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Roll bar design

Slimy38 - 18/4/23 at 07:23 AM

I'm looking at getting my roll bar sorted as a task, and I wanted to ask about the design. I have the two locations above the rear suspension for the main hoop, but the back end of my Healey bodywork doesn't give me an option for rear braces. Having said that, looking at the design of the Haynes roadster roll bar I'm still baffled how the rear braces actually provide much support, but I'm not an engineer!

If I just go for the single hoop, is there anything I can do to strengthen it in line with the car? Assuming that I can't brace it rearwards, and I want to keep the seating area as clean as possible.

It'll have the normal horizontal bar for seatbelt anchors, so hopefully that would help with any sideways movement should the worst happen.

nick205 - 18/4/23 at 09:45 AM

I'm not an engineer in that regard either, but it appears to me a roll hoop across the vehicle needs bracing to the rear to prevent it simply folding if the vehicle flips and drags forwards or backwards.

On 7 style cars:

I've seen some hoops braced to the top of the rear tub (above the fuel tank area) this looks feeble and pointless (IMHO).

I've also seen some hoops braced to the lower part of the chassis (either side of the fuel tanke area) on to plates. This looks more effective (again (IMHO)

If your car bodywork doesn't have space for this can you moify the rear bodywork to let roll bar rear stays through?

Can you modify the chassis (beneath the bodywork) to strengthen it and raise the mounting points higher?

nick205 - 18/4/23 at 12:51 PM

Some Cobra type cars have individual roll hoops for driver and passenger with rear braces - maybe a possible alternative for your bodfywork/chassis?

Sam_68 - 18/4/23 at 01:10 PM

If you have the ability to fix it both at the top of the rear bulkhead and somewhere lower down on the rear bulkhead (ideally at floorpan level), you can 'cantilever' the roll bar (and potentially triangulate it back on itself).

ETA:

Like this (the yellow points being where it is mounted to the chassis):

Description .

[Edited on 18/4/23 by Sam_68]

Slimy38 - 18/4/23 at 01:56 PM

quote:
Originally posted by Sam_68
If you have the ability to fix it both at the top of the rear bulkhead and somewhere lower down on the rear bulkhead (ideally at floorpan level), you can 'cantilever' the roll bar (and potentially triangulate it back on itself).

ETA:

Like this (the yellow points being where it is mounted to the chassis):

Description .

[Edited on 18/4/23 by Sam_68]

I like that idea (and I love the picture by the way, if I could get my car looking like that I'd be very happy!). I might be able to get it all the way down to the floorpan from the side, and would actually give a little bit of side protection. I don't know if the full cantilever would work, mainly because the yellow point in your example is directly above the rear wheel. But if I can mount the side bar high enough on the rear hoop, the angle down to the floorpan should provide triangulation without losing entry space. I don't have doors to worry about so I can make the bottom mounting a bit further forward.

[Edited on 18/4/23 by Slimy38]

Slimy38 - 18/4/23 at 02:01 PM

quote:
Originally posted by nick205
I'm not an engineer in that regard either, but it appears to me a roll hoop across the vehicle needs bracing to the rear to prevent it simply folding if the vehicle flips and drags forwards or backwards.

On 7 style cars:

I've seen some hoops braced to the top of the rear tub (above the fuel tank area) this looks feeble and pointless (IMHO).

I've also seen some hoops braced to the lower part of the chassis (either side of the fuel tanke area) on to plates. This looks more effective (again (IMHO)

If your car bodywork doesn't have space for this can you moify the rear bodywork to let roll bar rear stays through?

Can you modify the chassis (beneath the bodywork) to strengthen it and raise the mounting points higher?

The problem with modifying the rear bodywork is that for the width, I'd be modifying the boot lid rather than the body itself. They've given me quite a wide boot lid which is great for making it a reasonably usable car, but it limits options.

It's good you have the same opinion of bracing to the top of the rear tub, it's how it is in the book but there can't be a great deal of strength in those braces. It would be the same outcome if I modified the chassis to provide extra points, apart from the differential mounting cage there is very little structure at the back.

