Any function/ quality/ weight difference in the commonly available frame rail braces? - Miata Turbo Forum - Boost cars, acquire cats.

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Old 10-31-2011, 09:00 PM   #1
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Default Any function/ quality/ weight difference in the commonly available frame rail braces?

Is there any significant difference between the Boss Frog, FM, Boundary or V8R rails? Did I miss any? It looks to me like the Boundary rails are the cheapest, plus they're stainless, and they come with stainless hardware, where the others are painted mild steel with zinc hardware. Material isn't THAT big of a deal living in the South, but is appealing, because I'm a nerd like that.

Do the Boss Frog rails extend further up the frame rail than the others, or do they just have an offset to the sides of the rails? Any other significant differences?
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Old 10-31-2011, 09:44 PM   #2
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The FM rails are SS as well. I went with them for that fact, plus the fact that if I ever wanted to upgrade to the full butterfly brace it was possible.
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Old 10-31-2011, 10:43 PM   #3
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^ same here. The FM rails were the original ones as well, and I supported their R&D efforts by buying their product rather than a ripoff (same as I bought 6UL's rather than the ripoff Tire Rack wheels).

I'll eventually get the butterfly portion, but the rails do a great job by themselves.
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Old 10-31-2011, 11:18 PM   #4
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In reality the frame rails will help some with the intended purpose of chassis stiffening. But it won't in the area we needed it most, Torsional rigidity. There have been quite a few heated discussions about this recently on another forum, but what you really need is something like door bars that connect to the rocker panel or the firewall. You need to connect the front and rear chassis/subframes so as to increase torsional stiffness (which is the miata's big weak area). Anything other than closing off the U shape that our tube makes, is really just a band-aid and its usefulness is...not justified in most cases.

With all that said, I would go with butterfly bracing included. It will provide the most benefit really.
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Old 11-01-2011, 12:12 AM   #5
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if you do indeed go with frame rails, get the FM with butterfly portion. However, I am a firm believer that seam/stitch welding the tub would do much better than the frame rails
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Old 11-01-2011, 12:45 PM   #6
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1.) The 'frame rails' such as they are are the main source of chassis stiffness in the Miata tub. Supporting them will absolutely help. Especially mine, which have been a little mangled over the years.

2.) If I was going to pull the whole car apart to safely and correctly stitch weld it, I'd slice the frame rails open and sink rectangular tube into them to reinforce them, and build a real cage for it. I'm not a half-do-things kind of person.

3.) But, right now, I'm still regularly street driving this car, and I'm not willing to go without it for months to do the job right when I'm not convinced that I can even be competitive in my class. This may end up being a street toy again when everything is said and done, and it would be a waste (and a huge pain in the ***) to cage it just for it to end up back on the street.
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Old 11-01-2011, 02:45 PM   #7
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I have used both and door bars work much better than frame rail braces for the reasons 'Track' stated above. The only reason to use frame rails over door bars is if you cannot live with the door bar location for whatever reason. I have used them in a DD and in a track car. It is a minor PIA for a DD but not that bad and worth it IMO.

Also, if you already have a roll bar (which you should) like a Hard Dog, the door bars are easier to install than frame rails.
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Old 11-01-2011, 02:52 PM   #8
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*shrug* I used door bars for quite some time. they are great, the BF aren't perfect, but you could get a local shop to modify them and get em to the rocker panel.

The only disadvantage to door bars is they make race seat fitment difficult...Unless you are happy smashing your trans tunnel or better yet, cutting it to center the seat.

Anyways, focusing on the topic, the butterfly brace itself will provide the greater benefit of the two. while the Frame rails will be noticeable, since people tend to measure chassis rigidity based on cowl shake and vibrations over bumps. But those are not necessarily measuring to good ol' torsional rigidity. So try to stray away from the NVH way of measuring stiffness.

