# Speedometer/odometer car



## Paul Burch (Jan 2, 2008)

I'm thinking of a winter project to make a speedometer car/odometer car that would be calibrated for 1/29. I know I have seen an article or two on some circuits but can't find them. Any ideas?


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## Bruce Chandler (Jan 2, 2008)

You can do it VERY simply. I bought a cheap bicycle speedometer at Wal-Mart for under $10.












Basically, the bicycle speedometer consists of:

- a unit containing the battery and the display.
- a sensor that is typically mounted on the front wheel mount of your bike. A LONG wire connects this sensor to the computer. (I ended up cutting the wire as it was WAY too long.)
- magnet that attaches to the spokes, and triggers the sensor each time the wheel rotates 

When you setup a bicycle computer, you have to "tell" it how big your wheel is. Typically there's a table of values for the wheel diameters - and you enter this into the computer. There's also a mathematical formula you can use for other diameters.

Well, in this case, I'm using 26" scale wheels from Sierra Valley, so I entered the value for 26" diameter wheels into the computer. You would just need to enter the wheel size in 1/29 scale.

I mounted the sensor between the wheels. I didn't use the supplied magnet as it was too large, but used one I had bought earlier.











So, every time the wheel rotates, it triggers the sensor. The computer thinks it is measuring a 26" wheel - so everything is already scaled.

I get speed and distance in 1:20 scale. It also shows elapsed time, max speed, and probably some other stuff I don't remember.


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## Paul Burch (Jan 2, 2008)

Bruce,
Thanks for the info. I will check it out.


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## Dean Whipple (Jan 2, 2008)

I have a how to on my website.....*Make a scale speedometer *


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## Paul Burch (Jan 2, 2008)

Thanks Dean,
It should be really easy to do.


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## RimfireJim (Mar 25, 2009)

The bicycle speedometer is a lot easier and cleaner than the other solution I remember reading about in Garden Railways, which involved an inexpensive calculator and wiring the M+ function to a switch activated once per wheel revolution. It was basically an axle turn counter and you had to do the math externally.


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## Paul Burch (Jan 2, 2008)

Made one today. So much for a winter project.


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## Semper Vaporo (Jan 2, 2008)

Posted By Bruce Chandler on 05 Oct 2011 08:58 AM 
You can do it VERY simply. I bought a cheap bicycle speedometer at Wal-Mart for under $10.





Basically, the bicycle speedometer consists of:

- a unit containing the battery and the display.
- a sensor that is typically mounted on the front wheel mount of your bike. A LONG wire connects this sensor to the computer. (I ended up cutting the wire as it was WAY too long.)
- magnet that attaches to the spokes, and triggers the sensor each time the wheel rotates 

When you setup a bicycle computer, you have to "tell" it how big your wheel is. Typically there's a table of values for the wheel diameters - and you enter this into the computer. There's also a mathematical formula you can use for other diameters.

Well, in this case, I'm using 26" scale wheels from Sierra Valley, so I entered the value for 26" diameter wheels into the computer. You would just need to enter the wheel size in 1/29 scale.

I mounted the sensor between the wheels. I didn't use the supplied magnet as it was too large, but used one I had bought earlier.











So, every time the wheel rotates, it triggers the sensor. The computer thinks it is measuring a 26" wheel - so everything is already scaled.

I get speed and distance in 1:20 scale. It also shows elapsed time, max speed, and probably some other stuff I don't remember.




You need to be careful where you mount the magnet. Too near the edge and it may foul on the guard rail at a switch or pick a point, or snag and derail the car or break the magnet off the wheel.

I also had a problem with the magnet actually pulling the steel spikes out of the ties (I have hand spiked rail in my switches). The spike would then sometimes get wedged between the magnet and the sensor and either seize the wheel so it starts skidding or break the magnet off the wheel.

I finally attached the magnet to the axle in the middle so there was less chance of it hitting something or picking up metal debris. But then the magnet sometimes did not get far enough away from the sensor for the sensor to open after closing.... This was especially obvious at high speeds when the speed display would be come very erratic and usually go down when, by eye, it was obvious the train was going faster.


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## Bruce Chandler (Jan 2, 2008)

Good points! Those are the things we need to think about.


