cages and crumple zones

[kolosy]

so i had an oopsie in my other car on track, and while it did a great job of keeping me safe, it did get me thinking that i want a full cage in the e90.

one thing that popped up on another forum for me is the notion of energy dissipation through intentional deformation (crumple zones etc.)

if you go with a full nascar cage, are you working against factory crash testing? you're basically welding in what looks like a more rigid structure inside the car, which takes away the designed-in abilities to deform in predictable ways and absorb the energy of the impact that way.

... or am i overthinking it?

[DRLane]

Our cars couldn’t be further from a NASCAR car, if you want ultra safe take inspiration from Rally cars.

Retain the crash bars and don’t build beyond the firewall and you’d retain some of the crumple zones. But a well designed cage, with seats, nets, etc should be considerably safer for the driver.

[kolosy]

Originally Posted by DRLane

Our cars couldn’t be further from a NASCAR car, if you want ultra safe take inspiration from Rally cars.

Retain the crash bars and don’t build beyond the firewall and you’d retain some of the crumple zones. But a well designed cage, with seats, nets, etc should be considerably safer for the driver.

maybe nascar was the wrong term here, i just meant full cage including the piece that goes where the front doors are (don't know the proper term for it), but i get your point.

[RalliartRsX]

The cage is made to dissipate the forces throughout the structure and NOT crumple or bend.

Different requirements vs a road car as........road cars with cages are generally unsafe....so the OEMs have turned to crumple zones to absorb as opposed to dissipate around struture

[M3SQRD]

Crumple zones are designed to extend the duration of a crash. Impulse and momentum states

m1v1 + Integral(F * dt) = m2v2

If m1 = m2 then F = m x (v2 - v1) / dt

Crumple zones increase the time duration, dt. So the longer the duration of a crash, the lower the magnitude of the impulse force. Primary crumple zones are the front and rear crash structures which aren’t affected by the addition of a roll cage (it’s debatable whether a cage tied into the front strut towers alters the crash structure but it would have to be a massive frontal crash to significantly deform the strut towers).

The purpose of a properly designed 6-pt cage is to provide a survival cell to minimize potential injuries to the driver and passenger during a crash, assuming they are using a proper seat, 6-pt harness and HANS. The cage is stiffened around the doors (e.g., nascar door bars) to prevent side intrusion into the survival cell during a side impact.

So with a cage there are still crashes that will alter the momentum over a longer time duration (rate of change of momentum is equal to a force, Newton’s 2nd law) as well as absorb kinetic energy (Work-Energy equation, Work = F x distance = 1/2 x mf x Vf^2 - 1/2x mi x Vi^2 = 1/2 x m (Vf^2 - Vi^2 if m1 = m2 = m) by using the front and rear crumple structures. Additionally, there are side impact crashes that will convert translational momentum into rotational momentum (conservation of momentum) or convert translational kinetic energy (door bars do not deform) into rotational kinetic energy and impulsive moments x rotation (conservation of work-kinetic energy). If the door bars are elastically deformed in the accident then some energy is absorbed by strain energy stored in the deformed door bars (or any deformed portion of the full cage).

So a properly designed full 6-pt (or 8-pt if tied into strut towers) cage augments the primary crash/crumple structures by providing additional structure to protect the occupants from side impacts and roll over impacts that could collapse the roof at the A and B pillars. The addition of a full cage results in a car that’s overall safer for track use.

A real concern with running a full cage on the street is the possibility of hitting the upper and side bars of the cage in an accident while only wearing a 3-pt seatbelt and no helmet. Tube padding is too stiff and too thin to protect your head from an impact with cage bars.

[kolosy]

yeah, this would be going further down the rabbit hole and making it not streetable.

[M3SQRD]

Originally Posted by kolosy

yeah, this would be going further down the rabbit hole and making it not streetable.

A properly designed roll bar can provide added protection in the event of a roll over yet it’s still street-able. I have a custom BW welded-in roll bar in my e46 M3 track car with proper stiffening at each of the four support points. It doesn’t provide the protection of a full cage but it does provide roll over protection, proper use of race seats, correct usage of 6-pt harnesses with a HANS device and a rear coilover setup due to stiffening and strengthening the rear shock towers.

[M3SQRD]

It won’t let me post a third picture

[RalliartRsX]

I am going to go out on a limb here and say; most of the hard parker roll bars on the market today are junk........

Note: From someone who caged his E92 recently to do GLTC and went through how a cage properly distributes the loads/forces.

I still have pictures of that Mustang that rolled at Buttonwillow (? I know if was a track in CA) and the bolted in bar punched right through the floor plates and the driver and pass was lucky to walk away....

https://forums.pelicanparts.com/pors...-roll-bar.html

[M3SQRD]

Originally Posted by RalliartRsX

I am going to go out on a limb here and say; most of the hard parker roll bars on the market today are junk........

Note: From someone who caged his E92 recently to do GLTC and went through how a cage properly distributes the loads/forces.

I still have pictures of that Mustang that rolled at Buttonwillow (? I know if was a track in CA) and the bolted in bar punched right through the floor plates and the driver and pass was lucky to walk away....

I absolutely agree with your statement about OTS bolt-in roll bars not being functional/safe. You need to stiffen and spread the load out over a larger area, especially the main hoop connections to the chassis floor. IIRC, there is one OTS roll bar that has adequate support/load path at its four interfaces with the chassis but I’d still go for a custom welded-in roll bar. My e46 M3 roll bar interfaces matched BW’s full cage design used in their e90 world challenge touring car race cars.

[roastbeef]

i'm not disputing the safety of a properly fabricated cage, but alot of the time people seem to overlook the need for a head and neck restraint coupled with a proper harness bar.

people seem to like to throw around the term "half cage." i think its just a term we have grown to casually use, even though they are all marketed as harness bars. additionally, the only harness bar i've seen tested to NHTSA standards is brey krause.

secondary to all that- i don't think people are getting crushed in these cars, they are getting internally decapitated.

[sperkins]

FWIW,
I’m running the Pinnacle half cage/OMP HTE-R seat w HANS.
I believe the Pinnacle cage has the same mounting points as the factory GTS cage.
Obviously a weld in cage is ideal. My solution is better than “stock”

I feel a lot safer (and comfortable) during track days w/ this setup.

Rollover tests confirm that occupants die when the head impacts the ground through the window, not from the roof collapsing.