M4 GTS cage

[ngl]

Hi,

I'm trying to learn as much as possible about roll bar ( half cage ) design so I can make the best decision about installing one in my car, as it is now a requirement for the events I participate in.

Most OTS cages I found are tuning items ( bolted with 1 /2 screws in the car's body), will most likely rip their mounts off in case of an incident and will never pass inspection. The way cages are bolted in is very clearly regulated by FIA.

The M4 GTS cage mounts are actually overkill, since the FIA regulations mandated 3 bolts for the main hoop and 1 or 2 for the rear legs ( backstays ) but I guess it was never omologated because the main bar wall is too thin ( 1.5mm and regulations require a minimum of 2mm steel bar ).

I want to get mine designed based on the M4 GTS cage, and I was wondering if anyone ( GTS owner ) ever had the curiosity of looking at how they are attached to the car's body.

If anyone could share some pictures it would be huge help. Thank you!

[NYG]

I'm a structural engineer (specialize in steel design) so if you've got the rules/regulations available for me to look at, I can help you out with whatever you need (analysis/connection design/etc)

[ngl]

Originally Posted by NYG

I'm a structural engineer (specialize in steel design) so if you've got the rules/regulations available for me to look at, I can help you out with whatever you need (analysis/connection design/etc)

First, I want to say thank you for offering to help. The rules don't specify mounting points, because same rules apply to all cars from all manufacturers so I guess they can't cover them all with exact locations.

They only mention the following:

Main hoop and front hoop ( for 6 point cages ) need to include a reinforcement plate of at least 3mm and 120cm2 area ( as seen in drawings K52, K53, K54 ) in my first post and be secured with minimum 3x M8 bolts of 8.8 quality ( ISO ).

For the backstays ( rear legs ) they require the plate to be 60 cm2 and secured with at least 2x M8 bolt ( drawing K59 ) or one bolt in double shear ( drawing K60 ).

I have looked at the Teamshirmer cage design and I am considering using the mount points they used for backstays as I can understand where they attach (same place the real seat belts would be, so it should be reinforced from factory). They already have a M8 bolt, I just need to add the a second one.

Please share your thoughts on this.

I also like this design as it could allow me to use the rear seat space to transport tires when going to the track.

[NYG]

I'm going to take a look at this after work today.

[edwinm3]

That's awesome he's offered to help, outstanding!

[NYG]

Originally Posted by NYG

I'm going to take a look at this after work today.

Haven't forgot about you OP, left the office late yesterday.

[NYG]

Okay so I hope I'm answering the right questions for you, let me know if I can clarify. I'm not experienced with designing roll cages so the regulations/minimum loads/deflection should be implemented. I don't really know those requirements so I'm going to use what I have available to me from the SCCA. I'll try my best to illustrate it for you so that you can use the general information with multiple options. Regulations here on full cage race/formula cars is:

vehicle weight x 1.5 for lateral impact
vehicle weight x 5.5 for longitudinal impact
vehicle weight x 7.5 for overhead impact / roll over impact

With a half cage, we can disregard longitudinal for our discussion but even if these values are different in your case we can conclude that the connection design are governed by huge overhead impact and a significantly lower side impact.



This is the best photo that I was able to find of where the M4 GTS cage is mounted. (Ignore the clip point at the b-pillar, the image is too small)

It seems like all four connection points are on the lower frame rail. Main hoop at the center cross member and rear right in front of the wheel housing. Both seem like the logical connection points for base fixity/stiffness. The second cage you provided appears to connect at the seatbelt but there's a ton of structural disadvantages to doing this. You may get a strong enough cage but the car's frame is inherently weak at these locations. It can be done but less efficiently.

Using the M4 GTS and the M4 DTM as examples, they both opted to use an X brace for the rear supports and the main hoop (second cage has a single brace for the main hoop). This is more efficient for a couple of reasons:

1. X brace provides torsional rigidity.
2. X brace provides equal lateral stiffness on the passenger side.
3. X brace provides additional load paths and stability members to the frame.
4. X brace distributes loads equally from the center of rigidity to the supports.

It also looks like the second cage reverses the load path of shear and tension through the bolts. That's also doable but places a much larger requirement on bolt bearing/bolt shear/localized panel zone failure of the frame. The M4 GTS and the M4 DTM cage transfers vertical impact loads through uniform bearing of the plate on the frame. That means you're spreading that force equally over an area to reduce the pressure resulting in lower internal stress. The reinforcing plate probably helps prevent localized failure of the frame since it's made of a more flexible material. I highly doubt the intent is to put the bolts in double shear since there's more than enough with that many bolts in single shear.

The second cage has a more complex load path because the pipe has to transfer vertical load through an L shaped plate then into a bolt (or for shear). The reason for that is because the flexural stiffness of a thin plate is several magnitudes lower than the shear stiffness of a bolt. The stress in the bolt can't develop immediately. If this sounds completely foreign, I picasso'd this:



It's kind of terrible but I'm trying not to over complicate it while using high end software like ms paint.

