Audiablo

Okay, so we know a little more about automotive steel strength - but it didn't really answer my question: how would improving the frame rigidity soak up force such that the driver doesn't experience it?

I saw an old pickup race a course full of big bumps and deep ditches, and the frame, instead of passing the full force of these massive jolts to the driver, literally twisted out of shape - permanently. I can't help but think that if that frame was TWICE as strong, the driver would have experienced jolts TWICE has violent.

No one here, to my knowedge, has ever mentioned replacing mounts to reduce jarring - and I don't even know if you want stiffer mounts or softer ones...

audiconvert

Yep.

captainbilly

Here is something that we can sink our teeth into.

Finman

Most everything, including the steel or aluminum making up the frame of a car, acts as a spring when a force is acted upon it. Stiffness is the "force constant" of this spring, or a measure of it's resistance to compression or elongation. Something that is very stiff, transmits its force from the point it was acted upon to it's other end to the material that end is connected to. Obviously, aluminum and steel are both quite stiff when acted on directly at one end or the other of a straight rod, but are quite different when there is a bend in the material or the force is transmitted at an angle. Weight of the material vs a different material does not matter. Weight, structural shape, and other variables of the same material do matter in how stiff something is.
Torsional rigidity is simply the resistance of a material to twist vs. bending or compression. Weight, structural form/shape, material, etc affect torsional rigidity. Same materials of different shapes can have different torsional rigidity.
The joints of the frame are probably the most important regarding the perceived stiffness of a frame and the material, shape, density, and configuration of the frame is important in the torsional rigidity of the frame.
The goal of the frame, for the most part, is not to absorb forces (compress) or twist (torsion). That is the function of the suspension for the most part, and bushings and other easily (relatively) compressible or elongatable materials. The suspension ideally, should absorb all impacts, transmitting none to the "stiff" frame of the car (perfect ratio of spring stiffness and shock resistance), but also resist bouncing around or overcompensating for the impact(soft springs and non-resistant struts/shocks). As spring and shock stiffness increase, transmission to the frame increases which then tests the frames stiffness and torsional rigidity. If the frame is stiffer, it will transmit more of the force to the passengers. If it's torsional rigidity is increased, the occupants will notice less twisting sensation, but this force has got to be absorbed somewhere, ie. through the suspension, or to any place that can absorb it (soft, compliant joints of the car). For example, if someone exerts a force against a solid wall, the wall does not "absorb" that force, it transmits it--mostly back to the object that exerted the force (eg. through ball against a wall) and very slightly in compression of the wall.
A frame could be quite stiff, yet have poor torsional rigidity, or vice versa.
A bad or weak suspension will transmit greater forces to the frame esp when turning when the mass of the car is exerting force in the original direction, and may overcome the torsional stiffness, thus we sense twisting.
Basically, stiff, non-compliant suspension + stiff frame means occupant absorbs more impact.
Stiff, non-compliant suspension with weak, flexy frame, occupant notices frame flex.
Too soft, compliant suspension + stiff frame means frame bounces around too much.
Soft, compliant suspension + weak frame means occupants bounce around and when really get jolted notice flexy frame also.
Torsional rigidity comes into play mostly when the force change dirction (turns) when the frame is twisted. How much of this lateral force the suspension can handle determines how much twisting force is exerted on the frame and if it is too much the frame twists and the occupants notice.
This is much easier with pictures.
I doubt anyone made it this far, since this was quite a rambling. I need to find a physicist to explain this better.
Sorry

Finman

Screw what I said, as audiconvert says, if you can figure it out...it has been a while

Alec

I do know this:
Too soft, compliant suspension + stiff frame means frame bounces...
= 2.8.

It bounces like a speedboat if the road condition is right.

Audiablo

I think I get the fundamentals now with this issue.

<i>Everything bends. Steel, carbon, ceramic, everything. Some things just don't bend very much. </i>

So, I imagine if you have 3 foot long rods half an inch thick, one of steel and one of ceramic, you could compare them by trying to bend them. Support them between two chairs and add weight 5lbs at a time at the middle. As you add weight, the ceramic one will flex less than the steel one. But as you add weight, the ceramic one, while having flexed less, will suddenly snap where the steel one, being a lot stronger overall, can handle way more weight but continue to flex. Am I on the right track?

<i>A pickup truck, for example, has a ladder type frame. It's great for hauling lots of **** becuase it can flex and thus not break.</i>

Hmmm... I always wondered why they never attach the box to the cab... now I know! I recall an ad for a spiffy silver pickup going in slow-mo in rain with strobes flashing as it eats up big bumps in a curve, and you can see the box moving in relation to the cab...

Okay enough physics for one day - does anyone know how to calculate the angle in degrees between three points? I did great in trig but that was like 15 years ago!

Brian-PA

My tibia, femur, and hip have frame strength.My spine/joints have torsional rigidity.I've lost it !

eduardo

Let a, b, and c denote the lengths of the sides of a triangle. Let A, B, and C denote the angles respectively opposite to a.b and c. Then

c^2 = a^2 + b^2 - 2*a*b*cos(C);

^ stands for exponentiation; a^2 = "a squared"
* stands for multiplication

If you know the sides of the triangle, then you can compute any angle with

C = acos((a^2 + b^2 -c^2)/(2*a*b))

Bollinger

Most manufacturers don't use hydroforming at all, and of those that do, they only hydroform a few parts.

Hydroforming is slow, it it isn't used everywhere. It is basically used in two cases. One case is when you want a tube shape structure for rigidity. This is how the backbone of the Corvette is formed. It starts as a tube and becomes a squarish tube. The second case is where you don't need a tube, but putting in a tube can remove many many welds. These tubes typically much smaller. In a cradle on a certain GM car, they replaced three stamped and braced rails with a single hydroformed tube. So, instead of like 22 welds to connect 4 rails together, it takes more like 4 welds to connect a single rail across the top of a U-bent hydroformed tubs. This makes the part slightly lighter, somewhat stiffer, and much less likely to break or squeak since it has fewer welds.

The industry is still finding new uses for hydroforming, but basically no one has used it except in floor-pan level structures, so the number of hydroformed parts is still very low.