monocoque, spaceframe, or semi-monocoque (stressed-skin)?

[gr3pz]

From three types of chassis:

monocoque, spaceframe, and semi-monocoque (stressed-skin).

Which one is less weight, less cost, and mostly used in formula sae?

-gr3pz-

[gr3pz]

From three types of chassis:

monocoque, spaceframe, and semi-monocoque (stressed-skin).

Which one is less weight, less cost, and mostly used in formula sae?

-gr3pz-

[jack]

steel monocoque dude, steel is real...

[Z]

Since most FSAE'rs are out of town I'll have a go.


Least weight? If well designed they should all weigh about the same.

Least cost? For the Cost report it depends on how "creative" your bean counters are (see reports on current FSAE comp!). In real life it depends on your availibility of materials, fabrication skills, etc., (see below).

Most used? Probably steel tube spaceframe because that is the "default design" in the rules.

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My opinion of advantages/disadvantages:

Monocoque.
==========
For: Don't need additional bodywork. Possibly safest for driver. Can be relatively easy to build if done right (eg. production cars - I don't know if Jack was joking but I think a steel monocoque (~0.5mm skin) is actually a good option).

Against: A car chassis is mainly subject to "point" loads. Monocoques are best for distributed loads, as with planes or boats, but don't like point loads. So each point load needs a reinforcing patch, which can be a hassle. So can be a nightmare to build! Can be difficult to repair depending on construction method. Not really good around the engine because of poor access (to plugs, electrics, oil, etc...).

Spaceframe.
===========
For: Well suited to point loads. Can be done by "amateur handyman" using hacksaw, file, and any type of welding (oxy-acet.(braze or weld)/stick/mig/tig...), which is why spaceframes are so common in motorsport. Same skills can be used for suspension wishbones, miscellaneous brackets, etc., etc. Easy(ish) repair. Good access to engine, pedals, everything!

Against: Takes ages with hacksaw and file to prepare all the "fishmouthed" tube ends, ie. labour intensive and not suited to mass production. Still have to add some bodywork (for looks, and driver comfort/protection).

Stressed-skin.
==============
(Assume this is a simplified spaceframe (ie. not fully triangulated) that uses the bodywork/skins (say, 1mm aluminium sheet rivetted and optionally glued to main tubes) to complete the bracing.)
For: Easier than spaceframe 'cos less tubes and joints. Otherwise as spaceframe.

Against: Rivetted skins can loosen with time, but this is unlikely for short life FSAE car.

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I'm sure there are other pros and cons. Anyone else care to comment?


Z

[Denny Trimble]

That's a good summary, Z. Another benefit of spaceframes is that they are very simple to analyze accurately with FEA programs, using 2D line elements (beams). Our frame FEA ran in 30 seconds per design iteration this year, and the stiffness number was within 3% on our physical test rig.

Monocoques, especially composite monocoques, can be extremely difficult to analyze accurately. In a year and a half of attempting it, our guys never got a physical test that was close to matching the FEA.

I'm sure some other schools are better at composites than we are, and may have no problem with what I mentioned above. But, that's the experience our team has had.

[Cement Legs]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Z:

Spaceframe.
===========

Against: Takes ages with hacksaw and file to prepare all the "fishmouthed" tube ends, ie. labour intensive and not suited to mass production. Still have to add some bodywork (for looks, and driver comfort/protection).

Z </div></BLOCKQUOTE>

Generally I might agree with you Z but I found a way to get perfect fit cuts in a minimum amounts of time which would be well suited for mass production. Assuming all dimensions and angles were given, using a power hack saw and a mill I can finish about 20 tubes an hour if the end joints all lie in one plane or perpendicular planes and around 12 tubes in an hour if the planes the connected tubes are not as above. Having said that and knowing that our car used approximately 70 tubes, thats all tubes cut and ready in a day. (again - if all tube dimensions and angles are predetermined)

ps no hand file, just a little hand grinder work on some tubes that are not in perpendicular planes.

[gr3pz]

what if we attach stressed-skin to fully triangulated spaceframe? is it gonna improve the torsional rigidity?

how do you guys measure the torsional rigidity which acting on stressed-skin?

what is the best method for attaching the stressed-skin to chassis?

[Z]

Cement (or can I call you "Legs" for short? ),

Apparently there are some companies in UK that do contract fishmouthing with a CNC plasma cutter. Combine this with a good jig and robot mig welders and you've got mass production. For a one-off chassis a spaceframe, even with hacksaw and file, is probably quicker than a composite monocoque (requires mold construction first) or an aluminium honeycomb monocoque (requires lots of milling/drilling/trimming/gluing... aaarghhh!!!). Manual fishmouthing is a no-brainer and actually quite relaxing - no glues waiting to go off if the phone rings, etc.


gr3pz,

I see a "Stressed-skin" chassis as a monocoque that has its major "point-load-patches" joined up with the steel tubing. Only the main outlines of the chassis have to be done in steel tube - eg. rollover-bar/rear-bulkhead-frame, upper-cockpit-surround, front-bulkhead-frame, and maybe a middle-bulkhead-frame and two bottom-rails, if that makes sense. This type of chassis is possibly the easiest of all to do, and very efficient.

I don't think there is much point in fully triangulating a space frame and then adding stressed skins. Whichever material has the higher modulus (stiffness) will carry most of the loads, while the other material will be dead weight (or extra safety factor, if you want).

I think a steel/steel stressed-skin chassis is a good option. Some "garden sheds" now sold are built entirely out of 0.28mm galvanised steel. So use 0.3-0.5mm galv. steel skins, with some 1mm top-hat sections spot welded on for local stiffening, plus the rollover tube and cockpit surround spot or mig welded to the skins, and you have a similar construction to a production car. 2 square metres of 0.3mm steel skins weighs less than 5kg. The main advantage here is ease and speed of construction - like folding up some kraft paper and sticking it together. Very strong and stiff too, but may be noisy (drumming) unless you put lots of creases in it.

Pop-rivetting aluminium sheet to steel tube is sufficient (say, use 5/32" (4mm) rivets every 4" (100mm), or closer where higher stress). Search other threads for appropriate glues if you want them...

Z

[Colin]

Z
we've used CNC laser cutting for fish mouths in the past it is very acurate but due to the vatiations in cuts it's time consuming for the laser cutter as each new tube requries a new file to be downloaded into the machine, but would be great for a production run. As for good jigs we run full laser cut jig that slots togeather, it's fully reusable and cost us about $200, it also enabled us to build a new front chassie for crash testing in less than a day, i don't know much about monocoque's but i'm tipping a crash test chassie is going to take alot longer than that

[gr3pz]

Thanks Z,

I want to measure torsional rigidity of chassis and its skin, I'm thinking of using some kinda modelling (smaller) chassis and skin.

with torsion test machine, and by putting strain gauge on the skin surface which attach to loaded chassis.

we can get the modelling torsional rigidity, and eventually the modelling bonding strength of skin (using rivet, spot welding, glue, etc.)

by using dimensional analysis calculation we can get the actual torsional rigidity and actual bonding strength, so we don't need to use big heavy machine. what do u guys think?

-gr3pz-