truck tuning: consistent delrailment on a tight radius

[freedj]

I have an Atlas SD35 that consistently derails the long hood truck when running long hood forward through a specific part of a 10" radius curve made from flex.  I followed my track template to the best of my ability and used easements when drawing it up.  It LOOKS good to me.

It does not seem to derail running short hood forward through the same section.

I have put the calipers on the trucks between the flanges:

+/- 0.05mm

short hood
7.53mm
7.49mm
7.52mm

7.58mm
7.61mm
7.58 mm
long hood

The trucks have a 25.4mm spacing between the leading and trailing wheel.

All the wheels on both trucks pass the NMRA standards gauge.

When the long hood truck is trailing I don't seem to have as much trouble.  There are no joints in the track work where the derailment occurs.

I know that i am pushing my luck with a 10" radius, but its what would fit in my space.
I feel like regauging the center wheel in each truck to be slightly narrower (if possible) might help this situation, what is everyone else's experience? 
 
 

[carlso]

freedj,

I am, by no means, an expert but I offer this as a possibility.

Make sure the pickup strips that are attached through the tank area are straight and not pushed off to one side. The tab on the top of the truck must ride on these strips and not to one side. I have seen this happen and it caused derail problems.

Carl

[wm3798]

Also double check the trucks to make sure there's nothing binding a gear or causing some other thing to go bump in the night.  A tiny wad of ground foam or ballast caught in the gear grease can cause just enough of a wobble to make bad things happen.  Pull the trucks and clean the mounts thoroughly and see what happens.

Then rebuild your layout with minimum 12" curves!

If you have other 6 axle engines, see how they do.  I believe most manufacturers recommend a minimum 11" radius for the bigger stuff, although SD35s are relatively short wheelbase...  Consider also that the trucks are mounted off center, so the lead axle going in is going to be under slightly different conditions than on the lead axle of the second truck.  They are reversible, however, so if you pull the axles and reverse them, and you get the same result with the same truck on the lead, regardless of which way the hood is facing, then you've isolated the problem to the truck, and not the track.

Lee

[diezmon]

Is the truck installed in the right direction?   I put a loco back together once, and had one truck reversed, which caused it to 'pinch' more in one direction than the other.

[NtheBasement]

Two guesses: check the track gauge and check that the trucks swivel freely when the loco is placed on a flat slippery surface.

If you have ever taken off the trucks and the SD35 trucks have off-center contacts (some Atlas trucks are symmetrical), make sure the contacts are inboard of the tower, not outboard which places them too far from the contact strips.  Otherwise the contact strips, besides failing to contact, can keep the truck from swiveling.

If you are using Atlas flex or another brand where one of the rails is firmly attached to the ties and the other slides around, the sliding rail's top tends to tip toward the inside of the curve.  If this loose rail is the on the outside of the curve you can end up with too narrow a gauge when it tips.  Fix is to relay the track with sliding rail on the inside; that way if it tips you get too wide a gauge which is not a critical problem.

[randgust]

Since it's really easy to drop the trucks on any Atlas diesel just by loosening the frame screws, that's another thing to check.

Simply take that offending truck off and roll it through that curve with your fingers and see if you can feel anything binding, rubbing, etc.

If that passes, you're down to the relationship between the truck and the frame.  More finger checking on rotation and feel can help - hanging on the steps, fuel tank, pickups, or just the frame is slightly warped or too tight and the truck doesn't rotate freely.

As Lee says, it's also possible you've got a truck reversed, that can make weird things happen but you should be able to feel it on truck rotation in the frame.   There is a 'front' and 'back' to each truck and they are not symmetrical even though they may look it.    I typically mark my Atlas trucks with a knife point and draw an arrow in the bottom or an "F" so I can't accidentally get them turned around.

[wm3798]

They are not symmetrical in the SD35, but they can be swapped front to back.  You just have to keep the trucks rotated toward the coupler mount when you reinstall.  They are electrically symmetrical, so it won't run backward or screw up the a decoder.

Lee

[nickelplate759]

They are not symmetrical in the SD35, but they can be swapped front to back.  You just have to keep the trucks rotated toward the coupler mount when you reinstall.  They are electrically symmetrical, so it won't run backward or screw up the a decoder.

Lee

Swap the trucks, and see if the problem end is still the long end.  If it is, then the issue is the interaction between the truck and the frame (maybe the pickup strips several people mentioned).  If the problem is now the short hood end, then it's the truck that's at fault.
I note that you said all wheels are in gauge, but your measurements suggest that the gauge on the problem end is slightly wider than the rear.

Also, check your track gauge.  On a tight radius curve, you'll want it a micro-smidgen on the broad side.   If there's a spot that's just a little tight you could just file the inside of the outer rail a little bit and it might help.

