Wednesday, February 26, 2014

Load Paths

Today's article starts with this picture from a Baja competition but still quite valid even for Formula Student vehicles.

This is a regular double wish-bone front suspension with direct actuation, and there are a few things you can spot here that one can do better. The most striking thing however is the lower mount of the suspension spring on the lower control arm. The spring bears the weight of the vehicle, and also bears greater loads during load transfers associated when driving the vehicle. Since this is a Baja vehicle, shock loads associated with large bumps also would be seen by the suspension spring. These loads are first seen at the wheels and are transferred to the chassis via a path. Note that this path is known as the 'load path' - it is really a visualization of the stress flow. The load is seen at the contact patch, which loads the tyre > the tyre loads the wheel > hub > wheel bearing > upright > lower control arm > spring. Now for this suspension to survive this particular loading, it must be considered while designing all these sub systems. 

Let's look at the lower control arm in the front view. This control arm is really supported by the ball joint at one end and the chassis pick up points at the other end. The spring loads this control arm right at the center, which as you can probably visualize, causes some (or may be a lot of) bending. [Thought experiment] Give a thin meter long bamboo stick to a 7 year old kid (note she does not have an engineering degree). Ask her to break it, any which way she likes. What is she going to do? She is probably going to hold the 2 ends with her hands and try break it with her legs. Why did she not break it by pulling at its 2 ends? Why did she not compress it until it breaks?

Her intuition told her that the stick is most likely going to break in the first way. An engineer would call it bending load. There are probably 10 equations I can write here to prove this using a Solid Mechanics Text book, or a short simulation on a Finite Element analysis to prove this to you. But I am going to assume here that your intuition is in agreement with me.

Moreover, the bamboo will only break easily if the ends are held and the load is applied at the center. If the load were applied near the either ends, significantly more force will be required to break or even deform the bamboo. So if you want to break the bamboo apply loads to the center and in bending, if you want it not to break or even flex, then apply it near its ends.

Steel rods are not much different from bamboo [citation needed]. In a Baja car we do not need things to break or flex, specially the important suspension components. The lower control arm in this picture is a steel road in heavy bending load. It is going to flex a lot if not break. In fact this front suspension has 3 significant springs in series. 1) The Tyre 2) The Lower Control Arm 3) The suspension spring. I am willing to bet - the suspension design did not consider this extra spring and compliance in this calculation.

You should not therefore be surprised to see this picture of the same suspension after the car hit the first bump at the endurance event.

This blog post is originally written for the Formula Student India website and has been cross posted from here.