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]

Think of or go look through a junkyard and see how the OEs attach crossmembers and what they use for shapes. In your case, you are not using the crossmember to carry suspension loads or crash protection, so yours can be something relatively simple.

I would use 12-14mm bolts the cross bar can remain single shear up into the body (assuming you want to keep the factory orientation). Weld some 16ga reinforcements around the tunnel area. Then build some double shear saddle brackets for a each leg on the corner of the firewall. That will keep the whole structure up above or level with the floor pan to keep it away from road damage.

If you want to go whole hog, I would build a shear plate out of aluminum plate. It could double as engine mount and let it tie into a minimum 4 points along the base of the firewall and then on the lower longitudinal and suspension console. The problem will be how low the pan hangs in relation to this plate/firewall/suspension console.