[How do I?] Help Me Build My Bogies

Ok, I thought this was going to be easy and productive but it's becoming a nightmare. The numbers I'm getting just aren't realistic.

Here's my thinking:

I was going to simply calculate what the inner and outer diameters of a tapered wheel would need to be to make both wheels turn the same number of times on a 24" radius track. It turns out to be significantly different diameters. The larger diameter I would choose to be 1.25" (same as on the Bachman model I'm using). But then when I calculate what the smaller diameter would need to be it comes out to only be about 0.86", that would be one whale of a tapered wheel flat! Either I'm doing something wrong or this just isn't going to work out as nice as I thought it might.

Here's My Actual Calculations (done in a sleepy state of mind, so I may have made some major mistakes hopefully)

First, I decided to just arbitrarily choose a 90 degree curved section of track for the calculation. This makes calculations easy since it's just 1/4 of a circle.

So here's where I start:

The center line of curved section of track is 24" radius. The distance between the rails is 1.750"

Using those numbers the radius of the outer rail is just 24" + 1/2(1.750") = 24.88" radius
The radius of the inner rail is 24" - 1/2(1.750") = 23.13" radius.

Now armed with those radii I can calculate the length of those rails.

The circumference of a circle just Pi times the diameter. Since I'm only working with 90 degree turn I include a factor of 1/4. (i.e. I'm only working with 1/4 of the circle).

So to find the length of a rail I use the formula:

1/4 (one-forth of a circle) times (2 x the radius i.e. the diameter) times Pi.

So the length of the outer rail on a 90 degree turn is 1/4 x (2 x 24.88) x Pi = 39.07"
And the length of the inner rail on a 90 degree turn is 1/4 x (2 x 23.13) x Pi = 36.32"

So the difference in length of travel over the outer rail compared with the inner rails is 39.07" - 36.32" = 2.75"

This means that the outer wheel must travel 2.75" (almost 3 inches) further when going around these tight bends.

Now I choose an arbitrary large diameter for the bogie wheel. I'm using 1.25" since this is what the wheels on my Bachman train set measure to be. By the way, the smaller diameter is only about 0.015" smaller. Not much difference at all.

So onward with my calculation!

Now I figure out how many times the wheel will need to turn to traverse the outer rail.

First I calculate the circumference of the wheel. That's just Pi times the diameter. I choose to the diameter to be 1.25" so the circumference of the wheel is 1.25" x Pi = 3.93". In other words every time this wheel rotates one revolution it travels 3.93" down the rail (assuming no slippage).

Now I just figure out how many times the wheel needs to turn to complete a 90 degree curve. And that is just the distance of the rail divided by how far the wheel travels per revolution. Or 39.07" / 3.93" = 9.95 turns.

So the wheel has to make about 10 revolutions to go around a 90 degree turn on the outside rail.

So how to find out what the diameter of the smaller radius would need to be to keep the inside wheel turning the same number of turns we just use that 9.95 number of turns to figure that out.

The inner wheel must turn 9.95 turns to travel over the length of the inner rail which we had previously calculated to be 36.02". So we just divide "36.02" / 9.95 turns = 3.56".

So the smaller diameter of the bogie wheel needs have a circumference of 3.56"

Now we can calculate what diameter it must have.

The circumference of a circle is given by Pi times the diameter, so the diameter must be given by Pi divided by the circumference.

So we have Pi / 3.56" = 0.86"

WOW! That a lot smaller than the larger diameter of 1.25" In fact it's 1.25" - 0.86" = 0.39

