Cage Design, Are they really worth it in an auto-x Options

[914forme]

It is now offically cold here in Ohio, it will warm up a couple more times this year, so now it is time to ask questions and plan a little.

So is a cage really that inportant to an auto-x car. I was looking at the Spec Boxster build and noticed just how low the weight was on that really nice cage. Since in my class I can lose upto 100 pounds before gaining points, and moving up a class it makes for an interesting balancing act.

So cage <> No cage

I just loose a few pounds off the car to make up for the added weight of the cage.

So is it really worth it. I run the Engmann kit which was 15-16 pounds, the removal of all the floor tar took care of that. I actually lost ~7 pounds over all. I have seam welded and reinforced the rear arm mount points. Since I have take points for all that, and a cage is free unless I tie into pickup points, I think it is a logical step in the stiffness direction.

Since I don't live in the bay area, I can't drop the car off at TCdesign, and have it done, and RacerChris is also to far away, so I will have to either go bolt in, or have one custom built here locally. I know a few certified welders in the area, pipe fitters by trade for jet engines, and the others build Nuclear Power plants. But they are not cage designers.

[URY914]

My short answer: If this is only an a/x car and it is a rust free tub, I won't do it.

[nebreitling]

how serious are you about AX? i think it's worth it.

[J P Stein]

For my class I have to run one....and so does Paul (IMG:style_emoticons/default/biggrin.gif)
It makes a good place to tie the car together.
I'm doing some rework on my cage. When I'm done, pics will follow.

In your case, it does add weight (80-100 lbs)...not a good thing for a 95 hp motor.

[Jeroen]

unless you tie the cage to the suspension mounts, it'll do very little for extra rigidity of your chassis
and tie-ing to the suspension mounts may not be legals for your AX class or get you a shitload of points (check your rule book)

so... if you can't tie it to your suspension (because of the rules) I wouldn't do it

if it's for safety, a rollbar only will do just fine for AX

[groot]

Personally, I think the stiffness of these cars is pretty non-existent.... so adding a proper cage would benefit you more than the weight would hurt you... but that's just an opinion.

BTW... I haven't seen a bolt-in cage for a 914 that's proper. Not to be rude, but "Do it right, or don't bother."

[J P Stein]

Personally, I think the stiffness of these cars is pretty non-existent.... so adding a proper cage would benefit you more than the weight would hurt you... but that's just an opinion.

Not an opinion I'd argue with (IMG:style_emoticons/default/biggrin.gif)

[Brett W]

Yep I concur with Kevin and the others. These cars have a chassis stiffness level pretty low compared to other more modern cars. They also like to break in half right about where your feet are in the even of an accident.

A proper cage design will work wonders for suspension accuracy. If your suspension is moving all over the world, how can it accurately tell the car which direction to go?

The benefits will far out weigh the weight penalty. Plus you can go up to Time trials or Road racing with the proper cage.

[Aaron Cox]

brett, if could design a cage... where would you start?

lets ask you both scenarios,
suspension pickups, and just a cab cage...

[URY914]

My longer answer: Check the rules about tying into the suspension pick-up points. This will bump you out of stock and street prepared I believe. Also welded vs. bolt-in needs to be reviewed in the rules.

You could fab one that would really tie things together if you added some door bars. Thats where the car is at it weakest. You need to span those big door openings but getting in and out on a street car is a PITA.

If you have a main hoop that is bolted to the floor and at the seatbelt attachment points inside the targa bar with a diagonal bar (like an Autopower bar) thats the best way to start. Now add some doors bars.

If you do it right it will more than make up for it's weight.

[914forme]

I use SCCA and PCA for seat time, I run in a NASA Auto-x / Time Trail class called TTE, I am maxed on points in TTE. If I bump I have to go big and max out to the next level another 19 points to play with. But that takes the car from a semi drivable on the street to no way. It also means that an engine swap would be in my future, and more weight reduction.

I am a rather serious auto-xer, I really enjoy the sport and the people in the MVSCC, a real good group of car people that like to drive there cars in a fun safe environment.

So the short is I would go up a class if I had a nice 911 engine sitting in the wings, right now, it won't happen so I need a decent cage with just the inner cabin. I can not tie in, to the suspension.

BTW, Kevin I agree that the bolt in cages are not right. That is part of the reason I am looking for ideas, on design. Not many 914s locally anymore. (IMG:style_emoticons/default/sad.gif) Or at least that are running auto-x and not on jack stands.

[URY914]

One way to make a bolt-in cage better is to make a mounting plate wth nuts welded to it. Then weld this plate to the car. Your cage will be "bolted-in" but your attachment points will be MUCH stronger. I did this on my car and it was never questioned.

[Brett W]

I'll post more later, I have to run perform car surgery.

[Randal]

I AutoX my car for a year without anything before installing a cage after my first track days.

What I experienced was a huge difference in handling - having the cage as opposed to not having it.

That in my opinion is enough reason to put it in. However, you just never know what could happen, even on an autoX course. Having a cage is simply insurance for you. (IMG:style_emoticons/default/dry.gif)

An alternative would be a well constructed roll bar with front supports. But again a well placed cage really allows the car's suspension to work.

