Push rod placement

[pro.nub]

Hey guys,

For our FSAE car we have mounted the front pushrods to the upper control arm and have the shocks lying horizontally across the top of the chassis. The reason for the pushrods attached to the upper instead of the lower control arm is that we were originally going to use pullrods for the front but due to constraints with room, the shocks wouldn't fit underneath the car, so we kept the brackets for the pullrods attached and simply flipped the A-arms and put them on the opposite side of the car so that the brackets were sitting on the top of the upper control arm rather than the bottom of the upper.

While I realize that normally the pushrod is connected to the lower control arm, I am curious as to why and also from a design point of view, does it matter that it is connected to the upper control arm rather than the lower? The pushrod is approx. half the length than if it was connected to the lower control arm, meaning less chance of buckling.

Sorry if this has been asked before but I couldn't find anything on it.
Any help is greatly appreciated.

[pro.nub]

Hey guys,

For our FSAE car we have mounted the front pushrods to the upper control arm and have the shocks lying horizontally across the top of the chassis. The reason for the pushrods attached to the upper instead of the lower control arm is that we were originally going to use pullrods for the front but due to constraints with room, the shocks wouldn't fit underneath the car, so we kept the brackets for the pullrods attached and simply flipped the A-arms and put them on the opposite side of the car so that the brackets were sitting on the top of the upper control arm rather than the bottom of the upper.

While I realize that normally the pushrod is connected to the lower control arm, I am curious as to why and also from a design point of view, does it matter that it is connected to the upper control arm rather than the lower? The pushrod is approx. half the length than if it was connected to the lower control arm, meaning less chance of buckling.

Sorry if this has been asked before but I couldn't find anything on it.
Any help is greatly appreciated.

[ed_pratt]

pro.nub

yes there will be less chance of buckling the rod, but think about the efficiency of transmission of the forces from the unsprung mass into the shock.
If the included angle between pushrod and wishbone is reduced, what happens to your motion ratio? what are the forces in the bracket attaching your pushrod like?

Ed

[Bobby Doyle]

Draw a diagram of your suspension and do some simple statics. If you apply a load at the wheel, how does changing the position and angle of the pushrod affect the required pushrod force for equilibrium? What does this do to the reaction forces in your a-arms? http://fsae.com/groupee_common/emoticons/icon_frown.gif http://fsae.com/groupee_common/emoticons/icon_eek.gif

[TorqueWrench]

I got all excited from the title and thought someone was talking about pushrod engines...

[-francisco-]

Ed is right. if you changed the angle of the push rod you're gonna alter your motion ratio, the leverage you're applying on the shock and might have to redesign the rocker. The more horizontal (in relation to the A-arm) you put the push rod the more leverage you'll be applying, the softer the shock will be and you get more wheel travel.

[dazz]

Depending on the geometry, (substanial camber gain), the angle of the pushrod relative to the upper a-arm may in fact be pretty close to the angle made if it was connected to the lower a-arm. Like has already been mentioned, a couple of simple 2-d free body diagrams and a bit of year 7 level maths and you'll have your answers. (the bit that's not year 7 level is the application of the math, although most of the time it should be pretty obvious)
Note that with shorter links the motion ratio may change more as the suspension moves through its travel than it would with longer links. (ie the angles change, does this make your suspension rising rate, or falling rate, or a bit of both?) plot wheel travel 'vs' shock stroke. smiley face = rising rate. frowny face = falling rate. (shock stroke on Y-axix, wheel travel on X-axis)