My suggestion is to keep the reactions at the frame rails. If you do choose to go with the second cage then I suggest keeping the bolts parallel to the vertical force (7.5 x vehicle weight) and you'll have to reinforce the frame at the rear legs. The seat belt reinforcement is enough to resist failure from the accelerating mass of a passenger not 12,000 KG.

[ngl]

Thank you very much for this analysis. I suppose the second design is more of a harness bar than a roll bar.

There is another design I like as it doesn't require welding the back legs and seems to be closer to the OEM M4 GTS cage. It also connects to the chassis in the exact points that you outlined using that "state of the art software" so this should be the best compromise for a bolt-in half cage. I attached pictures.

I try to avoid welds as in the future a more advanced 8 points full cage with air jacks support will be installed, however at that point the car will no longer be street legal.

[NYG]

Even on your final cage, I personally prefer bolts over welds in most cases unless you're working with thick surfaces. I tend to prefer them in the case of dissimilar metals as well (your case) but I have to look at it from a high rise building perspective layered with expectations from weld quality + horrible accuracy of magnetic testing of welds.... but since you're dealing with a cage and will probably use a specialty welder, I'm sure my perspective wouldn't be applicable.

Be wary though, I spent a lot of time doing research on brittle failures (sudden rupture like glass) of crystalline solids (steel) and a lot of care needs to be taken in consideration of thermal loads from welding. When you spot load an elastic material like steel/aluminum/etc with heat the atoms will spread out in that area while the surrounding metal will be at a cooler temperature. Once that spot cools down suddenly you're going to develop a permanent tension force (residual stress) which lowers the stiffness of the material and can potentially create an embrittlement zone. The beauty of steel and most metals is that it will give you significant warning before it fails and in doing so it will get even stronger which will prevent sudden failures. That's why you only see concrete structures that have catastrophic sudden failures (not because reinforced concrete is bad but it's easy to over reinforce it and easy to overlook a really big mistake). Steel structures will never have catastrophic failures unless a connection fails.

[ngl]

The reason for going with a welded cage is for chassis stiffness. It will be done by a company called Ireco, they have 20+ years experience in building FIA roll cages, and even exported to the US.
That will however happen in a rather distant future as I need to send the stripped body shell to them ( different country ), no wires, no engine, no nothing. I have a friend that got his M2 caged by them and it is quite impressive. Hope he never gets to test how well it can withstand an accident.

I've read about the things you mention, but I don't have the expertise to understand it, and even the bolted cages usually have welded reinforcement plates so I think the only benefit you get from bolted is the fact that it can be removed.
I don't think you can have 8 point bolted cages though ( connecting the front struts to the cage ). I haven't yet tested this myself, but from everyone I know that did it, it changes the car handling completely and quite dramatically.

Regarding my current half cage, car is scheduled for the start of March. I also found a picture ( from the anniversary M4 GTS build by IND ) of how the M4 cage reinforcement plates connect to the car, but I would still like better / more clear pictures if any GTS owner would be kind enough to share.

[NYG]

Originally Posted by ngl

The reason for going with a welded cage is for chassis stiffness. It will be done by a company called Ireco, they have 20+ years experience in building FIA roll cages, and even exported to the US.
That will however happen in a rather distant future as I need to send the stripped body shell to them ( different country ), no wires, no engine, no nothing. I have a friend that got his M2 caged by them and it is quite impressive. Hope he never gets to test how well it can withstand an accident.

I've read about the things you mention, but I don't have the expertise to understand it, and even the bolted cages usually have welded reinforcement plates so I think the only benefit you get from bolted is the fact that it can be removed.
I don't think you can have 8 point bolted cages though ( connecting the front struts to the cage ). I haven't yet tested this myself, but from everyone I know that did it, it changes the car handling completely and quite dramatically.

Regarding my current half cage, car is scheduled for the start of March. I also found a picture ( from the anniversary M4 GTS build by IND ) of how the M4 cage reinforcement plates connect to the car, but I would still like better / more clear pictures if any GTS owner would be kind enough to share.

High fiving myself on guessing where it connects to. I never even looked at the F8x frame until this thread.

But yeah, remember a lot of aftermarket car parts engineering is brute force. I'm assuming when you say chassis stiffness, you mean joint stiffness. Which as a general statement is true especially if you match base metal strength to filler strength (25.4MM of E70 weld on carbon steel has ~18KN of factored strength versus a single M8 bolt which has ~14KN of unfactored strength). In solid mechanics your chassis will only be as stiff as the weakest member (frame) and although I don't know the full specs of the frame, I can only assume the base metal strength is extremely susceptible to rupture. My biggest issue with bolts would be slip considering how often flex occurs in the frame. I don't even think pre-tensioning would help. But yeah, you can achieve the same overall chassis stiffness using bolts on the cage but yeah that doesn't mean welding is bad/worse. It's probably the easiest.

I can't offer any help with more GTS pictures but here's a free bump.

*Also: It looks like they're using standoff base plates which will allow flex in the legs of the baseplate under load and deformation under extreme load. If I'm not missing anything here, that's a really good thing because under an impact load you NEED to dissipate that energy. Your rollcage only experiences tension and compression (theoretically) so it can't dissipate much energy. You can do so by purposely creating failure zones like using stand offs.