Monocoques vs. Spaceframe

[MikeWaggoner at UW]

I don't understand why so many teams still go with steel spaceframes. It's not much harder to design a monocoque, and the monocoques are a lot specifically stiffer/stronger.

Western Washington University FSAE
dot.etec.wwu.edu/fsae

[MikeWaggoner at UW]

I don't understand why so many teams still go with steel spaceframes. It's not much harder to design a monocoque, and the monocoques are a lot specifically stiffer/stronger.

Western Washington University FSAE
dot.etec.wwu.edu/fsae

[Richard Lewis]

Well its pretty straightforwards for us anyway.

1) We don't have a composites lab here.
2) We don't have the money to spend on CF. Our frame cost us under $200 canadian last year.
3) We can build our frame in 2 days. (not ideal, but we've done it)
4) We can repair our frame, or modify it very easily and inexpensively.
5) Cost analysis.
6) ...made for the weekend autocrosser. Who is much more likely to have MIG welder in their garage than an autoclave to make mods or repairs.

Don't get me wrong, I think composite tubs are great, and I'd love to have that option. But without all the fancy equipment and the big budget, its no where near as easy and a mild steel (or chromemoly) space frame.

-------------------------
UVIC Formula SAE Team
http://members.shaw.ca/drax77/UVICFSAEcar.jpg
http://www.engr.uvic.ca/~fsae

[Charlie]

I agree with those reasons, cost, repairability, etc. Also, it has been pretty well proven that a steel space frame can be made lightweight, and stiff enough for the suspension loads.

I see it as a similar arguement as turbo vs. NA. A turbo is ideal, you can get more torque in the unrestricted area, and maximize the air you can get through the restrictor overall. Comparing peak powers NA is close, but the area under the curve the turbo will be better (properly designed).

So every team should go turbo right? Not really, if you have a system that doesn't meet it's potential (because of lack of time & resources to perfect it) it often isn't any better than a optimized system of lesser quality.

If you go monocoque great, but you better be prepared to tread new territory and do it right the first time, or risk teams with the 'inferior' space frame pass you by.

I'd love to build a car with a monocoque, turbo, and wings. But not if it's slow! And definitely not if it never makes competition, or is unfinished and makes us look bad. I know we've got a ways to go before Auburn can achieve that level.

-Charlie Ping
Auburn University FSAE 1999-present

[MikeWaggoner at UW]

There's an image of complexity around monocoques that I don't think they deserve. Aluminum monocoques provide much of the benefit of composite tubs, but have the ease of buildability of steel tubeframes. It's simple to have bent sections at the corner and rivet two panels to the bent section (you keep doing this until you get box sections etc.). You can see this on old can-am monocoques and airplanes, it's relatively simple to do.
The main problem with monocoques is actually planning. Adding suspension and steering brackets etc. as an afterthought is difficult because you have stress concentrations on weak panels.
We're doing a carbon monocoque again this year for SAE, but I'm building an autocross racer of my own using these practices and am satisfied with the results (so far). A mass produced car that uses similar practices (although they extrude large aluminum sections instead of riveting) is the lotus Elise, and it has demonstrated extremely light weight and performance in a far more competitive field than FSAE is.
I will admit that changes and repairs are a little tougher to make to an aluminum monocoque (a little cutting an riveting), but the materials aren't very expensive ($150 in aluminum, $15 of rivets, an air-hammer gun $25-100). Also for mass production the panels could be cut out on a CNC router ($4,000), resulting in a very precise chassis that can be mass produced.

Western Washington University FSAE
dot.etec.wwu.edu/fsae

[Jeffrey Pace]

Look at the teams that are consistently in the top 10. Only a very small percentage use carbon fiber monocoques. Obviously a carbon fiber monocoque chassis, when designed and constructed properly, will have a superior stiffness to weight ratio when compared to a spaceframe. However, given the limited budgets, facilities, and experience most teams have, spaceframes will continue to dominate the competition.

[Lyn Labahn UW-Madison]

For two years now University of Wisconsin has ran a aluminum monocoque and chromoly steel spaceframe hybrid. We are very pleased with the results and we are using this design again for the 2003 car. I agree that most teams could probably do an aluminum monocoque if they planned right, but you better have a damn good idea what you are getting into and be ready to make a few mistakes along the way. Why don't we run a carbon tub? We do not have the resources to produce a tub that is up to our quality standards. Not to mention it is difficult to repair, and is extremely costly!

2002/2003 Team Leader

[sticks]

In general, I must agree with Waggoner, the monocoque is the way to a structurally superior car, whether it be a carefully planned aluminium skin or a properly layed out carbon tub. Naturally, the stiffer material will mean improved gains, given the assumption of proper associated strength and design. However, even given the FIA's contentions, it goes without saying that a composite tub will be more difficult to repair and modify. Modification can be a hinderance, but with the exception of first year schools, most teams have the knowledge and access to empirical data from past cars in which to base suspension and powertrain considerations. Also, as Ohio state proved this past year, a failure doesn't necessarily mean damage to the tub (I believe it was nylon blocks that were added to the underside of the tub, which proved significant upon the loss of a wheel in testing). Of course, this is a competition wherein the objective is a vehicle under $25,000 for the typical weekend autocrosser. But I believe it is far more important to learn or at least experiment with that which is not readily available in terms of the proven and reliable.
Carbon is in fact expensive, and it does take a significant amount of time to make proper bucks, molds, fixtures and such, and then to vacuum bag, autoclave (optional to extents) release and prep. I'm learning this entirely to quickly. But contrary to what I seemed to be observing in a few cases at competition, carbon is not the ideal material for all, or in fact, many applications. To schools without composites labs and autoclaves, these are relatively minor set-backs. But to those who insist and truly believe in the power of the spaceframe, obviously they have significant merit. Excuses, however, should not be the engineering background behind a design decision.

