The On-Line Magazine of Rideable Model Railroading
 NUMBER EIGHTY THREE

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May 06, 2007  

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Building a
"Personal Rail Vehicle"
 


Written by Jim O'Connor

You need something to ride the rails, but you don't have a lot to work with?  Here is a budget-minded way to get started.  This vehicle is also great for an established live steamer who wants something that's fun for a quick trip around the track.  Build a "Personal Rail Vehicle" or "PRV".   By definition, a Personal Rail Vehicle can only accommodate one person. You can think of it as a one-person Speeder, if you wish.   Like a speeder, you ride directly on the source of the motive power. 

This little PRV was designed with these points in mind:

  1. Small enough to fit in the trunk of a compact car.
  2. Light enough for a single person to carry and set up.
  3. Simple to build and maintain.
  4. Inexpensive enough to afford on a budget.

I wanted an electric drive for this project to keep it simple and transportable, and for that I would need an electric motor and a set of wheels with axles and everything in between.  Looking around the web, I saw various drive systems like a simple chain drive.  With a chain, you use a small sprocket on the dc motor and a large sprocket on the axle.  This gives you some level of gear reduction.  If your primary sprocket (on the motor) has 12 teeth and your secondary sprocket (on the axle) has 48 teeth, you have a 4:1 gear reduction.  You will need some major reduction since most dc motors run pretty fast, way faster than I needed.  Speaking of motors, where do you get one?  I "Googled" dc motors and came up with all kinds motors of various voltages.  Most motors large enough to power a person are at least 12 volt. I decided to use either 12 or 24 volts so I would be able to utilize "deep cycle" lead acid batteries which are relatively inexpensive.  I found some industrial gear motors with  built-in gear boxes that I figured I should be able to get to work. I found scooter motors with high RPMs that might be able to be geared down enough with chain drive.  I then stumbled upon a photo of a drive unit sold by Plum Cove Studios that was intriguing.

It was a dc motor with a small gear on the motor and a large gear mounted right on the axle. Motor and axle positions are "fixed", meaning they can't separate, causing the gears to grind. The motor was designed to run on 24 volts. The price wasn't bad either, around $400 in 2006.  Another plus was that the engineering had already been done. Since this setup fit my design criteria of small, light, simple, and cheap, I ordered it and decided to design the vehicle around the major component, the drive mechanism.

In an effort to keep it simple, I decided to try making the drive axle "fixed", meaning it would not use springs or be able to move to follow the track in any way.  This fixed arrangement worked well on a hand-peddled car that I built a couple of years ago.  On that hand car, the rear fixed axle was driven by a chain and the front axle was set up to pivot. This gives what's known as "three point suspension".  It's very stable, but a little rough riding, since you feel every rail joint on the track.

I made a wooden mockup to test the Plumcove motor using a 12 volt deep cycle battery. The only speed control was an on/off toggle switch.  My 12 year old son, Mike, was the willing test pilot.  We set the mockup on the rails with its wooden platform supporting both the battery and my son. We connected up the battery to the temporary wiring and threw the switch. It took off like a shot. It got up to speed in a few seconds.  It ran at about 5 miles per hour, a perfect speed for this little vehicle. It slowed a little going up hill.  When going down hill, Mike found if he kept applying power, the motor would act as a brake and limit how much speed he would gain.  When he turned off the switch, it picked up speed. It picked up enough speed to make it a little scary, so he mostly left the power on.  After about a half hour, we declared the test a success.  I knew we were on the "right track".

I looked on my favorite web site and other web sites for a speed controller for the PRV. Prices for dc motor speed controls vary widely.  I found a cheap controller kit for $20 that would work down to 12 volts and be able to control a dc motor up to 30 amps and met the "cheap" criteria so I ordered one to try.  After spending over an hour soldering resistors and chips to a bread board, I hooked it up for a test. It worked for only a few seconds and stopped.  Not worth the money or effort. I started to search again and this time I decided to try one of the speed controllers (model MC-7) featured on discoverlivesteam's supplier page from Diverse Electronic Services.  It was just $85 (in 2006) and able to handle much higher amperage.  I found out in my research that the control circuits on these boards don't function well down to 12 volts unless designed to do so.  If I wanted to use only one battery (12 volts), this was one of the few boards that could handle it.  The down side of this board:  it had no braking capabilities.  I felt this would be OK, since a friend told me about a simple electric braking system used on children's ride-on toys. Diverse also has a controller with regenerative braking that works on 12 volts, but it was more money.

