cymtriks
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| posted on 9/12/02 at 12:21 PM |
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chassis stiffness
How important is this exactly?
My analysis of most chassis indicates that many cars are getting away with a lot less stiffness than others. I've been surprised by the write ups on
the MK GT1 and the revised Midtech as, judging only by the available pictures, they aren't as stiff as they could be and so should be getting
comments like "a revised chassis is planed" or "chassis improvements are in the pipeline" but they get good reviews as they are.
This gives four possibilities-
1 the road tests weren't very demanding.
2 the pictures are misleading in some way.
3 torsional stiffnes isn't that important.
4 the rest of car design more than makes up for any chassis faults
Any thoughts guys?
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Spyderman
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| posted on 9/12/02 at 01:05 PM |
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Although I might not necessarily agree with your assesments of the two mentioned cars just by visual means, I would agree with you overall.
However I would rearrange your possibilities.
1.The rest of car design more than makes up for any chassis faults.
2.The road tests weren't very demanding.
3.The pictures are misleading in some way.
4.Torsional stiffnes isn't that important.
I would break it down to;
1. There have been many cars that have been quoted as handling very well, but have low torsional stiffness.
The humble Lous Elan was in it's day one of the most successful sports cars even though there were many other cars with higher stiffness. It was
it's power to weight that won it through.
2. How demanding can you be in someone else's car on the public roads in a short period of time?
3. The pictures are never going to show the details, only what is pleasing to look at.
4. See 1.
Personaly I would prefer a car that was more predictable, even if it meant being slightly less stiff. Making a car stiffer can destroy it's handling
qualities! Everything is a compromise!
My thoughts
Terry
Spyderman
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TheGecko
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| posted on 9/12/02 at 01:19 PM |
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My 2c worth.
I have always been told that a floppy chassis is not a replacement for some of the suspension. The best way to have predictable suspension is to make
the chassis as stiff as possible and then tune the suspension as required. If you can't predict what the chassis is going to do, you can't possibly
setup the suspension.
Torsional stiffness is a subject dear to my heart at present. In Australia, our chassis' must meet minmum torsion and beaming strength standards
which scale up with engine size. For instance, the standard Locost chassis is about 1/3 of the required strength in torsion!
My middy chassis will have triangular cross-section 'pontoons' down the sides to greatly improve torsional stiffness in the absence of a center
tunnel.
Hope this helps,
Dominic
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Alan B
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| posted on 9/12/02 at 01:40 PM |
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Guys, ever consider option 5?
5. The testers haven't a clue.
I wouldn't be the first time
I'm not being critical of either car in this case as I'm not familiar enough with their chassis.
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Alan B
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| posted on 9/12/02 at 01:48 PM |
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Better just modify that comment a little.
Most kit testers/writers are no more expert than most of us. Most are just enthusiasts who can string a few words together...being published in a
magazine (or a book ) does not make someone an expert.....I know you all know this, but it is worth repeating.
All IMO of course.
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ProjectLMP
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| posted on 9/12/02 at 05:27 PM |
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My personal opinion is that the importance of chassis stiffness increases with weight of the vehicle and stiffness of the suspension setup.
I think a lot of the BEC's get away with it because they are very light. Also, unless the chassis is really bad I doubt you can tell driving the car
on the road. Additionally, most people couldn't tell if it was the chassis or the suspension setup from my experience. There are just too many
variables involved.
Now if you have a very stiffly sprung car (i.e. race car setup) stiffness is very important to consistent behavoir. I have seen people chase setup
problems that in the end were a result of the chassis (and pickup points) flexing and not the suspension! Typical symptoms include anti rollbars that
have no effect no matter how big you make them.
I disagree that you need a flexible chassis for nice compliant predictable handling. This is a function of the suspension setup.
Home of the Astronomicalcost Mid engined LMP project
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cymtriks
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| posted on 10/12/02 at 10:28 AM |
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Thanks guys!
I was starting to think that something was wrong there! My analysis, as previously posted, indicated that the book chassis has between 1200 and 1400
foot pounds per degree of twist depending on the various options in the book. This is based on a simple finite element model which is probably
accurate to within, roughly, 5% but assumes a perfect assembly!
This stacks up reasonably well with statement that the book chassis has about 1/3 of the Oz requirement which I think is 3000ftlbs (4000Nm).
In case anyone is interested I think that the MK GT1 chassis would be vastly stiffer if the front suspension region was triangulated (the current
chassis appears to have no triangulation at all in this region) and a Y brace or diagonals were placed across the top of the engine bay.
The Midtech chassis could have double the stiffness if the engine bay was properly triangulated across the top. The midtech website pictures do not
show any diagonals over the engine bay top.
Would anyone like me to e-mail them a copy of my finite element results? I've written them up due to the number of questions I got about it. The word
doc has diagrams and covers spaceframes and ladder frames together with a list of high stiffness design features.
Thanks for the replies guys.
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philgregson
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| posted on 10/12/02 at 12:02 PM |
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Hi
I would be extremely interested in a copy of your results. It is something that I know little about but interests me greatly.
Cheers
Phil
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Spyderman
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| posted on 11/12/02 at 03:48 PM |
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quote:
Originally posted by philgregson
Hi
I would be extremely interested in a copy of your results. It is something that I know little about but interests me greatly.
Cheers
Phil
Yep!
I'd go along with Phil!
Terry
Spyderman
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MrFluffy
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| posted on 12/12/02 at 02:20 PM |
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Oooo good timing, can I get a copy from you, at least try to steer my layout in the right direction rather than just relying on triangulation in 3d
where possible..
Im probally going to want it more, because my crazed ideas call for a mid mounted v8 with a kitcar body, and the possibility of wider track, more
weight etc....
Email is phil@fluffycentral.com save you clicking thru the profile!
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