smiths chronometric tachometers

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smiths chronometric tachometers

Postby murph » Wed Sep 22, 2021 7:11 pm

Well to see if this can bring up an interesting conversation I know nothing about, and due to Dave's recent post in the craigslist finds let's see what we get.
I know for a fact that my restored 1962 xlch with late vin has an original chronometric nos tach from harley davidson on it because I bought it on ebay in its original box with all the mounting hardware. The RC# on the dial reads 1302/00.

Image

I also had an original paint 1962 xlch which was in my possession some time ago which also had the factory chronometric tach and interestingly enough the vin # was only off 6 digits from my restored 62. This also had an RC 1302/00

Now with all that said here is my original paint 1959 xlch that has the chronometric tach on it as well, yes I KNOW they never left the factory with the tach in 59 but the original owner did this prior to Harley Davidson and that's how the bike is staying. It's tach has an RC 83 on the dial. My question is What are the differences in the tachometers? My guess is ratio?

https://www.harleykmodel.com/gallery/19 ... index.html
Last edited by murph on Wed Sep 22, 2021 8:35 pm, edited 1 time in total.
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Re: smiths chronometric tachometers

Postby Ferrous_Head » Wed Sep 22, 2021 8:12 pm

I don't know a lot about tachos but pretty sure HD ran RSM's (Shallow Magnetics) on all road versions and RC's (Chronometrics) on race (XLR) bikes.. The Chronometric version (I've forgotten the number) is easy to spot as the dial sweep is upside down compared to the Magnetics.
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Re: smiths chronometric tachometers

Postby dr dick » Fri Sep 24, 2021 8:03 pm

murph wrote:Well to see if this can bring up an interesting conversation I know nothing about, and due to Dave's recent post in the craigslist finds let's see what we get.
I know for a fact that my restored 1962 xlch with late vin has an original chronometric nos tach from harley davidson on it because I bought it on ebay in its original box with all the mounting hardware. The RC# on the dial reads 1302/00.

Image

I also had an original paint 1962 xlch which was in my possession some time ago which also had the factory chronometric tach and interestingly enough the vin # was only off 6 digits from my restored 62. This also had an RC 1302/00

Now with all that said here is my original paint 1959 xlch that has the chronometric tach on it as well, yes I KNOW they never left the factory with the tach in 59 but the original owner did this prior to Harley Davidson and that's how the bike is staying. It's tach has an RC 83 on the dial. My question is What are the differences in the tachometers? My guess is ratio?

https://www.harleykmodel.com/gallery/19 ... index.html


a primer of escapements, hair springs, and timing of those:

cronos tachs dont have a 'gear ratio'.
instead they work by engaging/disengaging the indicator needle to the drive cable.
when the needle jumps (up or down) its connected directly to the cable rotations.
when the needle rests between jumps in disconnected from cable rotation.
so the when the connection time is great so is the needle sweep.
varying the connect-disconect time varies the needle sweep amount.
so given that you can print different background faces with different speed ranges.
print on the that face a code that says what the connect vs the disconect should be for a given cable rpm and you change the escapement to match.

escapment is time engaged vs time disengaged

so
for a given cable rotation rate (motor rpm) the tach needs to be calibrated to engage/disingage the needle rotation.
what cant be changed is that the needle rotation direction follows the cable rotation direction.

as far as the rotary orientation the face and needle thats easily set on assembly.

things change when talk goes to speedos.
for the odometer to be accurate the cable rotation per mile needs to be correct.
speedos faces will say how many cable rotations = 1 mile
then the escapement is calibrated to match 1 mile of cable rotation in 1 minute to 60 mph.

this is skimming of the whole picture.
those who want to fix old cronos-- the problem is allways the same. sourcing the small "swinging" pinion gear
i have no answer for this at the moment.
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Re: smiths chronometric tachometers

Postby Ferrous_Head » Fri Sep 24, 2021 10:57 pm

Ok. Pretty sure the RSM3005/00A was an option on the 62 and later CH's. The mag drive had to be changed to provide a drive.
These are 2:!.
The race versions I can't locate at the moment but were something like RC3012. (13 ?)

I have no idea what an RC83 would have been for.

Later XR's run a 4:1 drive.

Smiths made a zillion tachos for everything from Austin's to Zundapps. Well, not a zillion but a lot.
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Re: smiths chronometric tachometers

Postby hugoct » Sat Sep 25, 2021 11:31 pm

KRs 1952-1961 RC83 Smiths Chronometric
Sportster XLRTTs 1958-1961 RC83 Smiths Chronometric
KRs and XLRTTs !962-1964 92051-62 Smiths Chronometric
KRs, XLRTTs, and Iron XR750s 1965-1971 92051-65 Smiths Magnetic
Some race bikes are seen with Smiths ATRC series magnetic tachometers which at that time were the best that money could buy.
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Re: smiths chronometric tachometers

Postby murph » Tue Apr 26, 2022 2:06 pm

dr dick wrote:
murph wrote:Well to see if this can bring up an interesting conversation I know nothing about, and due to Dave's recent post in the craigslist finds let's see what we get.
I know for a fact that my restored 1962 xlch with late vin has an original chronometric nos tach from harley davidson on it because I bought it on ebay in its original box with all the mounting hardware. The RC# on the dial reads 1302/00.

Image

I also had an original paint 1962 xlch which was in my possession some time ago which also had the factory chronometric tach and interestingly enough the vin # was only off 6 digits from my restored 62. This also had an RC 1302/00

Now with all that said here is my original paint 1959 xlch that has the chronometric tach on it as well, yes I KNOW they never left the factory with the tach in 59 but the original owner did this prior to Harley Davidson and that's how the bike is staying. It's tach has an RC 83 on the dial. My question is What are the differences in the tachometers? My guess is ratio?

https://www.harleykmodel.com/gallery/19 ... index.html


a primer of escapements, hair springs, and timing of those:

cronos tachs dont have a 'gear ratio'.
instead they work by engaging/disengaging the indicator needle to the drive cable.
when the needle jumps (up or down) its connected directly to the cable rotations.
when the needle rests between jumps in disconnected from cable rotation.
so the when the connection time is great so is the needle sweep.
varying the connect-disconect time varies the needle sweep amount.
so given that you can print different background faces with different speed ranges.
print on the that face a code that says what the connect vs the disconect should be for a given cable rpm and you change the escapement to match.

escapment is time engaged vs time disengaged

so
for a given cable rotation rate (motor rpm) the tach needs to be calibrated to engage/disingage the needle rotation.
what cant be changed is that the needle rotation direction follows the cable rotation direction.

as far as the rotary orientation the face and needle thats easily set on assembly.

things change when talk goes to speedos.
for the odometer to be accurate the cable rotation per mile needs to be correct.
speedos faces will say how many cable rotations = 1 mile
then the escapement is calibrated to match 1 mile of cable rotation in 1 minute to 60 mph.

this is skimming of the whole picture.
those who want to fix old cronos-- the problem is allways the same. sourcing the small "swinging" pinion gear
i have no answer for this at the moment.



Thanks for the detailed response DR.
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