# Pony truck - springs or weight?



## DKRickman (Mar 25, 2008)

I'm seeking the opinions, advice, and collected wisdom of those with more expereince that myself. 

For a 2-4-0 bashed out of a Bachmann side tank 0-4-0, and using Sierra Valley wheels for the pony and tender trucks, would I be better off with some sort of spring centering/loading arrangement, or simply placing lead weights on the truck? I know that the former is prototypical, while the latter is more common in models. What are the pros & cons of each, from those who have tried both, or at least seen examples of both in operation?


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## DKRickman (Mar 25, 2008)

I thought I'd add a comment about my idea for a spring arrangement, now that I'm awake enough to think about it.  

I'd start by attaching a pair of square styrene rods to the bottom of the truck, level and parallel to each other. I'd then add a matching pair of rods to the chassis, also parallel, but with their forward end lower than the rear. Slipping a piece of piano wire through the holes in the center of the rods, and attaching them at one end with a setscrew, would create a simple spring. It would lightly supply centering pressure to the truck, and press down to a degree depending on the mismatch in angles of the rods and stiffness of the wire. The stifness fo the spring could be varied easily by replacing the wires. 

Does this sound like a workable idea?


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## East Broad Top (Dec 29, 2007)

If I'm understanding correctly, it does sound workable, though you may be limited on how far side-to-side the truck can swing. I'm also not sure you'd need to slant the upper square tubing. Instead, just bend the piano wire to match the alignment between the frame and truck. 

Personally, I wouldn't worry too much about centering on the pilot or trailing trucks. I designed my front/rear suspension with a means for centering, and found it didn't make a lick of difference in performance. I think it's more important to have some kind of downward force as close to the axle itself as possible. The locos with that kind of arrangement are far more reliable than those whose springs act closer to the fulcrum. Also, if possible, have (as you describe) two springs, so to apply force to both ends of the axle, not just the middle. That will help keep the wheel in firmer contact with the rail, and counter its desire to ride up over the rail. From a physics standpoint, the difference between a center spring and two side springs acted upon on the distances we're using isn't that great, but from a practical standpoint it makes a good bit of difference when going through something like a spring switch where the front pilot wheel needs to have enough weight to force the points apart. 

Here's a diagram of the suspension I designed: 









If I were to implement this again, I'd move the spring much closer to the axle. The tradeoff is that there's more swing, and you need to have a bit more length on the spring as it slides through the retainers in the side. This may or may not be possible depending on the room you have available between the frames. 

Later, 

K


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## Torby (Jan 2, 2008)

Why the centering spring? Seems to me you just want to hold it down. I have a couple ounces of weight on my 4-6-0's pilot. Nothing on my 2-8-8-2.


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## Stan Cedarleaf (Jan 2, 2008)

Ken......... In my first experiences with the B'mann 4-6-0, I just put lead weights over the front axle and that took care of most of the problems. Barry Olsen of Barry's Big trains then developed a pivoting front truck for later applications. I would think that added a bit of weight over the axle might solve your problem...


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## DKRickman (Mar 25, 2008)

Posted By Torby on 09/23/2008 1:03 PM
Why the centering spring? Seems to me you just want to hold it down.


Prototypically, the truck would help lead the engine into a curve by a swing bolster or (usually in England) spring arrangement. It helped the engine be stable at speeds above a crawl. I suspect that the same may be true in models, so I figured I'd ask from those with real world expereince. 

Posted By Stan Cedarleaf on 09/23/2008 1:54 PM
I would think that added a bit of weight over the axle might solve your problem... 


Stan, 

I don't have a problem, yet.  Of course, that's because I don't have the model built yet, and because I have no track on which to run it at the moment. Just trying to decide if it's worth the effort to design and build something more complex than a dead weight on the truck.


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## Greg Stevens (Jan 3, 2008)

Ken, 

In the first place, you are only talking about an ounce or less of weight. Not much for your loco to push, but it really helps when it comes to keeping the lead truck from climbing the rails. If you aren't a rivet counter, it shouldn't be a problem. Secondly, if you add the spring(s), it is only one more thing that you have to worry about adjusting and/or snagging something along the way. Plus, you would have to be very careful with your track. If it is not completely level and in gauge, you might have more problems than you want. I'm not saying that the spring arrangement isn't a good one, but I am a real believer in the KISS system. (Keep It Simple Stupid)


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## david bunn (Jan 4, 2008)

I've not used this method in large scale but it has worked very well in my 7mm models. The idea is to use a nylon cable tie as the connection between the truck and the pivot. The inherent spring in the material provides the downforce and also allows for the truck to move vertically by different amounts on either side.Cable ties are available in loads of different sizes so you should find one to suit most applications. 
Regards 
Bunny


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## W3NZL (Jan 2, 2008)

Its been my experience over the years that with the way oversize flanges 
that R typical of model train drivers, "steering" pilot trucks R totally 
useless... I weight all mine if possible, an if weighting isn't a practical 
idea, I use a leaf spring arrangement exerting downward pressure... 
Paul R...