[vinHonda]

Stiffer perhaps. Repair issues again....and to win FSAE, it's all about on track performance and explaining your design. Tub cars are great when you're in the professional business and throwing tonnes of $$$ at the projects.

For FSAE, I agree w/ hybrid. How heavy are other teams? I thought our car was pretty light in 02.

University of Toronto Formula SAE Racing Team
www.fsae.utoronto.ca (http://www.fsae.utoronto.ca)

[Nigel Lavers]

We all know that a superior frame would result if it were a properly design monocoque rather than a space frame, but the real issue here is being able to manage the design and construction of various vehicle systems within the time constraints created by annual competitions and bi-annual final exams!

I'm sure that once a well managed team builds a mononocoque leaving themselves with 2 months of testing time they'll out-race the space frames any day of the week.

But, because a simple space frame can essentially be built in under 2 weeks then I bet that the decision made by most teams /infopop/emoticons/icon_wink.gif will be to test the car for an extra 3 weeks rather than make it %50 stiffer and %10 lighter.

At least that's the way I see it...

[vinHonda]

I also see complexity in the autoclaving, and what about..... cost??

Following the spirit of the 'weekend' autocrosser..... we've stuck to the space frame w/ a lil help from the carbon panels....

but it's true, a properly done tub car is obviously better!! it's just easier to build a frame car and make some lil innovations like carbon panels etc.

i'm still interested in some of the weights out there.

University of Toronto Formula SAE Racing Team
www.fsae.utoronto.ca (http://www.fsae.utoronto.ca)

[Scott Wordley]

Mike,

If you guys are so keen to try and innovate then why don't you try building a set of wings for your car. You've already got the skills in using composite materials so it wouldn't be that difficult and I can guarantee that it will give you much more benefit than any weight saving you achieve by building a monocoque chassis.

Regards,

Scott "Maverick" Wordley &
Roan "Goose" Lyddy-Meaney
MOnash FSAE Wingmen

[Dusan]

OK, I'll admit that composites like carbon can be expensive, but having done both lay-up and welding, I think I did better learning lay-up so I don't buy the difficulty argument, at least for simple parts. I own a good TIG welder, but for what I paid for it I could build a few carbon cars, so cost seems a stretch.

Also I keep reading about composites labs? What's a composites lab? Sure a vacuum for bagging is quite nice and an autoclave can be handy, but I know folks who make professional body work and basically if you can do glass ... carbon seems to have a great mythos around it, I think born from missunderstanding.

Ease of repair is a good argument against it. It's hard to undo a lay-up. Still, Mike talked about aluminum as well, but the subject was monocoques. Obviously I am working on a momocoque car.

[Dusan]

Oh yes, I thought it would show up on my post but I am on WWU's FSAE team.

And regarding wings - not that I am against the idea, can you tell me why you are so sure wings would provide so much benefit on a vehicle that averages less than 30mph?

Dusan
WWU

[Scott Wordley]

I know that there is still a lot of scepticism regardings wings and I'm sure that there is nothing I can say to change anybody's mind on the issue.

When we decided to design an aero package for our car we looked at what other teams had done in the past and we knew that we could do it better. We decided to design for the maximum possible downforce within the constraints of the rules, that way if it didn't provide any benefit then we knew that nothing ever would. This turned out to be almost 100kg (220lb) at 60kph (96 mph), and we have around 50 hours of full scale wind tunnel testing to back up our initial design calculations.

This car came second to the Wollongong car (according to Carroll Smith the best SAE car he has ever seen) in the Autocross event by 0.04 seconds, despite being 50 kg (110 lb) heavier. We know that the car could have gone a lot quicker if our drivers had of learnt to drive a downforce car properly but I know that everyone else thinks that their car could have gone quicker too.

I don't expect anyone to listen to anything I have to say but I would have thought more people would listen to Carroll Smith. He's been saying for years that the first car to do aero properly is going to cream the competition.

I'm sure that once a few more teams start doing aero and start to win the competition then a lot of people will start to change their opinions on wings, but until then we'll just have to be happy with the advantage we have.

Regards,

Scott "Maverick" Wordley &
Roan "Goose" Lyddy-Meaney
MOnash FSAE Wingmen

[MikeWaggoner at UW]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>turned out to be almost 100kg (220lb) at 60kph (96 mph), and we have around 50 hours of full scale wind tunnel testing to back up our initial design calculations.
<HR></BLOCKQUOTE>

The average speed in Detroit for the WINNING team was less than 30 mph. Let's say you're cornering at 20 (being nice). That's less than a third of the force at 36 mph. So you're getting maybe 70 pounds of downforce in the corners while adding thirty pounds high in the air (raising your C.G.), increasing drag, and raising cost.

