How do you get it to go straight? Trial and error?
Hmm, I would suggest parallel axles, a rigid frame, and straight, equal sized wheels. If you have Electric Vehicle experience it helps enormously.
Yes on EV experience!. Three factors that affect steering, two of which are not so obvious. The obvious one is axles parallel when looking down from the top; if they're not, that puts a turn in. Its easy to make a jig for your side rails that sets the distance between the two axles exactly the same distance. Next is wheel diameter; if there is a difference, that will induce turning - toward the smaller diameter wheel. Again, pretty easy to make sure wheel diameter is the same (or in trial and error approach, to sligtly reduce diameter of the wheel its turning away from. Both these factors.....show up more if they're in the axle that's to the front. The front (which ever way you're going) has most of the steering effect, and the rear follows. I'll bet there is a significant difference in steering behavior going out and coming back.
The last factor comes from the fact that three points define a plane, and the vehicle has four wheels. Unless your axles are exactly parallel, looking at the vehicle end-on, one of the wheels will be somewhat up off the floor. It may not be apparent; may look like all four are in contact, but one will be more lightly loaded. Again, the steering effect this induces will mostly be seen when the axle w/ the lightly loaded wheel is at the front. There are a couple ways to.....factor this out, though dealing with this issue/effect is the hardest. The easiest is to set up a way that when you attach your chassis platform to your wheel side rails, both axles, with wheels in-place, are....there, and you attach (glue) the rails to the plate with all four wheels firmly on the ground. Of course, if you later adjust wheel diameter, it will throw things off. The harder approach is to put some kind of flexibility (i.e., suspension) in. One option - take a close look at the drive axle end of a radio controlled race car - is to have one where it can pivot- looking end-on, the ends of the axle can move up and down (EV from last year...). The other is to have one end of one rail flexible enough that the end can move up and down.... No details on this, though, till later in the season....
it can be done, and it does work
Getting it to run perfectly straight is, of course, not necessary - a little bit of curve in each direction, as long as you can stay in the 1.5m lane, and the behavior is consistent- you can adjust starting position along the start line to get back to the center point.