Roll cage design thread, apply within.

[Punter]

Righto,

Car is a ae101 20v.
Plan is to build a simple cheap (relatively) club car.

Haven't done allot so far,
strip the interior,
got the roll cage kit,
set drivers set position,
set main hoop position.

What I'm wanting is some input into the design of the rest of the cage.
The kit consists of the main hoop, both front bars, and the roof bar. All the straight bars are yet to be made, so the design is free to change.

Here's the car with the cage just resting in place (read lock-wired in place)

Final seat position is about 15cm forward of this, we had to pull out the center cross member and unfortunately I forgot to take more photos.
The seat is very low in comparison to other race cars, but there just isn't that much room in there for my head + helmet. It's certainly different to a mk1 escort.


Here's an MSPaint of what we've come up with. Went with the unusual side intrusion bars as this car has a very large door gap and feel this would be the best side protection.

Anyway, any useful feedback/suggestions?

[Mr Revhead]

a cross in the doors would be stronger
with the join of the cross out a bit from where the legs of the cross meet the hoops.
that way they support each other
plus have to be pushed straight before they can bend back in, thats hard to do.

a straight bar will bend easy when hit side on.

[Mr Revhead]

heres a way of doing what i mean

[Punter]

a cross in the doors would be stronger
with the join of the cross out a bit from where the legs of the cross meet the hoops.
that way they support each other
plus have to be pushed straight before they can bend back in, thats hard to do.

Not keen to have any bends in the intrusion bars, as a big hit from the front will be able to shrink the door gap length ways. (if you get what I mean)

a straight bar will bend easy when hit side on.

The triangle design is stronger than a cross would be at the center of the door, A cross is only one pipe think at the very center where as this is 2+. The foot at the door center will also tie into the bar under the front of the seat.

I had a cross setup up in the door and to be honest this triangle design covers allot more of my body when seated.

The big long doors are a bit scary.

[Punter]

That MSpaint is a little misleading, the mounting point at the door center would be much closer to the front of the car, and the top intrusion bar would be on a bit more of an angle, and so meet the front bar a little lower.

[Leon]

Harness bar in main hoop will give you a nice short belt span.

[Mr Revhead]

from your drawing the bars in the door look to be running dead flat
is that not the case?
its easier to bend a bend in the direction of the bend. but not against the bend.

[Punter]

Harness bar in main hoop will give you a nice short belt span.

Didn't think this would be possible to start with, but with the seat further forward it might just fit.

MSPaint update.

[IH8TEC]

wouldnt' that cross actually be stronger as it's not a "long" peice of tube only welded at the end? and the cross effectively makes a few triangles anyway.

would have thought a long straight one across the door would be a hell of alot easier to bend/break in a big crash than the cross, thats just me though?

[Mr Revhead]

thats what i was thinking
there would be fore-aft protection
but bugger all side impact

[Trls250s]

Cross is better from my teachings at uni.

Straight bar has the potential to flex inwards or outwards in the event of a frontal, and in the case of a side impact would be caparitively weaker compared to a X style member.

BUT the cross would idealy be made from 2 straight pieces with a notch either side then out of 4 independant bars.

Those are my 2 cents, but a professionaly opinion would be better.

[Distrb]

you're not going to put any crosses/diagonals or a harness bar in your main rollhoop?

i agree with revhead about the side intrusions, and did exactly that on my own cage.

[Punter]

thats what i was thinking
there would be fore-aft protection
but bugger all side impact

If you push directly side on, in the middle of the door gap, on the center of the cross, why would it be stronger the the design shown above.
To bend the bar inward you still have to move the main hoop and front bars the same amount, and you have to bend 2 bars (instead of one) plus the sill and floor.

I know crosses are very strong and we built my brothers car with crosses, but from my reckoning this design is stronger for side impacts.
Which, since I am sitting completely inside the door gap, is important.

Just as a reference here is my bothers cage.

you're not going to put any crosses/diagonals or a harness bar in your main rollhoop?

Probably will end up moving the horizontal belt bar into the main hoop, Will have to see after I've had another go at setting the seat position.

