which is a better place for the rear mounts?

 

I don't know about legality for drag racing, but both of those roll bars are not legal for SCCA Time Trials events. You cannot have curves (bends) in the rear down braces. Also, neither bar has a diagonal across the rear hoop. I only mention this because I see a cone in the middle photo of your car; I am assuming you want to run more than just drag racing.

 

thanks dude. i'm actually asking for which is structurally better irregardless of legality.
because i don't care about drag racing. and i will never "race" this car in a side by side environment. i only autocross and do HPDE's. thanks again.

 

Go to where the load comes into the chassis from the spring. On FB's its just inside the shock tower in the rear. On FC's it the tower itself because it is a coilover. I have never been under an FD so I can't say exactly, just look for where the spring load goes, that's the area that is holding up the car.

 

Wow you didn't read his post did you...

 

On a coil over type system, go to the towers if possible. Hit multiple planes with your pad.

Also, read your rule book. You say you're not concerned with the legality... you will be when they wont let you run. Harness bar, diagonal, straight rear hoop braces are all required for any kind of auto-x I've seen.

 

thanks dude but i race with SCCA, PCA and mostly with my local club www.motorsportsne.com (used to be called the M club) and safety equipment isn't required. so i don't need a harness bar, diagonal or straight rear hoop braces.

FYI i'm gonna stick with the bar i have and not get the weld-in custom one. I have the RaceShop bolt-in chromoly bar with dual swingouts, seat back bar which acts as a harness bar, and sort of a diagonal (main hoop to rear mounting).

 

For reference, even if you're not concerned about having the cage/bar meet some rules for a class of racing, it's a good idea to consult the rule book anyways, use it as a guideline for things like tube placements, bracing, and so on.

 

Strut tower on an FC for those FC owners reding this thread.

 

Right. The rollbar/cage isn't required, but once you have it, it has to meet specs.

Do what you want.

Straight bars transfer loads, bent bars act as springs. If you've got a choice of a straight bar vs. a bent one, go with straight.

 

Structurally the shock towers are much better choice. The added dividend is that your cage now also acts to stiffen the chassis structure.

 

check out this thread:

https://www.rx7club.com/showthread.p...highlight=cage

I was specifically asking about an fc, but I believe that the geometry should be similar enought that it will transfer over without any problems. In the end I went to the frame rails. I drilled a small hole into the shock tower side, and the metal wasnt really very thick, so that sold me on it.

 

All the more reason to tie the towers into the cage somehow; they are a weak point.

No doubt the cage in the thread you mentioned is "safe" and stiffens the chassis itself, but it's doing nothing to reinforce an area of the chassis that receives much of the suspension loads and that's a shame. This cage will do a great job of protecting the driver, but it's not taking full advantage of the performance gains that can be had by making the suspension pickups more rigid.

 

Here's mine. I love the job they did on reinforcing the whole tower.

 

What is the over all length of the plate that is welded around the strut tower? From where the top plate is welded to the side plate? Just curious. I did that for a car a few years ago and was found illegal because the over all length was over 12" by about and inch and the rule states 12"x10" maximum and 1 side no longer than 12 and no less than 4". Something like that. I had to cut the damn thing out and rebuild the strut tower. Really pissed me off!

 

Something Ive always wonderd. Is it better to have all the tubes come to one point, as in the above pic, or to say, spread the mounting points more around the shock tower? Is there any benefit to having all three tubes tie in at the same place? Seems to me that coming as close to evenly distributing the load as possible would be best... But then again I know almost nothing about cages...

 

Having all the tubes come together at one point will be better for transferring loads between the tubes, having them spread out will be better for transferring the individual loads to the chassis. It'll be stiffer to mount all the tubes together at one point.

 

also with most racing regulations you have a limited amount of material you can use as a pad, to attach the bars to the chassis, and a limited amount of attachement points.

If you were to split those bars up, and send them all independentlly to the chassis, they would be considered 3 points, as opposed to 1, and you would need a bigger pad than normally allowed

 

SCCA counts the "point" as the plate welded to the car. You can put as many tubes to that plate as you can fit. They limit a plate to 100 sq. in., minimum 2" wide, maximum 12" on a side. I like to call the clarification added making the 12" maximum my doing. For years it just had a minimum listed. I was building an IT car and the rules allowed only six points at the time with the unlimited tubes going to them. I called national and said, "So, I can have a mount 2"x 50"? Jim(tech king at the time) said you can for now... but don't count on that nex year... scribble scribble scribble heard over the phone...