SPEEDO ERROR

[NM Bonny]

I've got an OEM speedometer with what I assume is the original rear axle drive unit. I am running a 20T, # 57-1919 tranny sprocket and OEM rear sprocket (the only one that will fit the drum), so should be getting a slightly low speedo reading... but no! My speedo is reading far higher than it should--on the order of 30+ mph higher.

My rear tire is an OEM size 4.00-18. It's a modern tire, a Bridgestone Battlax BT46--but my BMW runs the identical tire, and its speedo is spot-on.

Any ideas...?

 

[Greyfell]

I've got an OEM speedometer with what I assume is the original rear axle drive unit. I am running a 20T, # 57-1919 tranny sprocket and OEM rear sprocket (the only one that will fit the drum), so should be getting a slightly low speedo reading... but no! My speedo is reading far higher than it should--on the order of 30+ mph higher.

My rear tire is an OEM size 4.00-18. It's a modern tire, a Bridgestone Battlax BT46--but my BMW runs the identical tire, and its speedo is spot-on.

Any ideas...?

Best of luck; I'm sure someone will speak up.

 

[Rudie]

should be getting a slightly low speedo reading
My speedo is reading far higher than it should--on the order of 30+ mph higher.

From about 66 onwards, 650 Triumph speedo driven by the rear wheel; sprockets don't make any difference; tyre on US market 650 (and 500) was 4.00x18 since forever, certification bodies like ETRTO (European Tyre & Rim Technical Organisation), US DOT mandate tyre overall diameters should remain about the same - i.e. Battleax should be a very similar overall diameter to the 4.00x18 Dunlop K70 Triumph fitted when the bike was new. Also, Triumph fitted the same speedo drive to UK & General Export 650's - that had 3.50x18 rear tyres before '71 - and all triples - that mostly had 4.10x19 rear tyres (some Hurricanes had 4.00x18, others had 4.25/85x18).

Chances are the speedo error is one or more of:-

. Wrong speedo - from the late 1960's, all 650's and 750's had a speedo. that went to 150 mph.

. Wrong speedo gearbox - 150 mph speedos must be driven by 1 to 1.25 ratio box (confusingly likely to be marked "15/12" ); otoh, 120 mph speedos must be driven by 1 to 2 box (again confusingly likely to be marked "2/1"). Boxes are marked on the face towards the wheel hub so you have to take the wheel out and pull the box off the drive ring to see the ratio marking ...

. Speedo. needs a service - the cable spins a magnetised disc inside the speedo, the magnetic strength drags the needle round the dial against the spring that returns the needle to zero. The distance from the disc to the needle is adjustable (with the speedo apart); as you probably know, distance increases or reduces magnetic strength rapidly, disc too close to the needle, disc magnetic strength would drag the needle further round than it should for a given speed.

. If you remove the speedo from the bike, turn the speedo upside down then right way up again and between dial and glass is filled with brass filings, cable has buggered the speedo. Cable inner just slightly too long for the outer and/or ends of cable inner not tapered correctly have been intermittent problems for decades. Either problem means, when the second nut on the cable outer is tightened, the incorrect cable inner applies axial force to the speedo, that does not have any thrust bearing to prevent the force pushing on the disc bush ... that is brass ...

 

[NM Bonny]

From about 66 onwards, 650 Triumph speedo driven by the rear wheel; sprockets don't make any difference; tyre on US market 650 (and 500) was 4.00x18 since forever, certification bodies like ETRTO (European Tyre & Rim Technical Organisation), US DOT mandate tyre overall diameters should remain about the same - i.e. Battleax should be a very similar overall diameter to the 4.00x18 Dunlop K70 Triumph fitted when the bike was new. Also, Triumph fitted the same speedo drive to UK & General Export 650's - that had 3.50x18 rear tyres before '71 - and all triples - that mostly had 4.10x19 rear tyres (some Hurricanes had 4.00x18, others had 4.25/85x18).

Chances are the speedo error is one or more of:-

. Wrong speedo - from the late 1960's, all 650's and 750's had a speedo. that went to 150 mph.

. Wrong speedo gearbox - 150 mph speedos must be driven by 1 to 1.25 ratio box (confusingly likely to be marked "15/12" ); otoh, 120 mph speedos must be driven by 1 to 2 box (again confusingly likely to be marked "2/1"). Boxes are marked on the face towards the wheel hub so you have to take the wheel out and pull the box off the drive ring to see the ratio marking ...

. Speedo. needs a service - the cable spins a magnetised disc inside the speedo, the magnetic strength drags the needle round the dial against the spring that returns the needle to zero. The distance from the disc to the needle is adjustable (with the speedo apart); as you probably know, distance increases or reduces magnetic strength rapidly, disc too close to the needle, disc magnetic strength would drag the needle further round than it should for a given speed.

. If you remove the speedo from the bike, turn the speedo upside down then right way up again and between dial and glass is filled with brass filings, cable has buggered the speedo. Cable inner just slightly too long for the outer and/or ends of cable inner not tapered correctly have been intermittent problems for decades. Either problem means, when the second nut on the cable outer is tightened, the incorrect cable inner applies axial force to the speedo, that does not have any thrust bearing to prevent the force pushing on the disc bush ... that is brass ...

Oh, duh... Rear wheel drive nullifies the tranny sprocket... it's not a tranny-driven speedo! More coffee!

Yes: the BT46's diameter is very close to the OEM tire, so that's not it.

As for the speedo itself, it is the OEM gauge, as the first photo shows.

On the drive box, the second photo shows it's a 1.25 box (took numerous strokes from the grease gun to force out 54 years of contaminated grease).

