Lets talk about chassis to roll cage gussets..

[bjhines]

I am trying to complete my cage install and send this thing off to paint.

 

I am just looking at some pics of cars that have significant chassis gusseting and I am wondering the value of this treatment and where it pays off most.

 

I have tried to be careful where I put the weight in my car. I am trying to keep weight off the front of the car and minimize the weight high in the car.

 

I have already gusseted the A-pillars and the dash bar. I am considering roof gussets front and rear. The roof gussets concern me because I have a cage installed in another 240Z that squeaks due to it shifting against the roof trim panel under load. I want to maximize chassis stiffness but I dont want to add a lot of top heavy weight.

 

 

Here is a pic of MY CAR with the front end gussets as it stands now.

I am considering extending the front roof gusset all the way across instead of just the middle section. I am also considering doing the same thing to the rear hoop(center or all the way across).

 

 

 

 

I have noticed that a lot of cars have A-pillar gussets but no roof gussets. I have seen cars with short tab type gussets instead of the punched sheet gussets that many use. I am providing some pics of gussets that I see in other builds. Some of these are Z cars and some are BMWs and others.

 

 

 

 

 

 

 

 

 

 

 

 

Here are some pics of a really complex Japanese 240Z buildup. These guys gusseted BOTH SIDES of the A-pillar tubes to the chassis. They also did some amazing things with sheetmetal in other parts of the car.

 

 

 

 

 

 

 

 

Here are some more examples of a Datsun with cage gussets.

 

 

[JMortensen]

I'm right at this stage too. Nice timing. I think that attaching to the A pillar does attach the cage to the roof for all intensive purposes.

 

That said my plan has been to attach the roll bar to the roof near where the hinges attach. I wasn't going to do a long strip with the punched and flanged holes, rather I was just going to do maybe a 3" section on both sides of the top of the hoop.

 

I don't know how much more benefit you get from running a 2' section of gusset rather than something like I'm planning at 6" long, but I'm guessing the difference in this part of the cage wouldn't be too great. You are after all just attaching to the roof, and we both have quite a bit of structure supporting the strut towers already, and thats where the major loads get introduced to the rest of the chassis (well there and the control arm bushing cups).

[260DET]

If the cage tubes are run very close to those parts of the car that you particularly wish to strengthen, then small lightweight tags can be used to bridge the gaps. Those holed strips are gross and unnecessary if the above is done, they also impede vision around the A pillars.

[bjhines]

I my particular installation the gussets are invisible from inside the car. In fact the exact position of the A-pillar tubes was carefully chosen to make them run inline with the factory pillars. They do not reduce visibility at all.

 

I have a hoop in another car that creaks and squeaks under load. I can stick my finger near the interface between the roof and the hoop and feel a significant amount of side-to-side movement between the hoop and the roof. In fact I would not cram my finger in between them while the car is moving. It is liable to get pinched if I hit a large bump.

 

I can still shift the hoop in relation to the roof with considerable effort by about 1/8" in the fully caged and partially gusseted race car. I can full well see the value of these gussets.

 

In relation to the types of gussets used; there are several types employed in my car and in the example pictures.

 

The punched strips are lightweight and have a great deal of strength in sheer. In fact, the Japanese 240Z has used them to encircle the roof. Assuming that the roof is stiff enough, that design will keep the upper cage tied to the roof in all the various ways it could move except up and down. I doubt that they will do much in a rollover which would compress them handily. They would however make it much harder to tear the roof off the car from above(for what it is worth). In this light they do not contribute much to safety, but they may do a lot for chassis stiffening.

 

The tab type gussets are often made of heavy guage material(>18g). They are often tied to structures that are formed using 22g or lighter steel. That is a problem with using small tabs of metal for gussets. They overload small areas of the chassis and form highly stressed areas that are subject to fatigue and work hardening. The strip type gussets have a significant advantage in that they tie long running lengths of metal together to better withstand sheer forces. The tab gussets have their place in designs that use them to tie heavier guage(or multiple layered) sections of the chassis to the cage. The tab gussets are often performing double duty as chassis gusset tabs AND mounting points for other parts of the interior equipment.

