Why is the roadster lighter than the coupe?

[Dammmittt]

Originally Posted by vicj

Not true. The coupes and roadsters are IDENTICAL in suspension parts and "other" body parts, whatever you mean by that. The spring rates, sway bar thickness, etc. etc. are all the same. The weight difference is simply the weight of the roof and glass. BTW, it's a steel roof, not aluminum. Again, this is a coupe derived from a roadster platform, not the other way around, as is usual.

Check part #'s...no reason for them to be different if they're identical...and the parts are different. Didn't I say the roof was steel?

And if the bodies were the same they'd look the same, by other body parts, I ment the hatch and roof spports, as well as the trunk...but maybe you think those look identical too?

Even the skeletons are different...

Z4R
http://www.realoem.com/bmw/showparts...47&hg=41&fg=05
Z4C
http://www.realoem.com/bmw/showparts...95&hg=41&fg=05

[vicj]

Sorry...I was mostly referring to the suspension parts. They are the same. They DO have different part numbers, but so do the different suspension parts for an E46 325, 328, and 330...even though, they too, are the same. Maybe there are different bolt holes. I'm not trying to argue with you, just point out that the weight difference is simply because of the coupe being derived from the roadster, and not the other way around, which is usually the case. Yes, the body skeleton is slightly different, but if anything, it should make the coupe lighter, not heavier. There's no mystery, it's just the roof and all the extra glass.

[Z4MMM]

Could it be that the roadster is the real sports car?

[vicj]

I don't even want to go there. I've never understood the chasm between coupe and roadster owners. It was far worse in the Z3 days, the coupe owners were absolute snobs...but for what reason, I could never figure out. For the Z4, there's only a small difference in the cars. The coupe is just the roadster with a top thrown on. The coupe is beautiful, in my opinion, but I haven't lived without a droptop in 20 years, and don't plan on starting anytime soon.

[Hawkeye]

Originally Posted by vicj

I don't even want to go there. I've never understood the chasm between coupe and roadster owners. It was far worse in the Z3 days, the coupe owners were absolute snobs...but for what reason, I could never figure out. For the Z4, there's only a small difference in the cars. The coupe is just the roadster with a top thrown on. The coupe is beautiful, in my opinion, but I haven't lived without a droptop in 20 years, and don't plan on starting anytime soon.

I think it comes down to good looks (coupe) vs. convertible (roadster). And since I live in Iowa and it is still 50 degrees somedays, the coupe is the best of both worlds for me

[ianbiz]

Originally Posted by Hawkeye

I think it comes down to good looks (coupe) vs. convertible (roadster). And since I live in Iowa and it is still 50 degrees somedays, the coupe is the best of both worlds for me

What's the temp today?

[Dammmittt]

I don't like the coupe is a roadster with a top thrown on...the coupe has twice the tortional stiffness of the roadster...saying the just simply put a top on undermines the engineering that went into it. As far as which is the real sports car...I know one goes round a track faster than the other

[vicj]

I think he was referring to the "true" definition of a sports car...being an open-topped car, that's all. Why so confrontational? Chill-out, they are both fun cars. Yours MAY get around a track faster, though I'd bet that's far more driver related than car related, but I sure know which one I'd rather be driving on any nice-weather day!

[Dammmittt]

Yes I like mine...and you like yours...so we're both happy.

As far as confrontational goes, I'm just saying that "throwing a top on it" isn't what happened...

[veryhuman]

Throwing a top on the roadster is a roadster with a hardtop... (Which IMO, will never look as good as a coupe)

[takedown8]

Originally Posted by veryhuman

Throwing a top on the roadster is a roadster with a hardtop... (Which IMO, will never look as good as a coupe)

You're right. It doesn't look as good.....it looks better!

[The HACK]

Originally Posted by Dammmittt

Part numbers are different...

Just because the part numbers are different doesn't mean they're actually different. The strut-hat from the MZ4 is identical to the E36 M3 strut hat but it received a different part number. And the MZ4R and MZ4C have the same identical suspension parts EXCEPT for the front swaybar. But not 15kg worth different.

The only thing different between the MZ4R and MZ4C, hardware wise under the skin, are the steering rack and the front swaybars. On the surface the rear quarter panel, the roof, and the truck/hatch are different.

The reason the coupes are heavier than the roadsters, is because the coupes are built from the roadsters using roadster designs but adding the roof and hatch. The structural rigidity difference between the coupe and roadster, is 32000Nm vs. 12500Nm. That's almost 2.5X more stiff. Compare to equivalent roadsters built from coupes, like the E46, coupe stiffness is 12000Nm while the convertible stiffness is 10500Nm. Simply put, the Z4 roadster was designed to be as stiff as an E46 Coupe, hence you add all the roof and hatch to the car to make it nearly 2.5X as stiff it's going to be heavier. The counterpart, E46 convertibles were made from an existing coupe body that's only so stiff, to cut the roof off of that car would likely result in a convertible that's as stiff as spaghetti boiled for 24 hours, so they had to add a ton of braces and reinforcement to stiffen up the chassis to a respectable number, resulting in a convertible being much heavier than its coupe counterpart.

