Cage Question

"whiskeybusiness NOW A DANGER TO CROWDS NEAR YOU" (whiskey-business)
08/31/2015 at 10:28 • Filed to: None

I find roll cages fascinating. They are aesthetically very appealing, make a properly equipped vehicle immeasurably safer with proper harnesses and helmets, and add structural rigidity to a chassis. However, I have come across an anomaly that seems to defy two of these aspects. See picture below.

Pivot Points. Why would a structural element in a car be allowed to radially move at key attachment points? Wouldn’t this reduce some of the benefits to safety and structure that they provide the vehicle? Or is there some downside to having a totally rigid cage, such as damage to the chassis, or possibly undesirable ride/handling characteristics?

Additionally, these are not uncommon. Many roll cage manufacturers provide optional pivot tabs for their products. Any insight would be appreciated.

Have a GT2 RS Roll cage for your time. A work of art.

DISCUSSION (25)

RamblinRover Luxury-Yacht > whiskeybusiness NOW A DANGER TO CROWDS NEAR YOU
08/31/2015 at 10:33

Triangulated structures are easiest to plan out and simulate when each element is only under tension or compression. It’s worth a longer explanation than I can give at this moment, but you don’t want elements to be under twisting loads or unexpected things happen. This simplifies loads on welds and a dozen other things.

wiffleballtony > whiskeybusiness NOW A DANGER TO CROWDS NEAR YOU
08/31/2015 at 10:36

Kinja'd!!!

Cage answer

whiskeybusiness NOW A DANGER TO CROWDS NEAR YOU > RamblinRover Luxury-Yacht
08/31/2015 at 10:36

So allowing things to pivot allow them to only be loaded in a linear manner, which is easier to simulate and prepare for the forces involved? Interesting, it makes perfect sense, but I’d never have thought of that.

crowmolly > whiskeybusiness NOW A DANGER TO CROWDS NEAR YOU
08/31/2015 at 10:38

That looks like a bolt-in roll cage, which is mostly for aesthetics. I can’t speak for all racing classes but I know many do not allow bolt-in stuff. Looks like the harnesses are floor-mounted which is a no-no too.

whiskeybusiness NOW A DANGER TO CROWDS NEAR YOU > wiffleballtony
08/31/2015 at 10:39

You sent me down the rabbit-hole and I’m not disappointed.

Kinja'd!!!

RamblinRover Luxury-Yacht > whiskeybusiness NOW A DANGER TO CROWDS NEAR YOU
08/31/2015 at 10:40

https://en.wikipedia.org/wiki/Truss

Goes into a bit more detail, but that’s basically what’s happening here. You don’t want one element to induce another element to buckle by twisting it or do a number of other things when it’s usually just smarter to plan out as a space-frame. Each element needs to do its own thing for things to stay strong (and predictable).

ChooChooMotherFudger > whiskeybusiness NOW A DANGER TO CROWDS NEAR YOU
08/31/2015 at 10:44

A lot of bridges and other structures around are like that.

Santiago of Escuderia Boricua > whiskeybusiness NOW A DANGER TO CROWDS NEAR YOU
08/31/2015 at 10:45

That looks like a drift car or something not legal for competition. A real safety cage will have everything welded together

whiskeybusiness NOW A DANGER TO CROWDS NEAR YOU > crowmolly
08/31/2015 at 10:45

It is indeed bolt-in, but the CR-X in which it is bolted is a street/track car that I assume has and would pass tech.

whiskeybusiness NOW A DANGER TO CROWDS NEAR YOU > ChooChooMotherFudger
08/31/2015 at 10:46

I had no idea. I guess Statics and Mechanics are gonna learn me good this semester.

whiskeybusiness NOW A DANGER TO CROWDS NEAR YOU > Santiago of Escuderia Boricua
08/31/2015 at 10:47

It is a street/track build featured on Speedhunters. Not full-on race car, some concessions made to streetability.

ChooChooMotherFudger > whiskeybusiness NOW A DANGER TO CROWDS NEAR YOU
08/31/2015 at 10:48

Studying engineering?

whiskeybusiness NOW A DANGER TO CROWDS NEAR YOU > ChooChooMotherFudger
08/31/2015 at 10:48

Yessir, mechanical. Recently switched from pre-med/biochem track and loving every minute of it.

wiffleballtony > whiskeybusiness NOW A DANGER TO CROWDS NEAR YOU
08/31/2015 at 10:50

10/10 would watch

Santiago of Escuderia Boricua > whiskeybusiness NOW A DANGER TO CROWDS NEAR YOU
08/31/2015 at 10:50

It would definitely add stiffness

crowmolly > whiskeybusiness NOW A DANGER TO CROWDS NEAR YOU
08/31/2015 at 10:50

Depends on the sanctioning body. The NHRA (as an example) would fail this thing for a multitude of reasons.

I want to say the SCCA says bolt-in only for some auto-x stuff but I’m not sure. Anything wheel-to-wheel usually is weld in only.

ChooChooMotherFudger > whiskeybusiness NOW A DANGER TO CROWDS NEAR YOU
08/31/2015 at 10:51

Cheers man. I am starting 4th of 5 years for Mechanical.

Don’t know if your school has one, but if they have a FSAE team I would recommend joining if you find this stuff interesting.

Nonster > whiskeybusiness NOW A DANGER TO CROWDS NEAR YOU
08/31/2015 at 11:01

In a well designed truss or cage type structure, the primary stresses are in the axial direction of the members. The cage in that pic is just a cheap, simple bolt-in cage. The attachment points that you’re referring to as pivot points don’t really pivot. Its only there so that the cage can pre-made then installed in the car without having to weld the cage while in the car.

A welded in cage will be stiffer as the welded joint will resist bending about that point, but if you’re getting significant bending stresses at a weld like that, your cage is probably poorly designed in the first place.

whiskeybusiness NOW A DANGER TO CROWDS NEAR YOU > ChooChooMotherFudger
08/31/2015 at 11:01

I’m currently trying out for my school’s Formula team. However, my school’s fairly recent transition from Baja as well as the team’s founding members graduating last year has been detrimental to the team.

whiskeybusiness NOW A DANGER TO CROWDS NEAR YOU > Nonster
08/31/2015 at 11:02

Gotcha. That makes sense; smaller bars with fasteners are easier to do at home than welding.

ChooChooMotherFudger > whiskeybusiness NOW A DANGER TO CROWDS NEAR YOU
08/31/2015 at 11:06

Ahhh ok. Also you have to try out?

We are entering our 7th year with fsae. Its a hard thing to do for new or smaller teams.

Nonster > whiskeybusiness NOW A DANGER TO CROWDS NEAR YOU
08/31/2015 at 11:18

Yep the door or hatch openings are only so big. I also wouldn’t completely discredit the use of a bolt in cage. A full proper weld in cage is better, but there are some good quality bolt in cages available for some cars. So if you want a half-cage to mount a harness to or to stiffen up an Autocross car then they serve that purpose well

whiskeybusiness NOW A DANGER TO CROWDS NEAR YOU > ChooChooMotherFudger
08/31/2015 at 11:40

Yes. There is a trial period where you shadow a current member and then have to design a and manufacture a part that will be reviewed by senior members.

ChooChooMotherFudger > whiskeybusiness NOW A DANGER TO CROWDS NEAR YOU
08/31/2015 at 11:44

That’s an interesting idea. We are a pretty small team so I don’t know if we could do that (or if the school would let us), but would free up dead weight.

gear5 > whiskeybusiness NOW A DANGER TO CROWDS NEAR YOU
08/31/2015 at 12:11

Also those belts are installed incorrectly...