Why Don't Model Leading Trucks Work?

Why Don't Model Leading Trucks Work?

Post by WMFisher » Fri, 21 Mar 1997 04:00:00



  On the 12"/ft railroads, the leading truck on a steam engine must
work to keep the engine from derailing by transmitting some torque
to the engine frame  on a curve.  (I suppose the smaller wheels have
less of a tendency to jump the track, or there wouldn't be any point
in having them.)  So how come almost all of the leading trucks
on model steamers are fastened with just one screw, so they can't
actually help keep the frame rotating into the curve?  Doesn't
the physics scale down?
  (One exception was the Varney Casey Jones I had in the 50's,
which had a very nice centering spring on its lead truck.)

 - Bill F.

 
 
 

Why Don't Model Leading Trucks Work?

Post by Peter G. Olivo » Fri, 21 Mar 1997 04:00:00



Quote:

>  On the 12"/ft railroads, the leading truck on a steam engine must
>work to keep the engine from derailing by transmitting some torque
>to the engine frame  on a curve.  (I suppose the smaller wheels have
>less of a tendency to jump the track, or there wouldn't be any point
>in having them.)  So how come almost all of the leading trucks
>on model steamers are fastened with just one screw, so they can't
>actually help keep the frame rotating into the curve?  Doesn't
>the physics scale down?
>  (One exception was the Varney Casey Jones I had in the 50's,
>which had a very nice centering spring on its lead truck.)

No.  To be perfectly brief about it.

Scale effects impact all kinds of things.  This is one example.  Another
issue is the effect on negotiable radius.  Anything over 8 drivers generates
serious limitations and even a poorly engineered six driver engine will have
problems.  Any resistance from the leading truck will only exacerbate the
problem.
--



 
 
 

Why Don't Model Leading Trucks Work?

Post by Paul Godfre » Fri, 21 Mar 1997 04:00:00


Bill,

I have one of those Varney Casey Jones 10 wheelers, it does have a
centering spring on the lead truck.  I don't run it very much though
because I have broken the stud which connects the side rod to one the
drivers.

Sorry this does not answer your question.  My physics knowledge is not to
scale. :)

Paul

 
 
 

Why Don't Model Leading Trucks Work?

Post by Daniel A. Micke » Fri, 21 Mar 1997 04:00:00


Quote:


> >   On the 12"/ft railroads, the leading truck on a steam engine must
> > work to keep the engine from derailing by transmitting some torque
> > to the engine frame  on a curve.  (I suppose the smaller wheels have
> > less of a tendency to jump the track, or there wouldn't be any point
> > in having them.)

Snip

On 12"/ft railroads, all of the wheels are equalized with levers to
conform to irregularities in the track.  The leading truck (or "bogie")
therefore bears a substantial fraction of the locomotive's weight, about
10% (The old 4-4-0s put about 35% of the weight on the leading truck.).
The truck was enabled to swivel, so that it would steer the locomotive
into the curve, just as the front wheels of an automobile turn to steer
the car.

Model locomotives do not have any equalizing suspension to speak of;
that is the reason that we use oversized flanges to operate on uneven
track.  Therefore the manufacturers of most of the steam models include
the trucks simply for cosmetic reasons.  So if you buy a 4-6-2 Pacific,
you could remove the leading and trailing trucks and make an 0-6-0 out
of it, it would work just as well.  

(Some modelers and some manufacturers do use some kind of spring
arrangement to simulate the equalization and to cause the steering
effect to become effective, but the work involved is somewhat
difficult.)

Dan Mickey

 
 
 

Why Don't Model Leading Trucks Work?

Post by bu.. » Fri, 21 Mar 1997 04:00:00


Sorry, I too cannot help you with the physics.  We do know that physics for the
prototype do not always apply to modelrailroading (e.g. cars rolling down a
hump yard, superelevation to keep cars on a curve).

As far as the lead trucks are concerned, I have had to modify several engines
where the trucks were attached with only a screw.  Two examples were a PFM SF
2-8-0 and 2-10-2.  This was done with the piano wire techniques that I posted a
few weeks back.  This technique does use the frame to both put downward
and center pressure on the truck.

