Weight vs. Torsional Rigidity

[Kevlar]

Now... we all know that cars are always getting bigger and heavier. Weight comes from not just adding gizmos and gadgets, but from also adding torsional rigidity to help reduce flex under load.

Looking at the way cars have grown, the E30 M3 to E36 M3 to E46 M3 and now E92 M3, weight has always gone up but so has performance. I'm not sure how this compares to other car manufacturers ... i.e. Porsche and its 911 over the years.

[Threxx]

Well, it has been a while since I got in depth on this topic but if I recall correctly - just paraphrasing from a couple of auto engineer, if you take a given structure and reduce its weight by half, its effective resonant frequency will double - such as if a structure would respond to a load at a rate of 20Hz normally - cut its weight in half and it will respond at 40Hz, which is to say the wave of response through the chassis will have a shorter 'wave length' (and height) and thus the whole platform will recover more quickly and with less of a noticeable 'unsettled' affect communicated to the occupants.

The catch 22 is that often times to increase bending and torsional rigidity, as you noticed, weight must be added - but the very weight that is added works against the effective frequency (performance) of the entire structure. Kinda like why often times, especially in past decades with fewer lightweight materials available for use in engine blocks and top ends, you could actually make a car with a motor so big and heavy that it couldn't out accelerate its own mass compared to a smaller and lighter weight motor.

Say if you add another 100 pound of weight to a structure to add additional sub frame connections in a unibody structure or possibly additional welded through cross members in a body on frame structure... you might also have to increase the size and weight of the rotors and calipers to stop, the transmission parts (clutch packs, etc) to accelerate, etc, etc... all of the parts have to get a little bigger and beefier to support that extra 100 pounds of weight in the frame/structure if they want to maintain the same performance and durability/reliability. When it's all said and done that extra 100 pounds may have compounded into an extra 250 pounds (which again, hurts the bending and torsional performance of the structure)... or else a drop in performance elsewhere.

It's a really interesting scenario with a lot of circular logic. The only real answer I think is continued development of new materials that allow improved performance with less weight.

I think I heard BMW uses a substantial amount of aluminum in the subframe design, which maintains performance with lighter weight, but has the drawback of very expensive and difficult repairs in the event of a frame damaging impact.

[RNeil]

Obviously, weight hurts performance. An Indy 500 car weighs much less than a 318.