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Discussion Starter #1
(1) The CF tub on the Alfa weight 150; The Elise aluminum frame/tub weights 150lbs. How is this possible? I was always suspect of the carbon fiber tub in the 4c because of the high weight. The car is bolted together in three sections. Front suspension structure made of aluminum, CF tub center, and rear subframe in aluminum. The Elise's 150lb frame accounts for the tub and the front suspension pickup mounts according to my sources. (the 150 may only be the tub?) The rear of the Elise is a high strength steel subframe which must be heavier than the 4C's aluminum, yet it probably is about the same due to the final weights of the car. I am just confused why the 4c is not at least 150-200 lbs lighter than the Elise that was first debuted in what 1996?

(2) The 4c weights 1973lbs in Euro-spec while it balloons to just under 2500 lbs in US-spec. (contested figure) How is this possible??? A few additional airbags and some more side impact bolstering cannot account for such a drastic increase.
Some reviews have the weight at around 2500, while others have the US-spec'd 4c at only 220 lbs heavier than the Euro-spec at 2194. What gives? The inconsistency is rampant.
 

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The Alfa 4C is MUCH stiffer by the numbers and presumably holds up to various crash test standards.

The weight has been a point of contention since the car's release. In fine Italian fashion the first released numbers were dry.

Road and Track weighed a couple US models and they were all in the 2450lb range. I think in UK tests the base Euro 4C (no AC, no radio) came in at 2,153lb wet.

Supposedly the US version is very different from the Euro version even down to the tub (which is supposedly 250lb in the US, not 150lb), in order to comply with various safety regulations.

And the lack of a base model in the US can't help - the A/C alone probably accounts for 50+lb of difference.
 

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Dual-clutch gearbox is probably adds 100 .lbs as well compared to an Elise.
 

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A word on aluminum parts. I'm going to be a little technical here, bare with me. This might be a surprise.

Aluminum's density is about 1/3 that of steel. It's modulus of elasticity and yield strength are also about 1/3 that of steel. So to build a structure or structural part from aluminum that has about the same properties of steel, you have to use about 3 times the thickness in aluminum. Hence, no weight savings. There are different alloys of aluminum and steel, so this is just a general comment, things can change somewhat with specific materials. When thinner aluminum is used, the design has to change to compensate for the reduced strength. This is why bikes made from aluminum have tube diameters 2 and 3x of steel bikes.

The real advantage of aluminum is that it's cheaper to form into odd shapes. So where it would be expensive to remove non essential material from a steel part to make it lighter, the aluminum part can be designed to have material only where it's needed and not where it doesn't. Disadvantage being that the material cost is much higher than steel.

So it's not a given that something made from aluminum must be lighter than a steel counterpart, don't feel bad about having steel a-arms in your Elise. Notice that Aston Martins and Ferraris have structures of mostly aluminum, but these are hardly light weight cars.

Carbon fiber - a whole different story.
 

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The torsional rigidity of the chassis could be vastly different. I'd suspect that it is, but have not seen published figures.
 

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A word on aluminum parts. I'm going to be a little technical here, bare with me. This might be a surprise.

Aluminum's density is about 1/3 that of steel. It's modulus of elasticity and yield strength are also about 1/3 that of steel. So to build a structure or structural part from aluminum that has about the same properties of steel, you have to use about 3 times the thickness in aluminum. Hence, no weight savings. There are different alloys of aluminum and steel, so this is just a general comment, things can change somewhat with specific materials. When thinner aluminum is used, the design has to change to compensate for the reduced strength. This is why bikes made from aluminum have tube diameters 2 and 3x of steel bikes.

The real advantage of aluminum is that it's cheaper to form into odd shapes. So where it would be expensive to remove non essential material from a steel part to make it lighter, the aluminum part can be designed to have material only where it's needed and not where it doesn't. Disadvantage being that the material cost is much higher than steel.

So it's not a given that something made from aluminum must be lighter than a steel counterpart, don't feel bad about having steel a-arms in your Elise. Notice that Aston Martins and Ferraris have structures of mostly aluminum, but these are hardly light weight cars.

Carbon fiber - a whole different story.
The strength of Aluminum or Steel all depends on the type and grade. If you're comparing 7075-T6 Aluminum to A36 Steel then the Aluminum actually has a greater ultimate strength and double the yield, yet is still 1/3 the weight. So your point is invalid.
 

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The strength of Aluminum or Steel all depends on the type and grade. If you're comparing 7075-T6 Aluminum to A36 Steel then the Aluminum actually has a greater ultimate strength and double the yield, yet is still 1/3 the weight. So your point is invalid.
Maybe you didn't read my entire post. Like I said, it depends on the alloy of each. But for every aluminum alloy you find, I can find a steel with a higher yield strength, so your point is invalid.
 

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A word on aluminum parts. I'm going to be a little technical here, bare with me. This might be a surprise.

Aluminum's density is about 1/3 that of steel. It's modulus of elasticity and yield strength are also about 1/3 that of steel. So to build a structure or structural part from aluminum that has about the same properties of steel, you have to use about 3 times the thickness in aluminum. Hence, no weight savings. There are different alloys of aluminum and steel, so this is just a general comment, things can change somewhat with specific materials. When thinner aluminum is used, the design has to change to compensate for the reduced strength. This is why bikes made from aluminum have tube diameters 2 and 3x of steel bikes.

