don't turn) all the motion will be transferred to the other axle and it will spin opposite of the one you're turning.
 
Will N said:
When you spin the one rear wheel and notice that the drive shaft isn't turning, what is the other rear wheel doing? If it is turning in the opposite direction of the wheel you are spinning, that is normal. Think about it, the spider gears can spin on their bearings without the carrier turning. If you turn one axle and hold the carrier still (so that the ring and pinion and driveshaft don't turn) all the motion will be transferred to the other axle and it will spin opposite of the one you're turning.
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In the first post he said "with one wheel off the ground"
 
Not sure what all you have, as mechanical brakes can't be "pumped up".
Sure sounds like the rear end needs to come apart.
 
It would be helpful to know what Intermediate gearbox is in the drivetrain. Some of these units might contribute to a problem like this. A photo would be a start.
 
rotorwrench said:
It would be helpful to know what Intermediate gearbox is in the drivetrain. Some of these units might contribute to a problem like this. A photo would be a start.
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In post #8 he said he had a Dual High.
 

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First of all, I am checking if the driveshaft is turning at the speedometer cable "port" at the front of the torque tube, so the fact that it has a dual high really has nothing to do with it, other than perhaps something having to do with shortening the driveshaft went awry.
Anyway, yesterday I checked with both wheels off the ground, turning one wheel does not cause the other wheel to turn in the opposite direction. Turning the engine causes the front of the driveshaft to turn but not the wheels. Clearly something has gone to heck in the differential.
 
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