That's a big difference. Way bigger than what Bachman is using. In fact, this difference would create a wheel that has a seriously slanted surface that rides on the rails.

~~~~~

Conclusion, either I've made some major mistakes here, or a 24" radius curve really is so sharp that it would require this steep of an angle to make the wheels run true.

If these calculations are right, I'd be better off allowing the individual wheels to just spin freely on the axles and not use a solid axle at all.

I may have made a major mistake in my calculations and methodology. I'm tired. I'm going to bed.

I was hoping to get a number I could work with, but a 0.39" difference between the large and small radius of a 1/4" wide wheel flat would be enormous. Seriously not good. Like I say, this Bachman model only has a difference in diameter across the flat of about 0.015". Hardly noticeable.

But now that you've mentioned this whole thing I'm seriously thinking of just making the wheels independently free to rotate however they like. That way the outer wheels can just turn more times with no problem.
 
James, you don't need to go through all of those calculations as that work has already been done, parts made, and tested over time. Simply refer to the GIMRA Gauge G wheel standards located at this link: http://www.wis.co.uk/andy/16mm/standardtrackdimensions45.html#GIMRA WHEEL & TRACK STANDARDS

A pair of wheels on an axle will find their own 'center' for any curvature of track within reason. 24" radius track is most likely a Bachman standard, and you should have no problems if you simply follow the tried and true dimensions given.
 
Hi Terry,

I know I shouldn't need to do my own calculations. But once you explained the physics of what's going on I couldn't resist the temptation to "tweak" the design specifically for the tight curves that I am using. Even though, as you say, the 24" radius is a Bachman standard. But after doing the calculations I quickly realized that the wheel taper would need to be extreme to truly all for both wheels to turn the same number of times around a curve. There's no way that I'm even thinking of changing the design to that extreme taper. The wheels would look silly, and probably wouldn't work very well anyway.

However, now that I actually see what's going on physically on this model with 24" radius curves it has me thinking that maybe I would do well to design in freely independent wheel rotation. Is there really any need to have both wheels "locked" together in terms of rotation? Independently rotating wheels would allow every wheel to turn as many times as it likes. It seems to me that this should actually reduce some friction. This would make the trains roll more easily. Be easier for the engines to tow, and potentially reduce wear and tear on the track. This latter point could be important when using wooden rails. If' I'm constantly "dragging" this wheels around the curves, they've got to be wearing down the rails too.

So rather than trying to deal with this by using custom tapers on the wheel flats, I might just design for independent free rotation of all the wheels. This way I can be sure that there's no unnecessary drag, friction, or "sanding down" of the wooden rails every time a train goes around a bend.

I just feel in my gut now that freely rotating individual wheels might be a worthwhile design feature. Otherwise, there going to be some necessary "dragging" going on.

In the real world I don't think this is a problem for actual railroads, because not too many real railroads have curves anywhere near as sharp as a G-scale 24"radius. Real railroads typically have very gradual bends. And specialty railroads that have sharp turns (like a logging railroad) might have actually recognized the need for independently rotating wheels? I don't know about that. But independently rotating wheels "feels" very inviting to me after hearing the physics of how railroad bogies negotiate curved track.

My gut is just screaming at me, "Free those wheels up to rotate independently!" I can't sleep at night anymore until I redesign these bogie axles to let the wheels spin individually.
 
I'm going to offer some unsolicited advice. Just build version one.

You can tweek and adjust afterwords, if you try to work out everything now you may never get very far.

That is to say unless this is the part of the hobby you really enjoy. Then plan away and keep us posted either way
 
I agree with Ralph. Try building a truck with independent wheels and see what happens. I've got a hunch that you may be over-thinking this and that independent wheels may not give the effect you desire, in addition to complicating the design and fabrication of the trucks. If there were an advantage to independently rotating wheels, one would tend to think that the full sized railroads would be using them. Small improvements in efficiency mean huge dollar savings to them. Remember that the two wheels being tied together (turning at the same speed) is part of what makes the axle/wheel assembly self center on the rails.
 
@Ralph,

I agree. I'm one who really likes to just dive in and work out bugs later. But then again, I have to confess that I've been bitten quite a view times in the past by using that strategy as well. I've learned the hard way that there is also some wisdom in taking the time to get it right before you build. So there's value in both of these schools of thought to be sure.

I have tons of things holding me up from the actual build in any case. Moreover, my first activity on the agenda is to just make the wheels, and I can do that much even before I decide whether or not to allow them to rotate individually. So at this point in time, the designing isn't exactly "holding up" the actual work.

Just as an update, the Aluminum Bar Stock has arrived. Also the 1/8" stainless steel rod for the axles has also arrived. So I have the raw materials ready and waiting for machining.

I'm currently waiting for some new bandsaw blades to arrive for cutting the Aluminum Bar Stock down to manageable pieces. Those bandsaw blades are scheduled to arrive by this coming Saturday. So hopeful, work on making wheels can begin shortly. I need to sharpen up some lathe tools for the job too. My shop is an absolute disaster area, and the past few days I've been working on cleaning it up and organizing things. But at my age, and imperfect health, things don't seem to progress as rapidly as they used to. :grin:

I've been going out to the shop with big ambitious and coming back in to lay down for a rest with some disappointment that I wasn't able to accomplish as much as I had first hoped. But I keep chipping away at this thing. :encourage:
 
By the way, I made 300 railroad ties yesterday. So the track is starting to take form. :)
 
terrywerm said:
I agree with Ralph. Try building a truck with independent wheels and see what happens. I've got a hunch that you may be over-thinking this and that independent wheels may not give the effect you desire, in addition to complicating the design and fabrication of the trucks. If there were an advantage to independently rotating wheels, one would tend to think that the full sized railroads would be using them. Small improvements in efficiency mean huge dollar savings to them. Remember that the two wheels being tied together (turning at the same speed) is part of what makes the axle/wheel assembly self center on the rails.
Click to expand...

I'll make some of each kind.

It's going to be a very long time before I'll be able to try the bogies out anyway. Before I can try them out I need to actually build the wooden track, lay it down, AND have a battery operated train to run on it. I'm pretty far from having any of those things available to be testing these bogies out on.

By the way, I'm kind of starting this "Bogie Project" a bit early. Precisely to give me time to work out the design.

I already have standard Bachman trains that I expect to run on my wooden track in the early going. Of course the Bachman engines need to be converted over to battery power and R/C control, which I haven't started in on yet either. So this is the very beginning of this whole entire project. It's going to be quite a long time before we see any actual trains running around the track.

But even if the standard Bachman bogies technical work (i.e. they don't derail and appear to be dependable), that still doesn't answer the question of whether freely rotating wheels might provide more longevity to the wooden rails in the long haul. What I can do is run a standard Bachman train for a while and take special note of whether it appears to be "chewing up" the curved track more so than the straight sections.

This is something that would not likely be an issue at all with metal rails. But with wooden rails it could be an issue if wheels aren't freely rolling over the tracks.

Then again, I could be imagining unnecessary nightmares. :eek:

But I think these ideas are worth considering in any case.
 
Loved the write-up Terri. I've been around trains growing up a city block from the Union Pacific railroad (our haunt for getting into trouble, squishing coins, hunting birds/rabbits and dirt biking), HO model trains and a visit to historic Rail Town park in Jamestown, CA, and I've never heard it explained that well. Good Job!

James, nice project! I too would suggest using Delrin for the all of the same reasons.
 
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