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## pete (Jan 2, 2008)

If you entered the size of a 26" wheel into the computer you are not getting a true reading. The wheel size for the g scale cars has to be calculated using a formula. I do not have the formula with me but i can get it tomorrow at the bike shop that i work at.


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## pete (Jan 2, 2008)

Go to deans site and he has the formula, enter the number you get and now you have the correct wheel size. Good luck.


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## Paul Burch (Jan 2, 2008)

I used the formula and method in Dean Whipples article to calculate mine. When I get a chance I will use the odometer reading to see how accurate it is and then adjust a little if necessary.


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## Torby (Jan 2, 2008)

The formula? Um, well, you get your caliper and measure the diameter of your wheel, then multiply by your scale. That's the number to tell the bike [email protected] url(http://www.mylargescale.com/Providers/HtmlEditorProviders/CEHtmlEditorProvider/Load.ashx?type=style&file=SyntaxHighlighter.css);@import url(/providers/htmleditorproviders/cehtmleditorprovider/dnngeneral.css);


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## Paul Burch (Jan 2, 2008)

Torby,
I think the number entered into the computer has to be based on wheel circumference. By your method what would you enter into the computer for say a scale 34" diameter 1/29 wheel?
I did what Dean said and used the circumference of my wheel as measured on the track which was 95.2 mm and multiplied by 29 to get an entry of 2791.


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## Bruce Chandler (Jan 2, 2008)

You could just use geometry: c=2 * pi * r


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## Paul Burch (Jan 2, 2008)

Lot easier to just roll it one turn on the track and measure ! I don't want a headache.


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## Semper Vaporo (Jan 2, 2008)

You could researchlook up the prototype's wheel diameter/circumfrence and use that. Then the speed and distance display would be perfectly as accurate as the manufacturer's (in)ability







to perform scale manufacturing!


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## Ray Dunakin (Jan 6, 2008)

This is a great idea! I want to make one for my layout someday. Then I could finally determine exactly how much track I have.


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## DKRickman (Mar 25, 2008)

Posted By Ray Dunakin on 06 Oct 2011 08:00 PM 
This is a great idea! I want to make one for my layout someday. Then I could finally determine exactly how much track I have. 
But, would you be measuring the inside or outside curves? Unless you have exactly the same amount of left and right hand curves, you would presumably need to measure the left and right sides, using a wheel which can rotate freely on an axle, then average the two numbers together.

You DID say you want an exact number, right?


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## Bruce Chandler (Jan 2, 2008)

Posted By Semper Vaporo on 06 Oct 2011 06:41 PM 
You could researchlook up the prototype's wheel diameter/circumfrence and use that. Then the speed and distance display would be perfectly as accurate as the manufacturer's (in)ability







to perform scale manufacturing!


That's basically what I did. I had 26" wheels...


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## RimfireJim (Mar 25, 2009)

Posted By Paul Burch on 06 Oct 2011 08:48 AM 
I used the formula and method in Dean Whipples article to calculate mine. When I get a chance I will use the odometer reading to see how accurate it is and then adjust a little if necessary.

The instructions on Dean's site say to use trial and error to adjust the wheel circumference value after a 260 ft. test run, guessing at how much to change it. One should be able to get there in one iteration by measuring the percentage of error on the first run and adjusting the w.c. by that percentage. It's a linear problem with straightforward math. Also, it would be much better to do the initial estimate of w.c. by measuring the travel distance over ten revolutions and dividing by ten, rather than basing it on just one revolution: one-tenth the error.


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## Torby (Jan 2, 2008)

The bike computer isn't smart enough to multiply by pi? Can't believe they'd expect the bike rider to do [email protected] url(http://www.mylargescale.com/Providers/HtmlEditorProviders/CEHtmlEditorProvider/Load.ashx?type=style&file=SyntaxHighlighter.css);@import url(/providers/htmleditorproviders/cehtmleditorprovider/dnngeneral.css);


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## Semper Vaporo (Jan 2, 2008)

Posted By Torby on 07 Oct 2011 10:52 AM 
The bike computer isn't smart enough to multiply by pi? Can't believe they'd expect the bike rider to do [email protected] url 

YEP! I have seen some that require the Diameter to be entered and some that require the circumfrence. Some are strictly INCH values and others Millimeters and others in Centimeters. I think I have one that will let you enter just about anything and it will figure it out for you. I had one that you had to enter the diameter in centimeters but would only display in Miles (and Miles per Hour)


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## Paul Burch (Jan 2, 2008)

You guys are getting way to anal with this !!!