 

 

The other types of gussets are formed 3D gussets that are used to box certain areas(especially corners). These can take many forms. They are often bent using 16g medium thickness metal. They can also be formed using heavy guage material that is welded together to form a 3D piece.

 

This particular gusset is formed with relatively light guage material. It looks like another tube runs down that path and is simply "boxed in" using curved flat plates. BUT it is actually nothing but sheet metal. there is only one A-pillar tube, the rest is nothing but a carfully formed piece of sheetmetal.

 

 

 

...

[JMortensen]

The one roof gusset I've seen previously was a center gusset about 8" long. That car was squeaky before and totally silent after the installation of the 4 point bar and some reinforcement of the A pillars (one side actually broke off the fender after an autox). I thought that gusseting at the sides of the hoop would be more stable and still feel it's better than in the middle, and John Coffey suggested attaching to the area where the hinges go as that is the most reinforced area of the roof. I may be wrong, but I was going to make my gussets with .060" sheet, and just do a couple 3" strips on each end.

[bjhines]

I think John C. is right about the corners being the thickest areas.

 

It appears that the original chassis design has a roofline tube made of multiple-boxed pieces of sheetmetal running up the A-pillar >>> along the side of the roof >>> and part way down the rear sail.

The center roof sections across the front and across the back are not as thick or complex as the upper sides of the chassis.

 

There is a MAJOR joint in the rear sails a few inches below the top corners. This area is filled with LEAD. There is another major joint at the base of the A-pillars. this joint is also filled with lead. The front joint is especially prone to rust and is a weak area on most 240Zs. There are several pieces of heavy guage metal that do not overlap. Some even have gaps as wide as 1/2", leaving only a thin section of metal joining 2 major sections of the car.

This is 1960s rollover protection. The A-pillar and sides of the roof to just above the rear strut tower is a surpisingly ridgidly formed section of the car. The rear section is supported by steel uprights attached to the strut tower and inner wheel wells. The front section is attached to the strong firewall and cowl box.

Unfortunately the front attachment is through a small "foot" of metal splayed out from the A-pillar base. The other problem comes from the fact that the rear sail is realatively flimsy between the strut tower and the taillight clip. The back edge of the roof is very ridgid. The joint between the rear sail and the back of the roof tends to pop or crack, it sometimes shows through the lead filler and quickly rusts away.

 

I am reinforcing these areas as I gusset the cage. That is where a lot of consideration is needed.

[bjhines]

That brings up another thought.

 

For anyone who shaves the drip rail... That is exactly the part of the roof that is so well reinforced from the factory. That lip is a major part of the roof structure. Anyone who removes this edge without a roll cage installed is seriously compromizing the roof in a rollover accident.

 

I think you could remove the lip and cap the roof edge to regain the strength. The cap would have to be 18-16g to do the job. That is no easy task to form a 3D cap piece(channel) that wraps around the upper and lower sheetmetal pieces, running the length of the A-pillar and roof..

[260DET]

Running the main bars close to the A pillar for example only requires something simple and light to tag it full length if required, something like a length of square tube. Easy and strong all ways.

 

The other advantage of snug fitting bars is that they intrude less into cabin space, a safety issue.

 

This site has lots of pics that may be of interest

http://e30m3performance.com/tech_articles/cage_symposium/cage_symposium.htm

[Pop N Wood]

Anyone who removes this edge without a roll cage installed is seriously compromizing the roof in a rollover accident.

 

You always make such strong statements. Can you quantify your use of the word seriously?

[bjhines]

I am pretty sure that tack welding these edges back together is not gonna give much of the original strength back...

