# Lower wishbone stress calculation problem

1. Mar 23, 2005

### brown

Hi,
I was wondering if some one could help me with my stress calculation problem.

I have designed a front suspension assembly for a locost sports car on pro/e. I would like to validate my design by performing some bending moment calculations of the part and then comparing the stress found in a 3g bump situation to that of the yield stress.
I have performed some of the calculations so far but don't know if i am going in the right direction and what to do next. I'm working in SI units.

I have simplified the v shaped wishbone to one length of tubing.
length = 0.3429m

One end is connected to the chassis and rotates about the bracket.

The other is connected to the lower ball joint that connects to the wheel and upright assembly.

The suspension spring and damper bracket centre is located 0.08407m from the lower balljoint.

The force at the wheel end is the reaction force of half the axle wieght + 3g
weight of axle = 300kg /2 = 150kg x 3g = 4414.5N
In a 3g bump the wishbone will rotate up wards and the damper would compress right up to the bump stops and so effectively would be stationary. therefore if clockwise moments = anti clockwise then..
4414.5N x 0.3429 = A x 0.2588

( 0.2588 is 0.3429 - 0.08407)

therfore A (force at suspension bracket =
5849.03N

I found the second moment of area to be..
I = 2 x10-8 m2

and y = 0.0127m

so if sigma = My/I then i should be able to find sigma and see if is below the yeild stress of the steel

This is how far i got with the calculations

Is the force at the suspension bracket correct?
What is M ?
I wanted the calculations to be for two bars side by side so to get the corect stress do i change the second moment of area to the value for two bars or,
perform the calcs for one bar and then x 2?

2. Mar 23, 2005

### FredGarvin

Suspensions are a complicated thing to anylize because of all of the forces you have to account for. Plus, you're doing static calculations. If you really want to look at the
actual stresses, you're going to have to do a dynamic analysis.

For the simplified model, I would look at modeling the tube as a beam with a ball and socket at one end andthe other free with a rigid support at the point of shock absorber attachment. I would also look at the moment you are using. At the point the strut bottoms out, you are not going to be seeing the axle weight, you'll be seeing the inertial load from the mass of the vehicle itself, plus the load created by the strut compression. You're going to have to estimate the force distribution to the wheels and then you will have an idea as to what the moment about the strut attachment point is.

See if you can't get your model to someone with ANSYS.