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FC front end chassis stiffeneing

Old 12-07-07, 08:56 AM
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FC front end chassis stiffeneing

I'm swapping engines in my FC and I can't use any off-the shelf strut tower bar so I get to build my own

So far the plan is going to involve some 5/16 or 3/8" AL plate and some 1"x2"by 1/8" wall oval tubing. I'll be linking the towers to the firewall and on the passenger side run to the firewall will be a provision for an engine torque brace mounted off the back of the intake mani. The bar linking the towers together will be arched slightly (no way around it unfortunately hence the overkill on the tubing) & I plan on using some QA1 rod ends to connect the bar with the plates so I can really dial in the length, however - how important or how much of a difference or is it worth it to incorportate a linkage adjuster such as the one pictured into the design @ both ends? With an adjuster such as that I would be able to install the bar with the car on stands & torque everything down. Lower the car and then add more load to the bar before tightening the Jam nuts. I also plan on building a lower arm bar as it has been proven that even on a street car with street tires there is flex there worth bracing against. Car is an S4 swapped which should be around 375 to the wheels when done, used for auto-x and I'll have some race rubber on track day. Thought or input is appreciated. I have a buddy that calls me Dr. Overkill; am I living upto my name here - thanks guys.

Attached Thumbnails FC front end chassis stiffeneing-adjuster.jpg  
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Old 12-07-07, 10:40 AM
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Hehe, I see a reference to my old thread: https://www.rx7club.com/2nd-generation-specific-1986-1992-17/how-build-lower-arm-bar-under-%2415-448638/

Making sure there's no slop is good, but I'm not really sure that pre-loading it is needed, but it can't hurt, so why not?

You should do a brake master cylinder brace while you're at it. I might just have to take a picture of mine and do a write up one of these days.
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Old 12-07-07, 11:46 AM
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Yes - I did come across that thread yesterday and was debating building the lower arm bar until I saw the bent AL. That basically sealed it for me.

I wouldn't use the adjuster for space reasons. I'll need at least 5" between the center of the heim joint to where the bar needs to be level and I'm not sure if I'll have the clearance. If it's a really good idea than I'll certainly try my best to do it but if it will not make a significant improvement over just the bare hiem joints than I'm not going to bother. Same for the lower arm bar. If it's a good idea to pre-load the bar than I'll buy those extra pieces but if it's not necessary than nevermind. I guess I could always put some pre-load on the towers by putting the Hi-Lift jack inbetween the plate and cranking a little before bolting the bar down.
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Old 12-07-07, 03:23 PM
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I would do my best to use straight pieces between the strut towers even if you have to build a monkey puzzle out of tubes. Anything bent it just going to be a spring. It may be a stiff spring but still a spring.

As for preloading, which way? Have you really thought about which way the struts move? They move in and out depending on what the car is doing. Turning they want to go up and out, braking they want to go up and in. They move all over the place sometimes toward each other, sometimes away.
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Old 12-07-07, 06:40 PM
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The bar when finsihed will be very R1esque with only 2 angles in it an 3 strait pieces; "arch" was probably a bad way to describe it - sorry.

Pre-loading the bar would basically force the towers away from eachother. Nothing more. Are you suggesting skipping the entire pre-load idea and just adjust the heim joints a wee long?
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Old 12-07-07, 08:16 PM
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Anything to spread the load across the car to the other tower, or even better the cowl, will help. Think in triangles and you can't go wrong.

If you build things accurately with little slop you'll feel the difference with or without any preload.
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Old 12-08-07, 03:10 PM
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Originally Posted by jgrewe View Post
Anything to spread the load across the car to the other tower, or even better the cowl, will help. Think in triangles and you can't go wrong.

If you build things accurately with little slop you'll feel the difference with or without any preload.
Thanks - I'll weld gussetts on the angles and the hiems themselves are adjustable to take up any slop so I guess I won't go bonkers trying to get the adjusters in for the pre-load. That's all thier purpose would be.
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Old 12-09-07, 11:57 PM
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Do you have a cage to back this up or just planning on moving the twisting somewhere else?
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Old 12-10-07, 08:09 AM
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street car so no full cage but there is a 6 point mating the rails behind the rear strut towers to the main hoop and bracing going into the trans tunnel. I did what I could reasonably do with a dual purpose car. A full cage = front kickers 1" from my head
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Old 12-10-07, 09:32 AM
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You're talking to a guy that streeted a full-on racecar for 3 years, and just built an FIA cage for a kid that's gonna street the car and probably never put it on a racetrack. With that in mind I possess no engineering degree, nor any real formal education. Without something to back that triangular brace in the back of the car, all you're doing is moving the twisting somewhere else. Maybe not a bad thing, but figure the Mazda designers *might* have a reason for letting it twist where it does... makes it a little more predictable. Maybe I'm just hopeful and moving it to the center of the chassis or the firewall is better, who knows. Just offering another opinion.
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Old 12-10-07, 12:27 PM
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It will help reduce flex overall and the firewall will be decently stiff at the attachment points. This can effect the handling balance of the car, but you want as stiff a chassis as you possibly can to minimize any 5th spring (chassis) effects.
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Old 12-10-07, 02:33 PM
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Minimally, at best.

Sheet metal isn't nearly as good at supporting torsion or shear. If it was, all our bridge and buildings would look like giant unibodies. They don't, they're "skeleton structures". I'm sure a tower brace would have a minimal effect on anything you put it on, the more flexible it is the more effect. If there was a properly designed 8 point cage behind it you might see reasonable results over stock.

But it doesn't really matter, lots of people use them and like the looks of them. It's this dude's car, not mine, and he can make it out of popsicle sticks for all I care. I'm just saying it isn't god's gift to chassis stiffness and it isn't.
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Old 12-10-07, 02:47 PM
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Sheet metal stiffness is all in the shaping. If you put 3D bends into it then it can be quite stiff.

Up near the top of the firewall it'll have some 3D curves to it, which gives it stiffness. It'll be more stiff side to side than fore and aft, but it'll still help keep the strut towers from moving around. Sure ideally you'd tie them into a many point roll cage for ultimate stiffness, but it'll help anyway.
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Old 12-10-07, 07:24 PM
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Beadrol some sheet metal and it becomes quite stiff. Back in the day I thought as well that the strut bars were gimicky until I drove a buddies car without and then with..... the next day I ordered one and I felt an improvement. Would it be better if everything was tied into som 1.75" DOM chromolly? Without a doubt but this is a dual duty car and that's out ot the question. At best I would redo add some swingouts but that still isn't going to stiffen the chassis nearly as much because the towers are tied to the firewall on the seam. As it turns out I don't have the clearance with the hood I thought I had and the bar won't have any bends @ all. It will be a strait shot across of 2x1x.125 wall Aluminum.

I'm willing to bet that if I ran a course without and then one with, I'd notice a difference. Black91 proved that there is flex between the rear pivots on the lower control arms - points that mount almost directly to the frame rails and I've felt a difference pre & post bar in the FC.
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Old 12-10-07, 10:38 PM
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You'll go on a track and tell me the difference you'll feel?

Man oh man, I think I need another RX7-Club vacation.
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Old 12-11-07, 07:01 AM
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To start off with,
Run lower arm bars and strut tower bars also.
Remove engine and spot weld the engine bay.

Remove wheels and weld wheel wells and strut tower box.


You can also Use the Convertible Subframe with connector.You will need to cut off the bolting points to the firewall/floor area and weld them onto your coupe or make custom mounting points.
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