Rear Axle Ratio?

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David R.
Posts: 82
Joined: August 17th, 2019, 1:52 pm
Body Type: Chassis
Model Year: 1930
Location: WV

Rear Axle Ratio?

Post by David R. » November 15th, 2019, 2:13 pm

Bevel gear rear axle: just about 3 1/4 turns of pinion shaft turns brake drum one turn. Is that a 6.6:1?

vonheine
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Joined: September 30th, 2009, 8:02 pm
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Model Year: 1930
Location: Massachusetts

Re: Rear Axle Ratio?

Post by vonheine » November 16th, 2019, 5:26 pm

(What you measured would be a ratio of 3.25:1) The standard Ford AA spiral bevel is a 6.6:1 ratio because it has 33 teeth on the ring gear and 5 teeth on the pinion. 33 divided by 5 equals 6.6

The high-speed Ford AA spiral bevel ratio is 5.14:1 because it has 36 teeth on the ring gear and 7 teeth on the pinion. 36 divided by 7 equals 5.14

The high-speed spiral bevel gear rear-end originally had a tag on one of the rear-end case bolts marked 7-36 indicating the quantity of teeth. This tag may not be present, but I have also seen 7-36 stamped on the edge of the case.

The gear ratio is always stamped on the face of the speedometer gear. You can look at it by removing the cable and gear from the torque tube.

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tiredtruckrestorer
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Model Year: 1931
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Re: Rear Axle Ratio?

Post by tiredtruckrestorer » November 16th, 2019, 5:53 pm

David,
Yes, you probably have a 6'6 ratio low speed. Try counting again and only leave the drum on one side turn. Either lock up the brake on one side, or if the wheels and tires are on leave it down so one side touches the ground and doesn't turn. The spider gears in the rear will mess up your count if you don't'.

Keith

David R.
Posts: 82
Joined: August 17th, 2019, 1:52 pm
Body Type: Chassis
Model Year: 1930
Location: WV

Re: Rear Axle Ratio?

Post by David R. » November 16th, 2019, 5:58 pm

I know and I have been scratching my head trying to figure out the math. For the rear axle in my truck, (5.14:1), with transmission in high gear, (1:1) it takes about 2 1/2 turns of the hand crank to turn the rear wheel one revolution.
On this spare axle I have, which I assume is 6.6:1, it takes about 3 1/4 turns of the pinion shaft to turn brake drum one turn.
So it looks to me like it takes about half as many turns as it should for the stated ratio? I haven’t figured it out yet. Something to do with differential gearing?

David R.
Posts: 82
Joined: August 17th, 2019, 1:52 pm
Body Type: Chassis
Model Year: 1930
Location: WV

Re: Rear Axle Ratio?

Post by David R. » November 16th, 2019, 6:01 pm

I did block one wheel in both instances. And chalked the tire and drum and counted carefully. I still haven’t figured it out, (nor has my engineer son).

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Neil Wilson
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Body Type: 82-A/89-A
Model Year: 1930
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Re: Rear Axle Ratio?

Post by Neil Wilson » November 17th, 2019, 5:53 am

David R. wrote:
November 16th, 2019, 5:58 pm
I know and I have been scratching my head trying to figure out the math. For the rear axle in my truck, (5.14:1), with transmission in high gear, (1:1) it takes about 2 1/2 turns of the hand crank to turn the rear wheel one revolution.
On this spare axle I have, which I assume is 6.6:1, it takes about 3 1/4 turns of the pinion shaft to turn brake drum one turn.
So it looks to me like it takes about half as many turns as it should for the stated ratio? I haven’t figured it out yet. Something to do with differential gearing?
Based on what you are doing, the number of turns are doubled to yield the gear ratio. So, "about 2-1/2 turns" = 5.14:1 gears and "about 3-1/4 turns" = 6.6:1 gears.

Note that the speedometer gear stamping, axle housing stamping, or axle housing tag can be used to determine the original axle gear ratio. Original gears could have been changed over the years.
Regards, Neil Wilson
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neil@aafords.com - use this email for contact
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