Aluminum plate vs mild steel plate in this application 3

You are probably right, the difference would not be noiceable. They are probably right, adding the plate would slightly increase the stiffness of the body. I agree about the letters.

Basically, without putting the thing on a stiffness rig it is going to be hard to tell, and on the track the weight reduction might overshadow the tiny stiffness effect anyway.



Cheers

Greg Locock

Excuse me gentlemen:

If I'm not mistaken Frank begins by stating:

" A Viper performance shop is claiming that by going from 1/8" mild steel, to 1/4" aluminum for the 15"w x 30"long underbody plate that comes on most if not all 1992-2002 Vipers that 'Our pan will increase the stiffness and rigidity of your frame ' "

If this is true and Al weights roughly 1/3 of steel and is 66% as strong (as per your statement) they are getting 2/3 of thye original weight and increasing the strenght by about 33% (2 times the thicknes x 66% strength ratio = 1.33)

So the rest of the statement is wrong or I got lost some where?

Double check your numbers, I don't know where the 1/16 th increase in thickness came from.

Cheers

SACEM1

FrankTHutchens/SACEM1,

Please do not confuse stiffness with strength. Also, in bending the equivalency formula is proportional to thickness^3.

Best regards,

Matthew Ian Loew
"I don't grow up. In me is the small child of my early days" -- M.C. Escher

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The 1/16" reference was saying if aluminum needs 1.5x the thickness of steel to have equal strength...which means 3/16" aluminum just to = the strength 1/8" mild steel piece. Then the 1/4" aluminum piece is only 1/16" thicker than that(!). So how much of a change/gain has been accomplished?

BTW the 1/4" aluminum piece is claimed to weigh 7lbs more than the 1/8" steel piece.

I just think claiming that if you buy this for $250 it's going to, "increase the stiffness and rigidity of your frame" and "reduce frame flex and rigidity when releasing the clutch @ 5-6K at the track in high rpm holeshots".. is wishful thinking - at best. (and just added weight)

Maybe if it was thicker with CNC'd webbing into it. Even then, with the bolts along the edges, and only along the side edges of the plate... (?)

Technically it could be argued that it's a wee bit stronger than the steel one (maybe), but functionally it's got to be 99% advertising fluff doesn't it?

FrankTHutchens,

For cantilevered bending (a reasonable place to start) the formula for deflection is:

[Δ] = 4(F l[sup]3[/sup])/(E b t[sup]3[/sup])

F = force applied at distance l
b = width
T = thickness
E = Elasticity Modulus

With this formula, one can figure out the comparative stiffness(not stress!) between the two components.

For stress, the formula is similar, but not identical:

[σ] = 6(F l)/(E b t[sup]2[/sup])


Again, it is important to understand the difference between stiffness (deflection) and strength (stress).

Best regards,

Matthew Ian Loew
"I don't grow up. In me is the small child of my early days" -- M.C. Escher

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

Here is what I came up with:


The aluminum plate at twice the thickness would be over 2.5 times stiffer than the steel at 2/3 the mass. This yields a specific stiffness of 4 times the steel plate.


Best regards,

Matthew Ian Loew
"I don't grow up. In me is the small child of my early days" -- M.C. Escher

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

Well, I'll be dipped. With 1/3 the elasticity of steel, just doubling the thickness makes it that much stronger...

So basically, for simple comparison sake, it's 2.5x stronger, or 4 x stronger?

Even so, I think the lettering carved into it and two sided (instead of four sided) mounting system make expectations of it having an real added effect when underneath a 3500lb car a bit thin.

Besides, metals online sells the same thing for $96.20 delivered instead of $250. Unpolished and unlettered...

FrankTHutchens,

FrankTHutchens said:

Well, I'll be dipped. With 1/3 the elasticity of steel, just doubling the thickness makes it that much stronger...

Click to expand...

It makes it that much stiffer. I emphasize that you really need to be more aware of your terminology. Stiffness and strength are not synonymous.

With respect to the figures I provided, the absolute stiffness is 2.67 (0.375[sup]-1[/sup]) x the steel design. The specific stiffness (stiffness divided by density) is 4 times the steel design.

Regarding the letters that are milled out of the plate, I guess you could assume that the thickness is not doubled but is slightly less and re-do the calculations.


Best regards,

Matthew Ian Loew
"I don't grow up. In me is the small child of my early days" -- M.C. Escher

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

Sorry, minor correction:

specific stiffness is stiffness divided by mass

Best regards,

Matthew Ian Loew
"I don't grow up. In me is the small child of my early days" -- M.C. Escher

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

I think, besides the mix-up of strenght and stiffness, there is also a misunderstanding creeping in this thread about the way this strenght/stiffness influences the body structure.