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## Torby (Jan 2, 2008)

In models, we don't use our pilot trucks the way they do in 12 inch scale.


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## FH&PB (Jan 2, 2008)

There's no reason why we shouldn't use our pilot trucks as in 12"/ft scale, though. They work just fine and do actually smooth out the ride some (you may not notice it on your 20' radius curves, but those with 4' radius probably will). 

Weights might do the job, if "the job" is just keeping the pilot wheels on the rails, but then you're just running a 0-6-0 with a problem-causing dingus behind the cowcatcher, rather than a 4-6-0 (or whatever). 

Go with springs. If you're building it yourself, build it right.


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## DKRickman (Mar 25, 2008)

Having finally got around to mounting the front truck, I've been working on the spring arrangement. I'll post pictures as soon as the glue cures enough that I can mount the springs and see how it works. I'm hoping to be able to test run the engine at docwatsonva's get-together this weekend, so I'll let everyone know how it works.


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## TrotFox (Feb 15, 2008)

Springs! For a host of reasons, not the least of which is because that's what I did on my Ruby! } ; ] 

http://1stclass.mylargescale.com/trotfox/pix/RubySuzy/ponypivt,jpg.JPG 

Trot, the out-of-touch, fox...


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## DKRickman (Mar 25, 2008)

Well, here's what I came up with: 









There seems to be just the right amount of downward pressure on the truck. Of course, it would be nice if I had any expereince at all with a large scale locomotive, so I would know what "just the right amount of pressure" really was, but it feels pretty good to me. 

It may be too stiff side-to-side. With the bare, unweighted chassis on the workbench, pushing sideways on the truck wheels will move the entire assembly instead of causing any significant swing. I'm hoping that with weight on the chassis, it will be enough to lead the engine without climbing the outside rail in a curve. I may have to restrict the curve radius, or swap in a softer spring.


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## DKRickman (Mar 25, 2008)

Here's a question for those of you who have experience in this such matters... 

How stiff is too stiff, regarding sideways motion of a lead truck like this? The rigid wheelbase is 70mm, and the total wheelbase is 157mm 

Does anyone know roughly what deflection I can expect on various radii?


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## TrotFox (Feb 15, 2008)

Kenneth, 

I do not have the info on deflection in curves. I work on a 'good enough' basis. } ; ] 

However, I must mention that I think your truck frame is too long. As it sits the wheel flange may try to climb a curve simply because it will present flange edge instead of flange side. I would suggest putting that pivot under the saddle, right between the cylinders. This will give the pilot wheels more turn in during a curve. 

Again, this is just my guesswork. I do like your spring application, though I suspect you are correct about the horizontal deflection being too stiff. 

Trot, the mind-full, fox...


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## DKRickman (Mar 25, 2008)

Trot,

I can now report that the truck works wonderfully. Total deflection, even on a 5' radius curve, is barely a few mm. The wheel is not perfectly parallel to the rail, but not badly enough to cause any problems. Turnouts were no problem either.

As for the length, there's a very good reason for building it the way I did - that's the forward-most point where I could conveniently attach the pilot! For what it's worth, I found a number of prototype 2-6-0 and 28-0 plans which show a similar pivot point. Of course, there's a difference between model and prototype. Bear in mind, though, that this is a very short model. The total wheelbase is only 157mm - only 8mm longer that the rigid portion of a Connie, and 24mm shorter than the rigid portion of a Spectrum mogul. In my opinion, the shorter the driver wheelbase on an engine, the more important it is to have a working pilot truck.


As a side benefit, the longer swing means that the wheels stay up front where they belong, rather than swinging back and hitting the cylinders.


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## FH&PB (Jan 2, 2008)

I just caught up on this thread, and my first impression of your photo was "that looks like it will work perfectly!" I'm glad to hear that it did. I think you'll be a lot happier with it that way!


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## Fritz (Jan 11, 2008)

Hi, 

Well for years I sometimes had problems with derailing pilot wheels on some locos. Mainly US-products on LGB track. Tried weight and springs. Sometimes heavier steel wheels. Wobbly diecasts or plastic does not work to well. 

Adjusting the gauge of the wheels helps most of the time. Seems that 45mm are not 45mms all over the world. Converting metric to imperial and vice versa seems not to be an easy task for some engineers. 

Have Fun 

FRitz / Juergen


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## TrotFox (Feb 15, 2008)

Good work and congratulations! 

I forgot to mention one of the reasons I like sprung over weighted... reduced inertia! A weighted truck could jump track due to the weight being accelerated upward by a bump A lighter, sprung, truck is less likely to do that due to the reduced weight and action of the springs. 

Have fun! 

Trot, the semi-werkin', fox...


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## DKRickman (Mar 25, 2008)

Trot, 

Thanks for the compliment. 

The automotive guys figured out a long time ago that reducing unsprung weight is a good thing, for exactly the reason you mention. Since physics doesn't really care whether it's working on a locomotive or a car, I figure that the same rules apply.


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