Frame Stiffening
 

I was randomly surfing parts sites to pass off the time while some computers where loading here at work, and I found something interesting over at Flying Miata. They had some frame stiffening parts thatwould tie the transmission tunnel together under the car. It also doubled as a guard for the cats running there.

Would something like this help out our old cars? I figure my frame is pretty weak by this point and it might be nice to build something like this up. OR, does this only matter when you are dealing with a roadster and dont' have the roof to stiffen the car up. I know the old mustang guys do this to their cars and it helps a TON.

I figure that I could take a bunch of mesurements and weld something up that might work pretty well.

Here is a pretty picture of what I saw.

https://www.flyinmiata.com/Store/images/13-69920.jpg

http://www.flyinmiata.com/index.php?...920%20%2090-97

I was wondering about this the other day. I didn't know anyone had anything like this but I did find an article about stiffening the frame on an older Mustang.

ive seen it custom done before.

That looks like someone has a plasma cutter.. Mustangs, the old ones like rx7's have whats called a " uni-body " it's not a full "framer " the car is part of the frame. I'm not sure about the MX5's . Sub-frame connectors are good for V8 powered uni-body cars. A V8 puts alot of torque on anything really. On rx7's with a rotory engine and no sun roof a front strut brace and a rear shock tower brace is enough. you really want the car to "flex " abit, it handles better then being "ridgid " one other way to stiffen up a uni-body car is to install a four point roll-bar, or better yet a six point roll-bar it looks cool and has two functions, safety and ties the car together.

I was wanting to stiffen up my car with tower braces front and rear, and I have a moonroof, but Lt. Dan informed me that a rear strut brace is unnecessary because of the fact that those're only meant for FWD cars like Honda's and whatnot due to the fact that their strut towers are so much taller and plus their rear ends are independent suspension versus our cars have a solid rear axle. But, a rear roll cage will help, I've seen it before and it looks really nice.

So, for those of us who don't want a cage or a rear bar taking up the hatch space, this might help a bit?

I honestly don't see why it shouldn't. Since the car body is part of the frame, it should help quite a bit. It should help the flex of the tunnel hump when the car shifts its weight.

Ummmm, no you don't. Flexing of the chassis is uncontrolled, unstable suspension. Think of it in terms of the car acting like another sway bar or spring. If allowed to flex, the chassis of a unibody will suddenly start to twist or flex causing all of your suspension work to go out the window as you cannot compensate for unknown forces causing the suspension to stop working and start flexing the now stressed chassis. This is of course a gross overstatement, as it is not like a street car is going to start flexing like a pretzel, but it helps me to think of things in exaggerated terms to understand little things, like the bit of flex in a stock street car. Now a car may RIDE better with a little flex, but not handle better. If that was the case, then race car engineers would not spend thousands of hours on stress jigs finding the forces acting on the car in various states of operation to stop them. the less the chassis flexes, the more the suspension can work as it is supposed to. That is why they make shocks for the wheels, not the chassis, as the wheels are supposed to move and make the car stick, not the chassis flex. The only chassis that handle better with some flex is a shifter kart, but that is a whole other story. LOL

As far as rear strut bars for cars like hondas that are FWD and independant suspension, well that is incorrect too. Solid Axle or IRS does not matter in this case, they are still transmitting forces the the strut towers through the springs, and these will still flex. See above as to why we do not want the strut tops moving around. As to why some people do not run a rear strut brace on a FWD is because most of the weight and forces is in the front, the rears are more or less keeping the rear bumper from dragging the ground. Check any good cage, and you will see some type of bracing to reduce rear strut tower flexing

Cheers,
Travis

Hey, I'm just quoting what I was told.

I guess my rollbar must be crap.

You're telling me that running an extra brace 3 INCHES higher than a bulkhead makes a noticeable dfference? The reason most people run the rear braces on the rollbar to the shock mounts is becaust it's one of the strongest places to tie in. The shocks do not locate the rear axle under the car, so why do we care how much the flex around?

Dan

IMHO

This argument needs to be clarified, are we talking street or race, because any flex in a race car is undesirable, the heavy duty spring rates, damper rates and use of rod ends makes any body flex reduce the effectiveness of any suspension tuning. But, in a street car the complete removal of the rubber suspension components will remove more flex than a stut bar can ever hope to remove.

I am far from convinced that a strut bar does much at all, until the point at which you need a roll cage then the strut bar becomes irrelevant anyway.

My rear suspension is about to be changed to have coil overs, this means I am replacing the original upper shock mounts with heavier and wider metal to cope with the forces from the spring relocation. When I build the roll cage it will be welding to these mounts eliminating 99.9% of the flex and seriously locating the rear suspension.

In summary any reduction in flex is good for performance tuning, but not needed in a street car.

Also from my understanding, metal doesn't loose strength unless it is strained so far that it fatigues. So even tho our cars are 20+ years old they are pretty well as strong as they were when made unless they have been in an accident or something.

LT. Dan I meant no offense in my response to you or your cage. But since I am talking to you and not your cage, tell it I am sorry too. :D BUt what I meant was you are not trying to reduce flex for the shocks, but where the spring are acting on rear of the chassis, the shock towers just being a convenient place to do this. Again as aussiemag stated, this will not become necessary until a certain level of performance has been reached. on a street car with street tires, I do not believe that you will introduce enough forces for this to become an issue. But if you have upgraded and tuned the rear end with springs, shocks, 3-link, or something then you may begin to induce flex from the increased pressure of the dampers, springs, traction, etc. On Sports Car Revolution they had a car on a flex jig at Multimatic and the owner was stating the importance of this. Now they had a street car on it with around 9500 lbs/* of flex. They wanted it around 15000lbs/* of flex which they stated as the base for good sportscars. Proof that flex is not wanted, how much this matters is an individual thing.

Cheers,
Travis

Metal doesn't loose its strength altogether untill its been fatigued. BUT, metal can become used to regular twisting forces that will weaken the structure overall. The slight twisting forces applied to the chassis of the RX-7 for those 20+ years does degrade the strength of the chassis.

You could do what I did to stiffen things up. I got some "frame rails" from a new mustang convertable. Got them brand new off the press. The tool and die shop my dad works at made the dies to stamp the rails. Anyway, I had to replace my floors in my 7, so I decided to replace the rails at the same time. Well, just recently I had my whole car up on jack stands and I noticed there was a little wobble on the stands, like 1 was higher than the rest. Turns out, 1 stand was on a piece of the floor that was higher than the rest, but not by much. My car was able to withstand flexing more than 1/4". Surprised me quite a bit. I knew the car was stiffer from stock, but I didn't think it would withstand flexing this much. But those rails are very beefy compared to stock. They are about 3" wide by 1.5" high and thicker metal than stock. They are probably .08" thick, mabey more.

I believe the stock torsional strength of the 7 in about 7800lb/*, or something like that.

Sure, it all depends on how much one wants to spend, race car , street car or what i like to call rx7's, performance street cars. on my 82 i fabricated a front strut brace and strut tower to firewall braces with backing plates. On the rear i fabricated a tubular brace that ties the port side and starboard sides together, right above the rear axle in the cargo area, centered in the rear wheels while each rear shock tower have there own tubular braces centered on the tops of the shock towers .one brace towards the center of the car above the fuel tank with backing plates and one brace facing towards the rear of the car with backing plates. the car has a sun roof, kyb's a rear sway bar still and 205/60/15 T/A's. Ask anybody who's ridden in my car that it "handles " like a go-kart. I would like to add some sub-frame connectors but thats later. But nothing can replace the performance that a well engineered 6 point roll bar produces. IMO