Speedometer drives and confusion.

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Speedometer drives and confusion.

Postby bobscogin » Mon Jun 10, 2019 9:48 am

I'm scratching my head over the difference in the 18/19" wheel and 16" wheel speedometer drives. My '63 Scat has 18" wheels and the drive gear on the front hub has 23 teeth, and my '63 and '65 Pacers with 16" wheels has 21 teeth. All of them have the same 9 tooth driven gear. Is this correct? If you look at it terms of the math, it works out. The difference in revolutions per mile between the 16" and 18" wheels is about 8 %, and the difference in the drive's gear ratios (2.33:1 and 2.55:1) is about 9% so the readings would be correct. Can anyone confirm that the 9 tooth driven gear is correct for both the 21 and 23 tooth drive gear, or is there another driven gear?

Bob
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Re: Speedometer drives and confusion.

Postby Don G » Mon Jun 10, 2019 1:20 pm

Crossing my fingers for an answer Bob.
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Re: Speedometer drives and confusion.

Postby Mutt » Mon Jun 10, 2019 3:13 pm

The 16" gear has a different part #., so it's definately not the same gear

Just ask a question:

Who that is looking at this forum owns a 1961 Super 10 or a 1962-65 Pacer? Can ya pull your speedo cable out and measure the gear diameter?
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Re: Speedometer drives and confusion.

Postby bobscogin » Mon Jun 10, 2019 4:10 pm

My Scat and the 2 Pacers all had a 9 tooth driven gear. I'll measure it and post. The drive gears measure as follows: OD of 21 tooth gear on the 16" wheel =1.933. OD of 23 tooth gear on the 18" wheel =1.906. In order to measure them I clamped a hose clamp around them so that my dial calipers wouldn't fall between the teeth, and then subtracted the clamp thickness, so I think my measurements are pretty close. It's odd that the gear with 21 teeth is actually a hair larger than the 23 tooth gear. I'll measure the driven gears I have and report back.

Bob
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Re: Speedometer drives and confusion.

Postby bobscogin » Mon Jun 10, 2019 5:57 pm

All 3 driven 9 tooth speedometer gears that I have measure about .513".

Bob
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Re: Speedometer drives and confusion.

Postby Mutt » Mon Jun 10, 2019 7:02 pm

are all the teeth the same thickness?
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Re: Speedometer drives and confusion.

Postby bobscogin » Mon Jun 10, 2019 8:09 pm

Yes, the teeth on all 9 tooth driven gears appear to be the same pitch/thickness. They mesh and rotate the speedo cable on both 21 and 23 teeth drive gears. Referring back to the OD of the drive gears, 1.933 (21T) and 1.906 (23T), that's a .027 difference on diameter, or .0135 on the radius. So, can the 9 tooth gear run on the smaller drive gear and just have .0135 less depth of tooth engagement than it does on the larger diameter gear?

Bob
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Re: Speedometer drives and confusion.

Postby Backyard Bob » Mon Jun 10, 2019 10:15 pm

Forgive me gentlemen but my experience says X # of teeth engaging X # of teeth= input vs. output rotation. Tooth thickness vs. tooth thickness is where the tolerances come in. My experience with HD is that they produced motorcycles for general consumption and didn't worry about close ratio fitting except for their Factory racing machines. Have you ever looked at the factory porting of any of their motors? They didn't even bother to grind out the casting flashing. I've bumped performance with a simple Dremel tool. So, would they worry about tooth thickness on a HD Lightweight? Feel free to beat me up on this.
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Re: Speedometer drives and confusion.

Postby Fatboyfreddy » Mon Jun 10, 2019 10:56 pm

Wow. I just wish I knew what the hell you alltalking about. I’m always amaz3d at the knowledge of people on here. I’m lucky to figure out how to turn my phone on or even to answer it!
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Re: Speedometer drives and confusion.

Postby bobscogin » Mon Jun 10, 2019 11:04 pm

I'm given to understand that there's another driven gear with more teeth than 9. 13 teeth I believe? That large a driven gear would throw the ratio pretty far off of what you'd get with a 9/21 or 9/23 combination. I'm beginning to think that the difference in the wheel sizes at 8% and the difference in the 9/21 and 9/23 gear pairs at 9% is not coincidental. I'm going put a little modeling clay between the teeth of each drive gear, roll the wheel a couple of revolutions and check the tooth engagement depth on each. Will report my findings.

Bob
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