E28 bmw tt1 roll cage

[Scott in VA]

This is my first car and has progressed over the years. I have taken on many large projects, this will be the largest I have done though. I'm looking to finally do a proper cage/chassis reinforment in this car to do track days as well as compete in nasa tt1 (looks like where it will fit in) 2750lbs no driver 500hp. I don't have big concerns about competitiveness, as this is for fun, and I enjoy the build. I know there are many people on here much more knowledgeable than myself when it comes to roll cage design, and I want this topic to focus on mostly that.

The biggest concern I have is the main hoop being so far back, and how to do the forward sections properly to account for this. As well as were to draw the line between weight/chassis rigidity and cage safety. I plan to tie in teh cage to teh rear shock tower as it's location is so convenient, and I plan to tie the cage into the front strut tower as well. I'm an experienced fabricator but a complete novice to cage design, beyond pics i've seen on the internet. I purchased a JD squared model 3 pipe bender, and will be using the 1.5" .120 wall tubing for the necessary cage pipes. I'll include the pictures of the interior currently, and if there is good info and such I will keep it all updated as I go along. The seat and shifter have been moved back approx. 1 ft to improve weight distribution and help give more pedal room (i'm 6'4") which I never had enough of previously.

https://www.google.com/url?sa=i&rct=...87638513914756
This car while having an extensive roll cage may be helpful, in the fact that the x brace and then the bar that ties the front hoop to the forward plinth box may be a good idea since my main hoop is going to be so far away from the front plinth.

Does anybody have a questimation of the amount of total tubing I would need to purchase, i'd rather have some extra than run out!

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[boostedinaz]

As a former cage builder (IG@boostedinaz) I don't like the minimalist approach. A lot of people get worried about weight but I would much rather have a stout and rigid chassis than save a few pounds. Don't think of a cage as just a piece of safety equipment think of it as the foundation for your chassis and your build. Since you have the car gutted now is the time to really do what you want. Front shock towers are a good call but I can tell you they will be a big PITA. I would also try and tie in rear subframe mounts as well. Now don't go crazy with tubes but more is better. I see a lot of "drift shops" do all kinds of stupid things just because they can. Every tube should have a specific role and reason for being in the car. Also, remember to make nodes with the tubes and make sure all tubes have a clean and clear load path. I am not familiar with the E28 chassis but these examples still apply.

Here is an E36 I did a few years ago. This car had a pretty extensive cage. Some people have said it is overkill but I disagree. As I said every tube has a purpose and a smooth load path. The rear subframe and front shock towers were tied into the cage. Since this car was going with coilovers we tied all the rear cage into the shock mount.


Here is an E30 with a similar cage. Again a lot of tubes but all with a purpose. The rear subframe and diff mount was tied in along with the front shock towers. You will notice that each car has an "FIA" bar. You mentioned that being a sedan the main hoop will be set back a good distance. That in turn make for a pretty long A pillar tube. The "headache" bar "FIA" are fantastic at supporting the A pillar. If you can both of those bars should land in the bend in the A pillar bar to help support and strengthen those bends.





Load paths are VERY important and also VERY over looked. When I browse Instagram I see all kids of "professional cage builders" with terrible habits. The biggest offense is misalignment of tubes and loads paths. This of it this way, if you are driving what is the best way to get across the street? Drive straight through or make a left then a right? Some thing with chassis loads.

This cage is dropped in order to weld 360* around each mandatory bar. You can also see that the top X and the short support tubes line up to help transfer the load. The second pic shows an FIA bar, door bars, and front shock towers all aligned.




No matter what you TAKE YOUR TIME. I can't stress this enough. If you've never built a cage it is going to take you awhile to get things to work the way you want them to. Take that time. It will help you learn a new skill but it will also make the quality of your work stand out. A quality cage doesn't just look good, it makes a race car more valuable, stiffens the chassis, and could safe your life.

[Scott in VA]

Amazing pics and information, I really appreciate that! When dropping the cage, I see the use of a plinth box in one of the vehicles, whereas in the e36 you did its simply welded to the body with supporting plates, to drop the cage did you simply put a hole in the floor first? Is there advantages/disadvantages to each way of roll cage connection.

I'm going to start planning the cage as well as get material. What amount of roll bar do you suggest I purchase, for exp like 60 feet of tubing. Should I buy some thinner tube for non critical tubes? like .080? I'll have to call around and try to find somewhere to purchase the tube.

I found a picture while searching for cage design stuff, I think this was interesting because its a very similar chassis and even seat position.
Larry_Chen_RED_e34_gatebil-22.jpg

[boostedinaz]

Amazing pics and information, I really appreciate that! When dropping the cage, I see the use of a plinth box in one of the vehicles, whereas in the e36 you did its simply welded to the body with supporting plates, to drop the cage did you simply put a hole in the floor first? Is there advantages/disadvantages to each way of roll cage connection.

