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There is a bolt-on chassis stiffener for the Spider that allegedly takes out the flex in the chassis and makes for a better handling car. Since they sell for about $470 from IAP plus shipping, I am hesitant to spend the money until I know that it is worthwhile. I have already installed the adjustable control arms and the front suspension is set up with slight negative camber. I do not race my '84 Spider Veloce, but I do appreciate precise handling and abhor chassis squeeks, rattles and cowl shake. That, notwithstanding, I love my Alfa. Any comments on this particular modification?

Desert Spider
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There have been several posts about the chassis stiffener. You might want to try doing a search, but as I recall, people have very positive comments about the chassis stiffener.
 

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I'd be interested in hearing why people think a roughly 1" square piece tubing running down each side of the underbody makes the spider frame significantly stiffer.
 

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Bruce,

It is not just two pieces of side tubing. The stiffener consists of four tubes which combine to form a square - the corner of which mates roughly to each of the four corners of the car.

I don't believe anyone has done any finite element analysis on the structure, but it is reasonable to assume that the box helps resist some twisting of the chassis in the same way a full roll-cage does to a coupe.

Anecdotal evidence seems to back this up - people have reported that jacking up one corner of their car before resulted in a visible difference to the shut-lines along the door due to chassis twisting, and this went away after installation of the stiffener.

As always - the post-purchase rationalisation of buyers probably factors into this - but I'm happy with mine.
 

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Nick,

I’m not asking for a major structural analysis job, just some common sense, which IMHO does not include “anecdotal evidence”. And I know there are more than 2 pieces of tubing associated with the frame stiffener but I also know that only 2 are involved with increasing the tortional resistance of the frame. For argument sake, let’s just assume the square tubing is cylindrical (cylindrical tubing is actually more resistant to torsional stresses than square tubing of the same size). The problem I have is that torsional resistant increases with the fourth power of diameter and decreases with length. This says that a 1” tube would be something like 1/16th as resistant to twisting as a 2” tube of the same length. When I look under a spider I see unit body boxes that are at least twice the size as the square tubing associated with the frame stiffener. To me, this suggests that the existing spider frame would at least 16 times as stiff as the frame stiffener. However, the weak/twisty part of the spider chassis is really just the section where the doors are. This is only about half the length of the frame stiffener so that adds another factor of 2 and the 16 becomes a 32. In other words, it looks like the frame stiffener might add something like 3% to the existing frame stiffness. Maybe a 3% increase is significant.
 

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First, let me say that if you install one of the chassis stiffeners, you will notice a difference before you get out of your driveway. They simply work.

Next, to the person wanting "plans" to build one: Don Ereminas spent alot of time doing R&D to make this part work and fit as well as it does. The units sold by IAP, Centerline, and other retailers all come from Don. Don't you think Don deserves some return on his investment, rather than having everyone just try to copy it to save a few bucks?

Joe
 

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JoeCab said:
First, let me say that if you install one of the chassis stiffeners, you will notice a difference before you get out of your driveway. They simply work.

Joe
I'd love to know that they work. I have one on my spider (thanks to the PO). Still, it looks to me like they wouldn’t do much to stiffen the chassis and no one seems to have anything remotely resembling objective data on them one way or the other. I just happen to think that if they really work, there should be some objective data that support this conclusion. To me it’s the difference between knowing something and believing something. I hope I’m not being obtuse.
 

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Bruce,
I, like you, would like to see all the graphs and statistics proving that these chassis stiffeners provide X% more rigidity than the basic chassis. But since it doesn't seem to be in the public domain, we'll either have to believe in word-of-mouth or try it ourselves. You, by dint of already having one on your spider, are in the perfect position of driving the same car both with and without one. If you somehow get the time to do a back-to-back comparison(removing the stiffener in between), I would love to hear the results.
 

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I was wondering about this brace. How much ground clearance would I lose with this device, if any? What about weight? I already have a bolt-in street roll bar, which I have heard will also stiffen the chassis. (Since I've never driven with the roll bar removed, I don't know what the difference is from stock). Have any of you added this brace after equipping the car with a roll bar? Any noticeable difference?
 

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Just a little more math in public; because I love mechanics of materials, and so rarely get to contribute anything to such a seasoned croud of alfisti.

It appears to me that the twist in the spider chassis would be coming from cornering forces applied to the front corner of the car, with the rear of the car remaining relativly flat (Its just the impression I get from looking at these things mid corner, The inner front wheel arch always looks to lift more than the rest of the car).

This would suggest that the bars that run crosswise in the stiffener get subjected to some sort of lengthwise bending moment, and the bars that run lengthwise, especially the bar outermost of the corner, get subjected to some torsional force (equal to the cornering force of the car as applied to the chassis stiffener where it bolts up, times the distance from that spot to the bars neutral axis).

If this is accurate, then two bars of the stiffener are bearing the most load during a turn, the front bar that runs crosswise in bending, and the outermost bar that runs lengthwise in torsion.

It kinda looks from the picture that the front bar running crosswise is a little bigger than the rear, so hopefully I'm on to something here...

The total stiffening benefit then would be the the sum of the resistive forces of both the bending and torsion.

Also, in comparing the stiffener to the chassis of the car, I agree with bruce, the formula I use for torsional stiffness(engineering mechanics of solids,Egor P. Popov 2nd ed. P.224) is:
tosional stiffness=(polar moment of inertia x shear modulus)/length
Where the polar moment of inertia for a solid cylinder(for simplicity) is = (pi*D^4)/32
D= diameter

But to make a fair comparison to the cars chassis you would have account for the shear modulus which, if sufficently large, could add some serious benefit. Lets remember that the spiders chassis has been in action for maybe thirty years, the material resisting these torsional forces has surely relaxed a bit and just isn't as stiff as it was when it left the showroom floor, Heck in some cases its practically rusted away, take into the accounts of fatigue, and the small diameter stiffener is probably significantly more rigid than the chassis, despite being geometrically smaller.

I'm sorry for bringing math to the board, and I certainly don't want to step on anyones toes, or sound like a smarta$$. But I'm a total nerd and can't help loving this subject.

If joecab knows the guy who designed this thing, then maybe we can get the designers opinion too?

All of that said, I do not own a chassis stiffener, have never sat in a car that did, and if I were to seek a solution for chassis flex it would be a rollcage/rollbar. Because it would seem to be a benefit to have the material higher up from the bottom of the car and also adding a degree of saftey while it keeps the car stiff.


:D I saw a guy on campus the other day with a tee-shirt that said "talk nerdy to me" I'm gonna have to try that with the ladies sometime :D
 

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Ah, but Sono, the beauty of the chassis stiffener idea is that it keeps the weight low to the ground, whereas a roll bar or cage is up high. Keeping the center of gravity low is better for handling. Besides, a roll bar isn't going to do much for rigidity in a roadster...you need to actually link the front and rear parts of the car to help with that, which is what the stiffener attempts to do.

That being said, I've never tried an Alfa with a chassis stiffener so I have no idea how well it works or not.
 

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I was trying to stay up late enough to see if anyone was gonna call me stupid, or insane... or both.

Looks like I'll have to wait until the weekend, where I will either be grasping at straws to defend my position, or try to make some graceful bow to one of the many, very knowledgable alfsti here.

G'night all...
 

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OOH a response!!!

Yeah, for a cage, i would do a bar under the dash, a bar across the rear seats(and over my head) and a two bars connecting them just far enough down so that I can somewhat cofortably get in, all four corners connected to the suspension mounts.
 
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