Matthew (as far as I understand) calculates the bending stiffness of the aluminium plate itself. However, the stiffness in this direction only decreases the 'membrane' effect of the plate, it does not mean anything to the (torsional) stiffness of the car's body as a whole.

The stiffness in shear direction (parallel to the surface) is much more important, and that is determined by the E-modulus and the thickness only (both linear).

Giel

The big picture for us is frame flex and rigidity when releasing the clutch @ 5-6K at the track

Click to expand...

This statement to me seems to indicate a bending mode rather than a torsional mode. Besides, the original poster's comments on the 1.5x thickness for equivalent stiffness is for bending modes. I was restricting my comments to the original question.

Best regards,

Matthew Ian Loew
"I don't grow up. In me is the small child of my early days" -- M.C. Escher

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

Giel;

I don't understand your term: (quote) " The stiffness in shear direction (parallel to the surface) is much more important, ......."

A plate is never going to shear fail parallel to the surface it would fail by buckling before or crushing, or you meant something else?

I agree with the second part: " ... that is determined by the E-modulus and the thickness only (both linear)."

I agree with Frank that the plate is only going to add in the stiffness if it is adequately fixed to the car frame, if not it will only serve underside protection purposes.

But I think the real difference can be deduced from the following statements:

If an aluminum plate 1/4" thick weights 7 lbs more than a 1/8" steel plate and both are the same area (15" x 30") then we find:

Steel plate 1/8" x 15" x 30" weights 15.8 lbs

Aluminum plate 1/4" x 15" x 30" weights 11 lbs

If aluminum plate weights 7 lbs more than steel plate then it weights 22.8 lbs

Then the aluminum plate is going to be roughly 1/2" thick and all our calcs are off target.

Somewhere lies the real answer but there is indeed a missinformation either in the weight or the thickness of the Al plate.

Cheers

SACEM1

A 15" x 30" aluminum plate that weights 22.8 lbs is

No, the shear plate comment was sensible. Take your structure, roughly a cuboid with a reentrant channel down one face. (Make this of matchsticks or drinking straws).

Now twist it. The reentrant channel needs a SHEAR plate to stiffen it in torsion.



Cheers

Greg Locock

Some notes: The OEM 1/8" thick plate weighs 6 lbs, the aftermarket 1/4" Al plate is claimed to weigh 13 lbs.

The lettering is painted, not machined into the aftermarket plate. When the vendor described the plate as 'CNC' I assumed the lettering had been carved into the Al plate. Now I wonder just what is CNC'd? The holes? I doubt he'd go to that much trouble for 12 holes.

I did a search and found one site that said AL typically has to be 1.5x as thick as steel to be of equal strength. I don't know that to be fact.


"Our pan will increase the stiffness and rigidity of your frame..."The big picture for us is frame flex and rigidity when releasing the clutch @ 5-6K at the track." Are the claims of the performance shop, and 99% marketing hype in my estimation. Mostly given how the plate attaches to the frame. What's a 3500car going to care about that narrow plate under it? (Especially at high rpm holeshots) It's not even boxed, just bolted along two sides near the edge(s).

I'm willing to give him his due if others more experienced can tell me there's anything to be had out of bolting up a plate 1/4" thick instead of 1/8". The OEM plate is merely a stone guard and not intended to stiffen/strengthen the chassis.

Frank:

If the plate is really 15" x 30" as you originally stated then if it weights in at 6 lbs then its only 0.47" thick.

The Al plate could weight about 11lbs so the statement of 13 lbs might be right if it has some ridges cast in.

Have a nice weekend

SACEM1

P.D. Greg: I don't disagree with you I just don't see it or visualice it as a shear force maybe you can elaborate a little more in order to get the idea right.

Thanks

The steel plate has a slight taper lengthwise but those dimensions are close. There are no ridges in it, it's a flat steel plate.

Sorry in the typing the thickness should be stated as 0.047" not 0.47"
and that is about gage 18 or 19

Cheers

SACEM1

I think it should be obvious that the only reason for selling this aluminum plate is to separate gullible Viper owners from their money.

- R

A plate is never going to shear fail parallel to the surface it would fail by buckling before or crushing, or you meant something else?

yes cast aluminum can, im working as a transmission man at the local AAMCO and have seen nuts do it to 4 wheel drive rigs.

also with the power the viper has why would you dump the clutch at 5 to 6k (hope you have a scatter shield) it seems like all you would do is waste tires.