This excellent posting appeared on another subject thread and was only warmly received.
I thought it deserved repeating here,...
Quote:
But the difference between 28" and 30" radius might just be enough to trigger a landmine. Here's where some of them are buried.
First landmine - Having watched your Youtube and found your track plan, can I suggest that you have a long read and a slow think about your staging yard and how this thread might relate to what you are planning. If you have to increase your track spacing due to sideswipe considerations though the 90 + 180 + 90 degree bends leading under the stairs then into the run along your wall, then a 12-track yard at "standard track spacings" might be unachievable. Long autorack and passenger cars will be the most problematic rolling stock for sideswipe conditions. You may have to increase your track spacing which will result in fewer yard tracks being possible on the same layout width.
Second landmine - With your track plan, the tightest radius will also change sides as you work your way around from the basement under the stairs to staging. Here's what I mean:
- From the basement 90 degree right turn (with tightest radius on right track)
- to head under the stairs 180 degree left turn ( with tightest radius on the left track)
- after passing the furnace 90 degree right turn (with tightest radius on right track)
- you may have to restrict which tracks in the yard you use for your autoracks to stay away from your planned 28" minimum radius.
- by rights the the tendency to derail should be reduced if you use the centre tracks for your autoracks and any passenger consists. These middle tracks should have a radius bigger than 28"
Third landmine - In-train forces and string-lining. With a 28' long train like you are planning wrapped around multiple +90-degree curves, you are facing probably the worst string-lining tendencies where a train "straightens out the bend" by string-lining and derailing. With long trains your in-train forces are significant and are added to by the lateral forces from the flange resistance from the track at the wheels when being dragged along the inside rail of tight curves. In the real world the tendency to string-line is expressed as an L/V ratio (Lateral forces caused by curve resistance versus the Vertical forces holding the wagon down on the rails. See the Force Balance under static conditions slide For L/V =0.82 in the presentation slide, the car stays on the rails. For L/V =0.83 the outside wheels will lift, the car will string-line and be over on its side. And that's an applied static load with the car not moving. The position is a whole lot more complicated when things start to move on rails.
Fourth landmine - The L/V calculation mentioned previously does not take into account slack run-in and run-out forces or the effect of a model head-end loco or helper loco "hiccupping" on dirty track or a track irregularity that could cause abrupt and sharp increases in in-train forces while your train is working its way through these curves. Have a look at the slide in the presentation for the lateral forces action on the lumber car on various curves. A 1= string-line derailment where the outside wheels lift from the track. Note the car is designed to take a buffing load off 62 kips (force unit) but a lateral force of 10 kips is starting to throw derailment events. At 20 kips sideways force you are in the dirt and over on your side on whatever radius curve. A model locomotive hiccup and the subsequent snatch as the power comes back might cause enough variation of the in-train forces to cause problems.
Fifth landmine - in the previous lumber car example, the curves in the presentation topped out at 12 degrees. A 28" radius in HO equates to about a 200' radius in the real world or a 28 degree curve. This is way tighter than a real railroad would or probably could operate an autorack and have it remain coupled and on the rails. The only way we can bend our models around curves that the prototypes can't is because of some smart design work by our model manufacturers, but there are limits. The increased sharpness of our model curves greatly adds to the sideways forces acting on your wheel flanges. And our model flanges are a lot smaller than the prototype's flanges.
Sixth landmine - model structural limitations with couplings - you are relying on both your couplings and bogies operating at the extreme end of their flexibility limits. There will be significant sideways force acting at the couplings. (remember that 2L in the 3rd landmine above?) This force is transferred to the wheels through the bogie pivot. You are relying on the couplings being able to move freely within their coupler boxes without them locking up. Any tightness of the couplings in the coupling box or elsewhere in the draftgear will add to the sideways force at the wheels.
Seventh landmine - bogie pivots - It the bogie swing is restricted by steps or under-gear, then you could be at a fail point before you get to 28" radius. To check for any fouling under your autoracks, accurately bend a piece of flex track that is longer than your autorack to 28" radius. This gives you piece of track that you can see through to check for anything underneath that could restrict your bogie swing and cause derailment problems. Remember to reverse the car, the bogie swing might not be the the same when pivoting the other side of centre.
Brian's first example was for the articulated autoracks. The articulation should prevent a derailment happening at the centre bogie, but the position of the outboard boogie pivots relative to the end of the car will have a massive effect on where the coupler head finishes up relative to the outside rail of the curve. This applies to both ends of a non-articulated auto rack. See the AAR Train Make-up Manual slide in the presentation and note the car dimension factors. In simple terms. you are in possible derailment trouble if the uncoupled knuckle head is close to or outside the outer rail on the curve.
Eighth landmine - your track laying is going to have to be near perfect though these 28" radius curves. a localised dip on the inside rail may be enough to allow a flange to lift and climb the rail resulting in a string-line derailment.
A suggestion - before you start build, can i suggest that you grab a couple of trestles or foldup tables and some plywood to mock up a test rig for what you are about to attempt out in the middle of your basement. You will need at least 28' of track BEFORE you start your 28" radius curves You can probably reduce the amount of straight track between the 90 degree bends and the 180 degree bend between them. You will also need another 28' of track after the curves to make sure your train makes it through OK. The in-train forces and sideways drag on your flanges will be at their worst as your locos reach the end of the last 90 degree bend with the rest of the train strung out though all the curves.
In closing - if you are going to push the edge of the envelope, sometimes it might help to have some idea where the edge of the envelope might be. Good Luck, (hopefully you won't need it)
