Old school speedometers

beetleman

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I am trying to figure out how old speedometers are matched to their drives, to provide correct speed readings.

For instance, with Dorcy, the face of the speedo indicates the wheel size it is intended for, i.e. 20'', 26''. In all the sets I found online, regardless of what the face indicates, the drives are the same - the 26 hole, 2.5'' diameter ones, with no stamping on them. So Dorcy made the adjustment to wheel size in the mechanism of the speedo head rather than the drive?
s-l1600 (1).jpgs-l1600 (9).jpgs-l1600 (2).jpg

I found a speedo by Cycle that has 20 stamped on both the speedo face and the drive. Note how this (slightly smaller) drive has 20 holes (not 26):
s-l1600 (4).jpgs-l1600 (5).jpg

So now I'm thinking wait, do the number of holes in the drive indicate the size of the wheel?
And to make things more complicated, I found a drive that is in fact stamped 24''-27'':
s-l1600 (3).jpg

Can anyone make sense of all of this? The Dorcy sets I mentioned are loose (no box) for sale online, so there's no real way to verify they're actually original sets and not just put together by a clueless seller. In fact some sellers even aren't ashamed to falsely state that these will "fit all wheel sizes"(which while they physically might, they won't provide correct readings).
 
I am trying to figure out how old speedometers are matched to their drives, to provide correct speed readings.

For instance, with Dorcy, the face of the speedo indicates the wheel size it is intended for, i.e. 20'', 26''. In all the sets I found online, regardless of what the face indicates, the drives are the same - the 26 hole, 2.5'' diameter ones, with no stamping on them. So Dorcy made the adjustment to wheel size in the mechanism of the speedo head rather than the drive?
View attachment 84553View attachment 84555View attachment 84554

I found a speedo by Cycle that has 20 stamped on both the speedo face and the drive. Note how this (slightly smaller) drive has 20 holes (not 26):
View attachment 84557View attachment 84558

So now I'm thinking wait, do the number of holes in the drive indicate the size of the wheel?
And to make things more complicated, I found a drive that is in fact stamped 24''-27'':
View attachment 84556

Can anyone make sense of all of this? The Dorcy sets I mentioned are loose (no box) for sale online, so there's no real way to verify they're actually original sets and not just put together by a clueless seller. In fact some sellers even aren't ashamed to falsely state that these will "fit all wheel sizes"(which while they physically might, they won't provide correct readings).
I think the gearing/dia is the indicator of wheel size to be used with it,the one that is 24-27" maybe accurate with one of the sizes but witch one lol,those types are not known to be accurate anyway rather just close to give an estimated speed.The Dorcy's may well have different ratio's in the head to work off the same drive.
 
If it's according to the drive size, why would they bother to stamp the heads with different wheel diameters? The fact that they do proves that the heads are different internally.
 
I am trying to figure out how old speedometers are matched to their drives, to provide correct speed readings.

For instance, with Dorcy, the face of the speedo indicates the wheel size it is intended for, i.e. 20'', 26''. In all the sets I found online, regardless of what the face indicates, the drives are the same - the 26 hole, 2.5'' diameter ones, with no stamping on them. So Dorcy made the adjustment to wheel size in the mechanism of the speedo head rather than the drive?
View attachment 84553View attachment 84555View attachment 84554

I found a speedo by Cycle that has 20 stamped on both the speedo face and the drive. Note how this (slightly smaller) drive has 20 holes (not 26):
View attachment 84557View attachment 84558

So now I'm thinking wait, do the number of holes in the drive indicate the size of the wheel?
And to make things more complicated, I found a drive that is in fact stamped 24''-27'':
View attachment 84556

Can anyone make sense of all of this? The Dorcy sets I mentioned are loose (no box) for sale online, so there's no real way to verify they're actually original sets and not just put together by a clueless seller. In fact some sellers even aren't ashamed to falsely state that these will "fit all wheel sizes"(which while they physically might, they won't provide correct readings).
I'm curious about this also. In the smaller drive with 20 holes, what is the number of teeth in the gear, and what is the final ratio in terms of wheel rpm ->cable rpm?
How does this ratio compare to the ones with 26 holes?
BTW I ran across a formula for many automotive speedometers, and I wonder if it applies to the design of any of the bicycle heads.
 
If it's according to the drive size, why would they bother to stamp the heads with different wheel diameters? The fact that they do proves that the heads are different internally.
Normaly the head doesn't have any gearing it's all at the drive,But since the Dorcy's use the same drive there must be somthing in the head to alter its ratio to match wheel size.
 
Normaly the head doesn't have any gearing it's all at the drive,But since the Dorcy's use the same drive there must be somthing in the head to alter its ratio to match wheel size.
On non-bike speedometers, and on my newer "sunlite" cheapo china speedometer, there is a worm drive in the head that spins the magnet very fast.
EDIT: I said that wrong. I believe the cable directly drives the magnetic speed cup, and the worm drive is a REDUCTION for the odometer. e.g. each time the worm completes a revolution, the odometer gear would move one tooth.
 
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Let's assume all the drives use the same size gear to drive the cable from the wheel. In that case, they could let 20 holes be for a 20" wheel, and 26 holes be for a 26" wheel since circumference is proportional to diameter.
If that were the case, I believe, as beetleman said, life would be great. The speedometer heads could all be designed the same, and there would be no need to put the size on the head. You'd just need the proper size drive gear for your bike wheel.
Since this makes so much sense from a manufacturing pov, the most logical explanation is that the parts have been mixed up in the loose parts box.

I hope other folks chime in with whatever examples or data they have. I'd really like to know for sure how this works and how standardized it is. Here is a link about how to choose a final drive gear for many automotive speedometers. In the US, the speedometer heads themselves seem to be quite standardized. Here's the formula (For GM, Ford, Chrysler):
(Number of drive gear teeth x axle ratio x tire revolutions per mile) divided by 1,001 = the number of teeth needed on the driven gear for accurate speedometer reading.
 
I have a new, sealed box of a Dorcy speedometer:
s-l1600 (6).jpg

I just opened it myself, so I know for sure it is original from the factory.
The face says 26, the drive has no markings but it's the regular 26 hole 2.5'' wide drive.

And there's an instruction sheet with it:
IMG_6511.JPG

So as you can see in the spare parts list at the bottom, there are 2 "Flexible Cable Assembly", 2 "Head Assembly", 1 "Drive Unit Assembly", and 1 "Washers and Bushings".
So we can deduct that both versions of speedometers used the same "Drive Unit Assembly"! Which must mean that the ratio difference is in fact determined in the speedo head!
 
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So we can deduct that both versions of speedometers used the same "Drive Unit Assembly"! Which must mean that the ratio difference is in fact determined in the speedo head!

Yep, that makes sense. So, on the 26 hole "Drive Unit Assembly", how many teeth are on the gear that the 26 hole ring engages with? Or if you can't see that without disassembly, how many revolutions does the cable make for one complete revolution of the drive assembly/bicycle wheel?
I'm curious if my Sunlite drive assembly is the same ratio.
 
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