Roll Bar/Chassis Bracing Tubing

[ls240z]

I am trying to get my roll bar/chassis bracing plan finalized, here is what I have come up with and I'm looking for your scrutiny and suggestions. I took info and inspiration from 74_5.0L_Z's build and numerous other's, but I have some questions. This car is going to be a summer weekend toy that I'll take to the track and autocross from time to time. I believe I have it to spec in all NHRA, SCCA, and FIA Canada rulebooks. Due to Canadian autocross rules, the minimum diameter/thickness is 1.75" x .120". Thicker than I'd like, but more what can you do.

 

 

Red = 1.75" x .120"

Purple =  bolted in 1.75" x .120"

Blue = mounting/spreader plates

Yellow = tubing of which both diameter and thickness I am unsure of

Lime Green = tubing I am unsure of even using or the final configuration

Dark Green = gussets

 

My questions are:

 

1. I know the knee bar should all be bent in one plane to keep it stronger, but I'm wondering if by gusseting it to the firewall/lower cowl box area will keep it strong and increase chassis stiffness?

 

2. What size/thickness is preferred for additional chassis bracing not outlined in the rulebooks? Is larger/thinner or smaller/thicker better? Can anyone suggest a size?

 

3. For the bracing between the rear hoop stays, I'm not sure if I should run an "X", a "V", or nothing at all? What am I looking at in terms of visibility vs. stiffening/strengthening?

 

4. Door bars, do I use them? Will they tie in the chassis as a whole a lot better than just the knee bar and main hoop attached to the rockers themselves? I'm keeping a full interior, so this might not be an option in terms of access/egress and my size (6'2", 200 lbs). If no, should I just run a sill bar between the knee bar and main hoop?

 

5. Tubular seat mounts, I'm sure I've read they should attach to the cage (I'm assuming the sill bars). I'm also assuming they terminate at the trans tunnel, welded to a mounting/spreader plate, is that correct?

 

As this is my first build, I'm still wrapping my head around some of the finer points of a full on build. I am trying to be thorough and do the job correctly the first time, and I greatly appreciate your help with that. Cheers.

[JMortensen]

Check with a tech inspector and see if this is legal for SCCA. I'm sure it is for autocross, but I would think that it wouldn't be for road racing, since you have no A pillar bar. Most track day type stuff doesn't have any requirement for a roll bar at all, unless you're in a convertible.

 

1. Are you trying to keep the stock dash? If you are, I don't think your supports to the firewall are going to happen. If not, I'd run a straight dash bar (at dash height) and then I'd have a second set of thinner tubing to do the firewall support. This will give you better protection if you get T boned, and the extra weight of the two tubes to the firewall won't be a big deal. Make your minimum requirement cage out of the 1.75 x .120, then do everything else thinner/lighter if possible. Thinner and larger diameter will be stiffer than smaller diameter thicker wall. I did all of my extra stuff out of 1.625 x .060. Same thing with the rear strut bar. Use whatever thinner tubing you decide on.

 

2.1.625 x .060, or 1.5 x .090, something in that general area is what I'd suggest. I picked by going to www.onlinemetals.com and checking weight per foot. For your yellow tubes I ran 1" square x .065, and I ran a tube from the TC rods to the xmember to make it a K member as well.

 

3. Not a fan of the X in the rear stays. I think you used to be able to do that instead of a diag in the main hoop, but not anymore. It's weight up high. I really like an X from the rear struts to the bottom of the main hoop and used that in my cage, but it's a real PITA to weld in. Had to cut holes in the fenders and dog legs to do it.

 

4. Yes, door bars should help a lot. One problem Z's have is flexing around the firewall. You step on the brakes, and essentially the car wants to fold at the firewall. Jack the car up and support it under the TC rods, and you can SEE the front end sag down under teh weight of the motor (at least you could on my 70). By tying the front and rear structures together, you'll prevent that beam flexing. Without it, you might have a stiff structure in front and another in the back, but nothing substantial to hold them together. I would put the front end of the door bars as high as you can, don't do it 1/2 way down the tube. Make that node the same one that goes to the firewall too. Every time two tubes hit a third, you should try to have them hit the target tube in the same spot.

 

5. Not so sure about seat mounts. I welded to the subframe connectors that I added. Not really seeing the benefit of welding a big spreader plate to the trans tunnel which is very weak anyway. Would make a lot more sense if you could put a connector tube across the bottom of the car, but most Z's don't have that option because the exhaust is in the way. 

[ls240z]

Check with a tech inspector and see if this is legal for SCCA. I'm sure it is for autocross, but I would think that it wouldn't be for road racing, since you have no A pillar bar. Most track day type stuff doesn't have any requirement for a roll bar at all, unless you're in a convertible.

I figured I'd be missing out on road racing without the A pillar bars. Not a big issue for me, I'll have my hands full even without other vehicles on track with me. This setup works for autocross, and if I go to the drag strip, it will work too. I'll need the roll bar since I will be going faster than 11.49, but not faster than 10.00. I can use a roll bar instead of a cage at those times with an "unaltered firewall, floor and body." I'll have to find out their definition of "altered".

1. Are you trying to keep the stock dash? If you are, I don't think your supports to the firewall are going to happen. If not, I'd run a straight dash bar (at dash height) and then I'd have a second set of thinner tubing to do the firewall support. This will give you better protection if you get T boned, and the extra weight of the two tubes to the firewall won't be a big deal. Make your minimum requirement cage out of the 1.75 x .120, then do everything else thinner/lighter if possible. Thinner and larger diameter will be stiffer than smaller diameter thicker wall. I did all of my extra stuff out of 1.625 x .060. Same thing with the rear strut bar. Use whatever thinner tubing you decide on.

