Board logo

Roll bar triangulation opinions
Slimy38 - 8/5/23 at 08:42 PM

following on from my previous post, I've mocked up a roll bar with bracing.








I've come from as high up on the hoop as I can get, then out to the side of the car before coming down the side in line with the steel. It seems pretty strong although I wonder about the angle of triangulation. I also think that having a bend in the side bar means that I'll need an additional fixing to the chassis. The bend is about 25 degrees.

What's people's thoughts on this?

[Edited on 8/5/23 by Slimy38]

Mr Whippy - 9/5/23 at 06:41 AM

Sorry zero out of ten. The angle is far too steep, the brace is not mounted to a strong part of the chassis, plus the brace is only supporting half the height of the hoop. It will add no appreciable support. Why can you not do the brace in the normal way?

russbost - 9/5/23 at 09:02 AM

I don't entirely agree with Mr Whippy, tho' it's certainly not ideal - it rather depends what you are trying to achieve .

If you want full protection like the "normal" race type cage then that's what you need to go for. What you have there would provide a little extra side/shoulder protection, something completely lacking in the standard 7 design & as it goes to around head height would certainly reduce the moment around the mounting point of the base of the rear hoop, so it would be less likely to collapse in the case of a rollover

It would also give added support if seat belts are to be mounted on the hoop. If you braced diagonally downwards toward the rear where the bend is that would add substantially as it splits it into 2 triangles & you could brace around the forward lower mounting point to stop that simply ripping off the lower chassis

I wouldn't say 0 out of 10, but far from perfect

As I say, depends what you want/expect it to do - why can't you go higher up the hoop?

Slimy38 - 9/5/23 at 09:12 AM

quote:
Originally posted by Mr Whippy
Sorry zero out of ten. The angle is far too steep, the brace is not mounted to a strong part of the chassis, plus the brace is only supporting half the height of the hoop. It will add no appreciable support. Why can you not do the brace in the normal way?


The bottom of the brace aligns with the corner of the triangle, here's a better photo;



I've not yet decided on how it fixes at the bottom, I've just used 'steel plate' (AKA piece of wood) for this example.

You have a good point about not getting all the way to the top, I'll have a think about that. I wanted it higher but I'm not sure whether the increased angle getting to the top would then make it less effective. I can't do the 'normal' brace because my car doesn't have the regular back end.

Slimy38 - 9/5/23 at 09:18 AM

quote:
Originally posted by russbost
I don't entirely agree with Mr Whippy, tho' it's certainly not ideal - it rather depends what you are trying to achieve .

If you want full protection like the "normal" race type cage then that's what you need to go for. What you have there would provide a little extra side/shoulder protection, something completely lacking in the standard 7 design & as it goes to around head height would certainly reduce the moment around the mounting point of the base of the rear hoop, so it would be less likely to collapse in the case of a rollover

It would also give added support if seat belts are to be mounted on the hoop. If you braced diagonally downwards toward the rear where the bend is that would add substantially as it splits it into 2 triangles & you could brace around the forward lower mounting point to stop that simply ripping off the lower chassis

I wouldn't say 0 out of 10, but far from perfect

As I say, depends what you want/expect it to do - why can't you go higher up the hoop?


This will be a road car, so I need to get a decent balance between form and function. The main issue is that I have no 'rear', the Austin Healey bodywork has no upper support for anything further back than the suspension tops. So I need to come forward.

I should mention it's missing the middle brace, I still need to include that as per the Haynes roadster design. But that's a horizontal bar, midway up. That'll be for the seat belt mountings as well.

Considering you've both said going higher on the loop I think that's what I need to address first.

Slimy38 - 9/5/23 at 09:40 AM

quote:
Originally posted by Mr Whippy
Sorry zero out of ten. The angle is far too steep, the brace is not mounted to a strong part of the chassis, plus the brace is only supporting half the height of the hoop. It will add no appreciable support. Why can you not do the brace in the normal way?



I'm thinking about alternative routes now. I could bring a bar down from the centre of the hoop on to the transmission tunnel, that would solve the angle but not the 'strong part of the chassis'?

I can't think of any other routes that would get me from the bar to something inside the car (without removing the passenger seat of course!).

I was trying to find some other examples and found this MX5 roll bar;



I guess that's similar in that the angle is too steep? It does get to the top though, I could try and end up in the same place with the side bars?

russbost - 9/5/23 at 09:58 AM

Where it mates to the lower front I would have a vertical plate welded to the end of the tube & add a matching plate welded vertically to the triangulation you already have
, if the side brace is triangulated from behind where it bends & you can get a little higher up the hoop & add a central support down to the transmission tunnel even if that isn't massively strong it all adds to the overall strength & stiffness - problem is, you don't want to finish up with it looking like a climbing frame!