Given what you've written, I'm guessing it'll be one of your last two tips. I've had the gauges out to clean them up while fabricating the dash, and didn't see any detritus you mentioned. So, I'm guessing it's number one: needs service.

I successfully rebuilt the mid-'60s Stewart Warner that I fitted to my '51 FL Pan-Shovel (3rd pic - next to a cheap aftermarket job), which sounds like the same basic design as the Smiths gauge. So, no biggie to adjust the distance... is there a spec for the cup-to-magnet clearance? Are there online pamphlets for these gauges?

Thank you, Rudie, as always.

 

[NM Bonny]

Ahhh... problem solved!

For some reason I don't know, the memory of replacing the guts in the Smiths speedo in my '67 Midget 'test mule' for Creative Spridgets (I put in '74 innards, as that was all there was available) popped into my mind... and I suddenly realized what's wrong with this Smiths unit.

Vintage Smiths speedos are cup-and-magnet units as Rudie described--but he left one thing out that I, too, had forgotten. In order to remove the guts, you simply pluck the needle off of the tapered shaft to remove the face, and so on.

But when you replace the guts, you have to calibrate the speedo. How do you know it's off? Your reading is either high or low. If it's reading high, like mine, the telltale sign of mis-calibration is that the needle is not resting on the peg at zero... and sure enough, I just looked and my needle is 'floating' 1/32~1/16" off the peg.

A 'floating' needle when the bike is motionless means there is essentially zero preload on the clock spring--which will net an excessively high reading. Believe it or not, setting the preload is very straightforward, and I'll do it whenever I get a moment:

- operate the vehicle until your actual speed reaches a steady 60 mph (easy to do these days with cell phone apps--or with a friend on a bike or in a car), then note & record your indicated speed (I seem to recall from research I did back then that calibrating roughly in the middle of the vehicle's operable range is best);

- remove the bezel or extract the guts from the rear, whichever works on your unit to access the needle;

- gently turn the cup--which is connected to the needle--until the needle reaches your indicated speed at 60 mph: in my case, about 82 mph. Be careful, as you're turning against the delicate clock spring--do NOT turn it the wrong direction;

- holding the cup steady, pluck off the needle, rotate it until it points to 60 mph, then gently press it home--doesn't take much pressure on that tapered shaft;

- gently let the cup return to rest the needle on the peg.

Reassemble the unit, and you're done! Note that this assumes a serviceable, lubed cable and an otherwise operable speedo.

 

[Rudie]

Vintage Smiths speedos are cup-and-magnet units as Rudie described--but he left one thing out that I, too, had forgotten.
calibrate the speedo. How do you know it's off? Your reading is either high or low. If it's reading high, like mine, the telltale sign of mis-calibration is that the needle is not resting on the peg at zero... and sure enough, I just looked and my needle is 'floating' 1/32~1/16" off the peg.

"You Learn Something New Every Day" As I posted earlier, I have always had speedos and tachos serviced by others, they have never come back with the needle not resting on the zero peg.

 

[NM Bonny]

"You Learn Something New Every Day" As I posted earlier, I have always had speedos and tachos serviced by others, they have never come back with the needle not resting on the zero peg.

Yeah, my sentiment exactly.

Professional rebuilders must have some way of setting the preload without driving the car. Perhaps a set input rpm spec, or [x] degrees of rotation on the cup....? Anyway, I think the driving method I recite is from an old article on rebuilding MG/Austin Healey Smiths speedos I have somewhere... and it worked for me. In the interim, I'll just watch the rpms to avoid tickets!

Should have the new 71" cable Tuesday, and the additional rubber for the shift peg Monday. Paint and supplies later in the week...

I'm going to try to find time to zip through the canyon with the GoPro today, so folks who haven't ridden one of these Meriden Unit 650s can get a sense of what they're like from the saddle.

 

[CarlS]

Ahhh... problem solved!

For some reason I don't know, the memory of replacing the guts in the Smiths speedo in my '67 Midget 'test mule' for Creative Spridgets (I put in '74 innards, as that was all there was available) popped into my mind... and I suddenly realized what's wrong with this Smiths unit.

Vintage Smiths speedos are cup-and-magnet units as Rudie described--but he left one thing out that I, too, had forgotten. In order to remove the guts, you simply pluck the needle off of the tapered shaft to remove the face, and so on.

But when you replace the guts, you have to calibrate the speedo. How do you know it's off? Your reading is either high or low. If it's reading high, like mine, the telltale sign of mis-calibration is that the needle is not resting on the peg at zero... and sure enough, I just looked and my needle is 'floating' 1/32~1/16" off the peg.

A 'floating' needle when the bike is motionless means there is essentially zero preload on the clock spring--which will net an excessively high reading. Believe it or not, setting the preload is very straightforward, and I'll do it whenever I get a moment:

- operate the vehicle until your actual speed reaches a steady 60 mph (easy to do these days with cell phone apps--or with a friend on a bike or in a car), then note & record your indicated speed (I seem to recall from research I did back then that calibrating roughly in the middle of the vehicle's operable range is best);

- remove the bezel or extract the guts from the rear, whichever works on your unit to access the needle;

- gently turn the cup--which is connected to the needle--until the needle reaches your indicated speed at 60 mph: in my case, about 82 mph. Be careful, as you're turning against the delicate clock spring--do NOT turn it the wrong direction;

- holding the cup steady, pluck off the needle, rotate it until it points to 60 mph, then gently press it home--doesn't take much pressure on that tapered shaft;

- gently let the cup return to rest the needle on the peg.

Reassemble the unit, and you're done! Note that this assumes a serviceable, lubed cable and an otherwise operable speedo.

Excellent information!