I am keeping the stock dash, so I'll have to have another look at what will fit. Would you say that as long as I manage to maneuver a bar under the dash it'll be a decent knee bar and better than nothing? 

2.1.625 x .060, or 1.5 x .090, something in that general area is what I'd suggest. I picked by going to www.onlinemetals.com and checking weight per foot. For your yellow tubes I ran 1" square x .065, and I ran a tube from the TC rods to the xmember to make it a K member as well.

Thanks for the recommendation. I am also going to look into your K member modification.

3. Not a fan of the X in the rear stays. I think you used to be able to do that instead of a diag in the main hoop, but not anymore. It's weight up high. I really like an X from the rear struts to the bottom of the main hoop and used that in my cage, but it's a real PITA to weld in. Had to cut holes in the fenders and dog legs to do it.

I was looking at your use of that X, and considered it if I didn't need anything between the rear stays. Since you're saying I don't, that's perfect in terms of visibility, especially when driving on the street. Another plus, I already have the "toolbox area" cut out and I'm 90% sure I'll have to be doing some work on the doglegs/ outer rockers regardless. 

4. Yes, door bars should help a lot. One problem Z's have is flexing around the firewall. You step on the brakes, and essentially the car wants to fold at the firewall. Jack the car up and support it under the TC rods, and you can SEE the front end sag down under teh weight of the motor (at least you could on my 70). By tying the front and rear structures together, you'll prevent that beam flexing. Without it, you might have a stiff structure in front and another in the back, but nothing substantial to hold them together. I would put the front end of the door bars as high as you can, don't do it 1/2 way down the tube. Make that node the same one that goes to the firewall too. Every time two tubes hit a third, you should try to have them hit the target tube in the same spot.

You have convinced me on the door bars, no sense leaving a major flex point between two structures. If running the style of door bars I've shown, is the sill bar still needed/recommended? I forgot to mention I'm installing Bad Dog frame rails and subframe connectors currently, if that makes any difference. I hear what you're saying about nodes, I'll definitely incorporate that. 

5. Not so sure about seat mounts. I welded to the subframe connectors that I added. Not really seeing the benefit of welding a big spreader plate to the trans tunnel which is very weak anyway. Would make a lot more sense if you could put a connector tube across the bottom of the car, but most Z's don't have that option because the exhaust is in the way. 

My reasoning behind the spreader plate (whether or not it's actually correct) was that the tubing would be less likely to tear the plate opposed to the thinner sheet metal on the tunnel. I suppose the tubular seat mounts johnc mentions in his list of chassis stiffening mods for street cars are of the kind you mention: solid tubing across the entire width of the car. I can't see that working, like you said, the exhaust (dual) will be in the way. Since that's the case, I'll likely go with what seems to be the norm on here: rectangle tubing that's cut on one side and welded to the floor.

 

I'll sketch another lovely paint jpeg incorporating these revisions. 

Edited January 31, 2015 by ls240z

[RebekahsZ]

Your door bars are out of spec for NHRA, which requires that the door bar pass between the shoulder and the elbow in the driving position. The NHRA door bar sucks for all other forms of racing. So, do what you want and see if you get away with it. In the meantime, I will be smacking my elbow every time I autocross the car, all because I have an NHRA legal door bar.

[JMortensen]

A knee bar will still help in a side impact. I'd run the door bars as far up the knee bar as you can.

 

The sill bar protects you from a low nosed car in a side impact, or if you go off the track or a road and hit a stump or a rock or something like that. Not strictly necessary, but a good idea.

[ls240z]

Your door bars are out of spec for NHRA, which requires that the door bar pass between the shoulder and the elbow in the driving position. The NHRA door bar sucks for all other forms of racing. So, do what you want and see if you get away with it. In the meantime, I will be smacking my elbow every time I autocross the car, all because I have an NHRA legal door bar.

Looking back at my picture, those bars are ridiculously low, I didn't realize it until you mentioned it Keith. I'm sure I could manage to set it up in a way to be NHRA legal and still have an ok time getting in and out. I was just looking for the opinion on that style (two bent, uncut door bars welded and gusseted).

 

A knee bar will still help in a side impact. I'd run the door bars as far up the knee bar as you can.

 

The sill bar protects you from a low nosed car in a side impact, or if you go off the track or a road and hit a stump or a rock or something like that. Not strictly necessary, but a good idea.

It makes sense, I just don't want to get caught up in the "tubing fever" and install more than I need. I'll likely add them, what's another ~20 lbs at this point?

[ls240z]

So here is version 2.0, hopefully utilizing your suggestions. Door bars are not accurate, only an approximation. They are subject to change to fit the door cards/be NHRA legal/be decent to climb over.

 

Red = 1.75"x.120"

Yellow = 1.625"x.060"

Edited February 1, 2015 by ls240z

[tube80z]

My personal thought is you'd be better off moving the cockpit front side of the cage to the firewall for you design.  Or another idea would be to add tubing that mimics the bracing of modern cars in the engine bay.  This would be similar to your cross tube in the cockpit.  Then you can connect the strut towers to this similar to most cage setups that go all the way to the towers.  And I'd add an inverted V that connected at the center of this bar and then went down to the engine bay frame rails.  you'd lose about an inch doing this of space behind the engine but I think it would be helpful.  You could also add doublers in the cowl that were either welded or structurally riveted. 

 

For a car with nice paint using rivets and modern structural epoxies might be an option.  More expensive than welding but probably less so than having to touch up the paint.  The plus would be not heating the metal enough to worry about oxidation starting.