As a review, the techniques works as follows:

1. Drill a hole toward the front of the truck and solder in a handrail stanchion
in the hole on the bottom side.

2. Cut a piece of brass stock into a square that is about 1/4" to 3/8."

3. Drill a hole in the center rear so you can attach the brass square to the
coverplate with the front coverplate screw.

4. Solder a piece of light piano wire to the square brass plate.  (Be careful
to use an old pair of cutters or Dremmel cutting disk, becuase the piano wire
is extremely hard)

5. Push the wire through the stanchion, and then install the square brass plate
with the coverplate screw.

6. Cut the piano wire so that it is long enough to work when trucks are to the
exctreme right and left.

7. Adjust the downward tension by bending the piano wire as needed.

Jim Budde
K SF & P RR


Quote:

> Bill,

> I have one of those Varney Casey Jones 10 wheelers, it does have a
> centering spring on the lead truck.  I don't run it very much though
> because I have broken the stud which connects the side rod to one the
> drivers.

> Sorry this does not answer your question.  My physics knowledge is not to
> scale. :)

> Paul

 
 
 

Why Don't Model Leading Trucks Work?

Post by Moritz Gretzsche » Fri, 21 Mar 1997 04:00:00


Quote:

>   On the 12"/ft railroads, the leading truck on a steam engine must
> work to keep the engine from derailing by transmitting some torque
> to the engine frame  on a curve.  (I suppose the smaller wheels have
> less of a tendency to jump the track, or there wouldn't be any point
> in having them.)

Indeed the big driving wheels have a bigger tendency to climb on the
flanges, as, when the flange touches the rail, this occurs under
a smaller vertical angle. Thats why shunter locos without leading
axles are limited to comparativly slow velocities.

Quote:
> So how come almost all of the leading trucks
> on model steamers are fastened with just one screw, so they can't
> actually help keep the frame rotating into the curve?  Doesn't
> the physics scale down?

They do scale down, but under quite sophisticated laws,
which cannot easily be applied to model railways.
I my company we run a 1:5-scale experimental bogie on a roller
rig, so we know the similarity laws quite well.
Masses, inertias, stiffnesses, velocities and so on
all have a different scaling factor, if the dynamical running
behaviour of the model shall be the same as of the prototype vehicle.
(This can be compared to the similarity laws used in aerodynamics
to reproduce the flow and turbulences. Perhaps you remember
the Reynolds factor)
For example, if the scale of length is 1:n,
the masses scale with n**2, the inertias with n**4,
the velocities with SQRT(n), and the stiffnesses with n.
So in order to reproduce the running behaviour of a 100-tons loco
running at 100 km/h, a HO-model would have to weigh about 13 kg
and to run at 11 km/h. You see that those parameters are very
different from those known on model railways.
So you would have to scale the density, which leads to a scaling
factor for the stiffnesses of (n**2) * (scaling of density),
lets say about 5000.
Assuming a stiffness of the bogie-centering-spring of 1.000.000 N/m,
the model spring would have about 200 N/m.
A lateral displacement of 1 mm would result in a force of 0.2 N
(corresponding to 20 Gramms).
This might be a sensible amount for true-scale curves.
But model railways run much, much narrower curves, so the
lateral force would cause a derailment, and not avoid it.

As mentioned above, the physics of model railways are completely
different of those of the prototype: The curves are narrover, and
the masses and inertias cannot be compared at all.
So the model vehicles have to be designed for their special
purpose, and cannot be a "scaled" copy of the prototype.
For most locos the guiding forces of the drive axles are
sufficient, and the leading bogies exist just for optical reasons.

Quote:
>   (One exception was the Varney Casey Jones I had in the 50's,
> which had a very nice centering spring on its lead truck.)

Fine. Of course thats possible, but if the bogie shall have a
real leading function, it has to be pressed to the rail with
a considerable amount of the locos weight, and not just by its own
gravity. So a suspension is needed, which is probabely very hard
to adjust, and might cause problems at the "bumps" and sharp changes
of gradient found on many layouts. Again, also the suspension cannot
be scaled correctly, and most model locos dont have any suspension
at all. And in case of a lead bogie suspension, the friction force of
the drive axles becomes less, and the traction force decreases.
Not to forget the danger of the drive axles to derail now.
And the mass of a model loco and therefore the vertical force is
already much to small according to scaling laws.

