ANTIQUE SPEEDOMETERS EXPLAINED

[Russ_Furstnow]

After reading a few items on the forum, I think it is time to explain speedometers in some detail.
First: No speedometer manufacturer ever expected their speedometers to be around in 110 years! Most manufacturers used pot metal in their speedometers (Jones and Corbin used only brass and steel), and this metal was a good metal at the time when considering cost and ease of manufacture. Pot metal does change over time (swelling and shrinking) which creates issues with rebuilding speedometers today.

Second: Stewart and Clark published a bulletin stating that a speedometer was accurate if the reading was plus or minus five miles an hour from the "real" speed. I have read comments about how a speedometer was three miles per hour off, using a GPS to determine accuracy, and the owner was upset about the accuracy of the speedometer. The diameter of the tire and the gearing on the wheel always affects the accuracy of the unit.

Third: Speedometers are fragile and intricate, BUT they were made by humans and assembled by humans, so there is no magic about assembly or disassembly of the speedometer. I feel that the more experience one has with disassembly/assembly/rebuilding, the better the outcome for the speedometer. I like speedometers that have not been "restored" previously because most of the time if the speedometer has been "restored" by a person who did not have the experience necessary to restore the unit, the outcome is not always positive.

Fourth: Cables and swivels seldom, if ever, cause problems with the operation of a speedometer head. If a speedometer is running erratically, the cause is normally in the head itself. I read a posting on this forum from a man who claimed that a little brass tab above the speed drum shaft caused "bouncing", and he claimed that the tab was not adjusted properly. The tab is there ONLY to stop the speed cup from coming out the of the lower jewel if the car hits a bump...nothing else.

Fourth: Have your speedometer reconditioned by a competent technician. I believe that Model T folks are great "tinkerers", but I don't recommend taking a speedometer apart and trying to restore without the knowledge, proper tools and extra parts necessary. Once a speedometer has been reconditioned by a competent technician, the unit should give good service and will not need any further work. If the speedometer does not work up to expectations, give the technician a call and get the problem resolved.
I hope this answers some questions, and if you have any further questions, I'm happy to answer them.
Russ Furstnow

[Dollisdad]

Thanks Russ. I know there are something’s that I just should not do. Opening a speedometer is on that list.

[Rich P. Bingham]

Thanks for that, Russ ! It clears up a lot of "normal" questions when a unit seems to be in trouble. It's good to know that the gearing, cable and swivel are not to blame if all seems well with them, I feel that end is within my grasp to maintain. Like Tom, I'll leave the instrument head alone !!

It's also interesting to learn what the original tolerance for accuracy was !

[TWrenn]

And that my friends, is exactly why I sent my speedometer from my '13 to Russ. THE expert! I would only make it worse, if not ruin it!

[Ron Patterson]

Russ
So true of many repair issues found in aspects of today's Model T Ford restoration hobby.
I particularly enjoyed your item three. I would dearly love to be the first person to ever open and work on any ignition coil. Just when you think you have seen it all, you find out your haven't when you take a look inside a coil. As an old friend of mine used to observe about Model T problems and said: "The darn thing is over 100 years old and 100 people have been trying to repair it; what are the odds they all actually knew or understood what they were doing?"
Thanks for your sage wisdom and guidance as it relates to 100 year old speedometers.
Ron Patterson

[Allan]

Russ, in your involvement with speedos, have you come across indicative prices for Stewart systems as opposed to a Corbin set. My gut feeling is there would be a premium for a quality instrument. Harley Davidson and Indian motorcycles generally have Corbin units, often 80 or even 100mph

Allan from down under.