[RedMist]

In a side impact you are putting a load into the bar at 90 degrees, its not going to hold in either of those designs. You are better off making the main hoop and the dash portion as strong as possible to prevent side impact damage. Or you could add to the existing brace and run seat rails transverse.
The cross is considerably stronger from a frontal or rear impact. Its going to brace the front and rear hoops from three points rather than the two on your design. Its obviously also adding considerably more weld area.

[matt dunn]

IMO your triangle intrusuion bar is not the best idea.

In a side on hit, the bars are only connected to the front bar and main bar by one welded place on each bar,

so in a hit if either of them are weakend, the whole intrusion bar is absolutly useless.

In a cross there are four points.

Also the way we do them now is two bars in an x, but neither are cut.
They are both bent in like a very wide V.
so one goes from the end up to the center and back down again,
and the other does the opposite.

When they are fitted they are tilted outwards a bit also, so if you are looking from birds eye view, the go outside the box so to speak and then back in.
thsi means that for the bars to come into the car, the front leg must move forward and/or the main hoop must move back.
You then run a bar from where the the intrusion bar meets the main hoop, back to the rear foot on that side, so the hoop cannot move back,
and the same at the front, from where the intrusion bar meets the front bar out to the strut tower.

Now for the intrusion bar to come into the car, you must strech the whole car.

hard to explain, but i cant draw, lol

And that's only a small part of roll cage bits and pieces.
Where the bar in and on what angle is as or more important than what it is made of.

[Mr Revhead]

^ yeah thats what i was getting at
just to lazy to type it all out

[TRD Man]

Punter, what you've proposed is fine. That doesn't mean it can't be better.
I like the design of Matt's intrusion bars but it's not practical to do this in every case.
I also share your view on frontal impact which is far more likely than a perpendicular side impact.

Rollcage design does incur an element of compromise as one considers strength & rigidity against requirements of homologation, weight targets and access/egress of occupants.

My last cage had a triangular intrusion bar much as you've drawn. We did this mostly because, as an aging competitor with a gammy leg, I would have had difficulty getting past a cross bar in a timely fashion. We also did this because we know that the sill is a very weak part of the car and can concertina by some length in even a light impact. And because, as a national championship class car, it needed to retain the door liner and this bar fitted neatly under the grab handle.

Far from being useless, it added good rigidity and in conjunction with the other bars served well in various impacts, of which we had a few.

I am a fan of triangulating wherever possible. With a big wide door opening like the Levin has I would put a brace from the bend at the A pillar down to the mounting by your feet and I would triangulate the joint between main hoop and foward bars.
Also, I do tend to have an opposing diagonal brace in both the main hoop and the rear stays rather than the cross brace your have only in the rear.

I also like a brace between the lower part of the main hoop. Normally this would also mount to the tunnel or floor. This aids the side strength others are concerned about.

Looking at your picture I wondered if you conldn't move the main hoop back a couple of inches which may allow the crossbar for a close mounted seatbelt as Leon suggested. This is the recommended practice for seatbelt mounting today.

[pc]

Also the way we do them now is two bars in an x, but neither are cut.
They are both bent in like a very wide V.
so one goes from the end up to the center and back down again,
and the other does the opposite.

This is my prefered side intrusion and If I was to get another cage would have this.
If racing amongst trees or power poles, side intrusion should be a concern. Likewise if racing on track with other cars t-boning happens too.

[Punter]

Well I think you guys have convinced me. Will go for a cross now, maybe with a foot in the center to tie in the sill/seat mount?
Is this legal? I'll check the book tomorrow.

Will have another go at the seat position tomorrow night to try get something in the main hoop. Can't really move the hoop any further back, The back of these cars are very short, and the low seating position limits my options as far as head/leg space.

[Punter]

Also the way we do them now is two bars in an x, but neither are cut. They are both bent in like a very wide V. so one goes from the end up to the center and back down again, and the other does the opposite.

I've seen that setup in a few of the more spendy cars, it's an excellent idea and allows for big pretty looking gussets.

Unfortunately I don't really have the motivation/patience to bend bars, and can't afford to pay someone to build it for me. hence buying the pre-bent bars.
(We did my brothers cage from scratch with a jack type bender, it was allot of effort getting the bends right.)