Steel Frame Design

[Denny Trimble]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Gareth:
...I like to wear ladies' clothing... </div></BLOCKQUOTE>

Hey Gareth,
Please don't quote me if you're going to edit what's between the quotes

And, our chassis was on the high end of both of the ranges I gave, physically tested by loading through the hubs and with dummy shocks installed. I'm sure you've seen the cad illustration I've posted here several times.

[kwancho]

Is that the one labeled UW 2005 Design Report? If so, that's the one I've been explaining a ton of concepts to my team with.

[Denny Trimble]

Nope, this one:
http://students.washington.edu/denny...DT-5-10-04.jpg

And I assume you're talking about our design report images, not our actual design report

[kwancho]

Ah, nice. I have the picture of the whole car, with blanked out frame and driver. Very informative.

Back on topic, what's the best way to do pathloading? Solidworks? I'm thinking about it for pushrods right now, but obviously we need it for the frame as well.

[Jersey Tom]

Doesnt matter what design software you use. Just have all your tubes come together at node points and make sure they are loaded axially, NEVER in bending.

[Gareth]

Sorry Denny, the stiffness claims comment wasn't directed at you or your team specifically. Few teams seem to look at how to properly load the chassis in torsion (proper constraints during fea and on the test rig), including mine. The real point of my comment was to emphasize that numbers made up to impress judges don't make the car go faster. When you run analysis or physically twist your car you've got to make sure you do it properly and without overconstraining it or else you'll see inflated, overpredictions of stiffness. This is something we've recently discovered and are on our way to remedying it.

We used to hold the rear wheels rigid (hubs bolted to fixtures, bolted to the floor) and then attached the front hubs rigidly to an I-beam on a pivot. Our analysis was setup in a similar manner. The problem is that this method over constrains the hubs and prevents them from moving laterally and longitudinally. It's possible (especially easy in FE) to properly contrain the suspension so that it has 1 degree of freedom (roll), driven by some load.

It would be really nice if there was a standard published on how to properly do a torsion test, and there might be, I just haven't looked. I know I've found it frustrating trying to compare numbers between teams because you never know how the test was done. Even if there was a standard, there's nothing stopping people from fudging the numbers anyway. Still, a standard would allow the judges to compare numbers more easily.

As for the teams with questionable designs and fast cars, I chalk it up to good driving and lots of setup. Not to knock the team, but there was a pretty big gap between RMIT's first and second driver at FSAE enduro this year. The rumor was that their first driver was an Australian karting champ...? It's a hard compromise for us engineer-types to deal with - you can build a mediocre car (again, not aimed at anyone specifically) and put a fast driver in it and be fast, but where's the fun for the engineer? I'd rather build a good car, even if it's a little heavy, that I can learn from and setup properly. I've heard that even some of the regular top teams have trouble getting their cars to respond to changes and I'm convinced it's because they're not stiff enough. The judges come around and lean our our wheels and complain about compliance every year and we keep making it better. We made a big jump in 05 and the new car has shown a significant improvement and I believe there's much more to be gained.

I try to keep the womens clothing to a minimum, but it ain't easy.

[Z]

Tech Tiger wants to build a "simple, all-steel ladder frame", and it has been recommended that he aim for torsional stiffness of 1500-3000 ft.lbs/deg, and chassis weight 50-100 lbs.

So, keeping in mind that the following are "back-of-envelope" calcs... (corrections welcome );

Torsional stiffness of a round tube; Kt = torque/twist = (G x Pi x D^3 x T)/(4L)

Where;
Kt = Nm/radian (/57 to convert to Nm/deg)
G = Shear modulus of steel = 80Gpa
D = diameter (metres)
T = wall thickness (m)
L = length of tube (m).

------------------------------------------------------------

EXAMPLE 1
=========
A ladder frame with two main tubes D=0.075m, T=0.0032m, L=1.5m (ie. similar to the 3"x2"x1/8" RHS side rails of hot rods).

Kt = 2 x (8 x 10^10 x 3.14 x (0.075)^3 x 0.0032)/(4 x 1.5 x 57) = 1,980 Nm/deg = ~1,430 ft.lbs/deg

Mass = 2 x Pi x D x T x L x Rho = 2 x 3.14 x 0.075 x 0.0032 x 1.5 x 7850 = 18 kg = ~40 lb

Extra X-members and shorter chassis between spring mounts will increase Kt. A pyramidal roll-over structure (like the yellow car) will greatly increase Kt. Flex in brackets and suspension will decrease real Kt.

Extra X-members, brackets, roll-over bar, etc. will add mass. 18kg above is for the "main structure". Total mass ~1.5x to 2x18kg, so 60-80 lbs.