Wings are a good idea when you have power to burn and are running at speeds you can get a large benefit from them. FSAE is not this type of class.

Western Washington University FSAE
dot.etec.wwu.edu/fsae

[sticks]

Actually, aero downforce, as drag, is a fuction of the square of the velocity. Reducing the velocity by half would return 1/4 of the downforce. Mother nature's kind of funny that way. Not to mention the adverse effects to the wing profiles (increased chord with thus reduced relatvent section thickness and reduced flow area) of a corning car due to cross flows over the wings. This is once place where very careful analysis if the upper echelon of formula racers end plates would be of substantial gain. Over the past five months I have run a series of 30% models through well in excess of 100 hours of tunnel time, including spinning scaled wheels up to speed. This has been done in a tunnel at a primarily aeronautics facility so I could run the free stream velocities high enough to achieve relevant reynolds numbers, as well as test for yaw effects. Out of this I have become increasingly convinced of the usefulness of carefully designed aerodynamics on a fsae car, if for no other reason than improved dynamic balance. I have also become frustrated with the required placement of the rear wing, which truly limits ultimate downforce capabilities due to divergent effects. But a well set up car, in my opinion, running regulation aeropackages, could have a very distinct advantage, even at the low speeds encountered. Well, good luck everyone with their aerodynamic endeavors.

[MikeWaggoner at UW]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Actually, aero downforce, as drag, is a fuction of the square of the velocity. Reducing the velocity by half would return 1/4 of the downforce.<HR></BLOCKQUOTE>

(20/36) squared = 31%

Western Washington University FSAE
dot.etec.wwu.edu/fsae

[Bob Wright]

If the average winning speed of the US comp is around 30mph, the 80hp and a car weighing around 500lb has grip as its main limiting factor. On corners there isnt much time when full throttle can be applied, and even on the straights there is still not much time when the driver does not have access to a slip ratio higher than optimal. When this is the case, we find that an excess of available power is best utilised to drive the car into the ground, thuss increasing the available grip. You can build wings for any speed if you crunch the numbers. We just found that are wings gave a huge advantage on the tightest track fsae has ever seen in Australia. It damn near made up for a 55kg weight penalty over the winner. We are fast approaching a drag vs fuel consumption optimisation as our main performance criteria, and as a driver it makes a BIG difference.

good work on the monocoque chassis though, I wish we had the time to have a go at one.

Bob Wright
Monash University
Australia

[sticks]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>(20/36) squared = 31%
<HR></BLOCKQUOTE>
Very true Mike, where in the hell was I educated. I was just spitballing, for example, but good point. Thanks.

[Mehul Botadra]

I guess we're going offtopic now!

[Hector]

That's about the deepest Necro-Threading I've ever seen.

[Wesley]

The OP kinda seemed like a troll to me. It's like a thread that starts off, "Why do you guys run Goodyears! Everyone knows Hoosiers are better!"

"gee maybe teams make a selection that isn't buying the shiniest parts they can find"

Quite simply, they're more competetive.

[Mark_W]

http://www.formulastudent.de/a...space-frame-chassis/ (http://www.formulastudent.de/academy/pats-corner/advice-details/article/pats-column-space-frame-chassis/)

and that is a design judge. its about the competition, not necessarily the engineering.

[bob.paasch]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Wesley:
Quite simply, they're more competetive. </div></BLOCKQUOTE>

7 years ago when this thread was started there might still have been a question. In 2009 however:

First place FSAE Virginia =&gt; 4130 spaceframe
First place FSAE Michigan =&gt; CFRP monocoque
First place FSAE California =&gt; 4130 spaceframe (but a CFRP monocoque won the overall dynamics)
First place Formula Student UK =&gt; CFRP monocoque
First place Formula Student Germany =&gt; CFRP monocoque
First place Formula Student Austria =&gt; CFRP monocoque

So for fun, what's the next major FS event that will be won by a steel spaceframe?

[Chris Allbee]

Mr Paasch,

That data is inconclusive at best. Aside from the over-all dynamics win in Cali, the competition win has far too many factors involved to say that they won because it was a CFRP chassis.

You could draw a corollary between the amount of research a team does into its design and construction of a CFRP chassis vs a steel spaceframe. I imagine you would see a fair bit more effort put into the CFRP designs. This will be looked upon favorably by the design judges, thus possibly tipping the scales in their favor under the design tent. However, were a team with a steel spaceframe to put forth the same thought and effort and document it accordingly, then it would be a more level playing field, design-wise, and the teams would be separated by their performance in the other events.

Simply put, the teams to which you refer won because they were a well-rounded and dedicated team. NOT because they built a car around a CFRP chassis.

[Wesley]

Didn't you know Chris, there was a big article published, it said that now correlation=causation. http://fsae.com/groupee_common/emoticons/icon_wink.gif

[Chris Allbee]

Aw crap! I always miss the important articles....

[Wesley]

A CFRP monocoque was second in Virginia!? I didn't know we had a carbon tub. Thats sweet.