The FM butterfly can be installed without a frame rail mod. I couldn't tell ya, but perhaps someone has installed it on the Boundry engineering one and posted about it? perhaps you could be the first one to do! Trail blaze my friend
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Old 11-01-2011, 03:46 PM   #9
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I'm not arguing that door bars would be beneficial, but if I'm going to do them/ put up with them, I'm going to just cage the car, which isn't in the cards right now.

Interestingly, my current Miata has been owned by enthusiasts and well maintained for effectively its entire life and only has 96k on it now, but it cowl shakes/ 65mph shimmies 200% worse than my beat down 200k mile 1.6L junker trackday **** did. But that has little to no bearing on whether or not I put chassis bracing in it, lol.
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Old 11-01-2011, 04:13 PM   #10
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yea, I have it pretty bad too (the cowl shake + 65 shimmy). Not sure why really, my car has 63k miles on it and is a 1999. I think its because I got an out of round tire.

My door bars helped a lot with the cowl shake.
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Old 11-01-2011, 05:11 PM   #11
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Buy roll bar before frame rail braces. Roll bar does much more
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Old 11-01-2011, 05:23 PM   #12
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I have a Hard Dog Ace double diagonal in it now.
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Old 11-01-2011, 07:43 PM   #13
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Quote:
Originally Posted by vehicular View Post
I'm not arguing that door bars would be beneficial, but if I'm going to do them/ put up with them, I'm going to just cage the car, which isn't in the cards right now.
Silly logic afoot here.

A street door bar is designed the way it is to not be a huge pain the the butt the way a cage is, nor is it unsafe for the street the way a cage is. There is no comparison between the two, and adding a door bar to your roll bar is not simply a hop-skip-and a jump away from having a cage. "If I did a door bar I'd just cage it" is ridiculous.

"street" door bars are simply the correct solution to stiffening the center section of a miata which will be used on the street. In other words, it is the proper tool for the job, whereas frame rails are not. You don't have door bars, so you don't know any better, but that's why people who do know better are trying to tell you this.

Add to the already good enough fact that door bars do the job wayyy better than frame rails the weight and cost factor - that you are basically throwing away the same amount of money and additional weight by adding a butterfly brace as you do for a door bar, and the choice becomes even clearer.

So, Mr. "I'm not a half-do-things kind of person", you sure are looking at a half-do-things kind of band-aid part, for the same costs as the do-it-the-right-way part.

-Ryan
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Old 11-01-2011, 08:00 PM   #14
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One other benefit of the stainless steel frame rails I didn't mention before was that after I lowered my car and started occasionally scraping on things, they did a nice job of taking the scrapes so the body's frame rails didn't have to. Painted aftermarket rails wouldn't hold up as well over time.
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Old 11-01-2011, 08:23 PM   #15
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+1 on there being a big difference between door bars and a full cage. Not even close.

Anyway, if you want frame rail braces for whatever reason that is cool. It is just the door bars will be a lot more effective for about the same cost and are less effort to install.
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Old 11-01-2011, 10:09 PM   #16
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Quote:
Originally Posted by ThePass View Post
Silly logic afoot here.

A street door bar is designed the way it is to not be a huge pain the the butt the way a cage is, nor is it unsafe for the street the way a cage is. There is no comparison between the two, and adding a door bar to your roll bar is not simply a hop-skip-and a jump away from having a cage. "If I did a door bar I'd just cage it" is ridiculous.

"street" door bars are simply the correct solution to stiffening the center section of a miata which will be used on the street. In other words, it is the proper tool for the job, whereas frame rails are not. You don't have door bars, so you don't know any better, but that's why people who do know better are trying to tell you this.

Add to the already good enough fact that door bars do the job wayyy better than frame rails the weight and cost factor - that you are basically throwing away the same amount of money and additional weight by adding a butterfly brace as you do for a door bar, and the choice becomes even clearer.

So, Mr. "I'm not a half-do-things kind of person", you sure are looking at a half-do-things kind of band-aid part, for the same costs as the do-it-the-right-way part.

-Ryan
Now, first of all, scooter, I haven't at any point said that I'm willing to spend money on a butterfly brace. I said I'm not willing to stitch weld the car, and I find bolt in door bars to be a pain to live with. Which I'm not, and I do.

