Structural Question, crossmember bolts in sheer or tension? Options

[cooler]

hello 914 structural experts,

I am planning to create a new crossmember to stiffen rear suspension and support engine. The 914 rear suspension console is well known to flex under racing stress and was reinforced by factory racing teams. I intend to address the flexing issue and create engine (Audi V8) mounting locations with a new crossmember and diagonal tension members. The attached photo shows the location of crossmember (red) and diagonal tension members (blue).

The crossmember spans between the suspension consoles, on center with the trailing arm pivot axis. The diagonal tension members span also from rear suspension consoles to firewall, near center tunnel. This assembly will need to be removable to facilitate engine removal and is intended to stiffen the entire engine bay/rear suspension.

I am struggling with detailing the crossmember bolted connections at suspension consoles. I assume each connections should include two bolts each but, should the bolts be loaded in shear or in tension? What size should the bolts be?

Thanks in advance

[Brett W]

Where are the engine mounts in relation to the rear suspension inner mounting ear?

Yeah if you have a standard production engine a fully stressed mount will definitely cause some negative distortion in the block. That won't end well.

However if you could use the oil pan, bellhousing adapter, front motor plate, etc to pass those loads through instead of straight through the block, you could use that components you have to carry for double duty.

One thing, that kinda matters, the rear inner pickup points only move in certain directions based on the loads from the tire. It really twists in plane and in a vertical plane. Your "dinky brace" to the tunnel will help the in plane twist. The Vertical you won't control with a simple crossmember. That needs to go to a cage which ties to the front of the chassis.

On normal street use there just isn't that much lateral bending of that console. So you adding a "straight" crossmember across the engine bad doesn't really gain you much.

Adding a flat shear plate across the whole underside of the engine bay, from firewall to suspension console and then use the engine mounts to stabilize the plate could be useful, but that may require you having a much shorter oil pan or machining the plate to allow the pan to sit below.

Another option. Machine the plate and oil pan into one piece. Basically the plate becomes wings under on the pan and ties all of it together across the engine bay.

OK I am getting a little outside of the realm of what normal people can fabricate.

Yes any of the aforementioned mounting methods will contribute to vibration being transmitted through the chassis.

[stownsen914]

Personally not a big fan of solid mounted engines. Ever drive a car with one? Think tuning fork.

If you use that cross bar, see if you can triangulate it. When I first built my /6 racecar I built the crossbar out of 1 1/4" .095 and of course it flexed. Same material triangulated, 20+ years later, still going strong.