I think for all these reasons most manufacturers dont do so,
as the centering spring is not really needed on models,
the effort would be considerable, and it would be a lot of
engineering work to be done to adjust the undercarriage
in order to work safely under all conditions.

--

http://www.op.dlr.de/FF-DR/dr_fs/staff/gretzschel/gretzschel.html
Deutsche Forschungsanstalt fuer Luft- und Raumfahrt
Abteilung Fahrzeug-Systemdynamik

 
 
 

Why Don't Model Leading Trucks Work?

Post by Gary M. Collin » Fri, 21 Mar 1997 04:00:00


Quote:

> (Some modelers and some manufacturers do use some kind of spring
> arrangement to simulate the equalization and to cause the steering
> effect to become effective, but the work involved is somewhat
> difficult.)

> Dan Mickey

Well, it depends, Dan.  I've done very effective springing/centering
jobs on 2-wheel lead trucks with two brass handrail posts and a length
of semi-hard brass wire (.020" handrail wire):

  Drill a mounting hole for a handrail post  in the loco frame (usually
the axle cover plate)on the centerline, about a quarter-inch behind the
lead-truck mounting screw.  Drill another one in the lead truck frame,
near the axle.  Both holes on the centerlines, of course. Solder a
length of wire that is longer than the distance between the two mounting
holes into one of the handrail posts.  Mount that post solidly into the
hole in the loco frame, with the wire pointing toward the lead truck.  
Bend the wire slightly downward (away from the loco frame), to provide
the springing action. Thread the other post onto the wire and mount it
(also solidly) on the lead truck.  The lead truck is now centered and
sprung, and will provide the leading function in proportion to the
hardness of the wire used.

--

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
"Jump, Sim!  Jump!" - J. L. Jones - Apr. 30, 1900

 
 
 

Why Don't Model Leading Trucks Work?

Post by Andy Sperande » Fri, 21 Mar 1997 04:00:00


What Jim Budde described is very similar to a MODEL RAILROADER article
by Bob Higgins in the March 1971 issue, page 40, "Truck centering springs for

That's always looked like a good idea to me, and I'm hoping to try it out soon.

So long,

Andy Sperandeo
MODEL RAILROADER Magazine

 
 
 

Why Don't Model Leading Trucks Work?

Post by Randy Gordon-Gilmo » Sat, 22 Mar 1997 04:00:00



said...

Quote:
>in having them.)  So how come almost all of the leading trucks
>on model steamers are fastened with just one screw, so they can't
>actually help keep the frame rotating into the curve?  Doesn't
>the physics scale down?

Bill,

Iain Rice's book _Locomotive Kit Chassis Construction in 4mm_ has a whole
chapter devoted to the construction and springing of leading and trailing
trucks ("bogies" and "pony trucks" in the UK).  He argues that the physics
certainly do scale down, and that the proper design of the trucks improves the
tracking of model locomotives, just as on 1:1.

Randy
--

Randy Gordon-Gilmore  // =   =  === ==  || ==   == == = || == == ==  = == =|
Benicia, CA, USA     /-O==O------------o==o------------o==o-----------o==o-'
                                -=[CB&Q 9900 "Pioneer Zephyr"]=-

 
 
 

Why Don't Model Leading Trucks Work?

Post by Janos ER » Tue, 25 Mar 1997 04:00:00


Quote:

> Iain Rice's book _Locomotive Kit Chassis Construction in 4mm_ has a whole
> chapter devoted to the construction and springing of leading and trailing
> trucks ("bogies" and "pony trucks" in the UK).  He argues that the physics
> certainly do scale down, and that the proper design of the trucks improves the
> tracking of model locomotives, just as on 1:1.

Randy,

Iain Rice's fellow, Guy Williams puts a different opinion in his book:
Scratchbuilding 4mm Engines (sorry, I don't have here the exact title). He states
that the "bogies" have little role in such a small scale, thus he prefers make
them as heavy as possible, but fixing only by a***through the truck's oval
hole. The same opinion can be read in Gordon Odegard's MR article-series. BTW the
task is quite complicated: you have to pull the truck to the center and at the
same time push it down to the rail. A simple pushing spring will push it _away_
from the center!

BTW, aren't both the above British gentlemen from Pendon ? :-)

Janos Ero

 
 
 

Why Don't Model Leading Trucks Work?