I'm going to start planning the cage as well as get material. What amount of roll bar do you suggest I purchase, for exp like 60 feet of tubing. Should I buy some thinner tube for non critical tubes? like .080? I'll have to call around and try to find somewhere to purchase the tube.

Good observation. When we would do base plates without boxes we would double base plate the bottom of the main hoop and A pillars. What we would do is make the normal baseplates and they would be final welded in. Then I would drill hole (usually 2" for 1-3/4" tubing) in the places I would want the tubes to land. Then a second piece of plate was tacked over the hole. I would then build the cage on that second plate. Once it was time to drop the cage Id break the tacks for the second plate and slide it out of the way, then the cage drops. After the cage was fully welded it would get pushed up and the second plates would go back in and get final welding.

The reason some of our cages had boxes was because of what was under the car. Just because I wanted a tube to go through the car at some point doesnt mean it was possible or easy. With boxes you only have to worry about welding in a specific area VS drilling a good size hole. With boxes you have to be careful because there are rules for them, usually they can not be over 100 sq/in in total area.

Before you buy or plan anything read over the rule book for you sanction body and your class. If you dont understand it ask someone. I can not stress this enough. It will determine tubing sizes, what you can and cant do cage wise, and if you car passes tech.

Tubing size usually depends on car weight. I wanna say in NASA 3200lbs and under you can use 1-3/4" .095, however, if the car weighs more than that the min goes up to .120 wall. The rule book will also show you a diagram that explains the minimum bars required also called mandatory bars. These mandatory bars have to be the size and thickness required. Non mandatory bars, like subframe tie ins, headache bats etc.. can be any size and thickness you want. Most the time when I would do non mandatory bars they would be 1-1/2 .095.

As for tube lengths its hard to say because it depends on how far you go with it, plus mistakes. I would say 100 feet of 1-3/4 DOM and maybe 40ft of 1-1/2 DOM would be a good start.

[Scott in VA]

1.500” x 0.120” Seamless Alloy (4130), Seamless mild steel (CDS Mechanical), DOM, or Docol R8 (only)
1.750” x 0.095” Seamless Alloy (4130), Seamless mild steel (CDS Mechanical), DOM, or Docol R8 (only)

I purchased the die for the 1.5" x .120 as I was originally told would be the better choice of the two.


I have been reading through the rules and maybe i'm being slow but this makes little sense to me.

15.6.17 Additional ReinforcementAny number of additional reinforcing bars are permitted within the structure of the cage provided that they areinstalled strictly for safety and do not violate CCR Section #15.6.2 Intent. This rule does not permitreinforcements in classes with spec cages.
15.6.2 IntentChassis stiffening is a side benefit of a good roll cage system, but it is not the intent of these rules. Parts of thecage deemed by the Chief Scrutineer, to serve no practical purpose other than chassis stiffening may beconsidered in violation of the intent of these rules.

Which makes little sense because where do they draw the line for that? I have looked at cages for Nasa and see dimple died plates all over, which are debatable to help safety.

[boostedinaz]

1.500” x 0.120” Seamless Alloy (4130), Seamless mild steel (CDS Mechanical), DOM, or Docol R8 (only)
1.750” x 0.095” Seamless Alloy (4130), Seamless mild steel (CDS Mechanical), DOM, or Docol R8 (only)

I purchased the die for the 1.5" x .120 as I was originally told would be the better choice of the two.


I have been reading through the rules and maybe i'm being slow but this makes little sense to me.

15.6.17 Additional ReinforcementAny number of additional reinforcing bars are permitted within the structure of the cage provided that they areinstalled strictly for safety and do not violate CCR Section #15.6.2 Intent. This rule does not permitreinforcements in classes with spec cages.
15.6.2 IntentChassis stiffening is a side benefit of a good roll cage system, but it is not the intent of these rules. Parts of thecage deemed by the Chief Scrutineer, to serve no practical purpose other than chassis stiffening may beconsidered in violation of the intent of these rules.

Which makes little sense because where do they draw the line for that? I have looked at cages for Nasa and see dimple died plates all over, which are debatable to help safety.

I've always built cages using 1-3/4" .095 as the mandatory bars. It's the way I was taught and I can't think of and significant reason to like one over the other, however, I would look at other classes and even other sanctionbodies rule books to make sure they will allow the smaller thicker tube. If you ever decide to change classes or race somewhere else you still want the cage to be legal.

