On Wed, 27 Mar 2013 13:28:25 -0700 (PDT), "
wrote:

On Mar 27, 11:50*am, Ed Huntress wrote:







most importantly, you'll see
the weakness in the vast majority of those frames: a lack of torsional
stiffness in the passenger bay. The Locost is weak in that area but no
worse than a Lotus 7.


Ed Huntress


I had an interest in the Locost a while back. i do not remember which
web site , but someone has analysed the original Locost frame and
published the design of a modified frame that is much stiffer than the
original frame.


Dan

Yeah, I think there are at least a couple of them. I haven't checked
into them carefully.

It had some obvious room for improvement but the torsional stiffness
issue is a chronic one with space-frame chassis that are narrow or
that have thin sides. That's most of them. There's just no room to get
a torsion-resisting structure in there, and if it's an open car, you
have no bracing across the top to compensate. A top, with triangulated
tubes, solves everything. A roll bar tricked with diagonals to the
chassis is a big help but a lot of racing class rules don't allow
structure that's obviously intended to stiffen the car and improve
performance.

One of the two original space-frame roadsters, the Mercedes-Benz 300
SLR race car from 1952, (the other was the Lotus 6 we've been
discussing) solved it as well as any car since. It consisted of a pair
of bridge-like tube-frame "boxes" in each sill. The result was a wide
and high sill that was hard to step over. When they applied the design
to their road-going coupe, the M-B 300 SL Gullwing, they had to open
the doors upward to make it practical to get in and out. You had to
hoist yourself by the door handle. g That was the entire reason for
the gull-wing configuration.

Another solution is a tubular box down the middle of the car, known as
a central torsion box. The British TVR used that approach and it had
pretty good stiffness. (Lotus did it in sheet metal in the original
Lotus Elan, 1961). It makes the car a little wider but today's car
shapes handle it with no trouble.

As I said, it's been a chronic problem from the beginning. It's led to
a lot of hybrid designs, starting with stressing the body skin on the
sides of the passnger compartment, as in the Lotus 7 and the Locost,
which helps a little bit; to the monocoque central bay on the racing
Jaguar D-Type (1954), made of magnesium alloy sheet (Elektron). Today,
with carbon fiber and tub-like monocoques, the problem is largely
solved. But not completely. It's still the weak link in an open car.

--
Ed Huntress