mingster

My buddy looked at my X brace and asked me: from which direction does the stress come from? I hadn't quite thought of that. For those who have the X brace, you know that the X brace has a flat horizontal steel tubing. My buddy went on to ask me: do you think that the stress put on the chassis (may be in thousands of pounds of force during spirited driving) will be from the sides going up and down or from the stress will be from the front to rear? (see below)



What do you guys think?

Now, I assumed that the stress will be somewhat vertical from the sides, say turning left and right the weight shift will cause one side of the chassis to experience more stress, and if you have the x brace, this is what it looks like:



so if the horizontal plane of the x brace was thicker, do you think it'll be beefier?

this is purely a technical question, physicists please help too

S2kRob

No the stress is from the sides. That cross brace would flex pretty easily if it had to withstand vertical forces. While I'm not totally up to speed on the X-brace and where exactly it attaches, this is what I do know.

An object in motion wants to stay in motion. Around a corner, you are accelerating the car in a new direction. Therefore force is needed. This force is transferred to the ground by the tires. The biggest cornering load is on the outside tires. So, most of the force required to turn the car is being generated by your outside front tire. The force is acting at the bottom of the tire, but the tire is attached to the car (via the wheel) in the center. This creates a torque. Large torques will actually very slightly cause the body to flex where the suspension mounting points occur. This is one reason why chassis rigidity is so important. You want to minimize this flex in order for the suspension to maintain its designed travel path. Where the cross brace comes in is that it ties the bottom suspension mounting points to the other a-arm mounting points, thereby boxing in the entire structure and making it much more resistant to flexing caused by turning. The forces transferred through the cross brace are primarily compression forces.

cmnsnse

Now this is the stuff I like

Ill go straight to my drafting board and draw some stuff up to help illustrate what happens . . .

Basically when cornering the force is presented up and aft from the front sus. A bit inwards too, the x brace helps reduce that inward movement.

What a question! now we need animated CAD . . .

mingster

so you guys are saying that the stress and pressure is from this (see the circle)???



and not applied from the sides in a verticle direction?

cdelena

The suspension should handle most of the forces up and down. The braces are there to make sure the platform the suspension is mounted to does not give side to side. I think your arrows are not accurate.

cmnsnse

heh its really quite complicated but the vertical stress is converted (the bend points act kinda like a fulcrum) to side stress at the top and bottom of the shock towers and inbetween.

mingster

all the arrows? or the up and down ones?

thanks!

mingster

can you illustrate please? i'm just curious to see if a beefier x brace would do anything

S2kRob

Highly unlikely a beefier crossbrace would do anything. Metal is very strong when it comes to compressive forces - forces that are trying to crunch the tube like an accordian. So, a thicker cross section metal than the Spoon system will just add weight with negligible effects.

josh3io

i think cmnsnse has the right point, but didn't paint the full picture. the forces on the chassis aren't quite side-side, but not vertical either. it's more of a twisting force since the front end experiences different vertical travel than the rear (fig 1). having the x-brace reduces the amount the frame can twist therefore causing reduction in vertical motion at the suspension (fig 2).

i studied EE, not ME so that's just what i believe from my 4 years of physics. can anyone agree with me?

i think to make it more effective you would need more front-back support on the X as circled in fig. 3.