[Brett W]

brett, if could design a cage... where would you start?

lets ask you both scenarios,
suspension pickups, and just a cab cage...

I agree with Paul, first figure out the purpose for the car, then read the rules. If you go building the pimpest cage in the world you might get bumped into my class, Super Production.

Now with that said. If you are moving suspension points, start with the pickup point location. Set that then move towards the middle of the car. Think Bridge. Then focus on the drivers cockpit area. Driver safety and comfort. Make it light. Depending on the rules use different size and shape materials to create different things. Many cages out their are way overbuilt. My race car has at least eight different tube shapes and sizes depending on load and rule requirements. Look at the chassis and see where there are "unboxed" areas. For instance look in the engine bay. See the area around the battery tray and relay board. That area is only supported on one side (the outside). You have to close that up or span that gap with some tubing. Also the harnessbar area on the firewall is relatively weak. Run a bar right underneath the engine lid mounts and run a bar underneath the engine shelf.

Moving towards the driver's area. I don't like NASCAR door bars. They are in my opinion rough on the driver in the event of an accident. They are also heavy and weaker than other designs. If you let the chassis absorb some of the initial impact and then let the cage take the remainder of the hit, the driver will be much safer. Think crush zones.

Also look towards your feet. Look at some of the pictures of wrecked teeners. Many of the drivers have had broken ankles and legs. This is the way the chassis bends if ever hit hard in the front. You need to brace this area. Many of the shitty bolt in cages are not braced to the front firewall.

Now move onto the front of the chassis. Again the area beyond the strut towers is really under very little stress. Use it as a crush zone. Build the majority of your structure into the are from the bulkhead back. Also you have to keep an eye on the tubes connecting to the strut towers. They add lots of strength up their but without any bracing down low only exacerbate the broken leg syndrome.


Lets cover the cab only setup. I am not a big fan of these but rule classes require them. Bolt in cages suck period. Don't waste your time. Always weld the cage in. Attach the cage on either side of the doors, but you nmeed to run some runners down to the wheelwells and to the front firewall. You have to do something to protect your legs. ALso make it as tight against the body as possible. You amy only be allow four-six points of contact, but if the A and B pillars are really tight. Push the main hoop as far back as you possibly can. Gives tall drivers room and gets the cage even further away from the bars.

Now I have some ideas for some non cage chassis strengthening, but it involves some pretty extensive fabrication. I am trying to find some examples of what I am talking about.

The rules will determine what direction you should go. If you are building a street car I am not a big fan of most cages. You head is not a helmet no matter how hard you thing it is. It will split open like a melon when you get in an accident, so you need to keep that in mind. Keeping your head as far away from the bars as possible is one of the marks of a good cage.

I did a cage in a GTI that put the main hoop and down bars almost three feet from the driver. The closest bar to the driver was the door bars. They were not NASCAR bars.

[groot]

Stephen,

I posted some pics of my cage construction in my blog. I certainly made some compromises on it for various reasons. If you ever want to drive up to Detroit on a Saturday, let me know.

Kevin's Blog

[914forme]

If you ever want to drive up to Detroit on a Saturday, let me know.

Kevin's Blog

Cool thanks, I am thinking around Detriot Auto Show time, two to three birds with on stone.

[jhadler]

I've considered it myself, but it really depends on the class and the rules.

In my class, I'm not allowed much (if any) actual weight reduction methods, and no specified minimum weight. As such, every pound counts. And a cage is a good 75-100 lbs of balast. Then again, stiffening up the chassis is always a good thing.

I have some ideas for an extended roll bar design, that should help with stiffening the chassis, but it won't do anywhere near as much as a full blown cage.

I'd be interested to hear if anyone has some good designs that aren't on the order of 100 lbs...

-Josh2

[Dave-O]

I've considered it myself, but it really depends on the class and the rules.

In my class, I'm not allowed much (if any) actual weight reduction methods, and no specified minimum weight. As such, every pound counts. And a cage is a good 75-100 lbs of balast. Then again, stiffening up the chassis is always a good thing.

I have some ideas for an extended roll bar design, that should help with stiffening the chassis, but it won't do anywhere near as much as a full blown cage.

I'd be interested to hear if anyone has some good designs that aren't on the order of 100 lbs...

-Josh2

Josh,
I'm in the same boat that you are. I would like to ad some more rigidity to the chassis but am hesitant to add any weight. I don't think you gain much (if anything) in rigidity using a full cage vs. a properly designed roll bar.

In your class you can have up to 8 mounting points and can weld them in. As best I can tell, I can have as many mounting points as I want it just has to bolt in. The rulebook give a lot of freedom in mounting plates in the car so I feel a "bolted" in roll-bar can add a lot of stiffness.

Here are a few ideas that i just whipped up.

The first one is the ideal 8-point mount.

The second one is a somewhat less intrusive 8 point mount.

The last one is a 6-point mount that is significantly less intrusive, yet ignores the weakpoint found by the front of the doors in our cars.

with the 8-point mounts you could further increase the rigidity by adding a bar across the top of the door (although it would be an exercise in flexability to get in the car) and/or a petty bar from the center of the main hoop to the passenger door.

[URY914]

David,
those are all good designs. On the third one, if you were to raise the mounting point of the bar at the front wheel you would stiffen that connection alot.