I I ordered the MC-7 board from Diverse Electronics and while I waited for the board to arrive, I worked on the frame.  I designed the frame to be large enough to hold the battery and not much more.  I used the three point suspension mentioned above.  For the front wheels, I made the pivot point below the axle (see left photo). This makes the frame of the PRV hang from the axle. It seemed like it should be more stable than having the pivot point above the axle. Having the pivot point below the axle IS more stable but I can't explain why that is. Truss bars are another feature of this frame. They add rigidity without much weight. I also built a wooded "crate" battery box.  I needed to hide the battery somehow, so I decided to make it look like the vehicle was carrying some sort of cargo.

For the modeling aspect of the project, I wanted this to actually LOOK like something in the real world. Perhaps a "Goose"?  I was thinking about how I could model the cab area and at the same time, was looking at some 1/8 scale stamped steel model trucks that have been featured on my web site's main page for years.  Could I use the cab from one of those?  It would save time.  They looked great, but the price was a factor.  I also looked at my son's GI Joe models.  He had two cool looking Jeeps that are about 1/6 scale or so.  I checked it out and the wheel base of the PRV would fit into the wheel wells of the jeep.  Mike offered to donate one of his Jeeps for the project.  The front of the PRV was designed to accommodate his Jeep, after some cutting. I then fabricated a wooden "crate" to cover the battery and control.  Our little PRV was a "Rail Jeep".  I don't know if any Jeeps were ever converted for rail use, but if one had, it might look like our little model.

To round out the project, I used a project box and 5K pot from Radio Shack as the hand held controller.  I also picked up a double throw switch for fwd/off/rev and a momentary contact pushbutton to control the electric brake.  The brake uses a double throw relay (from Grainger) to disconnect the controller from the motor and at the same time, short out the motor leads across 2 resistors.  This "dynamic brake" takes the voltage that a dc motor will generate when coasting and discharges the energy, as heat, into the atmosphere. The amount of braking depends on how much resistance you use.

When I take the PRV out, everyone comes by to see how something this small can possibly work. It's easy to drive and after about 30 seconds of instruction, you're ready to solo.  It's really fun to drive because the PRV gives you a strange sensation. It's like you're riding on nothing.  The entire vehicle is below you and out of your field of vision when you're looking straight ahead.  Since you can't see what you're riding on, it's like you're flying over the tracks.

Written by Jim O'Connor


the end

Did you build something like this?  I would like to hear about it.  email me

 

Here are some questions I've been asked about the PRV

Q. Is it strong enough to pull cars?

A. Uphill, I don't think it'll pull cars fast enough to make it practical.  If I had made it a 24V. system, it would have been able to pull a car or two.  If you added a second drive motor, you could pull a lot of cars, but then it would be a locomotive and not a PRV (personal rail vehicle).

Q. In some of the pictures, the PRV looks incomplete?

A. It is.  I was taking photos during development.  some of those are included here.  And I still want to add detail items to the Jeep.  Perhaps I'll change the crate too.

Q.  Who did the welding?

A.  It was me and no, I'm not blind.  I'm just a fair welder.  Also, I was fighting old welding rod and that didn't help.

Q. Can an adult ride on it?

A. Yes, the largest adult that's tried it weighed over 300 lbs.

Q. How long will it run on one charge?

A.  I don't know, I've gone 3 miles on it without any noticeable loss of speed. I've never had an opportunity to run it long enough to find out just how long the battery will last.  I'll let you know.

 

Write j.i.m.1.@.d.i.s.c.o.v.e.r.l.i.v.e.s.t.e.a.m...c.o. to ask about use of  this article.


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