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Old 08-30-2015, 05:11 PM   #1
bunnyc
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Default Rear wheels turn different RPM

Have my '29 Tudor up on jacks. Wheels and tires are not mounted.
Running the engine @1290 +/- 10 RPM in 3rd gear.
I have a laser tachometer I like to play with. The right drum turns at 334 RPM, the left at 133 RPM.
Brake drag doesn't seem to be a factor, I have the rods disconnected and the adjusting wedge backed out. Both drums read 84 deg after 5 minutes running, which is the present OAT.

Does the difference in RPM sound normal? I admit to ignorance. You can't hurt my feelings.
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Old 08-30-2015, 05:16 PM   #2
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Default Re: Rear wheels turn different RPM

If it were mine, I would reassemble and adjust the brake linkages Then I'd put the car on the ground and drive it.

You don't have a locked differential. The rear axles will spin at different rpms.
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Old 08-30-2015, 05:23 PM   #3
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Default Re: Rear wheels turn different RPM

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Originally Posted by Blessyouboys View Post
If it were mine, I would reassemble and adjust the brake linkages Then I'd put the car on the ground and drive it.

You don't have a locked differential. The rear axles will spin at different rpms.
Thanks. I learned something from you.
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Old 08-30-2015, 05:45 PM   #4
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Default Re: Rear wheels turn different RPM

With an open differential, bearings could be tighter on one side than the other allowing them to spin at different rpm as BYB indicated.
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Old 08-30-2015, 05:49 PM   #5
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Default Re: Rear wheels turn different RPM

I would not worry about that if you do not hear any strange noise. Turn by hand and listen as well, and see if you can feel any difference.
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Old 08-30-2015, 06:16 PM   #6
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Default Re: Rear wheels turn different RPM

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Originally Posted by PC/SR View Post
I would not worry about that if you do not hear any strange noise. Turn by hand and listen as well, and see if you can feel any difference.
No, I don't hear any strange noises. Just the wonderful sound of my engine at the tail pipe!
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Old 08-30-2015, 06:27 PM   #7
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Default Re: Rear wheels turn different RPM

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Originally Posted by Blessyouboys View Post
...You don't have a locked differential.
Boy wouldn't THAT be fun trying to go around corners with the tires turn the same speed...
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Old 08-30-2015, 06:54 PM   #8
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Default Re: Rear wheels turn different RPM

I am no master mechanic but simple differentials are designed so the power from the engine is sent to only one wheel at a time. The other just turns do to internal friction.

When you go around a corner the outside wheel has to travel a greater distance, so the wheels must be more or less independent of each other, yet designed so that either wheel can be the driven wheel. The driven wheel is usually the one that has the least resistance.

Hope that helps a little..
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Old 08-30-2015, 07:12 PM   #9
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Default Re: Rear wheels turn different RPM

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Originally Posted by Y-Blockhead View Post
Boy wouldn't THAT be fun trying to go around corners with the tires turn the same speed...
Yeah, you're right.
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Old 08-30-2015, 08:08 PM   #10
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Default Re: Rear wheels turn different RPM

Power from the engine rotates the drive shaft that rotates the pinion gear attached to the end of the drive shaft. The pinion gear drives the ring gear that is attached to the carrier assembly. Inside the carrier assembly are the gear ends of the two axles in mesh with the three spider gears.

When you are going straight down the road the pressure from the tires holds the two axles evenly to the spider gears and hence to the carrier. Both wheels are then driven evenly. When you go around a corner in either direction, the pressure on the tires changes and it allows the spider gears to drive each axle at a different speed. The guy that invented it had to be a genius.

I have taken a number of rear axle assemblies apart where some kid with a duck-tail haircut back in the 1950's spot welded the three spider gears to their hubs so that it locked the two axle together permanently. This was known as a poor man's posit-traction. It made it easy to lay a strip of rubber out in front of the high school to impress all the girls. The only problem was that when you went around a corner the outboard tire chirped.

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Old 08-30-2015, 08:14 PM   #11
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Default Re: Rear wheels turn different RPM

Thanks Tom.
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Old 08-30-2015, 10:27 PM   #12
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Default Re: Rear wheels turn different RPM

This has been posted here before but this will save you a search;

How a differential works
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Old 08-30-2015, 11:08 PM   #13
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Default Re: Rear wheels turn different RPM

Quote:
Originally Posted by Mike V. Florida View Post
This has been posted here before but this will save you a search;

How a differential works

OR......... You could watch the movie "My Cousin Vinny" which explains it all. Those of you that have seen the movie know what I'm talking about.
I still laugh after seeing it a thousand times.
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Old 08-31-2015, 10:57 AM   #14
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Default Re: Rear wheels turn different RPM

I believe the differential was first used in China in building the "South pointing chariot". That was over 3000 years ago. Those guys were smart.
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Old 08-31-2015, 11:15 AM   #15
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Default Re: Rear wheels turn different RPM

For years, I have noticed on most all of my cars, that the right rear wheel will break traction before the left rear wheel will. Also while running on jack stands, the right rear wheel will spin faster than the left rear wheel. I have accepted this phenomenon as normal. I think why the right rear wheel will break traction first is because of the lift that is given to it from the direction of twist from the drive shaft? While on jack stands, the reason why the right rear wheel spins faster than the left needs to be thought out. Drag is the only thing that comes to mind to give different spin speeds.

Last edited by SSsssteamer; 08-31-2015 at 11:23 AM.
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Old 08-31-2015, 01:21 PM   #16
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Default Re: Rear wheels turn different RPM

Quote:
Originally Posted by Idaho Brett View Post
OR......... You could watch the movie "My Cousin Vinny" which explains it all. Those of you that have seen the movie know what I'm talking about.
I still laugh after seeing it a thousand times.
Ha, ha...yeah....lovely Marisa explains Positraction
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Old 08-31-2015, 02:46 PM   #17
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Default Re: Rear wheels turn different RPM

Here's the clip from the movie where she explains it:
https://www.youtube.com/watch?v=6qgREMBTE28
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