GSpeed Roll Cage Kit Development

[Martyr03]

Subbed. Looking to cage the car soon, and i only live an hour away.

[philooo]

As usual, I just love to see the iteration in the design and finally feel that more bars is bette

What about NODAL POINTS ?

Everybody is talking about nodal point for cages. My understanding is that it is a good thing to regroup tubes into 'nodal points':

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instead of placing tube slightly at different places:
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For example bars coming from the rear suspension and going on top of the main hoop should merge on the opposite side as the bar going from main hoop to the A pilar.

Did you run some analysis on what difference it make to have one meeting 'nodal point' instead of 2 meeting points, like the one you have in your design. Not sure how you call your purple dots in your rendering ?

I would say that I have seen it both ways, and I am still unclear as to what perform better.

I would assume the force would transfer better with a point with symmetrical contacts (1 purple point), but then an asymmetric design (2 purple points) could allow some more distortion in between the 2 points and prevent tube sheer. (pure speculation on my end

I hope I am not confusing everybody

[GSpeed]

Correct. It's more structurally efficient to bring tubes together at one node, but occasionally (like we're doing) they don't meet at the same point for fitment reasons. That corner of the cage is pushed very tightly up into the roof of the car, and the A-pillar bar has to be moved outward a little bit to allow for the A-pillar bar to clear the chassis. If they were to meet at one node, the roll bar would have to be about 1.5" lower, which would also bring the A-pillar bar closer to the driver's head. Given our larger tube size, we can handle the slight shear loading put into the roll bar at that location. This is one of those things we're only able to do because we have a 3D scan of the car.

[OkieSnuffBox]

Originally Posted by GSpeed

Don't worry, we're still planning on making bolt-in cages. Not going to lie, though, people will expect prices to be much cheaper than it'll wind up costing us. It happens in every corner of this industry, everyone wants parts that say, "Made in USA!" but they don't want to pay what it costs to make quality parts in low enough quantities to fill these niche markets. This (tracking 86/FRS/BRZs) is definitely a niche market.

The problem with that, is for some reason the vendors in this market seem to think the same material and labor should be worth 2-3x what they are for other vehicles.

[GSpeed]

Originally Posted by OkieSnuffBox

The problem with that, is for some reason the vendors in this market seem to think the same material and labor should be worth 2-3x what they are for other vehicles.

Well sure, it's a problem of quantity. There just aren't as many interested buyers for this platform. It's a problem we all have to live with, and hopefully solve. If we can continue to push this platform as a cheap way to go fast on track, more people will buy them to modify, and prices will continue to fall.

Jake

[mrk1]

Originally Posted by OkieSnuffBox

The problem with that, is for some reason the vendors in this market seem to think the same material and labor should be worth 2-3x what they are for other vehicles.

That "some reason" is called economy of scale. Prices will go down as demand increases and parts can be made in volume. Pretty standard pattern with a new platform. I own a manufacturing business and I would be hesitant to crank out a million parts for a new market, especially when the parts would rust on the shelf. GSpeed is really doing some awesome work here and seems to have the process abilities to back it up. I'm following progress for sure.

[gatorbushmen]

subbed..... Definitely interested.

[JazzleSAURUS]

This is an awesome thread. Cage on.

[plucas]

Originally Posted by GSpeed

Alright, let's do some analysis here. First off, let's get one big thing out of the way.

This is not a quantitative stiffness analysis.

We do not know enough about the torsional behavior of the vehicle to get a hard, reliable number for the torsional loads a vehicle experiences on track. So we're going to pick a reasonable number, and hold it constant through multiple tests and see how the results compare.

What this means is the numbers do not match up to the real world. They are only useful for comparing between tests.

First off, we'll apply 1000 ft-lbs to the rear four mounting points of the cage. These loads are evenly distributed between the points, but the points are not constrained to each other. They can move around naturally.

The front two mounting points will be fixed. We will measure displacement of the lower left roll cage mounting plate, and use that to calculate an angular deflection, which we can use to compare different configurations.



The first test is a simple 6-point cage with a crossed main hoop brace, a double crossed main hoop, and full length harness bar. This is the lightest, simplest cage design, and will be the starting point for this study.

Deflection- 2.49°
Stiffness- 402 lb-ft/deg
Weight- 99.5 lbs

Next, let's add FIA style door bars:



Wow. Okay, door bars really help torsional stiffness across the car.

Deflection- 0.47°
Stiffness- 2146.6 lb-ft/deg
Weight- 125.6 lbs

That's an improvement of over 400%. So we're definitely adding door bars. Now, let's add some beams between the bottom of the rear hoop and the rear shock mounts. These types of bars are common on flexy unibody cars.



Deflection- 0.43°
Stiffness- 2321.9 lb-ft/deg
Weight- 134.05

So for an additional 8.5 lbs we pick up another 8% stiffness. Not bad.

Many series allow two additional chassis points on the firewall to protect the driver's compartment from wheel intrusion in accidents. Let's add those and see how much stiffness they add.



Deflection- 0.36°
Stiffness- 2758.1 lb-ft/deg
Weight- 135.52 lbs

An extra 1.5 lbs (neglecting landing plates, of course), and we gain another 18%. Pretty good for the negligible weight, and considering they're really there for safety.

Now, what about the different between FIA bars and NASCAR bars? Everyone knows NASCAR bars are better at protecting the driver in broadside impacts since they arc out away from the driver, but how do they affect torsional stiffness?



Deflection- 0.30°
Stiffness- 3297.7lb-ft/deg
Weight- 145.02

So they weigh an extra ten pounds more than FIA bars, but they add almost 20% more torsional stiffness! Sounds like a good deal.

This sort of iterative analysis is a bit of insight into how we design. Torsional stiffness is just one of many things to consider when designing a cage, but you can see here why certain things are the way they are.

Jake

Can you get a better picture of how you are loading the cage? I thought I understood the wording, but the picture look different than what I expected.

[GSpeed]

Originally Posted by plucas

Can you get a better picture of how you are loading the cage? I thought I understood the wording, but the picture look different than what I expected.

In that last picture, the pink dotted line? That's the axis around which the torque is applied.

My graphics card is woefully underqualified for Solidworks, and I get a lot of strange glitches. One of which is Simulation's External Load indicators showing up inconsistently, incorrectly, or not at all. Functionally, it's there correctly, but it doesn't always show the visuals right.

[GSpeed]

We're in the middle of moving our server to a new host, so our pictures will be down for a little bit. Sorry for the inconvenience.

[GSpeed]

Pictures back up! Thanks for being patient through this build, we're pushing hard on the Viper GTS build right now, and should resume work on the BRZ shortly.

[GSpeed]

Here's another update on the cage build.

The jungle gym of a rear cage:



Here's a shot of the harness bar offset, which allows for maximum seat room.



Door bars:



Although we won't be keeping them, it's possible to keep the factory glass windows installed, either for regulations or preference. We'll be removing ours, and making some brackets to fix the windows in a permanent upright position for transportation and storage.

Thanks for following this build! It's a slow winter for this car, but we're planning on doing some awesome things with this car when it warms up.

[Iwannajag]

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