Deutsch

The little bits can make a big difference sometimes. Here BMW M has the perfect oppurtunity to do a M coupe CSL w/ DCT transmission and the new 4.0 V8 to also be found in the E9 series M3 variants, that will also be equipped w/ lightweight construction materials. Even if they did this w/ the 3.2 still intact it could be one hell of a car that will make up for all the non CSL cars flaws.

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Centurion

Cabrios are often heavier but you have to bare in mind that the Z4 isn't a cabrio. It is a raodster which was constucted to have a folding roof. Constructing a coupe version of it will mean adding a huge metal roof and making the frame larger.

Deutsch

+ more rigid to cope w/ the power coming from the engine. The M coupe also has several more extensive modifications over the roadster. I'm sure BMW w/ out a doubt could have constructed something even lighter, but the buisness case for a model utilizing all new resouces would be a huge waste of money, what we have here is the cheaper more production effiecent alternative.

Matt

Great sig Deutsch!

If the coupe is heavier than the roadster I dont see it pulling a better time on the track....but i could be wrong..

Harry Plopper

You are wrong.

The aerodynamics is better on a coupe (0.35 vs 0.38) + much better torsion stiffness (32,000Nm/o!!!). These are quite huge benefits.

10kg more is almost nothing. It represents less than 1% (0.7% actually) of Z4's total weight.

Z4 M Coupe outperformed both Z4 M roadster & E46 M3 Coupe on Nordschleife.

Matt

^well thats good...Do you think it can beat 1.26 flat on the Top Gear track?

Matt

Is the first one the drag coeficient difference?? I've seen this (32,000Nm/o!!!). before but I have no idea what it means or what to compare that with....help plz

Harry Plopper

Yes. Drag coef.
Coupe: 0.35
Roadster: 0.38

Torsion coefficient (Nm/deg) shows the torque required to twist a body through an angle of one degree. The higher amount of torque is needed to twist a body for one degree, the stiffer the body is.

E.g. Torsion coeficient of Z4 M Roadster is 14,500Nm/deg (while Z4 M Coupe's is 32,000Nm/deg).



Consider a car with a weak chasis (with low torsion coefficient) which is easy to flex and twist under force.
If using stiff springs and dampers, the shock caused by road irregularities will be transferred to the chassis directly. The weak chassis will be twisted and bent, thus the suspension geometry will be reshaped, creating non-neutral steer and other side effects that is not the original suspension design intended to cope with. Therefore a weak chassis must ride on softer springs and dampers.

For the benefit of handling stiff springs and dampers are needed. So therefore we need a rigid chassis which could cope with the stiff suspensions without flex or twist.

So stiffer chasis allows stiffer suspensions, and stiffer suspensions offer better handling.

Matt

Thx dude...That sounds like an extremely rigid chassis/body. So because the M coupe is so much stiffer than the roadster, does that mean that BMW made the suspension firmer and more track oriented than teh roadster because the structure can handle the force??

Harry Plopper

Yep. Let's say M roadster is a raw cruiser, while M coupe is a raw racer.

M6 Cabrio case is different:
For instance M engineers have reinforce the M6 cabrio chasis' rigidity with using a carbon fibre in crossframes: making the car almost as rigid as M6 coupe, but at the same time not adding too much weight. That will allow M6 Cabrio to be on par with a M6 Coupe.

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