[Dammmittt]

The roof supports can't be what makes the car twice as stiff...doesn't add up.

[ianbiz]

Originally Posted by veryhuman

Throwing a top on the roadster is a roadster with a hardtop... (Which IMO, will never look as good as a coupe)

Roadster lambos look better than the coupes.

[swedish_z4]

is there diffrence in handling between the roadster and coupe? anyone has driven both and can compare?

[The HACK]

Originally Posted by Dammmittt

The roof supports can't be what makes the car twice as stiff...doesn't add up.

Yes it does. Take cereal box, cut a big open hole on the big side of the box. Take another cereal box of the same dimension and material. Try and twist and collapse both boxes.

Tell me which one is easier to collapse. The analogy is simple. That's equivalent to what they did for the E46 coupe and convertible. The convertible is like the cereal box with the top cut open. But BMW put a sh*t load of reinforcement in that open box to make it nearly as stiff as the closed box.

The E85/86 platform is the opposite. BMW built a very stiff open cereal box. Then closed up the box, hence the coupe is significantly stiffer than the E46 cereal box AND the Z4 roadster cereal box that it's built from.

[The HACK]

Originally Posted by swedish_z4

is there diffrence in handling between the roadster and coupe? anyone has driven both and can compare?

Yes. But not in the way you think about handling.

The major difference in how stiff the chassis is can not be appreciated from simply driving around on the street, but mostly apparent on the track. The stiffer chassis means less chassis flex under full suspension compression, less chassis flex means more sure-footed cornering at the absolute limit, more precise turn-in feel and more communicative chassis.

"Handling" is too generic a term. There's no such thing as a "better handling" car by definition. You can say car A as more grip than car B in a turn, but that may not necessarily mean car A "handles" better.

[Dammmittt]

Originally Posted by The HACK

Yes it does. Take cereal box, cut a big open hole on the big side of the box. Take another cereal box of the same dimension and material. Try and twist and collapse both boxes.

Tell me which one is easier to collapse. The analogy is simple. That's equivalent to what they did for the E46 coupe and convertible. The convertible is like the cereal box with the top cut open. But BMW put a sh*t load of reinforcement in that open box to make it nearly as stiff as the closed box.

The E85/86 platform is the opposite. BMW built a very stiff open cereal box. Then closed up the box, hence the coupe is significantly stiffer than the E46 cereal box AND the Z4 roadster cereal box that it's built from.

The analogy is simple, but also completely wrong in this case...

If you want look at it like this...the roadster body is like a big spring. Now, you're saying that the roof and hatch assembly is like adding a spring as stiff as the whole roadster body in parallel. I don't see it, maybe the engineer in me is thinking too hard...

[The HACK]

Originally Posted by Dammmittt

The analogy is simple, but also completely wrong in this case...

If you want look at it like this...the roadster body is like a big spring. Now, you're saying that the roof and hatch assembly is like adding a spring as stiff as the whole roadster body in parallel. I don't see it, maybe the engineer in me is thinking too hard...

The analogy is not wrong. It's just too simple for you to wrap your complicated engineering mind around.

If you don't want to believe me, that's fine. Structural engineering is all hocus pocus anyway. Imagine adding a cross brace in a square box, and the structural rigidity more than doubles.

[Dammmittt]

It's not hocus pocus.

First of all you're example weakens the cereal box...when in reality we're adding to it. As far as the cross brace goes, it depends on the thickness of the brace you're using...but again it's not really what's going on...and if it was, it'd be more akin to giving the Coupe anti-sway bars that are twice as stiff.

What you're saying is adding a thin roof doubles the rigidity of of a thick chassis/suspension. It makes no sense. There MUST be more that is different. And if you don't know what it is, that's fine, but I assure, that there's more to it than the roof, even if BMW hasn't told you what it is.

If the spring example doesn't make you happy, than take a cereal box (I'm assuming you know what that looks like) and glue a thin cardboard arch on top of it. Then twist a box with the arch and without...I guarantee you the arch isn't going to double the stiffness of the ceareal box...

[vachss]

Think about a car like torsional springs in series not parallel. The center section without the roof is much weaker in torsion than the roofed version (here the 3 sided vs. 4 sided box analogy is good). The torsional stiffness of the whole car is limited by the reduced torsional stiffness of the center section in the open roofed version.

[Dammmittt]

I think the best way to look at the car is a finite element model of beams welded together...but since we don't have access to that, we can look at it like a series of beams. If you look at the skeletons in my previous post alone, you see that there are plenty of horizontal supports already there..and the notion that the thin roof panel and supports are enough to increase the torsional stiffness by double just doesn't make any sense to me...especially when you see it decreasing only a 15-20% percent when hard top models are converted to convertibles.