 

 

The modern race car cages dont have very many bends in them. most of the tubes are straight. unfortunately most of the nifty car pictures cages are illeagal for the events I am running the car in. I am sure some exceptions could be made but the designs are not what SCCA wants to see.

[johnc]

On a number of cars, where the ruels have allowed it, I've added gussets and tabs to connect parts of the cage to areas of the chassis to help stiffen the chassis. With a properly designed caged the gussets are not needed to increase the strenght and safety of the cage itself. These extra tabs and gussets are used as chassis stiffeners and that's why, in many cases, they are not allowed or are counted as an attachment point by sanctioning bodies.

 

Another place to use gussets is to help strengthen the tube to plate junction when the roll cage tube is not normal to the plate surface. The gusset should not extend beyond the plate and typically is no more the 3" or 4" on a side.

 

Here's an example of chassis stiffening via tabs on a vintage 240Z (I just added some bracing to a customer built cage to get it past the tech guys):

 

 

[JMortensen]

I am pretty sure that tack welding these edges back together is not gonna give much of the original strength back...

I've actually cut and disected this part of the roof in the back. Haven't taken it apart in the A pillar or the front of the roof, but in the back I actually had the inside of the structure exposed. From what I could tell it looked like there was a roughly triangular shape to the... uh... "roof rail" (?) and it roughly followed the outside of the roof line then bent in towards the car. This piece was thicker than the sheet metal on the roof or in the sill area, and the sill area was spot welded to it. What I cut off was sheet metal and didn't look nearly as structural as the part underneath. When I patched the hole I felt that it was attached better than when I cut it off, especially considering the rust that was going on in there.

 

In my opinion the drip rail is not really the main structural part of the roof. The main part is the channel or roof rail, and the drip rail is just a convenient way to make a water tight seam, instead of having to seam weld the whole roof.

 

There are some pics of the roof rail in this thread:

http://forums.hybridz.org/showthread.php?t=107679

[heavy85]

I can stick my finger near the interface between the roof and the hoop and feel a significant amount of side-to-side movement between the hoop and the roof. In fact I would not cram my finger in between them while the car is moving. It is liable to get pinched if I hit a large bump.

 

Isn't that because the cage was not tied to the upper part of the shell? You already have A-pillars tied to the cage so this will not happen ... still trying to rationalize in my head how tying the halo to the roof helps stiffness IF you already did that with the A-pillars. Please help me understand the logic here as maybe I'm missing something?

 

Cameron

[JMortensen]

Isn't that because the cage was not tied to the upper part of the shell? You already have A-pillars tied to the cage so this will not happen ... still trying to rationalize in my head how tying the halo to the roof helps stiffness IF you already did that with the A-pillars. Please help me understand the logic here as maybe I'm missing something?

I would figure that if you flexed the rear strut towers that would be more inclined to flex the B pillars and shift the rear of the roof around the main hoop. If the defection was in the front struts I would figure this would be more inclined to flex the A pillars around the hoop. How much any one gusset contributes to a reduction in flex is anyone's guess. I haven't seen any numbers, anyone else?

 

I get the feeling that a lot of this is boy racer stuff (no offense intended John but you do see this in every "tuner" magazine on the shelf), and a lot of the Japanese roll cages I've seen were purely for show, but there are enough real race cars using these holed gussets that I have to believe that there is some merit to it. The question is: At what point does it become overkill?

[bjhines]

JM.. That is exactly what I started this thread for...

 

I realize that many of the cages the real race car builders on this site have fabricated DO NOT allow extra gusseting to the chassis. Most of the grass-roots style race cars are trying to keep the cage as a safety item ONLY. The whole idea behind Improved Touring is that the cars perform much the same as their street legal counterparts. A heavily gusseted chassis would not be anything like the street version of the same car.

 

It is pretty easy to add a lot of weight to little gain. The gussets we are discussing are high in the car and are putting weight where we least want it.