[Bruce Compton]

Hi Russ: Correct me if I'm wrong, but I'm assuming that I am the guy that you referred to as saying that "the tiny brass strip caused bouncing", so you may want to re-read my posts of Oct 30th and Nov 3rd under " Speedometer Bouncing". Actually it's pretty obvious that the brass strip is there to limit (not cause) vertical movement of the drum assembly, and in my speedometer the drum assembly was free to move vertically almost 1/16" because of the position of the brass retention plate. I'm also assuming that this area is the one mentioned in your 1995 book (page 50, section 14) that describes in detail the function and tolerances of this retention plate. The drum was bouncing around between 0 and 10 MPH with the car stationary and the motor running and the speedometer cable disconnected so I was pretty sure at this point that the problem was in the head. I really don't know what was wrong or confusing in my posts of Oct 30th and Nov 3rd, or, am I not reading your instructions properly? (I'm over 80 with eye issues). I am not capable of removing the odometer mechanism in this Model 102 that has the drum under the odometer, so measuring to get the 0.002-0.004" clearance you recommend is virtually impossible and I have to guess (feel). Anyway, I've almost got the bouncing eliminated but it still reads a tad low at higher speeds (over 40 MPH) so I'm assuming that the retention plate clearance is a tiny bit too tight. Thanks again for your explanations and contributions of your knowledge.

[Steve Jelf]

I'm curious about what happens, or should happen, when backing up. When installing the cable you spin the front wheel forward to take up the slack in the chain before you connect the cable to the head. 1 As the swivel turns in both directions, won't backing up reintroduce the slack? 2 does backing up have any effect on the odometer?

[Russ_Furstnow]

I'll try to answer these questions:
Allen: There is a difference in price for a Jones and Corbin setup, but I'm not sure if it is due to the quality of the speedometer or just the rarity of the units.
Bruce: I've read your posts, and the small brass tab above the aluminum drum main shaft has no effect on a speedometer that "bounces". There were many suggested solutions to correct your speedometer's problem, and that was what I was addressing.
Steve: Once the chain link cable is "pre-loaded", the sliding clutch allows the chain link to move up and down. The odometer can go in reverse if you drive far enough in reverse.
Thanks for the questions,
Russ Furstnow

[Bruce Compton]

Russ: Thanks for the response, but it now has me questioning my sanity. You have just said this morning that "the small brass tab has no effect on bouncing" and I'm reading directly from your book that this brass tab actually does have an effect on bouncing. On page 50, section 14 in your 1995 Speedometer book it says, and I quote ; "this plate is held to the drum bracket by one small round head screw and the clearance should be .002" - .004". This retention plate allows the speedometer to operate on very rough roads without allowing the aluminum drum to bounce, harming the unit. If this plate touches the shaft, the speedometer will run slow and be inaccurate." As found, in my speedometer, this brass retention plate was positioned to allow almost 0.060" of vertical movement in the drum assembly and it could actually rock back and fourth a bit sideways . After maybe 6-8 attempts I now have it very close to correct but as you warned in your directions, it's a bit too tight and the speed is reading low. Am I losing it or are we talking about some other area of this mechanism ?? Thanks again for your help.

[TXGOAT2]

.060 clearance is way too much. It is stated that .002 to .004 is the correct range. The brass strip should not touch the shaft. If your speedometer drum rolls or bounces when the car is NOT moving, it has to be because of vibration. The clearance at the brass strip being way too great initially may have aggravated that tendency to "bounce". The location where the speedometer head is mounted and the way it is mounted can affect the way the speedometer head vibrates. A short, stiff mount to a low vibration surface is best. The same is true of rear view mirrors, lights, etc. The speedometer head should be mounted rigidly at location that is not prone to heavy vibration. Alternatively, a well-designed rubber mounting that isolates the speedometer head from the car's vibration can be effective. If you walk around a Model T that is running at various engine speeds, you will notice that some parts of the car appear to vibrate very little, while other parts will obviously be vibrating at various frequencies and amplitudes. This is normal. The less vibration your speedometer is exposed to, the better.

[Bruce Compton]

I hear ya Pat. Everything has a natural frequency to accentuate vibrations and the fact that these speedos are hard mounted to the wood firewall makes the transmission of the vibrations even easier. Mounting one on rubber or small shock mounts might help even though you would still have the direct rigid connection from the cable. Anyway it's not the end of the world if it shakes some as it's still a Model T with all kinds of built-in traits that we all have come to expect and accept.