[Rex]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Chris Allbee:
That data is inconclusive at best. </div></BLOCKQUOTE>

My thoughts exactly - the question most certianly still remains, and those results, while they certainly spark a useful discussion, don't say anything to me except that we need more info. For example, what percentage of CFRP teams also have aero elements? I've been away from this competition too long to know offhand which of the teams from that list had wings, but it would be interesting to see that information posted right alongside the original list, along with other major factors such as engine choice, team structure, funding, etc to determine the cause of this effect. Who knows, you might find another relevant factor that is a better match for the trend! Or maybe not. But it would be fun to see.

So while the original statement regarding the next event to be won by a steel frame vehicle may indeed be accurate, the carbon tub itself may or may not be the reason.

[bob.paasch]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Wesley:
A CFRP monocoque was second in Virginia!? I didn't know we had a carbon tub. Thats sweet. </div></BLOCKQUOTE>

Shoot, my mistake. Got you guys mixed up with that team to the north of you. :^)

[Wesley]

It's cool, if we had the resources (team and otherwise) to tackle a monocoque, we likely would try.

[bob.paasch]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Chris Allbee:
That data is inconclusive at best. Aside from the over-all dynamics win in Cali, the competition win has far too many factors involved to say that they won because it was a CFRP chassis. </div></BLOCKQUOTE>

I agree that overall wins have many factors. If you just look at dynamic wins, the only one this year by a steel car was MST at VIR

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> Simply put, the teams to which you refer won because they were a well-rounded and dedicated team. NOT because they built a car around a CFRP chassis. </div></BLOCKQUOTE>

I agree with the first point, but not with the second. I agree 100% FSAE/FS is at its core a project management competition, and the top teams are there because they have good project management.

That said, a CFRP chassis offers provable performance advantages, although you need to have good project management to take advantage of them. More and more of the top teams are running a CFRP monocoque, with Uni Stuttgart the latest team to switch. I'm quite sure they didn't make the change without good technical reasons.

The three fastest autocross cars at FSG09 were monocoques. Good teams, but those cars are also bloody quick.

[bob.paasch]

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

For example, what percentage of CFRP teams also have aero elements? </div></BLOCKQUOTE>

Off the top of my head, Kansas is the only monocoque I can think of that runs wings. UTA, Maryland, Oklahoma and MST I think are all space frames. There are a few monocoques with undertrays, including Stuttgart.

At FSGermany, 21 of the 78 cars were CFRP monocoque. That is a lot higher % than at FSAE California and, I believe, FSAE Michigan. No wings at FSG that I recall.

[Mike Cook]

This thread is lame. I have yet to see any kind of actual real data showing laptimes vs. stiffness. I have read most of the SAE papers concerning chassis stiffness and I understand its effects well. Only if you take this argument to the very extreme might you be able to prove it (100ft-lb/deg space frame or something)... And the rules are written such that by meeting them, your chassis is most likely going to be stiff enough that you will not run into any problems. None of the five fsae cars I built had problems with being tuned correctly, and they varied quite a bit in stiffness. Our more recent cars have a very lopsided LLTD and our roll stiffness distributions are not very proportionally different.



It should come as no surprise that monocoque teams are winning, however. To even build one you need to have a well funded, organized, motivated, competent team. If they had put their effort into a space frame i'm sure it would be equally as good.

Bob, you are welcome to come to nationals and see how your moncoque fairs on a real autocross course where fuel economy doesn't matter http://fsae.com/groupee_common/emoticons/icon_smile.gif


on edit:

I've talked to one of the top guys at GM and he told me an interesting story about the development of the Camaro. He said they ran all the basic performance tests (skid pad, slalom, etc.) and then cut the whole top off the car and ran them again. The torsional stiffness was reduced a lot (although to be honest i'm not sure of the actual numbers). The actual times for the tests with and without the top were the same. The main difference was that the drivers complained a lot about the noticeable flex and noises associated with it.

[flavorPacket]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by rp:
That said, a CFRP chassis offers provable performance advantages </div></BLOCKQUOTE>

And these proven advantages are...?


<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">The three fastest autocross cars at FSG09 were monocoques. Good teams, but those cars are also bloody quick. </div></BLOCKQUOTE>

Autox is about driving talent more than anything, as is endurance. I submit that we could put Stuttgart's drivers in any number of steel cars and beat the pants off the entire field.

Also, to use this ridiculous logic, note that the 2nd and 3rd place cars in the dynamic events at FSG were steel cars.

[Rex]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by rp:
Off the top of my head, Kansas is the only monocoque I can think of that runs wings. UTA, Maryland, Oklahoma and MST I think are all space frames. There are a few monocoques with undertrays, including Stuttgart.

At FSGermany, 21 of the 78 cars were CFRP monocoque. That is a lot higher % than at FSAE California and, I believe, FSAE Michigan. No wings at FSG that I recall. </div></BLOCKQUOTE>

Good information - and I'm impressed at the high percentage of FSG cars with CFRP! Whether you think they give performance gains or not, that's a lot of teams getting lots of great hands-on experience!

Also good to know about the wings. I think accounting for these kinds of factors makes the discussion much more useful (although many other factors would be required to make this discussion conclusive).

