Trans mount design

There were a couple options for the trans mount.  I can summarize them into two categories as shown below in the diagram which is aft looking forward (gray is the transmission, red is the mount).  I won’t get into the details of the benefits for each except to say, that attachment to the frame rails is by far more typical. 

I expect vibe to be significant here, and I did run a quick FEA to check frequencies.  I will be making this mount out of 1 x 2 steel tube,  I had planned to drill through the outside face large and just bolt the inner skin to each location.  This makes much lower frequencies than if a sleeve:

Model was fixed at hole locations.  60lbm was placed on the lower section of the beam.

Mounting with 4 screws instead of 2 makes a big difference as well.  And wall thickness matters a lot.  I have .060 laying around.  I think I will order some .120 wall for this.

 

While I don’t expect a lot of load to transmit through this mount, the frame rail was not designed for this connection. Neither was the floor.  Simply bolting through both would crush the rail if I torque things down, and would not be secure if I didn’t.  Also, there is not really as much draft on the rail as it appears in my diagrams so it may not be well positioned to handle lateral forces.  I thought a while about how to address these issues:

Left:  Bolt (brown) head welded to floor (blue).  Transmission mount (black) sleeved with welded in steel bushing (gray).  Down side is possible rail crushing.  Bolt length is long and unsupported.

Middle:  Frame rail is bushed as well.  Bushing is welded to rail and ground flush.  Much improvement over version on left.  Bush ties rail and floor together and prevents crushing.  Downside is that trans mount slides over 4 studs at once, alignment may be tricky, and thread damage is likely.  Threads are difficult to repair or replace.  Also popping bushing welds is possible and would dramatically reduce the performance of this approach. Nevertheless, this is the way I would go if I didn't have the lathe.

Right:  Best.  Lathed part welded to floor and rail.  Low profile to minimize carpet bump inside car and reduce chance of tearing floor.  Internal threads address issued of design in middle.  Part would function as inteneded even without welding.