Roll bar vs. Roll bar

[dd74]

There are two roll bars I know of: one that mounts to the floor, and the second, which mounts to the sills beneath the doors.

My question is does the floor-mounted roll bar add the same amount, if any, rigidity to the chassis?

Thanks.

[Dantilla]

I've never seen any purpose to mounting a roll bar on the sheet metal floor. Less protection, and takes up more interior room. I don't see any positives.

If you're going to do it, may as well do it right.

I would make sure that it could be used later to complete a full cage without modification.

[jester911]

Here is a question that might answer your question.
Would you jack up your car by the floor pans?
Hopefully you said no, so why would you want to risk your life with them?

[petevb]

I'd say neither adds a lot of stiffness, but the floor mounted bar certainly adds less.

If you imagine the chassis floor by itself isn't stiff- you can imagine it as a piece of paper. Twist the ends and it flexes easily.
Now if you add the roll bar structure to the middle of that paper it will stop that one little area from twisting as much, but since you attach it to the paper with four little points, the paper will mostly just flex around those points, and it certainly doesn't help stop the rest of the sheet twisting.

Next version- bend the edges of your paper up at 90 degree angles, forming a U channel. This is more like the chassis floor and frame rails. Now twist it lengthwise- it's stiffer, but the tops of the paper "frame rails" are free to move in opposite directions to one-another, so it's not exactly stiff. Now if you add your roll bar attached to those vertical U walls, however, they mounting points don't flex the paper, and you're adding a little bit of real stiffness.

Next step- attach the roll bar not just at the bottom, but also at the top corners to the roof. Now you've closed the top of your U channel, and it begins to act more like a tube. Ever try and twist a tube? So attaching the roll bar at the top corners helps a lot, not just in the area where the bar itself is attached, but also the rest of the chassis- that's where you start to see real stiffness gains.

A floor mounted bar also attached at the top is probably stiffer than a sill mounted bar not attached at the top, but I'd go for sill mount every time.

[Racerbvd]

Well, Jester911 says it best with this;

Would you jack up your car by the floor pans?

And as one who has had both types of cages, crewed for pro teams have friends who build race cars for a living, DON'T GET A FLOOR MOUNT!!!!
As to whether or not it adds a lot of stiffness, well, I have found that it does, the car, with nothing else change was much stiffer.

[dd74]

Originally Posted by petevb

Next step- attach the roll bar not just at the bottom, but also at the top corners to the roof. Now you've closed the top of your U channel, and it begins to act more like a tube. Ever try and twist a tube? So attaching the roll bar at the top corners helps a lot, not just in the area where the bar itself is attached, but also the rest of the chassis- that's where you start to see real stiffness gains.

At this point, it seems as if you're talking about a cage, correct?

[dd74]

I do indeed have a floor-mounted bar - have for years - but have lately been considering its viability. With that, here's another question:

Should I just ditch it and buy a harness bar? I mean, if it's adding no rigidity to the car, it does add 50 lbs to the overall weight, which I'm certain I can do without.

Thanks.

[petevb]

Originally Posted by dd74

At this point, it seems as if you're talking about a cage, correct?

No, still just the bar. You can add tabs to connect from the upper corners of the bar to the roof or upper B pillars. If you find an early car with a bar but without this attachment you can easily see the flex two ways: Either grab the B pillar and bar with your hand and pull them together, or stick your hand between the bar and the B pillar while driving (don't really do this- it will hurt!).

-Pete

[jester911]

Are you using the roll bar to attach your harnesses? If so the roll bar is probably the lesser of the two evils. Unless you have the type of harness bar that the harnesses actually attach to instead of riding over it is not better. That type of bar means you end up with harnesses that are much too long to really be safe.

The bar you have in probably does add some stiffness. It just doesn't add a whole lot of safety because of deflection of the floor pan.

Besides that many organizations are tightening down on harness bar use including the PCA.
IMHO most harness bars will not be allowed at most track events at some point and rightfully so.

[dd74]

Jerry - no, I don't have a harness bar. Just the roll bar. But your point regarding the harness bar in conjunction with PCA rules is well taken.

My belts wrap over where they attach to the roll bar.

[jester911]

I use a Das bar that attaches to the seat belt points. It isn't the most ideal (welded in would be)
but it is PCA DE legal, it stiffens the car and the harnesses attach very nicely to it making it very good on the track.

You may want to consider doing something similar if you do enough track days to warrant it.
Another possibility is to have a harness bar welded into the bar you have. Just a thought.

[Cory M]

Originally Posted by dd74

I do indeed have a floor-mounted bar - have for years - but have lately been considering its viability. With that, here's another question:

Should I just ditch it and buy a harness bar? I mean, if it's adding no rigidity to the car, it does add 50 lbs to the overall weight, which I'm certain I can do without.

Thanks.

Don't ditch it. The rollbar gives a lot more addtional safety than a simple harness bar.

In gerneral I agree that mounting the bar to the sill is superior to the floor. That said, I have seen a few rolled cars with the standard floor mount Autopower bars/cages and in the wrecks the bars/cages did their job and protected the occupants.

[3.2 CAB]

What about one of those PVC pipe, truck headache rack/roll bar? I think J.C Whitney use to sell them. They looked like a real one, even came with the little round pad for it.

[dd74]

Originally Posted by 3.2 CAB

What about one of those PVC pipe, truck headache rack/roll bar? I think J.C Whitney use to sell them. They looked like a real one, even came with the little round pad for it.

Let's first see how that works in your car.

[TRE Cup]

David
yes you have a floor mounted bar, but you forget that we added (by welding) vertical supports from the bar to the adjacent inner part of the rocker panel. So any flexing from the floor mount is negated by what we had welded to the stronger part of the chassis.
You can add some more rigidity by having us add a B pillar connection between the shoulder harness mount and the bar. Then it would essentially have the same strength as a roll cage from the hoop side.
The rear extensions go to the wheel wells on your car (Autopower) which is actually acceptable to SCCA as part of their cage requirements. You do have a little bit of flex from these wheel well attachments, but not as much as you would think
As your car is getting more modifications, you may want to consider adding bars from the main hoop to the rear shock bridge; although this is somewhat radical with cutting thru the bulkhead to do so. Another mod would be to install a X brace on the rear section to the hoop. This is not going to be fun as access with carpeting and a welded in main hoop is difficult

[dd74]

Originally Posted by TRE Cup

David
yes you have a floor mounted bar, but you forget that we added (by welding) vertical supports from the bar to the adjacent inner part of the rocker panel. So any flexing from the floor mount is negated by what we had welded to the stronger part of the chassis.
You can add some more rigidity by having us add a B pillar connection between the shoulder harness mount and the bar. Then it would essentially have the same strength as a roll cage from the hoop side.
The rear extensions go to the wheel wells on your car (Autopower) which is actually acceptable to SCCA as part of their cage requirements. You do have a little bit of flex from these wheel well attachments, but not as much as you would think
As your car is getting more modifications, you may want to consider adding bars from the main hoop to the rear shock bridge; although this is somewhat radical with cutting thru the bulkhead to do so. Another mod would be to install a X brace on the rear section to the hoop. This is not going to be fun as access with carpeting and a welded in main hoop is difficult

So what I have is still better than not having anything? OK. I wasn't certain, and honestly, had forgotten the vertical supports.

I'd like to see the other welding you mentioned if you have photos, or when I stop by for the suspension work.

Thanks, Dave.

[TRE Cup]

When you come in , we can show you several cars here with varying degrees of bar and cage work
thanks