 

I noticed right off the bat that the front windshield upper lip is very weak... just leaning on the front edge of the roof causes it to bend and flex. the sides of the roof are strong(relatively speaking). The rear edge of the roof has a fairly substantial crossbar in it.

 

 

 

Another thought about this hot rod cage design is that we might be able to remove a lot of structure surrounding the roof and rely completely on the roll cage to support the car in a rollover. hell.. remove all but the outermost layer of roof metal and even that could be replaced with a carbon fiber roof cap. I bet I could pull 25lbs of roof metal off the top of the car and still have a roof shell...

Take a look at the major U-channel support that goes from the top of the rear strut towers to the front of the rear sail. That sucker was made to hold up the entire car.. It is dead weight with a strong roll cage in the car.

[heavy85]

I understand tying the cage to the A-pillars as its convenient and actually strengthens the downtube (good for safety and stiffness) but I'm still confused as to the front roof attachment. Thinking about what load cases you are trying to cover. Look at the ROUGH sketch below which is suppose to be a front view of the windshield opening and let me see if I can explain my thoughts. The top solid line is the roof, the dashed line under it is the halo, the verticals are the A-pillar / front downtubes acting as one member since you connected them, and there are corner gussets at the top.

 

EDIT: This didn't work out as it dropped the spaces in the middle but just picture all the right side vertical pieces lining up.

__________

l-----------l

l/ l

l l

l ________ l

 

My assumption is the load cases into the roof/halo in the view are 1) compression as in braking or 2) parallelograming as during cornering. You basically have two independent top members (roof and halo) which unless you 1) are into yielding (buckling case) tying them together would not provide much benefit and 2) the strength for parallelograming mostly comes from the corner gusseting and bending modulus of the structure which connecting the two I dont believe would provide much benefit either. So what input loads would benefit from attaching to the roof here? Maybe minor benefit but I'm not sure it would be measurable.

 

I understand attaching to the hinge area in the rear because there is not a real convenient way to attach to the B-pillars and the hinge area is already reinforced.

 

In the end it's not much steel but I try to think about what and there the input loads are and plan to react those loads with the bracing. Maybe there's more to the front roof than I realize ... maybe

 

Cameron

[bjhines]

I agree with your point about useless material high in the car.

 

One concern I had was that my front roof tube is bent forwards in the middle. It is situated in such a way that it is cantilevered forward of the upper corners of the A-pillar bends. The car's front roof lip is not that strong either. But I thought that connecting them across about 12" in the front might lend more strength to the front roof structure as a whole.

 

Another concern I have is overstressing the welds between certain sections of the car.

 

 

Here is an overhead rough sketch of the roof bars and gussets.

[S15 200sx owner]

Here are some pics of a really complex Japanese 240Z buildup. These guys gusseted BOTH SIDES of the A-pillar tubes to the chassis. They also did some amazing things with sheetmetal in other parts of the car.

 

 

 

 

 

 

Hey bjhines,

 

Do you have any other info pictures or links for this Jap 240z. It looks really trick, and i would love to see more detail of it.

[drmiller100]

Roll over protection and side impact are why you are "supposed" to have cages.

Real world says you can stiffen the car at the same time.

It seems to me the real goal when stiffening the car is to allow the suspension to work correctly. And, the thing the car needs the most help with is Twist between the upper strut towers, front and back. And, it seems to me the hardest one to stiffen is probably the tops of the front strut towers.

 

So how do you guys tie the top of the front strut tower to the top of the back strut tower?

[bjhines]

Ummm...

 

There are a lot of methods for doing this and different members have taken different approaches.

 

The most common approach is to use a triangle in the engine bay from tower to tower and diagonals to the middle of the firewall or the dash-bar.

 

Others have also used the door-X-bars and tubes running to the tops of the strut towers through the firewall and back to the rear towers.

 

The petty-bar and through-firewall center supports have also been used.

 

Other issues have come up that the floors and tunnel need a lot of reinforcement to keep the driver's legs from being pinned in a side impact.