And to Mike's point, we can probably agree that a CFRP can be made lighter for equivalent stiffness, but what kind of weight savings can be achieved at the minimum requirements of the rules (or, alternatively, at the minimum required stiffness for reasonable handling and tunability, if one believes that number to be different from the minimum stiffness provided by a minimum-rules-legal chassis)? And more importantly, how does that weight savings translate into dynamic event time decreases?

I agree this thread is lame - unless people start posting hard numbers and analysis. I have no such info to post, so I'll stop posting.

[Mike Cook]

Good point Rex. I forgot to mention that the obvious advantage is that a monocoque can be made a lot lighter than a steel tube frame, especially the way the rules are written! With that said, its still tough to argue weight vs lap time.

With all the steel tube frame cars I have built I'd love to give a monocoque a shot but I'm sure it is very difficult, especially the first time around.

[NRBaer]

I would like to add that, while we are not one of the teams that finishes at the top every year or one of the "well-funded" programs that you speak of, the Lehigh team has looked into doing a steel tube car both this year and last due to the rule changes and have found that it is quicker and easier for our team to create a VARTM CFRP chassis than a steel tube chassis. Once you get it done the first time it is a walk in the park.

[woodsy96]

I am of the opinion that you need to build a car to suit your team's strengths, and what resources you have. Regardless of which you choose, it is the quality of the finished product that will determine how good the car is.

For lots of teams, a space frame is therefore a better choice. The big advantage I see with it is that you can easily make changes to the chassis design as you go, and moving things like suspension pickup points is a lot easier. The disadvantage is that you need someone with sweet welding game (if you want a really good chassis).

For us, we have more people with composites game, so we make a monocoque. We also have access to an autoclave. The advantage is that anyone can lay down some fibers in a mould. It is harder to change chassis geometry, and there is a lot less design on the fly (unlike a spaceframe, where you go "Oh, I forgot about this part. Lets just weld another bracket on.")

[bob.paasch]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Rex:
And to Mike's point, we can probably agree that a CFRP can be made lighter for equivalent stiffness, but what kind of weight savings can be achieved at the minimum requirements of the rules (or, alternatively, at the minimum required stiffness for reasonable handling and tunability, if one believes that number to be different from the minimum stiffness provided by a minimum-rules-legal chassis)? And more importantly, how does that weight savings translate into dynamic event time decreases?

I agree this thread is lame - unless people start posting hard numbers and analysis. I have no such info to post, so I'll stop posting. </div></BLOCKQUOTE>

I'm not sure I can "de-lame" this thread, but I'll give what I've got.

In my humble opinion, the performance advantages in FSAE/FS for a monocoque lay in mass reduction, not stiffness increase. I don't think it's any coincidence that the competitive 150 kg class NA singles (RMIT, ETS, Delft and this year OSU) are all monocoques. When you've only 50 HP, every kilogram counts.

I think it makes a lot less difference when you have a 90 HP four. Stuttgart's 09 car is very fast, but so was their 08 car. And Ryan, I do recognize that their drivers are very good, each of them passed us in the FSG endurance. :^) (Congrats BTW on your 5th place finish at FSG)

With respect to stiffness verses performance, I think it would be very difficult to quantify this effect experimentally. I can image building a FSAE car with variable stiffness, but not a competitive FSAE car with variable stiffness. I agree with Mike, if you build a chassis that meets the FSAE rules, it will most likely be stiff enough. Stiffness increases were not our motivation for switching to a monocoque. When you're only 147 kg, you don't have to have a particularly stiff chassis.

With respect to mass verses performance, this is fairly easy to quantify. We did a day of testing last year with our 08 car (a 185 kg spaceframe four, similar to Michigan's 09 car) where we duct taped a 10 kg chunk of steel to various parts of the car, then ran acceleration, skidpad and autocross. Obviously, where we added the weight had a big impact. Not unexpectedly, adding 10 kg to the top of the main roll hoop decreased our accel times by a tenth of a second (nice dynamic weight transfer to the rear wheels), though needless to say this totally screwed the lateral handling. :^)
We did see a statistically significant increase in autocross times when adding mass at the cg. I'd give numbers, but they wouldn't really be transferable and I'd recommend other teams do their own testing, as easy as it is.

For us, moving to a monocoque was part of a five year plan. We made the change for both mass and manufacturability reasons. This first year the monocoque was somewhat more work than a steel spaceframe. We expect it to be less work next year now that we have a manufacturing process, and we expect to drop some more weight as well.

I think the most interesting chassis I saw at FSG was Vienna, with their CFRP tube spaceframe housing a KTM single. They need to get the mass of the rest of their car down a little more, as they're still around 190 kg, but an interesting idea.

[Zac]

Isn't the lightest car still a spaceframe design?

[Chris Allbee]

This is all silly conjecture anyways. The lightest car is one that isn't built. It would also be the cheapest and "greenest" (had to throw in a buzzword). And it would always be just as fast as you think it can be.