[mmagliaro]

Your biggest wheel guage variation is about 0.1mm, or only .003", so I doubt that's the cause of this.

I don't know that engine, so I can't help you there, but I agree with the others who advised you to check the track gauge with an NMRA plate.    If this is Atlas flex track you are using, one rail is fixed and the other slides in the ties, and you need to put the sliding rail on the OUTSIDE of the curve, especially on a curve that tight, because the sliding rail will push outward, making the gauge a tad wide, which is what you want.  If you put the sliding rail on the inside of the curve, it will make the track gauge too narrow and cause engines to climb up and out.

[Point353]

Is this the part of your layout where the derailment is happening?

[CRL]

Your biggest wheel guage variation is about 0.1mm, or only .003", so I doubt that's the cause of this.

I don't know that engine, so I can't help you there, but I agree with the others who advised you to check the track gauge with an NMRA plate.    If this is Atlas flex track you are using, one rail is fixed and the other slides in the ties, and you need to put the sliding rail on the OUTSIDE of the curve, especially on a curve that tight, because the sliding rail will push outward, making the gauge a tad wide, which is what you want.  If you put the sliding rail on the inside of the curve, it will make the track gauge too narrow and cause engines to climb up and out.

Agreed. That’s been my experience with Atlas flex track. Most try to put the sliding rail on the inside because you use slightly less flex track that way, but it will result in narrowing of the track gauge, especially during temperature changes. Put the sliding rail on the outside of the curve.

[nkalanaga]

I had an Atlas SD0, from the first run, that derailed on 11 inch radius curves.  After checking everything else, it turned out that the corner of the sideframe was hitting the steps, but only on one end.  The frame didn't want to stay centered in the shell, and the play was just enough that, at one extreme, the truck hit the step.  A little filing on the corner of the sideframe and the inside of the step solved the problem.  Since the offending portions were behind the steps anyway, nothing showed.

[wm3798]

Have you done the diagnosis?  I'm curious to find out what the hitch was...

[freedj]

Thanks for all your thoughts!  Some of these have turned out to be possible contributors although I do not yet have a resolution.

As @Point353 guessed, the derailment point is in my mini helix.

Both @nkalanaga and @carlso were onto something with truck mobility.  The SD35 is one of the new DCC + sound atlas locos so the trucks are wired directly rather than relying on the pickup strips, but it got me looking at the pickups of my other locos and one of those was out of position!  That loco runs much better now!  When i have the SD35 in my hand, i *can* make the truck hit the steps. Further investigation will be required here.

Since the trucks are wired in, and factory assembled, i doubt they are in backwards.

I was getting a big bump out of the loco at one spot in the curve and i assumed that was the derailment.  I found a bit of molding flash on the inside of the rail there and cleaned it off and now don't get that bump.  I was still getting the derailment though and so watched really carefully and found where it really IS derailing.   It is right at a track splice and of course it is at about the hardest spot to get to.  The spot is on the inside loop where the track splice is (push pin).  I've put the NMRA gauge on it and it all gauges out, but when i run my finger over it i can feel a fairly abrupt change in direction there.

I tried removing the around 4 "spikes" on the outside of the inside rail where the kink is to let that smooth out, but it didn't seem to help.

I am considering cutting that section out and replacing it, but it is right in the middle of a curve, with at least 30" between the spot and an access-able tangent track.  I could use some PCB ties to essentially do a small piece of hand laid track to replace that  section and smooth out the curve without attempting splices, or I could remove a much larger piece, say 180* of the turn so that I could splice both ends in.

I can only kind of see the kink with my eyes, but I can feel it with my fingertips.

Next up is to pop the shell of the loco and see if the problem persists.  If it does I'll do some further investigation on how freely the trucks turn.

Only one loco derails, and only going one direction, so I could just live with it and make it an operational restriction but I really don't like that idea, more locos are an inevitability and it'll be way  to fix now than after i've built up stuff around it.

If I have to replace the track section I am leaning towards the splice method, or failing that, just ripping the track out to the nearest module edge.  In fact, replacing to the nearest module edge would only take 2 pieces of flex.  That may be a way simpler way to go.

[ Guests cannot view attachments ]


 

[wm3798]

Hmm.  When I'm laying track on a curve, especially one that is going to be critically out of reach and difficult to maintain, I will always ALWAYS solder the track joint in a straight line and then bend it around the curve, loose rail on the outside.  Think of it as welded rail for the model world...  Staggered joints are nice when you can get to them, but for those hard to reach spots, a soldered joint is a highly recommended solution.  For expansion, once the line comes back to tangent, leave the first joint on the straight unsoldered.  You rarely need to have more than 2 lengths of track soldered together to make it around a typical model railroad curve.

Lee