P.Po

Gentlefolk,

I notice a lot of race cars (track and rally) these days have large gussets attaching the A-pliiar to the rollcage.

Why do they build the cages like this?

In pure race cars, like in Grand-Am, Speed Vision WC, and WRC rally, why wouldn't the factories simply build the cage right up against the A-pillar? Is it simply a strenght/saftey issue? Or does it make the car stiffer. To me you are also cutting out a good portion of the vision required to drive a race car....

Here is a link to what I am talking about ->
http://www.turnermotorsport.com/html...4_sears_17.jpg

Any suggestions would be appreciated.

regards,
patrick

James Achard

By moving the tube away and adding a gusset, you have effectively created a beam which is stronger. The larger the outside of the beam is, the greater the rigidity. There are probably some rules involved too in regard to removing interior/placement of cage parts that I don't know about.

Cheers, James

RedlineMan

Hey;

Most of us in the P-car world do not have the option of creating such beam structures because we simply do not have the interior space to work with. Too bad, because it is an incredibly easy way to massively increase the strength of the unibody without adding much weight!

M758

Note that some class rules limit the number and size of attachment points of a cage to the chassis. Thus not allowing such bracing.

P.Po

I don't see many cages like that in local and PCA racing. I would really like to study one of these fine gusseted cages up close. But John makes a good point... space is certainly an issue.

James, the beam comment really made a lot of sense. I wonder what the trade off is considering the driver is potentially losing vision in a lot of corners. If the car is stiffer, it handles better (generally speaking), but if you can't see or concentrate on where you are going does it defeat the value of the design?

Does anybody have any experience with a car like this?

How much does the added body stiffness (relative to an already well desgined caged car) really help?

Thanks for your responses! I learn way more theory then I need from you guys!!

-Patrick

Wormhole

I’ve always wondered the same thing. I understand it stiffens the frame, but how is it any stronger then mounting the cage flush with the pillar and then weld the two together?

RedlineMan

Pretty simple, John;

The height of a beam has great bearing on its strength. Its a cross section thing. If you took two tubes and welded them together, they would have far less strength than a beam constructed of the same type tubes with a sheetmetal web in between. The load that the beam structure of greater cross sectional height could carry would be far higher.

This can also allow designers to reduce the weight of beam structures by using thinner materials, but keeping the same load rating. Look at modern manufactured floor joists as an example. They are generally 2x2 wood on top and bottom with a waferboard center web glued in between. They can carry a much higher load than a regular 2x8(10, 12), but weigh far less.

The windshield pillar area is generally one of the weakest parts of a chassis, both torsionally and in compression. These beam structures increase both aspects greatly. If only all cars had enough interior space to utilize this, you would see a lot more of it.

Geo

While I've never sat in a car with such gussets, I would have to assume the builders were smart enough to place the gussets in the same line of sight with the cage tube. In that case it would have zero impact on visibility.

Another reason I could see doing this is to support an otherwise unsupported bend in the tube. For instance, if the upper forward tube of an X does not tie into the bend on the front down tube, you have a weak point at the bend. Install a huge, but low mass gusset and you've now supported the bend.

Adam Richman

I'd be surprised if that lower tube is the actual A-pillar tube. IIRC, there were several ALMS GTS cars that implemented a downward tube that went (straight in their case) from the horizontal top bar to the horizontal lower bar at an angle less than the A-pillar bar. Now perhaps this _is_ the A-pillar bar but I just can't imagine putting it in that shallow without a secondary A-pillar tube closer to the actual A-pillar.

One of the best reasons I can think to gusset the A-pillar (or B-pillar) tube to the shell is that it makes it significantly easier to make the bars to begin with. Although it may seem simple to bend tubes for a cage, bending and fitting them precisely enough to stitch weld the A-pillar bar to the A-pillar seems like a very tedious task (cutting cardboard cutouts of gussets is a whole lot easier and a whole lot less wasteful).

Also, it strikes me that having two sides welded to the A-pillar is more contact that one side (plane) ... but then again, thin sheet metal stitched to it w/ nothing to make it more rigid seems _less_ effective than welding the tube to the A-pillar. It is my understanding that is the precise reason for the punches (the ones that indent the sheet metal). When we made my gussets, I was told to try bending one that had a punch and one that didn't - the latter is notably more flimsy. So I am not certain that simply welding two sheets of metal from an A-pillar downtube to the A-pillar would do you that much as far as extending the rigidity.

In our case (haha, the we's = my cage builder's), we used both same-sized tube gussets in some places, the taco shaped sheet metal punched gussets elsewhere. The former were a lot more practical where getting TIG/MIG access to an entire side (or majority) of the taco (sorry, just easier to call it that) wasn't feasible. As for how much this sort of cagework adds to the car's overall rigidity, I can't say. We tried our best to look at it from the perspective of suring up any place that couldn't be triangulated otherwise (windshield rectangle, under dash rectangle, door openings - perhaps the thinking in the BMW ???). I won't argue with anyone saying that the ones on the passenger door X are doing much at all for intrusion but they do look pretty pimp :P

There are other types of gussets that are implemented - I personally question their value for added protection. I am sure that the sheet metal shim type gussets and small OD tube gussets do something for better suspension tunability but I don't know what they do for protecting the driver additionally. The thought of the small OD tubes for gussets to me looks like ready made hole punches for your cage or worse yet, free floating spokes if they don't snap and the cage deforms. I am somewhat curious as to how the shim types do much for protection as well but that's just me being a skeptic. Someone is now producing a large OD tube to small OD tube gusset - you have to give them your length requirements and I'd be all over these too as I think they definately would aleviate my concerns about some OD tubes.

At the end of the day, nothing I am saying should be confused for an expert opinion, its not. Its about safety and I get antsy saying what's right and what's not - just too freaking dangerous out there.

BTW Geo, it was nice meeting you at RA. Did they manage to get you that ride around?