[MH]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Zac:
Isn't the lightest car still a spaceframe design? </div></BLOCKQUOTE>

No, as far as I know Delft '04. 129kg.

cheers,
Miki Hegedus
Delft University of Technology 2001-2008

[JVC]

And there is a very good reason for that too: Anyone who paid attention during mechanics class should know that shear webs and the like are structurally more efficient than truss structures. Hence all commercial aircraft have an aluminium monocoque for an airframe, case in point. So if you want an efficient structure, go with a monocoque.

And as Boeing is proving right now with the 787, it makes no sense to switch to carbon, if you don't make use of the directionality of the material. If you are designing 'black aluminium' then just forget about any weight advantage over an aluminium structure.

Then about load paths. There are some people (I won't mention any names) that say a space frame is the best educational tool you can have, because you learn about load path management. I think everyone here has an intuitive feeling of what a load path is, however, nowhere in literature you will find a clear and concise definition of what a load path actually is. We all just rely on good old engineering sense, which is fine by itself. But I'm heading off track here. To come back to my comment on directionality of composite materials, you must have very good understanding of your 'load paths' in order to design a competitive carbon fibre reinforce monocoque. Thus making it even a better educational tool than a space frame. You get to learn about composites and load paths at the same time. And the structure is more efficient too. What more do you want?

Cheers,
Julien van Campen

[Chris Allbee]

JVC,

I'm sorry but I have to disagree. I see on an almost daily basis the results of people not understanding what a load path is. Granted, a car unibody is somewhat different than a carbon monocoque, but the principals still apply.

The ability to make odd shapes and complex geometries out of stamped steel and molded composites can be VERY confusing to someone who doesn't properly understand load paths, or more correctly strain energy distribution.

However, it is very easy to look at a spaceframe with a tube intersecting another tube and be able to visualize the results. Obviously the loads aren't going to travel through the air, they will have to go through the tubes. And as such, it is easier to see and interpret the effects of a badly designed load path such as a tube intersecting in the middle of another tube and not at a node.

That same concept becomes obscured and a bit muddled to many when considering a large sheet of carbon or steel intersecting with other sheets at various angles and radii. You will quickly find that there is no such thing as "good engineering sense", only those who understand the basics and those that don't.

As such, starting with something more tangible and "basic" like a spaceframe will enable more people to better grasp the concepts that will build the foundation of their understanding. And when they are ready they can tackle the more complex issues. People learn and understand at differing rates. And what may have taken you only a couple months or a year to grasp and master can take others many years.

The decision to go with a spaceframe or CFRP chassis is one that should be based on the teams current abilities and level of understanding. This will be different for every team. Provided the decision has been thought out and can be backed up with solid reasoning, then I would say that neither the spaceframe nor the monocoque are better they are simply the appropriate choice for that team at that time.

Remember that for all the fun dynamic events that take place, the purpose of this competition is to educate the next generation of engineers. And a good education starts with the fundamentals. Yes, its possible to build a CFRP chassis without understanding the basics, and the thing might even be fairly light and competitive. However, if you don't understand the WHAT and WHY behind everything you have done, then what happens when you get into the real world and find yourself in a situation where you can't just get on a forum and ask for the answer or go to a competition and photograph the object you want?

It just bugs me to see so many people say with absolute certainty that a composite chassis is the best way to go without considering the circumstances surrounding the competition and its intended purpose.

Whichever method will allow you to finish the project and learn the most is the best method to use. Period.

P.S. "Finishing the project" doesn't mean stopping once its built....

[flavorPacket]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by JVC:
you must have very good understanding of your 'load paths' in order to design a competitive carbon fibre reinforce monocoque. Thus making it even a better educational tool than a space frame. You get to learn about composites and load paths at the same time.
Julien van Campen </div></BLOCKQUOTE>

http://fsae.com/eve/forums/a/t...7348/m/953108962/p/9 (http://fsae.com/eve/forums/a/tpc/f/825607348/m/953108962/p/9)

OSU's broken monocoque had faster times than your monocoque at FSG. Thus, an understanding of load paths and composite structures is not critical to designing 'a competitive carbon fiber monocoque', as you say.

OSU, please don't take this as a dig against your team. We at Michigan were thrilled to see you bring the pace to keep up with Stuttgart, and you were the only US team to do so. We would have been happy to come in 6th and see you on top rather than come in 5th and watch your muffler fly off. You are not the only team out there that has to make tough technical calls in order to get things done and then pay a price for it. But you obviously came back strong from your failure and proved what you can do. We look forward to seeing you there again next year.

[Stocky Fast 1]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by MH:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Zac:
Isn't the lightest car still a spaceframe design? </div></BLOCKQUOTE>

No, as far as I know Delft '04. 129kg.

cheers,
Miki Hegedus
Delft University of Technology 2001-2008 </div></BLOCKQUOTE>

Penn I believe in Michigan was 275lbs or so, I think that converts to a little less than 129kg. Even if they were 295lbs that still proves the point that both steel chassis and carbon monocoques are competitive. Even 6 years after this thread was started.

