How much do diagonals benefit a roll bar?

In May, I bought a 2003 Miata. Just driving a convertible is thrilling enough for this gal. I will not be doing any racing, etc. I have been researching roll bars, including reading as much as I could find online, exchanging emails with Boss Frog and Hard Dog, and a lengthy conversation with the folks at Track Dog. I have a question and have not been able to get someone to commit. I figured a forum of experts would be my last stop before my decision.

Is there much value in the diagonals on a roll bar for someone who is driving only on the street, conservatively?

I am planning to buy a Hard Dog M2 Sport. It seems like it is the best combo of height and rear placement. I really like the look of the one without diagonals. Tom at HD says "We can't guarantee safety with anything we might build but my youngest son has an M2 Sport without diagonals in his Miata. I like that bar." How much support and function do the diagonals add? The bar is attached to the car the same way. The rollover crash photos of cars with that bar look like it holds up well. Do the diagonals aid in non-rollover crashes or in other ways?

Thanks for your advice. Go easy on me, I am new at this!

K

The diagonals add a little extra ridgity to the roll bar from what I've read, plus they look really cool. For a street car non diagonals are perfectly fine. For a track car the diagonals might give you a slight increase in chassis stiffness.

When I bought a roll bar I went with no diags. Can't see it with the top up and it doesn't block your rear window vision.

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Originally Posted by sammm

When I bought a roll bar I went with no diags. Can't see it with the top up and it doesn't block your rear window vision.

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That is the reason I think I will get the one without diagonals. I just want to be sure that I am not sacrificing safety.

What about the Hard Dog Ace?

It puts the bar farther back which could be a real benefit in a rear end accident which In my experience happens more often than rollovers.

http://www.bethania-garage.com/images/schematic_ace.jpg

http://www.bethania-garage.com/image...m1m2hcht_x.jpg

I have the Ace and like it but it is not as wide as the others so if you happened to get T-boned you would give up some benefit.........

No bar is the perfect combo of rearward placement, height, style, minimal interference with seat belts, good rear visibility, and minimal damage to preexisting car features. It is not an easy choice. I have been told that the Ace also interferes with that partition (what is it called?) that has the windscreen and the mesh pocket, and the M2 Sport does not. The Ace is further back, but it is and inch lower. William at Track Dog Racing and I talked a long time. I have decided that the M2 Sport is the best for me. Now I am just trying to decide on diagonals. I do not like the look of them but if they are substantially safer, then I chose get them.

The diagonals are there to reduce the risk of collapse in a roll-over. Its called triangulation. A triangle is inherently rigid where a square quickly becomes a trapazoid. For the same strength material, a rollbar that has triangulation will be much stronger than one without.

But for a DD, It probably does not matter unless you are street racing.

I have the ace with no diagonals and it works perfectly fine. As far as the height of the bar unless you are taller than me at 5'9" ish you are not going to have an issue with the height of the bar. So that is not a worry and also the ace is a bit cheaper.

Here is my ace bar
Attachment 6230

I'd say you are completely fine without diagonals on the street. Like previously stated its for rollover collapse.
I say save the extra money on the diagonals, get the one that fits your needs best. Any roll bar will be better than nothing! Make sure you get padding for it as well.

Diagonals add:

1. 5 HP per bar;
2. More real estate for go-fast stickers, which in turn add one HP per sticker.

So if you wanna be slow...;)

Just make sure your rollbar has more to push against than this guy's did...

This is William. We talked over the phone. I agree with the general consensus. If you prefer the no diagonals, get that. Diagonals make it stiffer but like I said before, I think all of the Hard Dog roll bars are over engineered and will provide adequate protection. While I think the diagonals make it stiffer, I wouldn't say its substantially better. I would be more inclined to say it is marginally better.

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Originally Posted by tailchaser

Just make sure your rollbar has more to push against than this guy's did...

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Fortunately all the Hard Dog roll bars mount to solid parts of the uni-body. Scary how that guys roll bar punched through the floor. I don't see any way for that to happen with a Hard Dog roll bar.

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Originally Posted by tailchaser

Just make sure your rollbar has more to push against than this guy's did...

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RIP mustang.

In a street Miata I want a "rollbar" with a cross brace between the seatbelt towers.
Seeing a NA with the door against the console is quite an eye opener. (no bar)
IMHO, Diagonals on a M2 Sport are a joke.
It just adds a single kink point for a substantial side impact.

I have been working on Miata's for many years and have seen plenty punched in the door.
Hell, William has totaled at least 2 Miata's like that himself alone.
I've seen very few rolled over.

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Originally Posted by KJTX

No bar is the perfect combo of rearward placement, height, style, minimal interference with seat belts, good rear visibility, and minimal damage to pre-existing car features.

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My $.02
I like the HD Deuce for a street car.
Its more attractive and less "Boy Racer".
It has the cross bracing and is the least intrusive into the cockpit.
I never know its there and never hit my head.
All the cross structure is hidden underneath the carpet with only the 2 hoops visible.
I've installed a bunch of them and even painted quite a few body color.

Track use is a whole different conversation.

Compare:
http://farm2.static.flickr.com/1351/...4d8dc4e2b9.jpg http://guillaume.lauraire.free.fr/MX...743-border.jpg
Ace has a crossbar but I would prefer it on the main hoop where its strongest.

But as stated there is no perfect bar.

I'm installing a hard core single diagonal right now for track use.

Pretty sure there is a deuce listed on CL right now. Save some bucks and shipping!