Frame cross member; removable or not

[GrUmPyBoDgEr]

The present topic on exhaust manifolds raised the question about the removable cross member on Spyder tubular frames.
I've often wondered how much "work" that cross member actually does?
Structurally it seems to mess up what could loosely be described as a triangulation.
A triangle in a space frame being the most rigid structure.
I would think that the cross member would put unnecessary lateral forces into the frame.
That's if there are any loads actually there.

What was the actual reason the designers decided to add that cross member?

Anyone out there with experience of running an Elan without that cross member?

Cheers
John

[Foxie]

Imho, it prevents the front lower chassis rails from buckling in the horizontal plane.

Check out Euler's Buckling Formulas

[rgh0]

I beleive the lower cross brace is mainly there to stop the chassis ( sorry sub frame ) side members from twisting and spreading apart at the lower edge under the weight of the engine which puts a twisting load onto them from the engine mounts.

If I have the weight of the engine in the car it is hard to fit the bolts in the removal cross member I have in the standard Lotus chassis due to this. The Spryder chassis in the Plus 2 appears to be much stiffer and I can more easily fit the bolts in the removable cross member in the same circumstances.

cheers
Rohan

[GrUmPyBoDgEr]

Sorry Sean, I don't have a copy of Euler's but no doubt my copy of Rourk would tell me the same.
Nevertheless formulas won't say much without vectors or loading figures which was why I was asking if there was practical experience out there.

Rohan, it seems that you have seen the deflection caused by the engine weight which is more than good evidence of the need for that cross member.

In retrospect, I guess that the Lotus frame is a bit "wobbly" in that area; the tubular Spyder frame probably not being quite so bad.

Thanks guys, my question was a bit hypothetical
John