For a very simple and easy to build frame, this Kt=~1,500ftlbs/deg is not too bad!

---------------------------------------------------------------

EXAMPLE 2
=========
Like the WWU "twin-tube", but made from 1mm thick (0.040") folded sheet steel. So 2 tubes D=0.15m (6"), T=0.001m, L=1.5m, plus the floor and lower side walls.

Kt = 2 x ( 8 x 10^10 x 3.14 x (0.15)^3 x 0.001)/(342) = 4,960 Nm/deg = 3,580 ft.lbs/deg

Say, total width of sheet 2 x (3.14 x 0.15) + 0.6(floor) = 1.6m.
Mass = L x W x T x Rho = 1.5 x 1.6 x 1 x 7.85 = 19 kg = 42 lb

Same comments as above for extra/less stiffness/mass, etc. (though floor included here in mass calcs).

5kNm/deg (3,600ft.lbs/deg) is a good place to start, and this chassis would be incredibly easy to make!

-----------------------------------------------------------------

EXAMPLE 3
=========
If two tubes are good, one must be better...

So same sheet of steel as above, but this time folded into a single tube D=0.5m (20"), T=0.001m, L=1.5m.

Kt = (8 x 10^10 x 3.14 x (0.5)^3 x 0.001)/(342) = 92,000 Nm/deg = 66,300 ft.lbs/deg !!!!!!!!

Err, is that stiff enough?

That's more than 20x the recommended maxiumum! But flex in brackets, suspension, etc. will considerably reduce the "real" (measured) stiffness (because springs in series...).

Mass = 19 kg = 42 lb, as before.

This chassis would look like a 44 gallon drum, and the driver would have to mount it like a horse! So to make it more realistic the same amount of sheet steel could be made more like the body of a regular FSAE car. The material cut out for the driver "access hole" would be used to stiffen the edges of this opening. Rear of "tube" should be "skeletonised" to provide easy access to the engine. Point loads from engine and suspension pick-ups should be spread by "reinforcing patches" such as bulkheads or top-hat section ribs, etc. (as I posted somewhere else...).

A big advantage is that no extra bodywork is required (except nose cone), so no hassles with attachments, bits falling off, etc. Also this chassis would be extremely strong - take a baseball-bat/sledgehammer to a 44 gallon drum to find out. 1mm sheet steel is very thick by modern production car standards. 0.5mm (0.020") would be thick enough and half the weight (but only 33,000 ft.lbs/deg ).

If anyone is thinking about doing this, don't bother starting with detail design, FEA, etc. Instead, spend one day making a rough mock-up. It is very easy (using tin-snips/electric nibbler, and hand held spot-welder/mig/tig) - a bit like origami. This will prove how stiff it really is. Feeding the point loads in is key, since flex here dominates the real stiffness...

Z

[Z]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by CMURacing - Prometheus:
that concordia car looks suspiciously like a brown go-kart to me... </div></BLOCKQUOTE>
Here is a quote from Racetech magazine (Aug/Sept 2002) covering that year's Detroit FSAE.

"The [Concordia] car's simplicity won the team the Cost Event but it did not impress the Design judges."

Who'd have guessed?

Z

[Denny Trimble]

Nice calculations Z, and teams should be doing these types of studies early on (now?).

For reference, I've measured the length of all the "safety tubing" on our car and come up with a weight list. These tubes could certainly be shorter if you weren't relying on them for stiffness as we were. Or, if you do an SSEF, you can use alternative side impact structures etc.

Tube Length Diam Wall Area Density Mass
inches inches inches in^2 lb/in^2 lb
Main rollhoop 97.4 1 0.095 0.270 0.283 7.45
Front rollhoop 47.4 1 0.095 0.270 0.283 3.62
Side Impact 158.6 1 0.065 0.191 0.283 8.57
RollHoopBracng 70 1 0.065 0.191 0.283 3.78
Front Bulkhead 44 1 0.065 0.191 0.283 2.38

total 25.80

Add in tubing to support the front bulkhead and protect the driver's legs; seat back support, brackets, most of the rear box, and there is a significant amount of weight that's not helping your torsional stiffness numbers. So Z's factor of 1.5 to 2.0 is probably close.

Also, for that steel torpedo monocoque idea, cutting a hole for the driver will take that 66k number and flush it down the toilet

I like doing these kind of calculations, but it can get ugly as you start to actually build a practical car. But, I guess that's the nature of engineering.

[RiNaZ]

Denny,
it might be obivous to everyone else, but what does SSEF stands for?