The real advantage of aluminum is that it's cheaper to form into odd shapes. So where it would be expensive to remove non essential material from a steel part to make it lighter, the aluminum part can be designed to have material only where it's needed and not where it doesn't. Disadvantage being that the material cost is much higher than steel.

So it's not a given that something made from aluminum must be lighter than a steel counterpart, don't feel bad about having steel a-arms in your Elise. Notice that Aston Martins and Ferraris have structures of mostly aluminum, but these are hardly light weight cars.

Carbon fiber - a whole different story.
Oh dear...

Your blanket statement that Aluminum's yield strength is about 1/3 that of steel is wrong. Look up the the values for the specific alloy used in the chassis, it's much better than, say, one third of A36 steel. We're not using aluminum because of its ability to be formed easier, there's nothing special about what is essentially welded panels and tubes. We're using it because the yield strength to weight ratio is better. Some bikes have wider tubes because a wider tube, while having the same crossection, will handle certain forces better. But they need to be oval shaped so you don't bump your legs into it, which costs more. The THICKNESS of the tube wall may indeed be larger, but that is a small fraction of the diameter.
 

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Oh dear...

Your blanket statement that Aluminum's yield strength is about 1/3 that of steel is wrong. Look up the the values for the specific alloy used in the chassis, it's much better than, say, one third of A36 steel. We're not using aluminum because of its ability to be formed easier, there's nothing special about what is essentially welded panels and tubes. We're using it because the yield strength to weight ratio is better. Some bikes have wider tubes because a wider tube, while having the same crossection, will handle certain forces better. But they need to be oval shaped so you don't bump your legs into it, which costs more. The THICKNESS of the tube wall may indeed be larger, but that is a small fraction of the diameter.
No, the blanket statement is correct and I clearly stated, general comment. Again, like I said the specific alloys can change that. The aluminum tube analogy is an example of how lighter weight is achieved through aluminum but the geometry of the material must be changed to make up for the strength deficit.
 

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Can someone point me to the thread re: torsional rigidity? Sounds fun.

(As an advanced R&D chassis engineer, I've run probably thousands of FEA and crash analysis jobs… I like to think that I have a somewhat developed sense of what structures will do, although there are always surprises.)
 

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Stiffness is not the same as yield, so both of you are invalid. (grin*)
Notice I said modulus of elasticity? E, Young's modulus, in other words, the property used to determine stiffness. It's not the only property you use to design a part though, is it?
 

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Notice I said modulus of elasticity? E, Young's modulus, in other words, the property used to determine stiffness. It's not the only property you use to design a part though, is it?
See, I knew I should have paid closer attention to reading the posts… I was in too much of a hurry to throw in my own smartarse comment :facepalm

It's great fun comparing materials for different applications… I guess that is what makes us engineers so interesting to the rest of the world. :crazyeyes
 

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Discussion Starter #16
I'm lost in this debate. Maybe I'll ask my brother about this subject, but in the
meantime I still don't understand why the CF tub isn't significantly lighter than
the Elige's AL tub.

This has yet to answered. Does the Elige's 150lb "tub" include the passenger
compartment and the front suspension pick-up points, steering rack et al.

US safety standards ruin our fun.

I'm surprised the US safety board did not require the FED Eliges to
have the S1's full sill instead of the accommodating notched out
variant. I wonder how much heavier the non-notched Elige sill is
compared to the notched?

-Robert
 

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sleepe -

If you designed two structures to do the same thing, in many cases a carbon fiber structure would be lighter than an aluminum one.

If you designed one structure to outperform the other, they could still be the same weight but one might be markedly stiffer.

Imagine two fishing poles, identical weight, one made of aluminum and the other made of carbon fiber. While they may weigh the same amount, the carbon fiber pole may be capable of landing a 50 lb fish where the aluminum one may only be capable of landing a 30 lb fish.

If the carbon fiber chassis doesn't outperform an aluminum chassis of the same weight, I would question why it was so heavy. But, if it outperforms the aluminum chassis, I would understand.

(This is all hypothetical, as I don't know the torsional rigidity of either the Lotus or the Alfa.)

Below is a photo that I grabbed off of the 'net, showing a tube frame chassis. Torsional rigidity is the measure of a chassis' ability to resist twisting (as shown in the picture). With some exceptions (karting comes to mind), more torsional rigidity is desirable for reasons I won't go into here. (Including: road "feel", suspension control, NVH (noise/vibration/harshness) considerations, "modes", etc.) In my former life I ran so many of these simulations that I used to fall asleep thinking in color.
 

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Thomas forget this crap go change your new son!
 

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first off- lets compare apples to apples. you can't buy a street legal new lotus elise or exile in the us. so there is not such thing a 2,000 lb lotus vs a 2,500 lb alfa. there is a 3,000+ lb lotus to compare it to if you like.... (although id say a bit different cars)

so your left to coampare it to.... a porsche? a bra/fr-s thingy? some other under 3k lb coupes that are quick handling. so not too much in the market there.

you can't even really compare it to a kit car - since a kit car is side stepping all the regulations that add a lot of weight into the Alfa.

so lets be factual here. isn;t the alfa the "lightest / fastest / cheapest" production sport coupe in the US?

and its a carbon fiber tub - whats not to like?!
 
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