Don't forget with the seat belt mounts the top point of the belt needs to be 450mm above the block they place in the bottom of the seat - that height is something it's easy to fall foul of on a roadster type car

Slimy38 - 9/5/23 at 11:01 AM

quote:
Originally posted by russbost
Where it mates to the lower front I would have a vertical plate welded to the end of the tube & add a matching plate welded vertically to the triangulation you already have
, if the side brace is triangulated from behind where it bends & you can get a little higher up the hoop & add a central support down to the transmission tunnel even if that isn't massively strong it all adds to the overall strength & stiffness - problem is, you don't want to finish up with it looking like a climbing frame!

Don't forget with the seat belt mounts the top point of the belt needs to be 450mm above the block they place in the bottom of the seat - that height is something it's easy to fall foul of on a roadster type car


That's great advice, thanks. As it is the whole of the side bar is hidden inside the bodywork, it's only actually visible from just above the suspension top. Triangulating the bend is a good plan and easily doable, it's behind the seat and out the way. I do just need to resolve the height, I'll see if I can get 'mockup version 2' up to the top of the cross bar.

And thanks for the block reminder, I'll be double checking that one as the cross bar goes in.

adampage - 9/5/23 at 03:25 PM

Hey Slimy38,
Just a thought but..... When you say "there's no rear, there's no upper support" - could you not go down to the top of the diff cage or top rear wishbone mounts, there must be some good structure there?
You could even do an X where a bar from the top-left corner of the rollbar goes down & back to the top-right of the diff cage and vice versa, crossing in the middle.

You might need 2 small holes in the rear bodywork to go down through there, but if you look at a Cobra rollbar / rollhoop it usually has something like that and you can make it look nice by finishing around the holes/mounts.

Then the only other thing I thought is you need to take care with a bar going out slightly wider than the roll bar and a little bit forward, because there's a risk that in an impact the driver's head moves around if you're thrown about a bit and you don't want that bar to be in range of banging your head on it. Might be worth checking by sitting in the seat and swinging around to see if you can whack it with your head......

Good luck, keep refining it!
Ad

Slimy38 - 9/5/23 at 03:58 PM

quote:
Originally posted by adampage
Hey Slimy38,
Just a thought but..... When you say "there's no rear, there's no upper support" - could you not go down to the top of the diff cage or top rear wishbone mounts, there must be some good structure there?
You could even do an X where a bar from the top-left corner of the rollbar goes down & back to the top-right of the diff cage and vice versa, crossing in the middle.


Interesting idea, and while there is some good structure there, it's also where the fuel tank will live;



But I do need some more support structure to finish off the tank 'shelf', I wonder how much extra work it would take to extend it enough for a couple of roll bar supports. I'll refit the tank and see if there is a straight line, thanks for the idea.

quote:
Originally posted by adampage

Then the only other thing I thought is you need to take care with a bar going out slightly wider than the roll bar and a little bit forward, because there's a risk that in an impact the driver's head moves around if you're thrown about a bit and you don't want that bar to be in range of banging your head on it. Might be worth checking by sitting in the seat and swinging around to see if you can whack it with your head......

Good luck, keep refining it!
Ad


At the moment I'm ok, the seats don't slide back far enough to be inside the bar hoop. But that is one of the worries I had if I bring the side bars up to the top of the hoop, at that point it might end up being in head range.

adampage - 9/5/23 at 06:27 PM

OK, so might not be possible but you never know!
Good luck with it, I bet it looks great when it’s done

adithorp - 9/5/23 at 08:29 PM

Not sure about the kink in the brace. Any force pushing the bar forwards would risk the brace bending at that point and thus let the roll bar collapse.

Slimy38 - 10/5/23 at 07:10 AM

quote:
Originally posted by adithorp
Not sure about the kink in the brace. Any force pushing the bar forwards would risk the brace bending at that point and thus let the roll bar collapse.


Yes, that's my worry too, I'm effectively introducing a weak point. There's a few options there though. First is what russbost suggested, triangulate it. That should be fairly straightforward, it's clear of the cabin by that point and I could get something there. The second is to add a second fixing to the chassis. That might not be as effective, it's a separate anchor point but there is still no substantial sideways resistance.

The third option is to connect the two sides, making a second roll bar hoop that lies just behind the occupants. Again it's in dead space so wouldn't be difficult to implement, but then I end up with a climbing frame as Russbost perfectly described it!

I am starting to think adampage's suggestion is going in the right direction. I do need to build a frame for the tank which would be triangulated around the diff cage by default. I just need to increase it's capacity to be a good enough fixing point for the roll bar back stays. They'll also need to be bolt in as they'll pass through the centre of the bodywork.

adampage - 10/5/23 at 08:07 AM

... have a diagonal or two, or an X in the bar itself, then the front or rear supports aren't the only thing. Bit like this...

Ariel roll bar
Ariel roll bar

Then the rear diagonals could be bolt-in as you say, and just pointed at a different location.