Sam_68 - 18/4/23 at 02:04 PM

quote:
Originally posted by Slimy38
... I don't know if the full cantilever would work, mainly because the yellow point in your example is directly above the rear wheel. But if I can mount the side bar high enough on the rear hoop, the angle down to the floorpan should provide triangulation without losing entry space.

You're working in three dimensions in reality, of course, so you have scope to move the yellow points in or out toward the centreline of the car (it depends on your precise chassis arrangement, but you might end up with transverse 'beams' where the yellow points are, bolted to the chassis, with the roll bar elements then welded to those beams.

Usual principles of spaceframe design apply: try to keep everything as well-triangulated as possible, and minimise bending in any given element.

Slimy38 - 18/4/23 at 02:40 PM

quote:
Originally posted by Sam_68

quote:
Originally posted by Slimy38
... I don't know if the full cantilever would work, mainly because the yellow point in your example is directly above the rear wheel. But if I can mount the side bar high enough on the rear hoop, the angle down to the floorpan should provide triangulation without losing entry space.

You're working in three dimensions in reality, of course, so you have scope to move the yellow points in or out toward the centreline of the car (it depends on your precise chassis arrangement, but you might end up with transverse 'beams' where the yellow points are, bolted to the chassis, with the roll bar elements then welded to those beams.

Usual principles of spaceframe design apply: try to keep everything as well-triangulated as possible, and minimise bending in any given element.

Oh of course, I still need the seat belt mounts on a transverse tube. That does make sense, thanks.

I think I'm going to get myself some waste pipe and gaffer tape and mock something up. I can see roughly where to go but I need the 3D view.

nick205 - 20/4/23 at 07:17 AM

An overnight thought...

Not familiar with your build, is it a kit or a self-design/scratch build?

If it's a kit, does the kit maker not offer a roll cage solution you could use?

When building my MK Indy (a kit) I asked them to weld a roll hoop to the chassis. Being a bit green at the time it didn't occur to me to get so rear stays welded in between the roll hoop and the lower part of the chassis rear (either side of the fuel tank location). If building again I'd be having that done as a minimum.

Another consideration is making sure the roll bar is high enough. Draw a line between the top of the roll bar and the top front chassis member and make sure driver/passenger heads are well below the line.

Slimy38 - 20/4/23 at 08:41 AM

quote:
Originally posted by nick205
An overnight thought...

Not familiar with your build, is it a kit or a self-design/scratch build?

If it's a kit, does the kit maker not offer a roll cage solution you could use?

When building my MK Indy (a kit) I asked them to weld a roll hoop to the chassis. Being a bit green at the time it didn't occur to me to get so rear stays welded in between the roll hoop and the lower part of the chassis rear (either side of the fuel tank location). If building again I'd be having that done as a minimum.

Another consideration is making sure the roll bar is high enough. Draw a line between the top of the roll bar and the top front chassis member and make sure driver/passenger heads are well below the line.

The chassis is standard Haynes roadster, adapted for an MX5 donor with the Saturn updates. The bodywork is an MK HSR Healey body, as the name suggests it's intended to fit directly on to an MK which means that it's needing some updates to fit on the Haynes.

With it being a Haynes chassis, any off the shelf roll cages are designed to fit the seven shape. The most common is the one from the book with the rear braces, but my car doesn't have the traditional seven rear.

I appreciate the reminder about height, I'll be starting with the height recommendation from the Haynes roadster book but I will double check it against the seat height.

Slimy38 - 2/5/23 at 07:32 AM

Just as a small update, what do you reckon to my 'prototype'...

The OD for appropriate CDS was 44m, so I thought this waste pipe at 40mm would be close enough to give me an idea of where I'm going. The blue marks show the minimum centre line radius that is allowed.

Now I can continue the process to see how to fit side bars. I can also measure up a cross brace, and thankfully with the seats in place and my noggin tested for size I reckon I will be just about ok if the worst does happen. I think I'd need a helmet but given that I've abandoned the windscreen I suspect a helmet will be needed anyway.