Second, stop and take an analytical look at what we're tying to do here. The objective is to stiffen the torsional connection between the subframes, accepting that everything else is secondary in chassis work. Now, if we're talking about the Hard Dog door bars, we're adding a link, connected on one end with bolts loaded in direct shear (in a chassis torsion case), bent most of 90 degrees, then connected on the other end to the floppy floor pan. Being that tubes are strong in tension and compression, and very not strong in bending, asking them to stop movement between two planes that are almost perpendicular to each other, without a direct strain path is foolish in and of itself. Asking them to do so through a bolted joint loaded in shear is especially foolish.

Now, the Boss Frog door bars might have some promise, by virtue of bolting to 4 separate points on the chassis, and having fairly nice shear plane supports. However, they're hardly a chassis designers wet dream, being as they still don't have a direct load path between any of the mounting points, and all of the tubes are loaded in bending, and are all dead headed into straight, unsupported sections of tube.

Frame rail braces, on the other hand are loaded compound torsion/ y-axis bending when the chassis is loaded in torsion (which is good, but not great, as thin C-sections are strong in direct bending, if not in torsion). They're supported by continuous contact to a directly loaded chassis member, along with multiple bolt points loaded in direct tension (which is great, since bolts loaded in anything but tension are a waste of time, along with bad engineering practice). I could stand for the FM rails to have a few more bolt holes, but that's easy enough to fix.

Having not seen the butterfly brace in person, I'm not 100% confident saying exactly how it's designed, but it looks like it's loaded in shear though the inboard rail brace mounting bolts (bad), and that it has only incidental direct load paths(not good either). I'm not thrilled with FM's design there, BUT I think the concept has merit. Closing the box on the trans tunnel can't hurt, but more importantly, the overall effect of chassis torsion is to try to drive the bottom corners (front right/ left rear, front left/ right rear) of the frame rails diagonally closer together. Connecting those points with a structure strong in tension/ compression would be a fantastic way to improve torsional rigidity.

Frame rails and the FM butterfly brace are hardly an end-all-be-all of chassis stiffness, but they're a hell of a lot better than silly bolt in door bars.
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Old 11-01-2011, 10:44 PM   #17
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You're thinking too hard and ignoring the most important piece of data: empirical evidence. Like I said, you don't have door bars, so you naively poo-poo them.

Trust me, I'm aware of every single compromise in the design of off-the-shelf street door bars. This is why I have a welded in 12-point... and why the door section alone ties into the car in 4 seperate reinforced points. But, I didn't recommend that to you because you want to DD the car.

I never said the off the shelf door bars were "a chassis designers wet dream". Indeed, they are full of compromise. However, they compromise so that you can use them daily without a hassle - which is a big deal to you. Furthermore, they do twice as much for the car than any frame rail reinforcement can. Simple as that.

-Ryan

Last edited by ThePass; 11-01-2011 at 11:20 PM.
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Old 11-02-2011, 12:32 PM   #18
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Uhhhh, tubes are monocoques. They're pretty darn strong in bending unless you dent them. Go ahead, try it with a beer can.

Also, the Boss Frog's have welded webbing in them, essentially creating big I-beams.

Think of it this way, our problem is that we don't have a roof. A cage is an excellent substitute for a roof. Serious door bars are pretty good too. Street door bars are a step in the right direction. Reinforcing the existing frame rails helps, but are just plain in a bad place as a roof replacement. The butterfly brace's main contribution is tying the corners of the car together in tension (with bolts in shear -- which, BTW, is well within the bolt's design strength).

Rule of thumb, the higher you can put your roof substitute, the more effective it will be.
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Old 11-02-2011, 03:32 PM   #19
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Quote:
Originally Posted by hornetball View Post
the higher you can put your roof substitute, the more effective it will be.
Bingo. Clearest/simplest layman's way I've heard it stated.

-Ryan
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Old 11-02-2011, 07:23 PM   #20
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Don't make me draw a free body diagram...
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