preliminary engine frame layout

After getting the headers, engine mounts and rack into place, I found that the subframe and header interfere pretty badly on the passenger side.  That is sort of strike two for these headers since they also limit how high I can mount the engine.  I ordered some block huggers last night.  We will see if they fit any better.

I had a plan to bring a tube under the engine and up to the other side (as shown in PVC in the last post).  That tube would hold the rack and both engine mounts.  Rather than one bent piece that would have to be bent very accurately I had a plan to weld up as shown below:

Proposed frame design.  Uses 1.125, .083 wall tube as a starting point.  The ears on the horizontal piece would hold the steering rack.

I am concerned that having the rack tied so indirectly to the suspension mounts could lead to relative movement between the two.  I made a quick FEA model to see if that was a valid concern. The cross piece below represents the engine.  It is essentially rigid.  The plot is displacement in inches.  Please note that the distortion is greatly exagerated in this plot.

There are two loads applied:
1) Lateral 2 G load.  I know we can only corner at about 1G, but wanted to account a bit for transients, and bumps that are unknown. Assume 450/32.2*32.2*2 = 900lbf lateral

2) Torque.  Torque is not what you might think it will be.  It is being reacted against the Diff, which means you get multiplication from the transmission.  Some of it will be resolved by the transmission mount, but because its footprint is so narrow it won't take that much.  Let's assume the engine mounts take it all.  Assuming max of 400 ft*lbf  and 3.5 multiplication in 1st gear we have 16800 in*lbf.

3) Axial and vertical load not considered.  The vertical loads are well resolved in this configuration.  The axial load from braking and accelerating would not be carried by this frame.

Already we see that this is an unacceptable amount of movement for the rack.  I don't know what acceptable looks like except that .125 inch won't be it.  Movement of the rack will make the car handling unpredictable.

Even with that information there is something to be gotten from the stress plot:

I am not that worried about the yielding shown.  I didn't pick cromoly for the model, I didn't model actual mounts, and I didn't include other stiffening members that will be present.  Observe the not surprising result that everything below the engine has no stress.  The engine is so much more rigid than the tube that all load passes through the block if the engine mount is stiff enough.  If I use rigid engine mounts, then it may be possible to triangulate from the rails to the mounts, and not bother with passing under the engine.  The under the engine piece can tie into the subframes more directly.  If I use rubber engine mounts, then this assumption may not be smart.