Help me understand body mounted vs truck mounter couplers

[Bart1701]

OK - I know what a body mounted coupler is, and what a truck mounted coupler is !

My question is around my Kato Mikado. I converted the truck mounted coupler to a MicroTrains 1129 truck-mounted coupler. When I pull my MicroTrains Heavyweight Coach ( with MicroTrains body-mounted couplers), there is one curve where I always get a derailment when traveling in a certain direction ( a curve that bends to the left).

The curve radius should be at least 11", if not larger.

When it travels the reverse direction through the curve (bend to the right), all is fine.

When it pulls freight cars with body-mounted couplers in either direction, all is fine.

I believe I have read that there can be issues when body-mounted and truck-mounted couplers are used together.

My Bachman Consolidation with a body-mounted coupler on the tender does quite well pulling the coach through the curve.

So, is my solution to put the MicroTrains 2002 body-mounted coupler on the Mikado tender?

Also, can someone explain what physical forces are at work that cause these issues?

Thanks,
Bart

[bbussey]

Unless you have easements going into your curved trackage, body-mounted couplers and truck-mounted couplers don't mix well, particularly on cars where the truck pivot is a distance away from the body-mounted coupler (such as the MTL heavyweights).  With truck-mounted couplers, the centerline of the coupler always remains over the centerline of the track.  With body-mounted couplers, the coupler swings out away from the track curvature until the truck reaches the curvature.  Truck-mount to truck-mount, or body-mount to body-mount, causes no issues because both couplers behave the same — either both staying on center (truck-mounted) or both swinging out (body-mounted).  Mixing types can cause derailments unless your radii is larger, probably 18" or better.

There are advantages of body-mounts over truck-mounts in general.  The push/pull load of the consist travels through the underframe on body-mounts, rather than through the trucks with truck-mounts which pushes (or pulls) the trucks to pivot against the rails.  Truck-mounted couplers are not conducive to pushing a freight consist through the multiple turnouts of a yard throat, for example, because the trucks will pick the points under load.  Body-mounted couplers also allow models to travel at the proper ride-height instead of being raised up to clear coupler-swing, and lower ride-height means lower center-of-gravity which translates to better tracking and greater push/pull load capacity.

[nkalanaga]

One can have the same issues with different length cars, even if both have body mount couplers.  Don't try coupling an ore car to an 89 ft flatcar if you have sharp curves.  The prototype has rules against that on sharp curves.  Basically, a truck mounted coupler is a very short wheelbase body mount, so it makes sense.

So, does a disconnect logging truck have truck or body mounted couplers? 

[randgust]

There's usually a pretty good reason for what's going on.    Bryan's right, but there are still more things to look for before you start major modifications.

The 1129 is a medium-shank T shank design.  What you want to look for is if one or both of the couplers are 'all the way to the wall' on side swing in that situation, which transfers the forces directly into the coupler box and pops the car off horizontally. 

That may or may not be true, on an 11" curve it very well could be.  There's several other things to look for....

1)  Coupler pin ABOVE the knuckle on either one hanging on something on the carbody (push trip pin lower/rebend)
2)  I'm assuming derailing to the inside - not outside - of the curve?   It doesn't take much on the inside of the railhead to pick and climb when side forces are involved from the couplers.   One sharp edge and you climb.
3)  You could try to find a longer T-shank coupler on the Mikado to fit that box.  Replace a medium shank with a longer shank.  It won't look as good but you might get enough additional swing to fix it.  That's standard approach on Atlas Geeps (bodymounts) with almost anything on a sharp curve.

I've had similar issues with MT heavyweights on 13" curves.  I'm mixing Rivarossi heavyweights (truck mount) with MT heavyweights and its causing some interesting issues at times.   they aren't derailing behind the tender, they are derailing between cars that otherwise had no previous problems.  It's not consistent, and the between-car clearances can cause derailments as well.

I really like the MT heavyweights but yeah, they will find spots to derail that nothing else will.   I don't like heavyweight truck-mounts because of what they do for appearance (no real car steps) and if all else fails, I'll convert the entire train to bodymounts except maybe the lead truck on the Rivarossi baggage.

Frankly, I've had  minimal problems with truck-mounts derailing over the years, and I'll lower a car and leave the truck mounts as the first approach.  As I use MT delay uncoupling, truck mounts generally do a better job of keeping the coupler vertically and horizontally aligned than a bodymount if you have ANY curves in the vicinity of coupling and uncoupling.    But for heavyweight passenger cars, that step thing is a big visual issue for me, along with the height of the cars, so in this case it's going all bodymount.

[bbussey]

... Frankly, I've had  minimal problems with truck-mounts derailing over the years, and I'll lower a car and leave the truck mounts as the first approach.  As I use MT delay uncoupling, truck mounts generally do a better job of keeping the coupler vertically and horizontally aligned than a bodymount if you have ANY curves in the vicinity of coupling and uncoupling.    But for heavyweight passenger cars, that step thing is a big visual issue for me, along with the height of the cars, so in this case it's going all bodymount.

Which is why, with delayed uncoupling, the magnet should be on straight sections of track where the couplers center.  The cars then can be spotted wherever with the couplers in "delayed" alignment.

[randgust]

Bryan, if I had a layout area the size of yours I'd agree completely.    To assure that you can properly uncouple and couple with bodymounts, you need the full maximum car length to both ends of the magnet area to make sure the carbody frame doesn't change the angle of the couplers - figure 190 scale feet of tangent.  With truck mounts, I can do the same exact thing in about 30 feet maximum for two coupled 89' cars.   

With passenger cars, I do little if any switching, so the body mounts are an easy choice.

My 'main uncoupler' (old Kadee electromagnet) for the entire yard is located between two facing-point turnouts - and if either car is on the diverging route in either direction swinging the body off centerline, it's just not going to uncouple.   Which basically shuts down the entire yard. 

I'm one of those guys that originally converted to N because there wasn't enough room to even set up a 4x8 sheet of plywood for a small HO layout - sold all I had and started over.   I could fit in a 3x6 HCD with N, that was my first.   Even today I'm only 5'6" x 8 feet, and that's all I can fit.   So while I philosophically agree completely with everything you say, I'd still rather stay with truck mounts than move to a bigger house!