door bars are worth every cent
12-09-2009, 10:07 PM
#65
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In my professional opinion as a chassis structural engineer a flat plate on the bottom of the car will do next to nothing for stiffness. a flat plate to resist parrelelogramming like where one rail shifts forward and the other one shifts back will also do next to nothing in this case as well because I don’t thing that is a weak mode for the chassis to flex in.
Closing the bottom of the transmission tunnel in a structural way such that it makes the trans tunnel behave like a big tube rather than an open channel will do quite a lot. But you still have that torsion bar concept going on like where the material in the middle of a torsion bar adds minimal torsional stiffness. Hollow bar and a solid bar of the same OD have very similar torsional stiffness and yet the hollow one is ¼ the weight. The trans tunnel being in the center of the car isn’t as effective as material further away from the centroid like near the sills.
The stiffness that the FM butterfly adds to the chassis is mostly from its effectiveness at closing off the bottom of the trans tunnel making it behave like a tube. The problem I see is it could be designed allot better and be lighter than what it is to do this. It needs to bolt to the car near the edge of the tunnel not out by the frame rails. The later model factory braces are much better at doing this than the FM peace. I saw it as way to much weight for the benefit.
Bob
Closing the bottom of the transmission tunnel in a structural way such that it makes the trans tunnel behave like a big tube rather than an open channel will do quite a lot. But you still have that torsion bar concept going on like where the material in the middle of a torsion bar adds minimal torsional stiffness. Hollow bar and a solid bar of the same OD have very similar torsional stiffness and yet the hollow one is ¼ the weight. The trans tunnel being in the center of the car isn’t as effective as material further away from the centroid like near the sills.
The stiffness that the FM butterfly adds to the chassis is mostly from its effectiveness at closing off the bottom of the trans tunnel making it behave like a tube. The problem I see is it could be designed allot better and be lighter than what it is to do this. It needs to bolt to the car near the edge of the tunnel not out by the frame rails. The later model factory braces are much better at doing this than the FM peace. I saw it as way to much weight for the benefit.
Bob
I suspect the reason FM did the butterfly brace the way they did is it makes it more of a bolt-in proposition once the frame rails are in. Although, the fab involved in attaching a pre-formed piece to the floor board at the trans tunnel transition would not be much more work than installing the frame rails IMO.
If it were not for the transmission being in the way, an easy, mass-efficient way to stiffen this mode would be to cross-brace the rails by tying the front end of the left rail to the rear end of the right rail (and vice versa) using the same simple crushed end tubes like those at the k-member and the diff. There is some risk of column bucking I suppose since they would be in effect simply supported, and have a large length to dia ratio. Hell it could even be braced with pre-tensioned cables for that matter, just like the bracing on a string-truss Newtonian telescope tube. Same situation, just tubular instead of planar.
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12-10-2009, 02:33 PM
#66
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If I follow you here, as a chassis engineer/ME albeit for spacecraft, I agree. Put another way, if it were not for the trans tunnel the existing floor board would do a decent job of stiffening the bending mode you describe. Thus, it could be alleviated by closing out the trans tunnel.
I suspect the reason FM did the butterfly brace the way they did is it makes it more of a bolt-in proposition once the frame rails are in. Although, the fab involved in attaching a pre-formed piece to the floor board at the trans tunnel transition would not be much more work than installing the frame rails IMO.
If it were not for the transmission being in the way, an easy, mass-efficient way to stiffen this mode would be to cross-brace the rails by tying the front end of the left rail to the rear end of the right rail (and vice versa) using the same simple crushed end tubes like those at the k-member and the diff. There is some risk of column bucking I suppose since they would be in effect simply supported, and have a large length to dia ratio. Hell it could even be braced with pre-tensioned cables for that matter, just like the bracing on a string-truss Newtonian telescope tube. Same situation, just tubular instead of planar.
I suspect the reason FM did the butterfly brace the way they did is it makes it more of a bolt-in proposition once the frame rails are in. Although, the fab involved in attaching a pre-formed piece to the floor board at the trans tunnel transition would not be much more work than installing the frame rails IMO.
