fatbillybob

Guys,

Do you have opinions on ways to tie cages into shock towers. I have seen metal plates welded to the top of the spring perches and then the cage welded to the metal plate. I have seen rollcage tubing dead ending on sheet metal of the shock tower with no apparent structural integrity, i.e. additional metal plate to thicken the tower area. I have seen criss cross rollcage tubes weled like a truss to the thin sheet metal of the shock towers. Any opinions?

RedlineMan

Sky's the Limit, man;

You can do it 6 ways from Sunday. PM me if you are really getting serious here.

Adam Richman

FBB, my opinion is that the how is less important as the why. I wouldn't get overly concerned w/ tying into a shock tower if the suspension location point is not being aided by it. Strut tower location (if allowable per rules) makes a whole lot of sense to me for locating cage tubes. But as for the how, I can't visualize a solution that doesn't use a backing plate in one form or another (but John's right, there are myriad ways you could do this and nothing is necessarily right or wrong - execution seems to be a big determinant on that). Either way, I would have serious concerns for any solution that tied directly into sheet metal w/out backing.

RJay

Here's how mine were done (I dunno, I'm an idiot, but they at least look good to me):

Front tower, pretty much finished:


Rear tower prior to weld up:

fatbillybob

Well...maybe I did not ask the right question because I really did not know how to ask it; and I still do not but will try. Are there cage builder rules to how you build a gage beyond the many who say things like, "I've been building rollcages for 20 years" (so believe me) type statements. Is there any real diagnosis and treatment planing of a chassis? For example, lets say you lift your car from the right front with a jack and you can see body panel gaps change shape or put a targa top car on a 4 point lift and watch the gap of the targa top open when you lift it. What do changes in specific body gaps (like the targa top) have to say about the structure of your car other than it is a noodle and we need to do somwthing about it with a properly fabricated rollcage? Is the standard 4 point interior box cage (+ knee bar, +door bar,+overhead cross bar) tied to the frame exterior surronds (basically left front foot area, right front foot area, left rear butt cheek area, passenger right rear butt cheek area) with 4 more points ending in each shock tower just always "good enough"? Are all cars internal structure basically so similar that the typical cage you see all you need or do you keep adding bars until you see no body panel gaps change upon lifting each corner? Leave sanctioning rules out of the discussion this is just a general question. O.K. that is alot of crap coming out of my head I understand. Can anyone make any sence of my sputtering?

RedlineMan

RJay -

FWIW - Blowbag comments from a jaundiced-fabricator-hardass-know-it-all with no formal training of any kind:

- Not particularly space efficient (a personal bugaboo of mine).
- Not necessarily DIRECTLY reinforcing the "vectors of force" that the suspension generates.
- Does not appear to offer a lot of multi-angle, torsional rigidity to the mounts.
- it certainly will be FAR stiffer than stock.
- Much cheaper to build (??) than all the stuff that runs through MY head, which generally includes nesting tubes into existing sheetmetal structures.
- Very nice welds... better than mine! Course, I'm too lazy to TIG, and MIG is so much faster, but "sloppier."



I would URGE you to have them plate the tops of those rear shock towers if they didn't already. Heavier springs, and even stiffer damping, will blow the tops right off!

James Achard

Billy Bob,
There are several ways in which one can asses a certain chassis and build a proper cage to deal with the inherent issues associated with the car in question. Your example of a Targa is a good one from a problem solving standpoint. When I build a cage I use a CAD program to help me decide where I need to add strength and where I don't(see picture below). I can also perform FEA on the model to see how different changes will affect the stiffness and strenght of a cage.

This is just one of the tools available to a fabricator during the design process. Previous data on a car and studying actual crashed cars go along way in helping one make the right decision when building a cage.


Cheers, James

RedlineMan

FBB -

First you have to formalize your intent. If you just want a little roll protection for DE, "any" cage will do. If you want speed protection, then you need more. If you want a forgiving chassis, leave it alone. If you want a sharp instrument, start triangulating and tying in. If you intend to run VERY stiff spring rates, you need some real torsional rigidity to have any repeatable tuning success. This means some full three-angle triangulation of the suspension points into the cage.

What car? A 944 is a fairly stiff chassis, but does benefit from stiffening in the more extreme cases. A pre 964 911 is decidedly NOT. A 964 is pretty decent, a 993 far stiffer yet, and a 996 is borderline exceptional.

