A Speedometer for model trains makes Speed Matching locos real easy.
An easy HO speed calculator - The number of inches traveled in five seconds = speed in miles per hour.
I need to speed match my locos, so that the displayed Speed Step on my NCE Procab throttles, makes all my locos run at the same “MPH speed”. That is Speed Step 10 equals 10 scale Miles per Hour. This will make “rear end banking” over my Liverpool Ranges, much easier. Each operator will know that setting their throttles to 10 out of 28, they will know their locos are supposed to be going at the same speed 10 MPH, hopefully reducing the chance of a “dreaded” trip to the cement floor of any wagons and locos.
Maximum speed for my locos will be
30 MPH. I don’t need to operate my locos/trains any faster even though many
locos/trains operated prototypically much faster than that, but have you ever
operated a model train at 50 - 60 scale MPH, it just flashes past. This is not
my idea of operating trains.
I used to use a stopwatch and “time” the loco through a 3 or 6 foot of straight section of track and adjust CV 5 and CV 6 to get my speed matching right.
Soundtraxx decoder equipped locos that don’t support CV 5 and 6, I would use Decoder Pro. It was quite easy but never completed it for all my locos.
Recently (May 08) a topic on the Yahoo NCE DCC group about speedometers caught my eye. So I built one after purchasing a PRO DIGI-X5 Cyclecomputer. I now only use the stop watch to calibrate the speedometer. Connect the Speedo S Wagon to the loco/train and adjust CV 6 and 5, or use Decoder Pro to adjust the User Speed Table, so that the locos speed is in SCALE Miles Per Hour as shown on my Procab display. That is at Speed Step 10 out of 28, the loco is running run at 10 MPH, at Speed Step 20 of 28, the loco is running at 20 MPH etc.
Making the Speedo S Wagon.
I used a rigid 4 wheel S Wagon for the speedometer car because fitting a wiper and contact or a magnet and reed switch to a fixed axle installation is much easier to setup instead of to a swinging (moving) axle in a bogie. The wheels of the S truck are 10.5 mm in diameter, close enough to 36 inches for the real thing.
Mark I: My first attempt at making the speed
sensor was using a piece of 1 mm styrene rod glued to the axle acting as a cam,
causing a piece of .015 Phosphor Bronze wire to touch a piece of brass plate,
making one pulse per revolution, shown at the right.
Connected this to the cyclecomputer and calibrated it with the 2873 figure discussed in the Yahoo discussion - the figure for a 36 inch wheel. My cyclecomputer will accept a number up to 3999, more than enough for my purpose. Depending on which type of cyclecomputer you purchase and your gauge, according to others, you may not be able to get your figure. Check this out when purchasing your computer.
Connected the Speedo S Wagon behind
a loco, increased the throttle until 10 MPH was displayed and ran the
loco/speedo through my Speed Trap two pins, 6 feet apart on a straight bit of
track, I timed it - 35 seconds. That’s close enough to 10 MPH (35.6 secs for
HO) for me, that I got from this from Joe Daddy’s Speed Converter.
Now having got success with my first attempt, I thought I would try and improve on the 80% readings.
Mark II: Using a Jaycar Reed Switch, Part No. SM1002 and a small trimmed down to fit magnet glued onto the axle as shown in the above photo on the R/H axle. The reed switch did NOT operate no matter what orientation I had reed switch in and I was as close as I could get. With the axle trimmed a little, there is only 4.00 mm to play with here, so the magnet clears the points/turnouts. I think I needed a stronger magnet.
Mark III: A stronger magnet and another S wagon! Occasionally I use my Wand to reset my QSI equipped Eureka AD60 Garratts. They are always lying around usually attached to some metal object, so I sure I wont miss one. I cut 6 mm off one of my Wands, glued it onto the axle that I had filed a flat onto it for more clearance. This was Wand magnet feels so much stronger and the installation proved that.
I had just removed some reed
switches from a Trainorama 49 that I had just sound chipped and modified the
marker lights for better DCC control instead of using the lighting board on. I
used one of these.
Locating the reed switch. With a multimeter connected to the reed switch and selected to the audible continuity tester and the magnet and axle positioned as shown in the photo, I moved the reed switch away from the axle/magnet until there was NO audible warning. Moved the reed switch back till I got continuity again. Moved the axle/magnet 180 degrees and bingo no audible warning. Super glued the reed switch into position.
Connected the bicycle computer, blue tacked it into the S wagon and tested with the same 2873calibration figure. Tested and the readings were steady with the occasional lower reading appearing. This indicated a missing pulse giving a momentary lower reading. 80% steady reading with a constant 10 MPH was once again, good enough for my speed matching.