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## Gary Armitstead (Jan 2, 2008)

Posted By Paul Burch on 07 Oct 2011 02:46 PM 
You guys are getting way to anal with this !!!


Amen to that Paul!


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## Semper Vaporo (Jan 2, 2008)

Posted By Gary Armitstead on 07 Oct 2011 02:51 PM 
Posted By Paul Burch on 07 Oct 2011 02:46 PM 
You guys are getting way to anal with this !!!


Amen to that Paul!









That is the whole idea behind a hobby... something you can throw yourself into to delve into the parts that you enjoy as deeply as you want to enjoy it.

Are we not anal about getting the right cars to be pulled by the right engine? Are we not anal about getting our track right so the trains don't derail? Are we not anal about getting the right plants right where we want them? Are we not anal about keeping the weeds at bay? Are we not anal about getting couplers that work with each other? Are not anal about getting the "correct" power to run our RRs the way we want to run them (track power, battery, live steam, live Diesel, clockwork, rubberband, a gerbil and a cockroach)? If someone want to delve into measuring the speed and distance of a model train then they have every right to discover whatever meets their need. How does it work, how do you calculate it, how can "I" do it, what would make it better, easier, or deserving of bragging rights ??? Nothing whatsoever wrong with this discussion!


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## Totalwrecker (Feb 26, 2009)

I think you need to use the cube root to find scale speed... Mass X Energy =Speed 
We all know to find scale mass (wt) one uses the cube root... 
A scale 1/29 mile is 182' and a scale hour is 2.068655 minutes 
I hope I cleared this up. 
Smiling John


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## Paul Burch (Jan 2, 2008)

Got a chance to do some testing with mine today. Made some readings against a measured distance. I only ended up tweaking it by about 1.5% from my original wheel roll measurement. Accurate enough for me. Now I'm going to install an ampmeter and voltmeter in the car. All my loco batteries are plug-ins so it will be easy to make a cord with a battery plug and new jack for the battery. When it is plugged in the meters will be inserted into the circuit. Then I will have a car that can measure mph,distance, battery voltage and current draw at any given speed.


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## NTCGRR (Jan 2, 2008)

When things "click" or fall into place, it happens fast.


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## SD90WLMT (Feb 16, 2010)

Ok Paul - you only need one more "Test Toy" then!! With what you have you could figure out everything, except what it takes to pull a string of cars. 

You need a "Tractive Effort Test and Research Vehicle", it would be pulled behind a loco, and could be used to measure the dynamic load abilities of the loco, while it is in motion, using a 50 pound fish scale and give direct reading while you follow along with it. You can than compare the volts and amp loads required to move it under a given load. 

I have built a car such as this, it is a glass fish tank with a drag wiper "dragging on the rails" and is adjustable to increase the moving drag, or resistance. It carries 4 bricks for ballast, to keep the drag under pressure. The fish scale mounts to a sliding coupler mount at the front only, fixed at the rear. 

But wait - there's more,............. 

It will also do something it was not designed for.... 

You can measure the load of a string of cars, or rolling effort to move and maintain moving them! IT will change with curves and grades, and again you can compare with your V & A readouts. This is accomplished by lifting the drag arm off the rails, and direct reading between the loco and cars only. 

Lots of fun and educational for me!! I learned that my SD70Mac could pull 8.5 pounds at max load., running at about 7.0 amps and 12.0 - 14.0 volts, and pull 70 cars on a 2.5 percent grade. The 8.5 pounds should be equal to over 100 cars on level track, btw. 

Dirk - DMS Ry.


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## sandbarn (Feb 13, 2010)

Once you've done that to just need to connect the outputs to a wifi transmitter and send it to your laptop so you don't need to follow it around your layout.


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## Greg Elmassian (Jan 3, 2008)

Dave Bodnar made me a wireless speedometer car, the electronics are in a USAT hopper:










The base station shows speed in SMPH, and track voltage:










It also has a U shaped hoop that you can put over the track and measure passing trains.

As with anything Dave makes, it works great. I requested the walnut ends.

Regards, Greg


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