Tight Truss Rods / Fix

[Sherwood]

Have been having problems with bowed underframes of the new low frame type caused by the trussrods being too tight.  It makes the ends of the underframe and coupler pocket droop down and leave a gap to the body.

I have found this on the replacement underframes and some but not all of the cars that come with the lower frame.

My fix so far is to put a piece of sheetmetal between the truss rods and the underframe as a heat shield, and then heat them with a heat gun to soften them.  They are stretched by putting the heat shield between them and the frame.  Of 5 I did, on two of them I overdid it and melted them - it is touchy.  For those I cut off ones from underframes I was replacing.

Any better ideas, and has anyone found replacement frames without this problem?  The older cars and some of the new ones like the pulpwood car have a softer material that stretches and does not cause this problem.

[NarrowMinded]

Boiling plastic make it soft enough to bend and stretch, ive done it for forming strene, not sure if it would work for what you are doing, it's an idea...

NM

[mabloodhound]

Try this by setting just the truss rods into a container of hot tap water for awhile and see if they will stretch.   You can even make a holder for the frame to keep it so it sets just into the water.   I don't suggest using boiling water as it is too hot and the plastic guys also recommend only hot tap water.

[Sherwood]

Thanks to all for the advice - after destroying another tight truss rod trying to control a heat gun, I was able to get just the right amount of permanent stretch using water that just stopped boiling.  Hot tap water was not enough to do it for the particular material for the ones I tried, although there does  appear to be a material change beween the older and newer cars.

I put a piece of slightly curved sheet metal between the truss rods and the queenposts which stretches the truss rods and even flexes the whole frame.  By suspending it into the near boiling water so only the truss rods were submerged, they softened up and stretched enough so that when I took out the metal after cooling they were about right.  They don't droop noticeably but don't have any tension left to warp the underframe.  To finish up I put the frame on a pair of trucks and set a bean can on it for a few days to work out the bow from all the time spent with tight truss rods.

[NarrowMinded]

Glad it worked out...

NM

[Sherwood]

After much experiementing (I have some new stlye underframes that don't have homes yet) I found the best fix is patience - if I put a flat piece of metal (expansion slot cover from computer works perfectly) between the truss rods and the top of the underframe so they are stretched quite a bit and leave it for days, they end up stretched sufficiently (platic creep) while they are not trying to warp the car in the wrong direction during the process.  They look good in a day or two but return slowly towards their original length, so it takes a few days or more to get enough permanant stretch.

I then leave them on a pair of trucks with a can of beans flexing the center down for another week then it finally comes out flat and does not leave a gap between the draft gear box and the car body.

Would have been a lot easier for bachmann just to have made the truss rods slightly longer to accomodate the new longer path length...  (I noticed that the new ones were the same length when I took one off an old underframe to replace one of the new ones I had melted.)

[lvrr325]

They just have pins at the ends to hold them on.  Lop the pins off one end.  If they sag, some super glue or ACC should hold them fine.