If you are developing a pre 964 911 chassis, for example, and want to run stiff springs, you WILL have to stiffen things up dramatically. You will only get a very limited amount of tune-ability out of such a chassis before the changes simply cause chassis flex instead of dynamic handling change. Been there!

Plating mount points is generally a good idea. A strut tower is a fairly robust piece to start with, and if you will be tying in multiple tubes that will spread forces far and wide, plating is not a necessity (although I'd probably do it anyway). A rocker sill may or may not be more than one layer of sheetmetal in a given spot. Either way I would probably plate them anyway.



I have no idea if I'm scratching any of your itches here, but it's a start.

RedlineMan

Oh sure, James... be that way;

Can I post a picture of the seat of my pants to illustrate MY vast engineering capability?

Corvette?

RJay

Unfortunately, I don't have any photos of the finished rears, the picture is from an interim state, I believe the top was grabbed and more metal added. I'll make sure I check into it. Thanks for the advice!

OTOH, perhaps I should take extreme offense at your remarks and call you out to a track challenge. That way you have to go to LRP next month or risk public humiliation

poorb0yw

LOL !!

fatbillybob

Yes you guys are scratching my itch. You see I am better at car improvement than driver improvement. Quite frankly either my car is slow or I am slow not sure which. How much cage is enough is a good question. Cages add weight but stiffness makes suspension parts work as intended. I drive a wet noodle. It is a tired 348 targa Ferrari with 300hp at the crank when new and now gutted out with a cheap Ferrari bolt in cage with total weight of 2685lb dry. I would buy a Porsche it is a better race car but once you have a beat racer ferrari no one will buy it...so I'm stuck with it. This car is a noodle and nobody has any real experiance racing them. So I am after the lightest stiffest magic rollcage. I run unsanctioned multi-marque club race group events about 6 times a year so there are no rules. So my problems are that Ferraris work best with stiff front springs and soft rears dispite the 40/60 rear weight bias. The front shock tower is about as thin as a potato chip bag. It flexes. For any of you who have jacked a BMW m3 from the front corner knows how stiff those cars are stock! Well my Ferrari is the reverse of that. Think 1980's technology. Then even balanced on 4 points on my lift the targa top opens up upon lifting. Add to that the rear frame is a bolted subassembly for easy engine removal. That unit flexes too.

John...I love your photos. Pictures are worth 1000 words. You have some very severe tie in points and I get the point. I like it. Do you have an average metal plate thickness you like to use to beaf up a shocktower? 1/16" plate 1/8"? more???

James...is there a way we could work together using FEA to come up with something more than a seat of the pants design? What type of data do you need? There is zero data from Ferrari, and no one has ever seriously raced one of these cars. There is no crash data for one of these either. I think I can design a stiff seat of the pants unit but the key is to use the least number of tubes.

I love the pictures keep them coming.

John on a similar note do you 911 guys have problems with upseting the car's weight balance during a race by putting the fuel cell in the front bonnet? My fuel tank is right behind my seat and right at the CG. So I fear removing the stock tank and moving a fuel cell to the front bonnet 911 style. Nowever, if my stock tank was gone then it would be easier and better to triangulate off my main rollbar to the rear subassembly and could achieve better initial weight balance which would change during the corse of a race. Life is a trade off what do you think? 348s are alot like 308's that more people are familar with. 348's are just a bit stiffer and more horsepower and generationally better like '75 911s is to 911SC.

RedlineMan

Actually, I like 3/16 if the surface is such that this thickness can be formed to fit it. And BIG plates too, not just an inch bigger than the tube. Spread the load far and wide, don't you know?

It would quite depend from whose pants said design sprang, eh?

Well, I've never OWNED a 911, but you are making a valid point in any event. We never really had too much trouble with this in the PCA GT3S 911 in reality.

I can say that the type of move you mention works wonders in a 914, and that is a car more dynamically similar to your Pastawagen than a 911. The usual problem with mid-engined cars is getting them to turn in. No weight up front usually means huge understeer. Simply moving the battery in a 914 shifts 2-3% of the total weight, which is not insignificant. Therefore, I would not necessarily assume moving the cell to under the front bonnet would be detrimental.

Incidentally, while not on the track, I DO have some minor Ferrari experience.