[J.R.]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Stocky Fast 1:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by MH:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Zac:
Isn't the lightest car still a spaceframe design? </div></BLOCKQUOTE>

No, as far as I know Delft '04. 129kg.

cheers,
Miki Hegedus
Delft University of Technology 2001-2008 </div></BLOCKQUOTE>

Penn I believe in Michigan was 275lbs or so, I think that converts to a little less than 129kg. Even if they were 295lbs that still proves the point that both steel chassis and carbon monocoques are competitive. Even 6 years after this thread was started. </div></BLOCKQUOTE>

Wasn't PSU's car a hybrid chassis? (Steel tube with stressed carbon panels?)

PS. they were 289 lb (131 kg)

[D Collins Jr]

Wasn't Penn State's frame primarily titanium?

[JVC]

Let me set something straight. I never said teams should go with a composite monocoque, but only that in potential such a structure is superior to a space frame. What is best for a team, indeed depends on a lot more factors.

Nevertheless, I need to disagree with you Chris. As the story of OSU proves proper load introduction is very important in composite design. This can be just as insightful for students in terms of 'load-paths' as a space frame, only the emphasis is somewhat different.

And to flavorPacket: there are many factors that make a car go fast, a monocoque is just one of them. Just imagine how fast OSU would have been otherwise. A real shame their muffler fell off.

Julien

[MH]

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

OSU's broken monocoque had faster times than your monocoque at FSG. Thus, an understanding of load paths and composite structures is not critical to designing 'a competitive carbon fiber monocoque', as you say.

</div></BLOCKQUOTE>

Sorry, but you're wrong Flavorpacket. OSU was only quicker in AutoX. Both our drivers were quicker in Enduro. We kept track of the laptimes. Not to take anything away from OSU, though...very good team.

cheers!
Miki Hegedus
Delft University of Technology 2001-2008

[flavorPacket]

Never said anything about a specific event, Miki. OSU also placed higher in skidpad and acceleration. Regardless of the details, the fact that OSU was competitive with a broken monocoque still stands.

[JVC]

That OSU drove with a repaired (rather than broken) monocoque, only means their car was heavier than it should have been. Or put more appropriately, that it could have been even lighter than it already is.
Competitiveness doesn't boil down to one thing. A structurally efficient chassis is just one of the contributing factors. So it is possible a team is very good regardless of chassis design, but with a properly designed chassis they can be even better.

Julien

[bob.paasch]

First, on behalf of OSU's team, I'd like to express our sincere appreciation for the complements for our car, our team and our performance this year. It is really gratifying to be mentioned in the same sentences with the University of Stuttgart and TU Delft. Thank you all.

I would, however, like to correct some misconceptions about the 09 OSU car and team. First, we are not a rookie team. OSU built its first FSAE car in 1987, and we have been competing continuously since 1995. We have a lot of experience with steel spaceframe cars, our 08 car was a 182 kg four cylinder steel spaceframe. The 09 OSU team was a veteran one with extensive experience in suspension and chassis design. The team does understand loadpaths.

As explained previously, our chassis failure in California occurred during the brake test, wherein a tire pressure adjustment outside the normal range changed the natural frequency enough to put the front suspension into a forced resonance situation, manifested by severe wheel hop. This put dynamic loadings on the front suspension far above what would normally be expected in the competition. The failure initially occurred in our lower A-arm attachment points, though the buckling of the front A-arms also delaminated the top attachments. The design of the monocoque hard points and resultant distribution to the rest of the monocoque were inadequate for this dynamic loading situation. We could have done a better job of analysis, but I expect that few teams have calculated or measured the loads occurring during wheel hop. We did do the typical load analysis for lateral acceleration and steady state braking. The composite team took loadpaths into account in designing the carbon orientations. To state that we were competitive without understanding loadpaths or composite structures is not entirely true. We admittedly have more to learn about design and analysis of monocoque hardpoints. :^) :^)

Our car was not "broken" for the California dynamic events but it was certainly sub-optimal. The crude front A-arm repairs left us with essentially no caster. The monocoque repairs moved the the pickup points and changed kinematics.

Before FSG, new A-arms were built to the original dimensions, and the upper attachment doubler plates were removed. Our lower attachment points were still shifted down by the 1.5 mm thickness of the aluminum doubler, but kinematics were only slightly compromised. The repairs increased the mass of the car by about 3 kg.

The 09 OSU car was not competitive solely because of the monocoque chassis, but neither was it competitive without the team understanding loadpaths and chassis design. In my humble opinion, the 09 OSU car was competitive primarily because of the efforts of our chief suspension designer. That the suspension still worked well despite the monocoque repairs and kinematic changes is testament to the quality of his design.

Still, I agree with Julien, a monocoque chassis has the potential to be superior to a steel spaceframe. The "provable performance advantage" is that it can be made stiffer for the same mass, or lighter for the same stiffness. It is not the answer for every team. It is most likely not the answer for a rookie team, or even a veteran team still struggling with project management. It was the answer for our team, this year, and for the foreseeable future. We would not have had the lightest car at FSG09 with a steel chassis.

[Shin]

The number of teams using Monocoque chassis had been increasing in the world.

But I don't know why most of teams in USA still using steel chassis.
You know, So much user of steel chassis is in Japan also. In this case, there are two reasons why Japanese teams using steel chassis.

One is easy, they don't have enough stuff to make CFRP.
Second is the shape of the course endurance. It have long two straights. So that, most important performance is not Total weight but engine power.