Bit of head-scratching in 3D to do
Ad

motorcycle_mayhem - 10/5/23 at 06:48 PM

For want of a better description, what is there in the 'A' pillar region?
If there's a substantive structure around that bulkhead, then two forward stays could connect to it. They'll have to take a slight bend, but if substantive tube I can't see an issue. For how this works in practice, look at the Sylva Riot. This came with either a single unbraced hoop, a bolt on set of forward stays (triangulated down to the rear bulkhead floor) or a set of forward stays welded in, without the triangulation bit. Superb design.

Slimy38 - 10/5/23 at 07:12 PM

quote:
Originally posted by motorcycle_mayhem
For want of a better description, what is there in the 'A' pillar region?
If there's a substantive structure around that bulkhead, then two forward stays could connect to it. They'll have to take a slight bend, but if substantive tube I can't see an issue. For how this works in practice, look at the Sylva Riot. This came with either a single unbraced hoop, a bolt on set of forward stays (triangulated down to the rear bulkhead floor) or a set of forward stays welded in, without the triangulation bit. Superb design.


I kind of understand what you mean, in my case there is no A pillar in the conventional sense but I do have some scope to build something out. Looking at the r1ot compared to the bodywork that I'm using, I could hide some decent metalwork inside the body;



The right image shows the body coming up quite high all the way to the bulkhead. I wonder whether I'd still have trouble on the backend though, trying to get the bar high enough on the hoop to be effective. The r1ot comes up halfway but it is pretty much a straight line from the front.

Slimy38 - 11/5/23 at 08:50 PM

OK, here's attempt two, heading backwards this time;



This is just a temporary structure to gauge where the roll bar will end up, essentially I'm aiming for the bottom rear corner of the tank. The roll bar can be straight, the only thing that I'm slightly worried about is that it starts at the hoop bend (the blue tape). I don't think that will make any difference though.

So this weekend I'm going to build a platform that is secure enough for the tank and triangulated enough for the roll bar. In terms of triangulation, I'll be coming down diagonally to the lower suspension point like this;



and just to confirm the last point of what Mr Whippy suggested, here's a side on pic for the angle;



This angle is 35 degrees, with MSUK yearbook specifying a minimum 30 degrees I'm happy the bar will give enough support.

I'm still in two minds whether to keep the other bars, they do serve a purpose for side impact and they're reasonably hidden.

adampage - 11/5/23 at 09:25 PM

I think that looks good, plenty of rear diagonals are near the bend, and if you can have the forward pair too for side impact then that’s great

I wish I had front bars on mine

Ad

James - 12/5/23 at 08:18 AM

How about something akin to Hicost's original design?

Not ideal have the forward support in the middle of the bar I guess but perhaps better than nothing.

[img] The Beast revealed! :-D
The Beast revealed! :-D [/img]

(Does this photo work?)

Cheers,
James

Slimy38 - 12/5/23 at 08:25 AM

quote:
Originally posted by James
How about something akin to Hicost's original design?

Not ideal have the forward support in the middle of the bar I guess but perhaps better than nothing.

[img] The Beast revealed! :-D
The Beast revealed! :-D [/img]

(Does this photo work?)

Cheers,
James


Aha, I was after an example picture of the middle support, I could only see small ones in the '£250' book. Thanks for that, now I can see how it might look. I'm not sure about the use of flat bar but each to their own I guess.

adampage - 12/5/23 at 01:25 PM

If you search for petty strut there’s a few Caterham and Westfield examples online, angled, long, short, removable, etc.

Slimy38 - 12/5/23 at 01:42 PM

quote:
Originally posted by adampage
If you search for petty strut there’s a few Caterham and Westfield examples online, angled, long, short, removable, etc.



Excellent, thank you very much! I didn't realise it had a specific name, I can see lots of different options now.

adampage - 12/5/23 at 04:07 PM

Either that or go forward to create a front hoop, but that's a bigger investment / project!

James - 12/5/23 at 07:54 PM

quote:
Originally posted by Slimy38
quote:
Originally posted by James
How about something akin to Hicost's original design?

Not ideal have the forward support in the middle of the bar I guess but perhaps better than nothing.

[img] The Beast revealed! :-D
The Beast revealed! :-D [/img]

(Does this photo work?)

Cheers,
James


Aha, I was after an example picture of the middle support, I could only see small ones in the '£250' book. Thanks for that, now I can see how it might look. I'm not sure about the use of flat bar but each to their own I guess.



Hicost's was definitely 2" or so round tube, not flat bar.

I had a number of extremely memorable trips in the 450bhp beast! lol

Slimy38 - 13/5/23 at 07:24 AM

quote:
Originally posted by James

Hicost's was definitely 2" or so round tube, not flat bar.

I had a number of extremely memorable trips in the 450bhp beast! lol


Ah that makes more sense, must just be how my brain is interpreting the photo.

Slimy38 - 15/5/23 at 11:11 AM

Rear brace option;






So this takes care of the platform for the tank and some reinforced plates for the brace. It's only clamped in here, it will be braced downwards for reinforcement but I think this is what I'll go for.