This week, we got around to doing the calculations for Ackerman steering, which cemented down rough dimensions for the frame. Then, we could start CADing our kart.
Steering Calculations + Finalizing Frame dimension:
This link is great: http://www.idsc.ethz.ch/Courses/vehicle_dynamics_and_design/11_0_0_Steering_Theroy.pdf
With Ackerman steering, when the steering wheel is turned, trapezoidal linkages ensure that each wheel turns a different angle. Like so:
So, if we wanted a low R, which determines how sharply our vehicle can turn (!), our front wheels would have to turn through large angles d_o and d_i. We didn’t want to have crazy large angles and decided on a 45 degree max turning angle (for deltas). Measuring the L, R, and t of the example go-karts from previous years lying in the IDC lab, we got ranges of L: [30-50 in]; R[48-72 in]; t[18-32 in]. We wanted to keep a pretty high t, to be stable, so we settled on values of L = 30 in; R = 48 in; t = 32 in. Those are the dimensions that we designed our kart around! We also started (finally!) CADing our frame, with these dimensions.
We also proceeded to calculate all the ackerman angles that would fit the length/width of our kart + our optimal turning Radius R (lowered to get sharper turns). I CADed all the pieces in Solidworks and played with it to ensure that the wheels turned around different radii. It looks pretty cool!
Physical HW: We scaled down our linkages and lasercut them out! Here are some pictures!
Steering Components: We also looked at the components that would make up steering. A brief lecture in lab informed us that people made their steering blocks in two different ways that basically differed by the placement of the bearings. One method required the screw to move along with the wheel. The other method (on the right in the diagram below) had the steering plate and blocks rotating around the screw. Picture from Charles’ powerpoint below.
Looking at both of these designs, we weren’t sure which plan was better, so we spent time on Saturday in lab looking at the various steering geometries of the other karts. We decided that we did want to use ball bearings ($2.50 each from Charles, $10 on Mcmaster o.O) to have our turning be very small. I also started to design and dimension a steering assembly, based on the bearings in upright, but we are going to ask Charles about the drawbacks/benefits of each plan, if there are any.
Pictures of our CAD:
I modeled a person sitting in the go-kart with approximate dimensions of someone 5’3″ (that’s me). As you can see…our kart is pretty short. Our feet will be hanging off the end, and our COG will be near the front.