Now I want to look at our expectations for the driver. We need our system to enable drivers to perform at their best, and steering effort is important when assessing the comfort of our design. For this analysis, I will disregard friction due to bearings and bushings. In essence, I'll look at tire forces (use these to find forces in the tie rods) and aligning moment.
The max force in our tie rods is current 200 lbf (done during suspension analyis), and I'm confident that the force
contribution from aligning moment has a larger impact on steering effort. I'll start with this assumption and come
back if my process seems to show otherwise.
Now, I don't currently have aligning moment fits, but I have the following data from the FSAE TTC:
My current plan is to find a normal load I want to analyze and use the discrete FZ slice that works best for that target FZ. To select this FZ, I want to get the combination of FZ and SA that has the highest possible aligning moment (this will give a worst case steering effort). To do this I've taken the GGV sweep from earlier and plotted FZ vs SA for the front tires. I've included this plot to the right:
When looking at the MZ vs SA plots from the TTC, I notice a couple of details. First, all of the peaks occur well within 0 deg and 10 deg. Second, peak MZ shifts outward as MZ increases. Third, the peaks for positive and negative SA aren't the same (to be safe, I'm going to take all MZ magnitudes at the greater peak). Now I'm going to sweep the FZ vs SA plot with these constraints in mind. I'll take the maximum total FZ that occurs on 0 ≤ SA ≤ 10 deg. I've included this state to the right:
Now I need to approximate the MZ for the conditions above. It looks like FZ will shift to around 100 Nm as we approach 1500 N. I can include a rigorous analysis of this later, but for now this approximation seems reasonable. Keep in mind that as FZ increases, we also shift the peaks outward. Therefore, I'll approximate both aligning moments as 80 Nm or 700 lbf-in. Now I need the corresponding suspension member forces for this state. I've solved for these forces and included them to the right (note that I only need the tie rod forces for this analysis):
Now I have everything for the steering effort calculations. Here's the process I plan to use:
1. Calculate force due to the aligning moment (divide MZ by distance to tie rod mount)
2. Subtract force from the tie rods (axial force on the tie rods will oppose the direction of the moment)
3. Sum forces from the left and right sides of the vehicle
4. Convert force on the rack to moment on the column (Force * (C-factor / 2π))
These calculations give me a steering effort of 14.7 lbf-ft at the steering wheel, or a force of 17.6 lbf on both sides of the steering wheel. Note that this calculation is done without tire scaling. Scaling will drop the steering effort below 10 lbf-ft and the required force below 10 lbf, but I want a worst case scenario for force calculations, so I'll use the 14.7 lbf-ft value for now.