From reading those rules it seems they are talking about spec classes and their limitations. As an example both the E30 and E36 cages I posted would not be legal in Spec E30 or Spec E36. Since both cars have extra tubing like shock tower tie ins, rear subframe ties in that dont add to the safety of the car they are not legal. Those cars were built for the GTS class and their rules basically say you can do whatever you want cage wise, so I did.

I also don't like dimple die plates, but they do have their place. If you make the cage fit the contours of the chassis really well you don't need dimple die plates. With fitment like that you just weld the tubes directly to the chassis. This does require a LOT of work because now you're getting into 3 and 4 bend A pillar tubes. If you look at my IG this time last year I did a Honda fit cage and the A pillars were a very odd shape but they fit really well.

[nik77356]

I actually have a caged E28 that I am building to race in the NASA ST classes. I don't have any pictures with me right now to post, but I will try to post them tonight or tomorrow, along with how I got around the challenge of the main hoop being so far back. Very cool to see another one out there!

[Scott in VA]

Thanks for the input guys, and i'll check out some of the other racing bodies rules as well. (scca and nasa show 1.5" .120 tubing so i'm good there) I'm focusing right now on the main hoop area of the design and the possible difficulties with it. So the biggest issue I can see is once again how far back the main hoop is. Does the main hoop have to be vertical? If so what would you consider to be the minimum distance from the seat itself? Can I run it darn near against the seat, and run a bar that kind of loops back purely for the harnesses? The seat is approx 2 ft from the rear shock tower, if the main hoop was 3 inches from the seat, the bar going from the top of the main hoop to the rear shock would be at a steep 50 degree angle. Is this an issue? I tried to take better pics to focus on this one part.


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[Scott in VA]

7.2 NASA CCR Section 11, 15 and 18 ExceptionsAll of the rules listed in the NASA CCR Section 11 and 18 will apply, except, the followingrules will supersede those in the CCR:CCR 15.6—Roll cages may be built to provide an unlimited amount of chassis stiffening.Any number of cage mounting points may be used above the six (6) minimumrequirement, and, any number of additional tubes may be used above the minimum withadditional attachment points to the body, including tubes that penetrate the firewall.

So no worries with cage design for doing time trial, go the tubing bender mostly setup.unnamed.jpg

[Scott in VA]

Here is an album of current pics. Got the bender all setup with the hydraulic ram. I got the metal, reinforced the rear strut towers, bent the main hoops and held it in place with magnets. I think i'll bring the forward hoop mounting place just in front of the door line as you can see me holding a piece of pipe in one of the photos. So it doesn't interfere with the dash mounting and hood opening, as well as the door harness there. http://s271.photobucket.com/user/Som...ry/roll%20cage

[XXLBIMMER]

I would try to move the A pillar upright as far forward as possible. Still being in the door opening, also plan out where the dash bar is easiest to run. It sucks to get everything tacked and fitted and then there is no easy way to go across the base of the dash.

[Scott in VA]

Thankyou for the reply. I will move it as forward as possible but still have the wires able to make up the pillar. If I were to put the pillar uprights as you were saying, i would need to bend the bar to put the dash bar where its needed. Is this an issue, do you try to keep the dash board bar straight or is a little angle on either to push it towards the firewall acceptable?

[XXLBIMMER]

It's not really an issue. But, it is considerably stronger with no bend in it. Picture getting t-boned or sliding sideways into a barrier/wall. If the bar remains straight the distance between both A pillars( read: where your legs are) remains the same. If it needs to be bent try to keep it minimized. Also keep it as high as possible, your shins are down there and you don't have anything keeping your legs from hitting that bar. Lots of FIA padding ( NOT pool noodles!!!!) I try to incorporate a foot rest into the tube from the A pillar to the firewall. I'm not a fan of the boxes at the base either as it can become an ankle breaker in a wreck.

Thankyou for the reply. I will move it as forward as possible but still have the wires able to make up the pillar. If I were to put the pillar uprights as you were saying, i would need to bend the bar to put the dash bar where its needed. Is this an issue, do you try to keep the dash board bar straight or is a little angle on either to push it towards the firewall acceptable?

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[aeronaut]

Subscribed for the learning opportunity.

[Novablue454]

This is an excellent thread!

[boostedinaz]

I also agree move those A pillar tubes as far forward and as far out as possible. I would move it forward and up onto that little ledge on the outer sill. You can make a box thats top will sit on the sill and come down to the floor pan as well. This is gonna help push the tube forward and outward. This will help with interior space but also allow you to weld the cage to the shell better if you can get the fit correct.

[Scott in VA]

So I made the forward hoops, and the bar at the top of the windshield, i'll include some pics. So far things are going okay, and it's coming together decent. I made some mistakes with the offset angle at first, but now I think its decent.