Nova Chassis Bracing Ideas --Feedback Please

[JMarsa]

I'm looking for some feedback on some bracing ideas I have for my Nova. It's a cherry rust free 40K mile car. The car will see 1-2 track days per year and be my autocross beater. The pics below are not my car but it's the same model. My goals are:

1. A strong platform for the new subframe to do its business.

2. Reinforcement for the stresses from the relocated rear suspension.

3. Strengthen the body shell and tie everything together.

4. Add torsional rigidity.

I've got a HTH truck-arm kit like this (http://www.hotrodstohell.net/truckarm/truckarm_tri_fives/tri_fives_1.gif) for the rear and will be doing a custom subframe with the frame rails extending all the way to the front leaf spring mount. The extended rails will mount to the two stock mounts under the car and will use a modified & beefed up spring perches much like a bolt in sub frame connector would. The frame rails will be tied together with a middle cross member that will also be the truck-arms forward mount. There will also be a rear cross member in front of the axle with an integrated driveshaft loop. Behind the axle there will be some trunk floor bracing for the Panhard bar.

Under the hood will be tie bars running from the front frame rails to the firewall. My car does not have AC or power brakes so there will be more room than appears from the pics. There will be a bolt in cross bar to tie them together like a NASCAR chassis.

I'm contemplating the need for the 4 pt. cage. If I do it, it will have a cross bar and diagonal behind the front seats and also a cross bar in the trunk. I'm not sure if I need any of it though. The truck arms push the car from the middle cross member. I'm not sure how much the 4 pt. will aid the body in rigidity over and above my other bracing mods.

All comments are welcome.

--JMarsa

[MrQuick]

looks good. but you will not have to go out so far. Its better to have a tighter "base". front hoops at or before UCA mounts. rear hoops between axle and where rear leaf perch was... good plan so far.

[astroracer]

I have to go along with Vince and recommend keeping the cage "lever arms" short. Tieing the rear bar to the front will help with flex and don't tie the body into the cage. Doing that just transmits additional flex into the body which will possibly pop front and rear windows and keep doors and trunk lids from opening or closing.
Mark

[JMarsa]

I have to go along with Vince and recommend keeping the cage "lever arms" short. Tieing the rear bar to the front will help with flex and don't tie the body into the cage. Doing that just transmits additional flex into the body which will possibly pop front and rear windows and keep doors and trunk lids from opening or closing.
Mark


Thanks for the feedback guys. I'll shorten the legs front and rear. :icon996:

Mark,

I've got a mint interior and have no intentions of having door bars, a front hoop or cutting up the firewall. I guess I'm looking for the 2nd best way to meet my objectives given those constraints.

If I do th 4 pt. I was just going to weld it to the floor in the cabin and the trunk and not tie it to the sub-frame since I want it (the sub) to remain removable. I only want to go through the 4 pt. trouble if it really buys me the strength I'm looking for (my peviously posted objectives). Can it really have the opposite effect if i install it this way?

Regards,

--JMarsa

[parsonsj]

Well ... another option (not better, just different) is to keep those long cage "lever arms" and add some intermediate braces between them and frame rails. If you end up with something bracketing both front and rear axles, you can use those cage tubes to hang suspension components ... allowing you to have lots of suspension travel and really slam the car.

jp

[astroracer]

Thanks for the feedback guys. I'll shorten the legs front and rear. :icon996:

Mark,

I've got a mint interior and have no intentions of having door bars, a front hoop or cutting up the firewall. I guess I'm looking for the 2nd best way to meet my objectives given those constraints.

If I do th 4 pt. I was just going to weld it to the floor in the cabin and the trunk and not tie it to the sub-frame since I want it (the sub) to remain removable. I only want to go through the 4 pt. trouble if it really buys me the strength I'm looking for (my peviously posted objectives). Can it really have the opposite effect if i install it this way?

Regards,

--JMarsa

Beefing up the front sub and adding a roll bar in back, without tying them together, will gain you nothing except added weight. All it will do is create a "hot spot" right in the middle of the car that will focus all of the flex into that area.
It's like building a bridge with either end made from steel I beams and the middle from plywood sheets. Yea, the front and back will be strong with no flex but the structure is only as strong as the weakest link and your weakest point will be the floor (the plywood in my analogy) right behind the front seats.
Adding a cage and tying it and the front and rear subs together is really the only way to spread the load over the entire structure. It can be done without a major tearup to the interior if you take your time and fit the cage carefully.
Mark

[JMarsa]

Mark,

I might be way off track here. But I'll try to explain my logic. :hammer:

If I brace the frame through the use of cross-members and body mounts I'm using steel sections and the body shell to strengthen the frame.

If I install a cage that is triangulated via a diagonal cross brace it will tie together the left and right sides behind the seats and distribute stresses. It will also distrubute stresses from the rear downtube areas foward and vice versa.

I actually consider the area behind the seats a strong area given the amount of right angles in the sheet metal that joins the floor to the interior walls, inner and outer rockers, inner and outer wheel-houses to the quarters, and to the trunk floors where the front spring perches are. The trunk floor is actually double skinned because of the stamped frame rails that are welded to the bottom layer which has a void where the shock mounts are and then a second skin to cover that.

If I welded the cage directly to the frame I would not utilize the existing strength of the floor and the body structure. My idea ties the frame to the floor (which has the shell for strength) and the floor to the cage which in turn ties in the trunk area.

Am I way off base here?

--JMarsa

[astroracer]

https://static1.pt-content.com/images/noimg.gif
Crossmembers will help the lateral stiffness but they will do nothing to support the car fore and aft. I marked up your picture so you can see what I am referring to. Like I said in my previous post...
"Beefing up the front sub and adding a roll bar in back, without tying them together, will gain you nothing except added weight. All it will do is create a "hot spot" right in the middle of the car that will focus all of the flex into that area."
The hot spots will be in the area around the base of your cage and the base of the firewall. Adding bars to triangulate that gap is the only way you are going to supprt the chassis so it doesn't buckle at either of those points.
You are right when you say the floor of these cars is strong but, when you start strengthening other areas to the point where their strength is greater then the floor, it will fail, sooner or later...
Mark

[JMarsa]

Mark,

You know what they say about the worth of a pic...I misunderstood you. :pat: I thought you meant the "hot spots" would be between the rear wheel well and the trunk.
Thanks for your input.

I really don't want a caged car....I don't want the weight or tearing up the inside to install it.

--JMarsa

[farmer]

I would add a bar in back of the front wheel. you have added some stiffness to the middle of the car with the full frame rail the best way to add more stiffness is to add more depth to the frame if you run a door bar from about 6" up on your roll-bar forward and bending down to about seat height and forward to just inside the kick panel with a brace the the sub-frame you will add a lot of support to the middle of the car. It would be stiffer with the bars that astroracer suggested but you wouldn't have to have a full cage.
Russ Allison