Based on the answers so far, I see consensus around a few points:a. Move center of mass further back to effectively increase the height of the incline (Thanks Mark Eichenlaub). Shorter wheelbase also helps with this.b. Use smooth, larger wheels to reduce friction on axles (less axle rotations) and track (knobby wheels suck energy into other directions). Larger wheels might also reduce loss of energy from the transition from the slope to the plane by smoothing the transition. (Thanks John Morris and Jingyee Chee)c. Keep braces perfectly parallel to reduce axle friction from misaligned braces (Thanks David Eaton)What other recommendations do you have?One of the major tenets in physics is that, ignoring air resistance and other similar factors, all objects fall at the same rate. See the video below to see a feather and a ball bearing fall at the same speed.Therefore, if friction, air resistance, and any other factors other than gravity were absent, the vehicles would fall at very nearly the same rate (there are some wheel rotation issues which I'll mention a bit further down).
However, there are several other factors present. The first is air resistance. None of those vehicles are significantly more streamlined than the others, and the difference in air resistance at the speeds they will achieve is likely minimal, at best. However, if one is fighting a lot of drag, it will be slower.Another factor is friction along the wheel's axle. Again, for this situation, the difference between the vehicles is likely negligible since they are all similar in design. If axle friction is a major concern, then the weight of the car and the speed of revolution will make a difference. Most likely, a lighter car with slower axle revolutions (bigger wheels have slower revolutions to cover the same distance) would have less friction, though there are a number of ways that can be affected.And yet another, and probably the largest factor is energy lost from the shape and texture of those wheels. The big wheels have a rough surface compared to the smaller wheel. While rolling, those big wheels probably generate more noise and vibrations, and may suffer from some slipping along the track.
The noise, vibrations, and slipping all require energy that would otherwise go to the car's speed.And lastly, the different wheel sizes have an effect on speed due to the differences in there rotational properties (e.g. moments of inertia) and how those translate to forward motion. I was going to talk about this in more detail, but Jingyee Chee already did in another answer, better than I would have.Overall, I would guess that at the speeds your dealing with, when combined with the largely similar designs of the cars, they are all nearly identical in terms of speed and time to finish. If I were to make my own car aiming for the highest speed, I would probably use the smaller, smoother wheels to negate energy loss from the big wheel's texture and to better translate the wheel rotation to linear motion. I would use a flat bar for the body (like the lower 2) to limit air resistance as much as possible. And I would use as few parts as possible to keep the car light and reduce axle friction.
It's tough to be more specific than that from the picture alone. There are a lot of factors at play, and they all mix together to make a fairly complex scenario. In the absence of specific measurements and data, the best bet is to experiment as you are doing.The newest addition to the grand list of Coolest Things Ever is being unveiled in New York today: the Lego X-Wing, the largest Lego model ever built. The model of the classic Star Wars fighter being unveiled in Times Square has a wingspan of 44 feet and comes complete with R2-D2 and a full range of sound effects. It’s a super-duper-sized version of Star Wars Lego starfighter set #9493 and was made with 5,335,200 Lego bricks. That, according to Lego, makes it the largest model ever built, eclipsing the Lego robot at the Mall of America by some 2 million bricks. This replica of the Rebel Alliance dogfighter is 42 times the size of the Lego version we’ve all built and a bit bigger than a real X-Wing. (Yes, yes, we know they’re not real.
Just go with it.) The X-Wing Luke Skywalker and his fellow rebels flew was about 41 feet long, 2 feet shorter than this Lego masterpiece. The X-Wing was built at the Lego Model Shop at the company’s facility in Kladno, Czech Republic. It took 32 “master builders” (Note: This is a real job, and we’re preparing our resumés.) 17,336 man-hours to construct the X-Wing. Plans for the model were created using Lego’s proprietary 3-D design software, and the construction team had to work with a team of structural engineers to ensure that the model was safe, master builder Erik Varszegi told Wired. “This has been a wild and exciting project for us, and it’s taken an international team of designers, engineers, structural consultants, model builders, and logistics personal over a year to bring this model from a conception to reality,” Varszegi said in an email. “In one respect, designing it was the ‘easy’ part, as we were creating a scaled version of an actual toy construction set.”
Once completed, the model — which weighs 45,980 pounds — was broken down into 34 pieces to be shipped to New York by boat (the voyage took two boats two months to complete). Lest the West Coast feel left out, the starfighter is coming to Legoland California after its tenure in Times Square and will remain there until the end of the year. Twenty-three tons is a whole lot of anything, especially Lego bricks, but much of the model’s weight lies in the model’s steel skeleton, which it needed in order to withstand the vibrations from the subway that runs beneath Duffy Square in Manhattan and any seismic activity it could face in Southern California. While a slightly larger-than-lifesize Lego X-Wing would be awesome even if were made for no particular reason at all, the model was built to celebrate the forthcoming premiere of the latest Lego Star Wars series The Yoda Chronicles — a three-part animated miniseries launching on Cartoon Network next Wednesday. That’s the official reason this was built, but we wouldn’t be surprised if Lego made this thing just for the thrill.