The winner FSAE Japan 2009 is Tokyo by around 300kg total weight car.

I know steel chassis has many advantage but are there clear reason why steel chassis are major in Formula SAE USA?

[The AFX Master]

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

The winner FSAE Japan 2009 is Tokyo by around 300kg total weight car.
</div></BLOCKQUOTE>

Well, in such a heavy car, not only the chassis can take all the blame.. if you're talking about the car alone (no driver), 300kg (661 lbs) is huge for a F-SAE car

[exFSAE]

<LI>Lighter not always = better.
<LI>I have yet to see a team put some math behind a well-designed space frame and a well-, designed monocoque and be able to accurately justify one or the other by the numbers. Show me how much rigidity each has (in whatever direction is most appropriate), and show the effect each has on suspension performance. Even if it's just total roll rate distribution.

There is much handwaving when it comes to this subject.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">I know steel chassis has many advantage but are there clear reason why steel chassis are major in Formula SAE USA? </div></BLOCKQUOTE>

You just answered your own question. There are many advantages.

[TorqueWrench]

Our team had crunched some numbers on a hybrid structure (combination monocoque and spaceframe) simply due to easier manufacturing and our limited resources. I remember the guy running the numbers telling me there was not much room for it to be an improvement over a spaceframe.

I keep telling my team this: "Until you can make a 55 lb roll cage (including tabs) with near perfect load paths, I don't want to see a monocoque considered."

EDIT: Should point out it wasn't CF we were analyzing.

[The AFX Master]

If F-SAE is settled around a learning environment (F-SAE is not a racing series), then i don't see limitations in order to create things. Yes, i know that the team that better exploits its resources and manpower wins,not only the team with fancier gizmos, But if you can build a car that works with those flashy bits, you'll learn something.

Do you learn something about composites at uni?.. no, then you go and put an effort to develop composites parts in your car.

The same with stuff like wings, Turbos, fancy homemade electronics, Ground Effect and so on.

The goal here is to learn all corners of engineering life.

[jrickert]

Ask your self this. Is your frame the weak point of your car? This was the question posed to me by a prominent race car designer with many Lemans/indy/etc cars under his belt.

Do whatever suits your resources better. At the level of optimization that 75% of teams are at i believe that track time, good drivers and time for static events will do you more good than any fancy bell or whistle.

Are cf monocoques cool? yes! Do they have the potential to be lighter and more rigid? Yes! Will they automatically own? No!

A well tested and tuned car with good drivers will do better than other cars regardless of frame type. If your team has oodles of experience with one type of frame but has not MASTERED the fundamentals they would be best to master the fundamentals first.

That said the spirit of FSAE is learning and fancy technologies can be a great learning experience. Depending on your mindset and priorities the learning experience alone might be worth it. CF is not black magic.

I do believe however that design judges are susceptible to being wowed by bling.

[PatClarke]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">I do believe however that design judges are susceptible to being wowed by bling. </div></BLOCKQUOTE>


Oh yeah?? http://fsae.com/groupee_common/emoticons/icon_smile.gif

Pat

[js10coastr]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by PatClarke:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">I do believe however that design judges are susceptible to being wowed by bling. </div></BLOCKQUOTE>


Oh yeah?? http://fsae.com/groupee_common/emoticons/icon_smile.gif

Pat </div></BLOCKQUOTE>

Oh the stories I hear about the baja judges... none in the FSAE series.

[Muad'Dib]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Shin:
The number of teams using Monocoque chassis had been increasing in the world.

But I don't know why most of teams in USA still using steel chassis.
</div></BLOCKQUOTE>

I'm convinced that the reason for this is money. Although I'm basing this on no actual evidence it seems like a lot of the European teams have very large budgets. (One of our alumni is at grad school at a german university. Supposedly their budget is around $500,000 when you factor in free autoclave time from professional sponsors). The type of sponsors the european teams get just seems rather out of the grasp of most US teams (TU Graz with red bull for example). When you have this type of funding and a large team there is a lot you can try.

Oh and before someone yells at me I'm not trying to provide excuses or take away from anyone's successes. I have huge respect for the big budget teams because a huge budget with idiots behind it goes nowhere.

[TorqueWrench]

This has been said in here before, but this is my view on anything on these vehicles. You have a finite budget to spend on the car. Identify the areas that are most affecting your performance/points and allocate funding accordingly. On most of these cars, I see the money that could be spent on a monocoque coming to much better gains being dumped into the suspension (this is a handling competition for the most part), brakes (the much neglected system on our team), or powertrain (I like shiny toys).

That being said, I know there are teams out there that have refined their suspension setups and powertrains to the point that the benefit/cost is now skewed to the chassis being lighter as there is always a point of diminishing return on the other systems. For them, it makes perfect sense to go to a monocoque.

As I have also said before, I don't think you can design a good chassis without understanding load paths. This is much easier to learn with a steel spaceframe as you can physically see the paths the loads will take. Until recently you average person had no ability to FEA a monocoque (most still don't as I don't trust Solidwork's composite FEA), so this would have increased development cost to teams due to iterative design. I have seen some scare pictures float around of composite tubs with cracks at corner radii because the person who designed the chassis didn't pay close enough attention.