If it were not for the transmission being in the way, an easy, mass-efficient way to stiffen this mode would be to cross-brace the rails by tying the front end of the left rail to the rear end of the right rail (and vice versa) using the same simple crushed end tubes like those at the k-member and the diff. There is some risk of column bucking I suppose since they would be in effect simply supported, and have a large length to dia ratio. Hell it could even be braced with pre-tensioned cables for that matter, just like the bracing on a string-truss Newtonian telescope tube. Same situation, just tubular instead of planar.
Other than the sills, the trans tunnel, and the wimpy so called frame rails it already is a flat plate trough the center of the car. To make it stiffer it needs added section modulus in the vertical direction to reduce vertical bending and torsion. A flat plate structure on the bottom of the car dosnt give section modulus in the correct direction. Spindly tubes replicating the flat plate is harldy better. Door bars do ad a huge amount of vertical section modulus though not as much as a real roof or a cage would. Also Converting the Trans tunnel from an open hat section to a tubular section also would significantly change the section modulus in both vertical bending and especially torsion.
Mazda I think was pretty on track on the Mazdaspeed as seen in the pic but I still think it can be done better.
Bob
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12-10-2009, 03:13 PM
#68
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I think there both too heavy. Door bars are lighter and they do more.
Bob
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12-10-2009, 04:14 PM
#74
Go Park your car on some flat ground takes the roof off or put the top down open both doors and look at it from the side.
Other than the sills, the trans tunnel, and the wimpy so called frame rails it already is a flat plate trough the center of the car. To make it stiffer it needs added section modulus in the vertical direction to reduce vertical bending and torsion. A flat plate structure on the bottom of the car dosnt give section modulus in the correct direction. Spindly tubes replicating the flat plate is harldy better. Door bars do ad a huge amount of vertical section modulus though not as much as a real roof or a cage would. Also Converting the Trans tunnel from an open hat section to a tubular section also would significantly change the section modulus in both vertical bending and especially torsion.
Mazda I think was pretty on track on the Mazdaspeed as seen in the pic but I still think it can be done better.
Bob
Other than the sills, the trans tunnel, and the wimpy so called frame rails it already is a flat plate trough the center of the car. To make it stiffer it needs added section modulus in the vertical direction to reduce vertical bending and torsion. A flat plate structure on the bottom of the car dosnt give section modulus in the correct direction. Spindly tubes replicating the flat plate is harldy better. Door bars do ad a huge amount of vertical section modulus though not as much as a real roof or a cage would. Also Converting the Trans tunnel from an open hat section to a tubular section also would significantly change the section modulus in both vertical bending and especially torsion.
Mazda I think was pretty on track on the Mazdaspeed as seen in the pic but I still think it can be done better.
Bob
That underbody has more skid plates then a Jeep Rubicon!
I'm not sure how tight Miata's normally are, but I get about an inch of flex front to rear when I jack the car up just infront of the back tire or just behind the front tire as a "door bar car". Doing the same on an Fbody produces around 10 inches of flex.... No joke
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12-10-2009, 04:44 PM
#77
By the time I transferred over what I hadn't finished and added new stuff it was just as long...a full page.
My wife was laughing her *** off at me...but said she'd help me out by getting me a Hello Kitty steering wheel as a stocking stuffer.
Car sure will be nice when I'm done though.
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12-10-2009, 04:55 PM
#78
I know this is probably short-sight, since one's interests and tastes change over time, but does anyone else feel like they will keep their Miata forever? Mine is rust free with a straight body. Motors and parts are plentiful. I don't see any automakers building a similar car that would be within my budget. Within a few years it'll be virtually worthless, so there wouldn't be much point in selling it.
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12-10-2009, 05:08 PM
#79
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I might sell the one I have but I will always have a miata. Well let me put it this way, I'll always have a cheap sports car...the S2000 is getting cheap. Lets move this discussion to m.net 
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12-10-2009, 05:11 PM
#80
I'm actually planning on it. I stock backup parts second only to rharris. I've got 3 workable turbos for my oddball setup, and can almost switch straight back to stock.
$150 local jy 1.6Ls keep me happy.
But we know how these plans tend to change. If it's based on a Subaru boxster motor I can only imagine the aftermarket support would be great.
Tokyo 2009: Toyota FT-86 Concept Unveiled - egmCarTech
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