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The Lenz Effectt

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Mechanical speedometers all operate under the "Lenz Effect" in which a non-ferrous metal like aluminum or copper is pushed by the invisible field of a magnet that doesn't make contact with it. I had always known speedometers used a magnet that caused an aluminum cup to spin but couldn't understand HOW when aluminum isn't magnetic. Last year I had to calibrate my speedometer because it's non-permanent magnet had lost it's strength and in the process I learned a LOT about speedometers. To learn more Google "Lenz Effect You Tube" to see some videos about the Lenz effect and how an "Eddy Current" is used in industry.

As I didn't have access to the equipment necessary to adjust the strength of my speedometer magnet I intentionally killed it's strength then epoxied two tiny Radio Shack permanent magnets to my (now dead) magnet then calibrated the mainspring to the strength of the magnets. I won't go into the details about everything I did but in the end it reads within about 99% correct at all road speeds.
 

kpic

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It's called self-induction.

A lot of older vehicles use gears.
TCI Speedometer Gears and Accessories | TCI

# Drive Teeth x Axle Ratio x Tire Rev. per Mile = # driven teeth
1001

Electronic speedometers work in a completely different way. Small magnets attached to the car's rotating drive shaft sweep past tiny magnetic sensors (either reed switches or Hall-effect sensors) positioned nearby. Each time the magnets pass the sensors, they generate a brief pulse of electric current. An electronic circuit counts how quickly the pulses arrive and converts this into a speed, displayed electronically on an LCD display. Since the circuit is measuring the number of wheel rotations, it can also keep a count of how far you've traveled, doubling-up as an odometer (distance-measuring meter). Electronic speedometers can also display speeds with analog pointers and dials, just like traditional eddy-current speedos: in that case, the electronic circuit drives a highly controllable electric motor (called a stepper motor) that rotates the pointer through an appropriate angle. Electronic speedometers are more reliable and compact than mechanical ones and the motion sensors can be any distance from the display that shows you your speed, making them suitable for any kind of vehicle from a bicycle to a 40-ton truck!
How do speedometers work? - Explain that Stuff

Most designs I've done since the 1980s used the Hall-effect. ABS uses the Hall-effect. This is also how shaft speed of the rotor group in a turbocharger can be monitored. Typically for testing as the driver could careless.
The Hall-effect is also used for camshaft position timing for injector pulse timing,

Basically there is a tone ring and a pickup. They can sense MPH or shaft speed differential. Inexpensive and effective. Wickedly accurate at low or high speeds which is a good thing.
Passive and Active ABS Wheel Speed Sensor Function


 

Mac

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Are you kidding me? They're GEAR driven? Where are these gears located?

Not sure for the automatics (I'm guessing it's the same) but for the 4 speeds, the gears were at the side of the transmission. The speedo cable came down and screwed onto an adaptor which then bolted to the tranny.

Mac
 

GTR1999

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Muncies speedo for corvette is on the passenger side.

Here the tail has been removed and you can see the drive gear shrunk on the main shaft. 69-70 used a plastic slip fit gear with clip.

separatetail.jpg


TH400 on Drivers side

TH400%20rebuild%20finished_zpsazoh1rsf.jpg


newth400stand0004.jpg


Different drive and driven gear for each gear ratio Some of the BW Super T10's used a 90* reduction gear as well
 

Mac

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Are you referring to the nylon 15 tooth drive gear and 44 tooth driven gear I installed to compensate for my TH700R4 and 3.70 gears? Oh, THOSE gears! Yes, those gears will spin my speedometer cable 1000 rpm @ 60 mph but the purpose of this thread is to tell you guys what happens inside the speedometer HEAD. The cable-driven magnet that is spinning 1000 rpm @ 60 mph and causing the non-magnetic aluminum cup that surrounds it to rotate about 50-60 degrees. You know, the LENZ EFFECT that causes a non-ferrous metal to move when a magnet is passed by it. Watch the You Tube videos on the "Lenz Effect" to see how it is used in mechanical speedometers.
 

GTR1999

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I don't know that I've ever seen a polished transmission case before... Very pretty but not factory correct... :chuckle

Mac

Yeah I know but I like the way it came out. I did it by hand with scotch brite wheels.
 

Tom Bryant

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Yeah I know but I like the way it came out. I did it by hand with scotch brite wheels.

I did this to my alternator on my stepside being careful to brush it all in the same direction. That was a few years ago and it still looks great. You can see about half of it in the picture.

Tom

ry%3D400 Exhaust.jpg
 

kpic

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Are you referring to the nylon 15 tooth drive gear and 44 tooth driven gear I installed to compensate for my TH700R4 and 3.70 gears?

The right answer depends on the tire diameter.

BTW, they are 3.73 gears or 41 on the ring and 11 on the pinion.

41/11=3.727272727 etc. So most folks call it 3.73. 3.70 would be 40.5/11. ;)

Point is, you are not only dangerous; you're boring. One thing for certain you are not is a mechanical engineer.
 

SVO

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Those transmissions look too clean. How do ya keep your fiberglass from rusting if the transmission isn't leaking oil? :D
 

Antz81

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Are you referring to the nylon 15 tooth drive gear and 44 tooth driven gear I installed to compensate for my TH700R4 and 3.70 gears?

I assume your running 225/70R15 or 255/60R15 tires.
Maybe my math is off but but shouldn't that be a 41 tooth driven gear? or a 16 tooth drive gear?
With the gears you have your speedo will read about 7mph slower than your actual speed. Let me guess that's what you were trying to correct for?:chuckle
 
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I have a You Tube video on metal recycling and it shows small pieces of shredded scrap metal riding along on a conveyor belt. At a certain point the metal reaches a powerful spinning magnet UNDER the belt and all aluminum and brass pieces get thrown up and over a barrier and into a separate bin because of the powerful eddy current under the belt. It's that same eddy current that makes your speedometer magnet rotate the aluminum cup with the indicator needle attached.
 
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more-drama-plz-smiley-emoticon.png Geeze, following you two is better than watching Comedy Central....
 
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The right answer depends on the tire diameter.

BTW, they are 3.73 gears or 41 on the ring and 11 on the pinion.

41/11=3.727272727 etc. So most folks call it 3.73. 3.70 would be 40.5/11. ;)

Point is, you are not only dangerous; you're boring. One thing for certain you are not is a mechanical engineer.


But I'm running Motive gears that are based on a 10 tooth pinion gear. In my case I have a 10 tooth pinion gear and 37 tooth ring gear which equals 3.70. The old Perfection American and Zoom gears were also based on 10 tooth pinion gears so their ratios were 3.50, 3.70, and 3.90.
 

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