As I did not want the magnet to magnetize the reed switch, I installed a keeper attached to a rod through the handbrake supports on the S wagon. Painted the lever white. Now this also provides a brake for the wagon as my layout is not too level especially in the upper deck (1420 mm high) Werris Creek Yard where I do all my speed trials.
Finished Speedo. My Reed Switch Speedo
going through the speed trap with the Procab displaying SS 10 and timed at 35
seconds. Ready to speed match all of my locos. That’s another story. Also shown
in the photo is the white magnet/brake release handle in the released position
vertical for hold.
The 4.0 other figure on the Cyclecomputer, is an Odometer providing a scale miles figure, that may be of some use for us modellers, when boasting about the size of our layouts. When not in use, my Cyclecomputer displays the time.
The S wagon being a small wheel base wagon, there was no room to place a slot in the floor that would have interfered with the reed switch, I had to lay the Cyclecomputer on its side. I hope this will get through my tunnels, when installed. All my playing with locos is on the upper deck (1420 mm) so setting the display vertical was better than laying the cyclecomputer on top of the S wagon, but now I walk around with a twist in my neck!
My Suggestion for a Speedometer Wagon: Build the phosphor bronze wire and contact version instead of the reed switch version, unless you have a really strong magnet or a Wand that you don’t mind trimming up a little. The trimmed Wand still works.
The Conversion Factor.
The speedometer works by programming the cyclecomputer with a figure that corresponds to the circumference of the wheel in MILLIMETRES. This will provide one pulse per revolution of the wheel The correction factor is:
Diameter of wheel in mms (Inches X 25.4) X 3.1416 (Pi) =
36 x 25.4 x 3.1416 = 2872.675 making it 2873, the figure discussed in the Yahoo discussion for a 36 inch wheel.
For my own calculations for my S Wagon:
Using Mopak Mike’s Scale Converter a 36 inch wheel that is what’s on my S wagon equals .41 inches that equals 10.41 mm. (.41 X 25.4).
10.41 x 3.1416 x 87 (scale) = 2846
or measuring my S wagons wheels at 10.55 mm and using this figure I get 2883.5 (10.55 x 3.1416 x 87 = 2883.5.
So 2873 is in between my above two figures of 2846 and 2883, so Ill stick with what I have already used 2873, because Im too lazy to change it after writing up this web page. Its close enough, its not that critical. These are examples on how to work out the calibration factor for your speedometer and cyclecomputer and the size of the wheels you’re using.
Speed Matching Locos.
Speed matching locos can be as easy or as hard as you want. It depends how finicky you are and why you are speed matching.
A common reason for speed matching
is for when you consist locos together. Using CVs 5 and 6 just takes a few
minutes when using POM. While this is good to speed match, it is not critical,
the locos are coupled together, they will go the same speed.
Some decoders for example, Soundtraxx that are installed in most of my locos, don’t support CVs 5 and 6. With these decoders youll have to make a Speed Table and Decoder Pro is great for this. An example of the Motor Control page is shown at the left with my SS as MPH Speed Table.
For my locos to run at the scale MPH indicated in Speed Steps on the Procab, Ill use Decoder Pro and tailor a User Speed Table using the Match Ends Tab and the Speedometer to get the same speed. On the final, write to the decoder, depending on the customers needs, I may write the last few CVs (CVs 26, 27 and 28 to provide the maximum speed that the loco is capable by progressively increasing the last CVs to a value of 255. This table will provide everything, but for me I will limit my locos to 30 MPH.
This NR example the loco uses 3/4s of the throttle travel, to operate the loco up to 25 MPH and the rest to get it to 70 MPH. I don’t know how fast an NR in real life goes, but this is as fast as I can get it to go with my NCE 12.3 Volts DCC at the track with an 1156 in series for Power/Short Management and my bridge rectifier Ammeter wired in all the time. Bypassing the Ammeter and 1156, I have 14.1 Volts DCC on the track and the top speed of the NR is 86 MPH. Toooooo fast for ANY train on my layout and at this speed with my track laying skills, the loco and or wagons will not be on the layout for long.
Parts.
Apart from the cyclecomputer, youll already have this stuff lying around in your box of goodies.
Suitable wagon you can go overboard here, but keep it simple.
Cyclecomputer any one that you can calculate with 2873 for 36 inch wheels.
.015 inch phosphor bronze wire
Brass flat strip for fixed contact
1.0 mm styrene rod
More reading.
Bob Backways Speedometer Car 1999 article.
Graeme Browns photos of his Speedometer wagon. Youll have to a member of Yahoos NCCDCC group to view them.
Commercial Speedometer Cars.
http://www.cp4e.com/trsped.htm
http://www.trainspeed.com/TrainSpeed1.htm