View source for Wikispeed Frame Review

[[File:framecritique.jpg]]

From 3/27/12 wikispeed blog:
http://www.wikispeed.com/wikispeed-team-blog/safety

>>>
...while we have no reason to suspect a safety flaw, we must not ever stop
examining for one, and must always take advantage of knowledgeable folks
offering to help collaborate with us on safety systems and testing.
>>>

Since safety is the issue, I’m happy to be taken advantage of.

In my opinion, the highest priority weaknesses in the Wikispeed chassis (which
I note from the videos is now the OS Car prototype chassis as well) stem from
strain loads at welds, and a general lack of triangulation.

I don’t have much data, but working from the materials specs, dimensions and
processes shown on http://opensourceecology.org/wiki/Wikispeed_SGT01 the
chassis is made from 6061 T6 tubing, and from notes in this seven-person
discussion group I understand it is TIG welded with heat treating post-weld.

The material issue is that the 6061 alloy with T6 heat treatment is a
relatively strong and rigid material, particularly relative to the same alloy
in an untreated state (T0) and welding cancels the T6 treatment in the
vicinity of the weld, taking it back to T0.

To compare the metal’s mechanical specs before and after welding, see…

http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MA6061t6
http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MA6061O

…but to summarize the significant differences, T0 has 20% the yield strength
(the load it can take before permanently deforming) of T6 (8000 psi vs. 40000
psi) and 40% the ultimate tensile strength (18000 psi vs. 45000 psi).

In this chassis application, the difference in ultimate tensile strength is
greater than the specs show—more like 14000-15000 psi vs. 45000—because since
the small welded section fails before the large unwelded section even takes a
set; the welded section starts stretching at 8000 psi and as it stretches, it
necks down and the metal gets thinner. For example, each side of the 4” x
.125” wall square tube has about 1/2 square inch of cross section, but once it
starts to bend at the weld, the weld on the outer side of the tube begins to
stretch and thin. Once it thins to, say, .100” wall, there’s only 2/5 square
inch of cross section, so while it might take 9000 pounds of tension to tear
that weld all else being equal (18000 psi x ½ square inch = 9000 pounds to
failure) it’s not so equal once the weld begins to yield, and if it stretches
down to 2/5 of a square inch, 18000 psi x 2/5 square inch = 7200 pounds to
failure, for an effective ultimate tensile strength of 14400 psi if you’re
calculating from pre-stretched dimensions.

The lack of triangulation problem is it makes the wikispeed frame the opposite
of a space frame — instead of having loads carried through tubes in tension
and compression, many loads are supported by the stiffness of the tube
junctions, which since the junctions are T0 instead of T6, are not up to the
task if faced with unusual loads, such as getting hit by another car. For
example (I’ve attached a drawing; if your ISP blocks unexpected attachments,
click http://dl.dropbox.com/u/66433621/StrainSketch.jpg to see it), if you do
simulations where both the top and bottom frame rails get struck at the same
time and with the same force, the transverse tubes in the section behind the
cockpit are going to show compression loads and the upright tubes are going to
show little or no loads at all, but if the impact is off center or the initial
strike is to the upper rail (as is common in car accidents—the bumper hits
first, followed by the grill, and the lower portion of the struck car isn’t
hit until the striking car’s suspension reaches the strikee) then the tops of
the 12” high uprights move before their bottoms do and there’s a 3 to 1
mechanical advantage acting on the 4” junction between the uprights and the
frame rails.

Each junction can support roughly a 1300 pound load (8000 psi x ½ square inch
divided by 3/1 mechanical advantage) before the tube junction starts to yield,
but once it goes it’ll go fast, because as one side of the tube weld crumples,
the 4” fulcrum gets effectively shorter. For a BOE calculation, each junction
may be good for as much as 2000 pounds before failure in this side impact
example, and there are four junctions resisting the load (the two uprights
welded to the frame rails, and the two transverse tubes welded  to the frame
rails) so the assembly might well withstand an 8000 pound load, and assuming
the lower portion of the frame carries 2/3 of the impact resistance (all of
the passenger loads, most of the wheel loads, most of the inertia load of the
powerplant) and the car weighs 2000 pounds fully loaded, it’s good for a 6G
impact (8000 / (2000 x 2/3) = 6).

However, if the analysis and simulations have been done with the assumption
that the chassis is all one material (which it is chemically, but not
functionally) one might be off by a factor of 5, and believe the chassis is
sufficient for a 30G impact.

This is a back-of-envelope strain analysis and is surely wrong in details, but
it’s close. It might take a few more than 6 Gs to penetrate the chassis, but
if you get hit from the side by a pickup, SUV or crossover car, you’re going
to have it in your lap before you hit 10 Gs. The chassis will fail by tearing
at the circled areas shown in the drawing, long before the shear strength of
the aluminum (12000 psi, fyi) becomes a factor.

The simulation graphics on the various Wikispeed videos show the chassis as a
monolithic structure, but it’s really not one. At each weld, the yield
strength is 1/5 the strength of the surrounding tube, and as the saying goes,
a chain is only as strong as its weakest link.

Here are some ways to alleviate this:

-- Either heat treat the entire chassis after welding OR make the chassis out
of 6061 T0 to begin with. I don’t know which would make the safer chassis. The
T0 chassis would be more expensive to operate, because relatively minor
accidents would bend it, but with T0 having twice the elongation at break as
T6, T0 would take twice the impact to reach the same G loading and it might
absorb more total energy before failing. But as it is now, T6 for the most
part with small areas of T0, that’s the worst of both worlds—it won’t bend
more than its T6 major parts and won’t tolerate higher loads than its T0 welds.

-- Bond or rivet (or both) shear panels over every exposed rectangle in the
chassis. You need to leave the biggest rectangle open (it’s the one the
passengers get in through) so add another transverse tube as far back into the
cockpit as you can, and put a shear panel between that tube and the upper
transverse tube at the front of the car. Alternatively, you could triangulate
the open rectangles by welding diagonal tubes between one corner of each
rectangle and its opposite corner.



--
Jack McCornack  http://www.kineticvehicles.com