Post by Reginald Barro » Wed, 26 Mar 1997 04:00:00


Quote:


>> Iain Rice's book _Locomotive Kit Chassis Construction in 4mm_ has a whole
>> chapter devoted to the construction and springing of leading and trailing
>> trucks ("bogies" and "pony trucks" in the UK).  He argues that the physics
>> certainly do scale down, and that the proper design of the trucks improves the
>> tracking of model locomotives, just as on 1:1.

>Randy,

>Iain Rice's fellow, Guy Williams puts a different opinion in his book:
>Scratchbuilding 4mm Engines (sorry, I don't have here the exact title). He states
>that the "bogies" have little role in such a small scale, thus he prefers make
>them as heavy as possible, but fixing only by a***through the truck's oval
>hole. The same opinion can be read in Gordon Odegard's MR article-series. BTW the
>task is quite complicated: you have to pull the truck to the center and at the
>same time push it down to the rail. A simple pushing spring will push it _away_
>from the center!

>BTW, aren't both the above British gentlemen from Pendon ? :-)

>Janos Ero

I believe Bob Darwin explained how to make prototypical performing
lead and trailing trucks in MR around 1961.

Reg Barron

 
 
 

Why Don't Model Leading Trucks Work?

Post by Terry Fly » Wed, 26 Mar 1997 04:00:00



Quote:

>> (Some modelers and some manufacturers do use some kind of spring
>> arrangement to simulate the equalization and to cause the steering
>> effect to become effective, but the work involved is somewhat
>> difficult.)

>> Dan Mickey

>Well, it depends, Dan.  I've done very effective springing/centering
>jobs on 2-wheel lead trucks with two brass handrail posts and a length
>of semi-hard brass wire (.020" handrail wire):

Snip

I had a brass 2-8-0 with this type of springing. It caused the front
bogie to derail. Adjustment was difficult to get right, so I removed
the wire spring, now have no trouble with derailments and have
increased the reactive effort of the model slightly.

Terry Flynn.

 
 
 

Why Don't Model Leading Trucks Work?

Post by Terry Fly » Wed, 26 Mar 1997 04:00:00


Quote:


>said...

>>in having them.)  So how come almost all of the leading trucks
>>on model steamers are fastened with just one screw, so they can't
>>actually help keep the frame rotating into the curve?  Doesn't
>>the physics scale down?

>Bill,

>Iain Rice's book _Locomotive Kit Chassis Construction in 4mm_ has a whole
>chapter devoted to the construction and springing of leading and trailing
>trucks ("bogies" and "pony trucks" in the UK).  He argues that the physics
>certainly do scale down, and that the proper design of the trucks improves the
>tracking of model locomotives, just as on 1:1.

>Randy

I certinally cant feel or see the improved ride of models that are
sprung. I does improve electrical pickup and allows fine scale flanges
to be used.
 
 
 

Why Don't Model Leading Trucks Work?

Post by Terry Fly » Wed, 26 Mar 1997 04:00:00


Quote:

>  On the 12"/ft railroads, the leading truck on a steam engine must
>work to keep the engine from derailing by transmitting some torque
>to the engine frame  on a curve.  (I suppose the smaller wheels have
>less of a tendency to jump the track, or there wouldn't be any point
>in having them.)  So how come almost all of the leading trucks
>on model steamers are fastened with just one screw, so they can't
>actually help keep the frame rotating into the curve?  Doesn't
>the physics scale down?
>  (One exception was the Varney Casey Jones I had in the 50's,
>which had a very nice centering spring on its lead truck.)

> - Bill F.

The flange on the railway wheel steers both the model and the
prototype. The reason why the leading bogie wheels are small is so
maintance is easy and the wheels clear the locomotive cylinders. The
main reasons to use a trailing bogie is it allows a larger fire grate,
and fire box. The main reasons for the leading bogie is to limit out
of ballance forces at speed from the recriprocating pistons,  decrease
flange wear on the leading drivers and decrease maxium axle load. All
these reasons are not a problem with HO scale models, therefore
springing bogies on HO scale model steam outline models is usually a
waste of time. The only necessary case is when you are using bogies as
extra electrical pickups on small tank engines.

Terry Flynn.